Bipedal Animals

The term bipedal comes from Latin and loosely translates to “two feet.” Bipedalism is a form of locomotion for organisms that live on land and can be in the form of running, walking and/or hopping. Bipedalism can also describe animals that stand on two feet but do not necessarily walk or run that way. Some animals walk on two feet all the time while other animals are quadrupeds and do it only intermittently. Bipedalism offers several advantages to species. For example, in humans, walking upright raises the head which gives a better view for seeing dangers and approaching predators.

Evolutionary theories about how and when animals became bipedal are abundant, with at least 12 ideas currently being studied. One theory explains that humans may have developed bipedalism in order to carry food to share with other group members.

Amphibians

There is no evidence that any amphibians, either modern day or in the fossil record, are or ever have been bipedal.

Reptiles

Lizards often adopt a bipedal stance for running away from predators. One example of this the spiny-tailed iguana which is known as the world’s fastest lizard. The fossil record shows that the first bipedal animal on Earth was a reptile known as Eudibamus which lived about 290 million years ago.

The image above shows the first known bipedal animal Eudibamus which lived about 290 million years ago.

Birds

All birds display bipedalism. This makes sense because of their evolutionary relationship to the dinosaurs who were mostly bipedal.

Mammals

Nearly all primates are capable of bipedalism, although most spend the majority of their time on all fours. Primates move bipedally but they also use bipedalism to stand up on their hind legs to reach food and look for predators. Some examples are baboons, bonobos, chimpanzees and gibbons. Other mammals such as beavers, raccoons, mice and rats squat on their back legs while eating, and raccoons and beavers walk bipedally when carrying things. Other bipedal mammals are rabbits, bears, meerkats and ground squirrels. Kangaroos and wallabies are examples of bipedal marsupials.

References

• Bipedalism. (n.d.). In Wikipedia. Retrieved July 19, 2017 from https://en.wikipedia.org/wiki/Bipedalism

Bipedal Animals

Tundra Biotic Factors

The tundra is a biome that does not have many trees because of short growing seasons and low temperatures. The three types of tundra on the Earth are the Arctic tundra, alpine tundra and Antarctic tundra. There are a variety of biotic factors that are characteristic of each type of tundra. Various bacteria and fungi are important biotic factors that are common in all tundra biomes.

Arctic Tundra

Polar bears are one of the most recognizable animals on the Arctic tundra. Other animals include the Arctic fox, the Arctic wolf, reindeer, musk ox, seals, the Arctic hare, the snowy owl and lemmings that live underneath the snow. Peat soils, cotton grass, rushes and mosses like Sphagnum are typical of this area as are willow trees and sunflowers. There are some species of Arctic insects like black flies and mosquitos as well as spiders that have evolved and adapted to the freezing conditions.

The image above shows a mother polar bear Ursus maritimus with her offspring. Polar bears live in the Arctic tundra within the Arctic Circle.

Alpine Tundra

Resident mammals in the alpine tundra include mountain sheep, mountain goats, ibex, chamois, wildcats, marmots, ground squirrels, jumping mice, pikas, rabbits and birds like the snow goose, the ptarmigan and owls. The alpine tundra has few, if any, trees because the altitude and soils do not support their growth. The few spruce, fir, pine and willow that live in the transitional area (about 330 feet higher than the lower forest areas) are stunted in their growth and scattered around. Other plants in the alpine tundra are cotton grass, lousewort and various shrubs. Alpine insects include black flies and mosquitos.

The image above shows the alpine tundra area of Glarus, Switzerland.

Antarctic Tundra

Seals are one of the few mammals that live on the Antarctic tundra due to the physical isolation of the area that has occurred over millions of years. Adélie and Emperor penguins and seabirds like petrels, terns and gulls populate the area. Survival is challenging for plants on the Antarctic tundra, but there are close to 400 species of lichens, 25 liverworts, 100 mosses, about 700 species of algae and two species of flowering plants (Antarctic hair grass and Antarctic pearlwort) that have adapted to live in the rockier areas of the biome. The Antarctic midge is the only insect that lives on the continent.

The image above shows Emperor Penguins Aptenodytes forsteri on the Antarctic tundra.

References

• Tundra. (n.d.). In Wikipedia. Retrieved July 11, 2017 from https://en.wikipedia.org/wiki/Tundra


• Tundra. (2017, July 11). In Encyclopedia Britannica online. Retrieved from https://www.britannica.com/science/tundra

Tundra Biotic Factors

Head

Head Definition

The head is the part of most animals, located at one end of the body, which contains the highest density of neurons, and often other sense organs. In humans, the head contains the brain, which is encased in a protective skull, and the eyes, ears, nose, and mouth.

The image below shows the head of llama. The eyes, ears, nose, and mouth are clear. The brain is found within the skull, which is between the llama’s ears.

“You lookin’ at Me!?”

The head is thought to have evolved through a process called “cephalization.” This term derives from “kephalo” for “head.” It is thought that the clustering of nerves close together enables faster and more complex processing of information.

This gives organisms advantage by allowing them to respond faster and more intelligently to their environments. As a result, over time neurons tend to become clustered close to one end of an organism.

Having sense organs near the animal’s nerve center also improves reaction time by allowing impulses from these sense organs to reach the brain almost instantaneously after perception.

Vertebrates like humans, invertebrates such as insects, and complex members of the animal group called “invertebrate chordates” which have neither spinal cords nor exoskeletons, all tend to have heads where a large cluster of neurons is bordered by sense organs.

Function of the Head

The head typically serves as the information processing center of the animal. Information from the sense organs and the body is relayed to the brain, which is the nerve center found in the heads of complex life forms.

Even very simple organisms often have a point where their nerves concentrate, along with any cells they may have such as photoreceptors.

The brain can process information about the environment to make decisions, store memories and skills, and make changes to the animal’s behavior and emotional states.

The head also often contains the primary organs for vision, hearing, smell, taste, and in some organisms, antennae.

Examples of Heads

Humans

The human head contains the brain, eyes, ears, nose, and mouth. The front of the head, which is hairless and contains the eyes, nose, and mouth, is referred to as the face.

The human brain is tuned to recognize small differences in the bone structure of the human head, allowing humans to recognize other individuals based on their facial features.

Bees

Like humans, the head of a bee contains two eyes and a mouth that’s used for tasting and ingesting food. However, the bee also has antennae, which are sensory organs that humans don’t possess.

Antennae are very sensitive to vibration, and also to pheromones, which are chemical signals put out by other insects.

This is an example showing that even when organisms do not have the same sensory organs, they still tend to cluster their sense organs near the brain for efficient processing.

Hydras

Hydras are very simple aquatic organisms that have neither skeletons nor exoskeletons. Hydras are thought to be a very old species, having first appeared long before the evolution of the body structure seen in most animals.

But even hydras have something that could be called a head; on one end of their bodies, a cluster of nerves and light-sensitive cells serve as primitive “eyes” and information processing center.

Quiz

1. Why do most animals have a head?
A. Because having a central nerve cluster allows them to process information faster, and make more complex calculations.
B. Because having sense organs located close to the brain allows for faster reaction time.
C. Because organisms that can process information faster have a survival advantage.
D. All of the above.

Answer to Question #1

D is correct. All of the above are advantages to having a head which concentrates neurons and sense organs!

2. Which of the following is NOT part of the human head?
A. Brain
B. Eyes
C. Ears
D. None of the above

Answer to Question #2

D is correct. All of the above are part of the human head!

3. Which organisms would you NOT expect to have a head?
A. Insects
B. Vertebrates
C. Simple aquatic organisms
D. None of the above.

Answer to Question #3

D is correct. Most members of all of the above classes of organisms have body parts that qualify as a head.

References

• Cobb, J. B. (2009). Back to Darwin: a richer account of evolution. Cambridge: International Society for Science and Religion.


• Wake, M. H. (1979). Hyman’s Comparative Vertebrate Anatomy (3rd ed.). Chicago and London: The University of Chicago Press.

Head

Brain

Brain Definition

The brain is an organ that coordinates nervous system function in vertebrate and most invertebrate animals. The brain is typically located inside the head, within a protective covering such as an exoskeleton or skull.

In humans, the brain weighs about three pounds and consumes a stunning 20-25% of all the body’s energy!

The brain is primarily made up of neurons, which send nerve impulses and store information, and various support cells which nurture, insulate, and protect neurons so they can do their jobs reliably.

Neurons are an extremely high-maintenance cell-type, requiring large amounts of oxygen and fuel to keep them alive. In order to produce action potentials quickly, allowing for thought, movement, and other survival functions, neurons create an ion gradient that must be actively maintained at all times.

If the brain is deprived of oxygen or cellular fuel such as glucose, the ion pumps which maintain this ion gradient will shut down. This will lead to an influx of ions and fluid into the cells, which will actually cause the neurons to burst open.

This is why the brain is so vulnerable to oxygen deprivation. For a human, brains cells can begin to burst and die within minutes of oxygen deprivation.

Because it is so vital to the body’s functioning, brain tissue is separated from the blood stream by a “blood-brain barrier” through which only certain substances can pass. The blood-brain barrier filters out bacteria, some viruses, and some chemicals while allowing nutrients and oxygen to reach the brain tissues.

Brain Functions

The brain is involved with virtually every aspect of our experience. It is the fascinating arena where human experience meets biology.

Our experiences and feelings are processed, stored, and sometimes created by physical and chemical processes in the brain. Our thoughts and feelings can be measured as action potentials; our memories and personalities have physical form as synapses, which are branches that connect nerve cells to each other and determine how they interact.

Our brain perceives colors, sounds, and sensations. It perceives and creates emotional states. It contains our motor skills and our language center. It also releases hormones that regulate unconscious functions of our bodies. A part of the brain called the brain stem even sends nerve impulses which control and maintain our breathing!

Some general categories of functions of the brain include:

• Receiving and processing sensory information


• Directing movement through nerve impulses


• Directs breathing through the brain stem


• Helping to maintain the body’s homeostasis


• Helping to direct the body’s reproductive cycle


• Forming and storing memories


• Storing skills and conceptual information


• Creating, processing, and regulating emotional states


• It is thought that one particular part of the brain may create consciousness by unifying all of these functions into a single matrix. This is a relatively new finding, and needs more study.

Because the brain’s functions are many, it’s easiest to discuss them in more detail by looking at the function of each of the many structures in the brain.

Structure of Brain

The brain is a complex and intricate organ, with numerous functions. The easiest way to list those functions and discuss how they are accomplished is by studying the structure of the brain, which is the “machine” that accomplishes all of these functions.

Neurons

To understand the function of the brain, it helps to understand the structure of neurons. Just like other cells, neurons have a nucleus, cytoplasm, and a cell membrane. But unlike other cell types, neurons also have long, long arms called “axons,” and they constantly create and destroy tiny intercellular connections called “dendrites.”

The structure of a neuron can be seen here:

“Dendrites” can be thought of as the neuron’s receivers. They receive input from other nerve cells, to be processed in the cell body. This input can be received either in the form of a nerve impulse – that is, a direct electrochemical impulse – or in the form of neurotransmitters, chemical messengers which interact with receptors on the receiving cell. Neurons can “decide” whether or not to fire an action potential in response to an action potential or neurotransmitters received from another cell.

The means by which dendritic inputs are processed by the cell are not well understood. Some nerve cells appear to use simple summation – that is, the amount of stimulating and inhibiting input is added up by the cell when deciding not to fire. But other cells may fire different patterns of action potentials depending on which dendrites are stimulated, suggesting that there may be additional internal processing.

The “axon” of the neuron is the part of the neuron that fires an action potential of its own, if the cell “decides” to do so. Axons can be long, spanning the whole length of the brain or even the length of an arm or leg! They are insulated by a special layer of lipids called a “myelin sheath,” which prevent the ions that carry the nerve’s signal from leaking out as the nerve impulse travels the length of the axon.

What all of this means is that neurons in the brain are able to receive input from many other neurons and make “decisions” about what sort of action to take. This complex web of input, processing, and firing is what allows our brain to turn simple colors and lines into images of faces that we recognize, among other wonders!

Within the brain, there are numerous distinct structures that perform unique tasks. For the sake of brevity, we won’t talk about every single one of them here. Instead, we’ll focus on the major regions of the brain and the functions that these regions perform:

Frontal Lobe

The frontal lobe is part of the cerebral cortex. This cerebral cortex, or “cerebrum” is the largest part of the human brain, and is thought to be the most recently evolved.

Most other animals have a much smaller cerebral cortex than humans. In humans, the lobes of the cerebrum are responsible for “higher” tasks such as thought, language, action, and impulse control.

The frontal lobe is the region of cerebrum found at the front of the head, immediately behind the eyes and forehead. It contains the regions of the brain that can perform math and speech, as well as those responsible for planning, problem-solving, regulating the emotions, and making conscious decisions.

People with damage to the frontal lobe from injuries can show traits like volatile emotions, lack of self-control, and lack of socially appropriate behavior. They can also have trouble solving problems and making and sticking to plans.

Some neurologists have gone so far as to assert that, due to its connection to language, math, problem-solving, emotional regulation, and conscious decision-making, it is the frontal lobe of the brain that makes us uniquely human. Other neurologists, however, say it’s far more complicated than that!

Temporal Lobe

The temporal lobe is where our brain processes sounds, including the sound of speech. It is found on either side of the brain below and behind the cerebral cortex. A good reference point for the location of the temporal lobe is the place where the hinge of your jaw meets the braincase of your skull.

The temporal lobe contains complex circuitry for analyzing the sounds we hear for pitch, tone, and meaning. It even sends auditory data to the limbic system to determine a sound’s emotional content, and to the language center to determine its verbal content. The temporal lobe can even determine roughly where a sound is coming from through triangulation, by comparing when the sound reached one ear vs. the other.

The temporal lobe can temporarily store auditory memory, and may play a role in the formation of long-term memories through its connection to the hippocampus.

Parietal Lobe

The parietal lobe is located on top of the brain toward the back. It spans roughly from the top of your head to about halfway down the back of your skull. This lobe processes sensory input from the body, and also contains the circuitry for movement.

Once, it was thought that there was really only one sense contained in most parts of the body: that of touch. Now, however, we know that there are at least two distinct senses: touch, and proprioception. Proprioception uses motion and position sensors in the body to tell us where different parts of our body are in space. This is essential to allowing us to execute complicated movements, and to move at all without losing our balance!

The parietal lobe also contains circuitry which can process visual input from the occipital cortex to help us recognize faces and objects.

Occipital Lobe

The occipital lobe is the smallest in the cerebral cortex. It is located at the very back of the head, near the base of the skull.

The occipital lobe processes visual information. The optic nerves from the eyes pass deep into the brain, through a processing center, and finally deliver their information to the occipital lobe, which decodes visual information into colors, shapes, and objects.

Because we have two eyes facing the same direction – a trait called “binocular vision” – our occipital lobe can produce a three-dimensional image of the world by comparing the slightly different views from our two eyes.

The occipital lobe sends visual information through many steps of processing, ultimately linking up with memory circuits to allow us to recognize objects, people, and places in our environment.

Cerebellum

We are now leaving the cerebrum – the most recently evolved part of the brain – and moving into older structures. The cerebellum is a small structure at the base of the brain, directly below the parietal and occipital lobes. It is responsible for regulation of movement, posture, and balance – very important functions for any organism to have!

People with damage to the cerebellum can have difficulty walking, executing complex movements, and even standing. We often take this little part of our brain for granted, but walking on two legs is no easy task!

Limbic System

The limbic system is sometimes referred to as “the emotional brain. It lies at the center of the brain, with the cerebral cortex wrapped around it and the cerebellum tucked behind it. It is an evolutionarily old structure; it is also extremely vital. The limbic system includes:

• The hippocampus, which creates and stores memories. This structure nestled in the center of the brain has connections which can stimulate most other brain regions, allowing us to recall the sights, sounds, emotions, and other aspects of events in our past.
  
  It is not known exactly how the hippocampus creates memories, or whether it is truly the storage site of memories. Some studies have suggested that people with hippocampal damage cannot form new memories, but may still be able to access memories from before the damage.
  
  It is thought that the amygdala can affect how the hippocampus stores memories, resulting in stronger and more vivid encoding of memories that take involve fear, trauma, or other strong emotions. This may have given our ancestors an evolutionary advantage by ensuring that they avoided dangerous or harmful situations in the future.


• The amygdala monitors and helps to create emotional states.
  
  Emotional states are now thought to be a team effort between the brain and the body. The activity of the amygdala is influenced by cues from the body such as heart rate, posture, and adrenaline. But the amygdala also influences the body in return by triggering fear responses when a threatening sight, sound, or other stimulus associated with a dangerous or painful memory is detected.
  
  The amygdala can also send signals to the hippocampus which cause a memory to be encoded more vividly if it is made under circumstances of acute fear or pain. This is thought to be a survival adaptation to allow us to avoid fear and pain more effectively in the future. It is also thought to be the reason why traumatic memories tend to be very vivid, and can sometimes be “triggered” by similar sensory stimuli in conditions such as PTSD.
  
  The amygdala is most often said to be related to fear and pain, as these are some of the easiest emotional states to identify and are commonly studied by psychiatrist and neurologists seeking to help people heal from trauma. However, it is possible that the amygdala may also play a role in positive emotions that is not yet well-understood.


• The thalamus acts as the “switchboard” for the cerebral cortex. All sensory information except for smell passes through the thalamus before it goes on to the cerebral cortex processing centers. This might be why you cease noticing some parts of your environment, such as the way your clothes feel against your skin, while stimuli that are new or important to what you are doing attract your attention.
  
  It is thought that the thalamus might help the brain to “decide” which sensory stimuli to pay attention to. This might assist us with survival by ensuring that we prioritize relevant things in our environment, while ignoring parts of our environment that are not affecting us at the moment.


• The hypothalamus is a tiny structure located under the thalamus. It plays a vital role in releasing chemical messages from the brain to the body, which regulate many of our body’s involuntary functions.
  
  Chemical messages released by the thalamus include messages which make us hungry, thirsty, and sleepy; messages tell our kidneys when to conserve water; and messages that can effect our emotional states.
  
  Problems with the hypothalamus can result in a wide array of illnesses where organs do not function as they should, even though the organ itself is healthy and undamaged.
  
  Organs which need signals from the hypothalamus to function properly include the adrenal glands, the thyroid glands, the kidneys, and the reproductive organs.

Brain Stem

The brainstem is responsible for the most basic of life functions. Its duties include causing the diaphragm to expand and contract so that we can breathe; regulating the heartbeat; and regulating blood pressure. The parts of the brain stem include:

• The midbrain. This intriguing structure assists with many purposes, and may have been relied on more heavily by our very ancient ancestors early in our evolutionary history. It plays a role in vision, hearing, eye movement, and body movement. Its most crucial functions include:
  
  The midbrain contains the substantia nigra, which produces all the dopamine used by the motor system to allow movement. Parkinson’s disease is most typically caused by deterioration of the substantia nigra, resulting in a lack of dopamine in the motor cortex.
  
  The midbrain also contains the superior colliculus, which has a remarkable ability. Some people who cannot see because of damage to the occipital lobe can perform basic visual tasks using the superior colliculus. Although the superior colliculus does not consciously register visual data, the body seems to be able to use it to move appropriately!


• The pons is involved in auditory processing, motor control, and sensory analysis. Perhaps its most unique function is its role in sleep.
  
  During sleep, the pons sends signals to the rest of the brain which activate the processes of REM sleep – making dreams, as well as the consolidation of learning and memory possible!


• The medulla oblongata is responsible for maintaining our breathing and regulating our heart rate. It also contains cells which can detect poisons in the bloodstream and trigger vomiting in response.
  
  When death occurs from brain injuries, it is most often because the brain swelled to the point of crushing the brainstem, which lies underneath the other brain structures. Disruptions to medulla activity can cause breathing to stop, resulting in death.

This is why doctors recommend that patients with certain serious head injuries be awakened every few hours when they sleep. In cases of brain swelling and internal bleeding, unconsciousness typically occurs before breathing stops. Discovering that a person with a brain injury is unconscious and cannot be woken up can sometimes allow doctors to take action to prevent death from medulla compression.

Clostrum

The “clostrum” is a part of the brain which was only recently discovered, and about which little is known at present. It is intriguing because some scientists believe it might be the part of the brain where input from all of the above functions are combined into the experience of consciousness.

For many years, the answer to the question “how does consciousness occur?” as “we have no idea.” It still is – scientists don’t know how exactly the clostrum might produce consciousness – but with the clostrum’s discovery, at least one small piece of the puzzle has been solved.

Previously, a puzzle for doctors was that there seemed to be no single area of the brain that interfered with consciousness when damaged. Damage to different areas of the brain could cause many different symptoms, but people would continue to seem awake and aware unless most or all of the brain stopped functioning. What part of the brain, then, was responsible for consciousness?

The very existence of the clostrum was missed for many years because it is tiny. The clostrum is a thin sheet of tissue lining each hemisphere of the brain, which receives inputs – and sends outputs – to virtually every part of the brain.

While attempting to treat a patient with epilepsy, it was quite accidentally discovered that disrupting the activity of her clostrum resulted in a cessation of consciousness. She did not react to, experience, or remember anything from periods of time where her clostrum was being disrupted.

All of these discoveries have been made only in the last few years, so much more research is needed. But that makes it a very exciting area of research to follow!

Two Hemispheres

One of the most remarkable and underappreciated things about our brain is that it has two hemispheres. Our cerebral cortex is essentially separated into two halves, each of which have very similar wiring. The two halves of our cerebral cortex can only communicate with each other directly through the corpus callosum – a band of fibers which sends information back and forth between the two sides.

They can communicate very basic information, such as emotion and survival, indirectly through the limbic system and brainstem which receive input from both hemispheres.

For many years this was thought to simply be a biological oddity, but recently scientists are beginning to think that it’s very important to who we are. Our brain hemispheres often have slightly different wiring, which gives each one slightly different abilities. In most people, for example, a speech center is only found in the left hemisphere; the right brain may have little or no language capability, but is more sensitive to the emotional content of sensory stimuli.

It’s not quite as simple as the pop culture myth that math and science live in the left brain, while art and music live in the right. But it is true that the different brain hemispheres have some different abilities – and may even have different desires, and come to different solutions when solving problems.

One patient whose corpus callosum had been cut to control severe seizures was interviewed by scientists. He was found to have some language function in both his left and right brain hemispheres, which allowed each side to be verbally interviewed separately. This was done by only letting one hemisphere “see” or “hear” the questions, since each eye and each ear send their sensory input to only one hemisphere of the brain.

The results were rather astounding! This patient’s right brain hemisphere gave different answers from is left brain when asked about his ambitions, political feelings, and religious beliefs.

Later experiments showed that other patients with their corpus callosum cut would display similar “differences of opinion” between their brain hemispheres, such as having each of their hands try to solve a puzzle in a different way. Sometimes the two hands would even appear to fight over the best solution, undoing each other’s work!

For most people, the two hemispheres are united by a corpus callosum which allows us to take both of their perspectives into account. But the implications of the split brain for healthy people are still being studied!

Quiz

1. Which of the following is NOT a function of the brain?

A. To process and integrate sensory input

B. To permit movement

C. To coordinate essential life functions

D. None of the above

Answer: D. All of the above are functions of the human brain!

2. Which of the following is the part of a neuron responsible for firing an action potential and sending a message to other neurons?

A. The dendrites

B. The nucleus

C. The axon

D. The myelin sheath

Answer: C. The axon is the part of a neuron that fires an action potential.

3. Which of the following symptoms might be expected of someone with frontal lobe damage?

A. Blindness

B. Deafness

C. Lack of impulse control

D. Inability to regulate bodily functions

Answer: C. Impulse control takes place mainly in the frontal cortex.

References

Shepherd, G. M. (1994). Neurobiology. New York: Oxford University Press.

Brain Structures and their Functions. (n.d.). Retrieved July 14, 2017, from http://serendip.brynmawr.edu/bb/kinser/Structure1.html

Sporns, O. (2010). Networks of the Brain. MIT Press.

(n.d.). Retrieved July 14, 2017, from http://www.nytimes.com/health/guides/disease/hypothalamic-dysfunction/overview.html

Kandel, E. R., Schwartz, J. H., & Jessell, T. M. (1995). Essentials of neural science and behavior. Norwalk, CT: Appleton & Lange.

Rogers-Ramachandran, V. S. (n.d.). When Blindness Is in the Mind, Not the Eyes. Retrieved July 14, 2017, from https://www.scientificamerican.com/article/when-blindness-is-in-the-mind/

Brain

Blood

Blood Definition

Blood is the body fluid in humans and other animals that delivers the essential materials for life to the body’s cells. It has sometimes been called a fluid “tissue,” because like solid tissues it contains several types of cells which perform complex functions for the human body.

The components of blood are produced mainly in the bone marrow, where special cells produce red cells, white cells, and platelets. So-called “blood cancers” such as leukemia are actually cancers of the bone marrow. As cancerous tissue replaces healthy bone marrow tissue, healthy red blood cells, white blood cells, and platelets cannot be made.

Despite looking like a simple red fluid, blood is as complex as any tissue in the body. Here we will discuss its functions, its components, and some clinically important characteristics of blood.

Function of Blood

Important functions of the blood include:

Bringing Vital Substances to Cells

Complex multicellular organisms need complex circulatory systems; that’s because we have many cells, and these cells have high metabolisms.

Without highly efficient means of delivering vital substances like oxygen, water, and nutrients, complex and active life forms like ourselves could not exist.

Some of the vital substances which blood delivers to our cells include:

• Oxygen – Near-constant supply needed for cellular respiration.


• Water – Correct balance needed for enzymatic activity to proceed smoothly.


• Nutrients – Fuel for cellular respiration, and necessary materials for cellular maintenance.


• Biological building blocks – Molecules out of which replacement parts and daughter cells can be made.


• Chemical messages from other cells – Allows body cells to alter their activity appropriately in response to environmental changes.

Blood also performs other important functions for our bodies, including…

Removing Dangerous Wastes

Most living things produce waste products that, at a certain concentration, become toxic to their own cells. Multicellular organisms with high metabolisms like us have had to find a way to deal with all those waste products in order to allow many cells to live together in a single organism.

We have our liver and kidneys, which break down toxic substances into harmless substances and expel them from the body in the form of urine. Our blood carries toxins from all of our tissues to these organs, where they are processed and removed.

Our blood also releases unwanted gases in the lungs, where they are exchanged for fresh oxygen.

The blood keeps our cells safe by carrying all of these waste products out of our tissues and to the correct processing and elimination organs. Some waste products our blood helps us get rid of include:

• Carbon dioxide gas – Byproduct of cellular respiration, stops cellular respiration and causes acidification of blood if not removed.


• Excess water, salt, and other substances – Too much of a good thing can be a bad thing.


• Debris from dead cells – Cells die regularly, especially red blood cells which are not made to live longer than two weeks. Dead cells release toxic substances as they break down.


• Toxic waste products of metabolism – Some forms of routine cellular metabolism produce highly toxic substances that can be safely swept away by the blood to the liver and kidneys.


• Toxins we ingest in our food and water – Our liver and kidneys aren’t fool-proof, but they can handle some toxins we might encounter in the environment.

In addition to transporting substances to and from cells in other organs, the blood also contains its own cells and performs its own unique functions. These include:

Contains and Transports Immune Factors

Our blood contains antibodies and white blood cells which fight viruses, bacteria, and other invaders. Without these vital cells, we would quickly succumb to infections and die.

Our white blood cells even fight cancers that originate within our own bodies. It’s thought that most people develop cancerous cells at some point in their lives – but in most healthy people, the immune system destroys them before they are noticed. That’s why people with immune disorders have a higher chance of developing certain cancers than those with healthy immune systems.

The lymphatic system is also important for the movement, storage, and creation of immune factors.

Contains and Transports Clotting Factors

One of the most serious risks of injury to our body is the risk of blood loss. Because all of our organs, including our brains, rely on constant blood flow to stay alive, loss of large amounts of blood can be devastating. This is the most common cause of death from trauma.

Fortunately, our blood has a response system in place for when we are injured. A combination of cell fragments called platelets, chemicals called clotting factors, and other components of the blood work together to form blood into solid clots and scabs to stop bleeding.

Our blood clotting system cannot save us from large injuries, such as those that result in ruptures to our arteries. But their power can be seen in cases of people who do not have properly working blood clotting system.

People with certain cancers, vitamin deficiencies, and other diseases have blood that does not clot normally. These people can bruise and bleed with no apparent cause, and sometimes die from minor injuries or apparently spontaneous bleeding.

This happens because their clotting systems are not functioning properly. We can be thankful that most of our clotting systems do!

Components of Blood

There are several major components of the seemingly uniform liquid that is our blood. When centrifuged, the components of different densities separate to look something like this:

Here we will discuss the most vital components of blood, including serum, white blood cells or “leukocytes,” red blood cells, and platelets.

Plasma

Plasma is the liquid which carries the red blood cells, white blood cells, platelets, and other substances found in blood. More than half the volume of our blood is composed of this fluid.

Our blood plasma is mostly water, but it also contains salts proteins, and other substances, which can make it appear thick and syrupy even when the red and white cells have been filtered out.

One important protein, albumin, exists in part to keep the blood thick and syrupy. This ensures that the blood does not leak out of our vessels and into tissues, and slows bleeding when we are injured.

Other substances that can be found in the plasma include:

• Antibodies, which are proteins that attack invading pathogens


• Clotting factors, which prevent bleeding


• Hormones, which are chemical messages sent between different tissues in the body


• Electrolytes such as salt


• Nutrients such as sugar, vitamins, and minerals


• Lipids including cholesterol

So even this seemingly simple fluid is a veritable stew of the ingredients for life! But it could not do its job without…

Red Blood Cells

Red blood cells can be thought of as the cargo ships of the body. They are small, numerous cells which are specifically designed to carry oxygen from the lungs to cells, and carry carbon dioxide back to the lungs to be expelled when we exhale.

Red blood cells contain hemoglobin – a protein which is beautifully tailored to bind aggressively to oxygen in the lungs, and then release it and pick up carbon dioxide at a slow, steady rate as it passes through the body.

Hemoglobin is a pigment which changes color slightly, depending on whether it is bound to a molecule of oxygen or not. That’s why blood drawn from veins, which carry oxygen-depleted blood back toward the lungs, is a dark red that can appear almost brown. Blood drawn from arteries, which carry oxygen-rich blood from the lungs to the tissues, is a bright red.

White Blood Cells

White blood cells perform both immune and clean-up functions for the body. Like red blood cells, they are made by stem cells in the bone marrow.

There are many types of white blood cells, which play many different roles in immune response to infection and injury. Some types of white blood cells include:

• Neutrophils – Target bacteria and fungi.


• Eosinophils – Target larger parasites such as those which cause malaria. Also play a role in allergic inflammatory responses.


• Basophils – Release chemicals that enhance inflammatory responses.


• B Lymphocytes – Release antibodies and assist in activating T cell lymphocytes.


• T Lymphocytes – Different subtypes help the immune system learn to “recognize” new infection so it can target it; help immune system to activate in response to infection, then return to normal after infection has passed; target virus-infected and tumor cells.


• Natural Killer Lymphocytes – Target virus-infected and tumor cells for destruction.


• Monocyte – Migrate into tissues and mature into macrophages, literally “big eaters,” which engulf harmful cells and cellular debris and destroy them; some mature into Kupffer cells, which live in the liver and break down and recycle dying red blood cells.

Platelets

Platelets are cell fragments – bits of membrane-bound cytoplasm – which stop bleeding by clumping together to form clots and scabs seal wounds. Like red and white blood cells, they are made in the bone marrow. Cancer of the bone marrow may prevent production of properly functioning platelets.

Platelets have two states: active platelets, which are prepared to create blood clots, and inactive platelets that do not clot. Under normal circumstances, the endothelial lining of healthy blood vessels produces chemical messages that tells platelets to remain in their inactive form, so that they don’t form clots inside of healthy blood vessels.

Under normal circumstances, platelets are activated when a nearby injury starts a chemical cascade that urges platelets and other nearby clotting factors to activate. These factors then release clot-promoting messages of their own, encouraging more clotting factors to join their growing clot.

Platelets can sometimes be incorrectly activated when endothelial lining is damaged and does not produce the usual inhibitory messages for platelets. This can happen in people with some metabolic disorders and some forms of cardiovascular disease.

Blood Types

In early medical history, it was thought that blood transfusions from one person to another might be impossible. This was because when this was attempted, most test subjects died.

In time, however, scientists discovered the existence of “blood types” – a few basic proteins that coat the surface of our blood cells, helping the immune system to differentiate between our own blood cells and foreign invaders.

Attempting to transfuse blood of an incompatible type into a person can cause a clotting reaction, which may be fatal. Fortunately, today doctors have rapid tests to determine a patient’s blood type, and store blood bags for transfusion sorted by type so that patients are assured of getting a compatible treatment.

The three common blood type protein markers recognized by science are called the A, B, and Rh proteins.

The A/B protein group can give rise to blood types A, B, AB, or O. There is no “O” blood type protein – instead “O” is the blood type used to describe people who have neither A nor B marker proteins.

Each of these blood types can also be positive or negative for the Rh protein, leading to blood types such as “AB+” or “AB-.”

The blood type “O negative” is known as the universal donor. Because it does not have A or B proteins and is negative for the Rh protein, people of any blood type can receive O negative blood without having an adverse immune response to foreign proteins.

Unfortunately, people with “O negative” blood type also have the narrowest selection of possible donors for themselves. O negative people cannot receive any blood that has A, B, or Rh proteins; they can only receive blood from other O negative people.

When platelets are deficient or absent, the results can be seen in an increased probability of catastrophic bleeding events, including spontaneous bleeding and bleeding from minor injuries.

Quiz

1. Which of the following is NOT a function of blood?
A. To transport oxygen to our cells
B. To transport water to and away from our cells, as needed
C. To break down toxins into harmless substances
D. To fight infections

Answer to Question #1

C is correct. Blood cannot break down toxins; however, it can deliver toxic substances from our cells to the liver and kidneys, which can do this job.

2. Which of the following is NOT likely to be found in a blood sample?
A. Red blood cells
B. Plasma
C. White blood cells
D. Bone marrow cells

Answer to Question #2

D is correct. Although bone marrow cells are the sites of production for red blood cells, white blood cells, and platelets, bone marrow cells themselves are not normally found in the blood.

3. Which of the following is NOT likely to be found in blood plasma?
A. Salt
B. Digestive enzymes
C. Hormones
D. Albumin

Answer to Question #3

B is correct. Digestive enzymes are not normally found in blood plasma. The other items, however, are all common components of plasma!

References

• Laki, K. (1972). Our Ancient Heritage In Blood Clotting And Some Of Its Consequences. Annals of the New York Academy of Sciences, 202(1), 297-307. doi:10.1111/j.1749-6632.1972.tb16342.x


• Haubrich, W. S. (2004). Kupffer of Kupffer cells. Gastroenterology, 127(1), 16. doi:10.1053/j.gastro.2004.05.041


• Wilson, J. H., & Hunt, T. (2002). Molecular biology of the cell, 4th ed. New York: Garland.


• American Society of Hematology. (2014, March 29). Retrieved July 11, 2017, from http://www.hematology.org/Patients/Basics/

Blood

Liver

Liver Definition

The liver is a vital organ found in humans and other vertebrates. It is a large organ, with its major lobe occupying the right side of the abdomen below the diaphragm, while the narrower left lobe extends all the way across the abdomen to the left. The liver is the dark pink organ in this image:

(click to play animated GIF)

The liver performs many vital functions without which humans cannot survive.

Presently, there is no way to replace a liver with an artificial one in the long-term, although partial liver donations in which a donor gives part of their liver to someone in need of a transplant have been successful. The liver has a remarkable regenerative capacity, and can grow back most of its tissue if it is removed or damaged.

Despite the success of partial transplants in some cases, liver failure is an important cause of death among people with chronic alcohol and drug use.

Liver failure can also occur accidentally as a result of an accidental overdose of acetaminophen – a medicinal compound found in many over-the-counter and prescription medications. Because acetaminophen is found in so many different medications, people often don’t realize that two or more medicines they’re taking together contain the same compound, which can overwhelm the liver and lead to potentially fatal liver damage.

This is why it’s always important to tell your doctor about all medications you are taking, and check the labels of different medications for common ingredients. We will talk more about acetaminophen toxicity, and how to avoid it, below.

Liver Function

The liver serves many vital functions in the body, including:

Detoxifying the Blood

The liver’s most well-known role is as a detoxifier of the blood. It contains cells with special enzymes that can break down toxic substances into non-toxic forms.

These enzymes explain why certain medications, foods, and supplements can interact with each other. Some liver enzymes break down multiple types of toxic substances; if the enzymes are “busy” with one substance, they might not be able to break down the other substance as they usually do.

This is why you should always tell your doctor about all medications and supplements you are taking, and why some medications require that you avoid certain substances like alcohol or grapefruit.

Without these enzymes to break down toxic substances, the body slowly poisons itself. It doesn’t even need to consume anything toxic from the environment – the chemicals produced by the body’s own cells are sufficient to cause fatal toxicity over time.

Fortunately, the liver is very good at what it does. We rarely have to feel the effects of these toxins, unless we ingest large quantities of substances that can damage the liver, such as alcohol, acetaminophen, or anti-freeze.

Our livers can also run into trouble if we contract viruses that damage liver cells, such as hepatitis.

Making Blood Clotting Factors

The liver uses Vitamin K to produce proteins that are important for blood clotting. Without these proteins, the multi-step process of blood clotting may not be able to get started.

This is why people with severe liver disease or Vitamin K deficiency often develop bleeding disorders. With the body unable to clot to repair even tiny, routine injuries, people with these conditions can appear to bruise and bleed for no reason.

This can be a very serious complication in the treatment of severe liver disease, since transplantation is a surgical procedure with a risk of severe bleeding.

Making Digestive Chemicals

The liver produces bile, which is a little-known but vital ingredient to the digestive process. Bile helps the body to break down and absorb fats, and also uses it to help get rid of certain waste products.

Problems with the liver are occasionally signaled by changes in fecal matter resulting from a lack of bile in the digestive tract. That’s one reason why it’s recommended to see a doctor if you experience changes to the color, consistency, or frequency of your bowel movements that last for several weeks.

Making Energy from Protein

Under normal circumstances, the body tries not to digest proteins for energy. That’s because there are so many better uses for protein and its building blocks, such as making enzymes and other essential cellular machinery.

However, under starvation conditions where there are not sufficient stores of carbohydrates or fats to meet the body’s needs, the liver can turn amino acids into fuel for our cells to make ATP.

The liver does us a double favor here. Not only does it make it possible for our cells to survive off of proteins – it also detoxifies the toxic byproduct of this process, which is ammonia. The liver turns ammonia into urea, which can be safely eliminated by the kidneys, before releasing it into the blood.

Storing Glycogen

The liver also serves another purpose that is useful when food is scarce. It stores carbohydrates in the form of a high-density, high-calorie substance called glycogen.

Our body’s normal order of priority for digestion is: carbs, fats, protein. Our body will metabolize any carbohydrates we eat first; if it runs out of carbs, then it will turn to metabolizing our long-term fat stores.

But in between those steps, it has the liver’s glycogen. This acts as a “quick release” fuel that is easier to release and replenish than fat. Only after depleting our liver’s glycogen reserves will our body normally start digesting fat.

Breaking Down Red Blood Cells

Red blood cells die more often than any other cell type in the body. This is because red blood cells do not have nuclei, so they cannot make their own proteins. When the proteins they had at maturity wear out, they break down and are replaced by new blood cells.

Without the liver, this process would lead to severe toxicity in the blood. Dying cells release toxic compounds, which must be processed by the liver in order to keep the body safe.

Thanks to the action of the liver’s cells and enzymes, the materials from dying red blood cells are broken down into harmless forms, or even recycled for later use in new red blood cells.

Producing Hormones

The liver produces several chemical messengers that help the body to coordinate its activities. These include:

• Insulin-like growth factor 1 – A hormone that prompts tissue to grow, and is especially important in childhood.


• Thrombopoietin – A hormone that tells the bone marrow how many platelets to produce to help with blood clotting.


• Hepcidin – A hormone that tells the body whether iron should be absorbed, or eliminated as waste.


• Angiotensinogen – A pro-hormone that increases blood pressure.


• Carrier proteins – The liver also produces carrier proteins, which bond to other hormones including sex hormones, thyroid hormone, stress hormones, and vitamins and minerals to specified destinations.

Liver Location

The liver is found within the lower ribcage, under the diaphragm which separates the heart, lungs, and stomach from the abdominal cavity.

It is located in the upper abdomen, with the bulk of the liver contained on the right side of the body. Its smaller lobe extends all the way to the left side of the diaphragm.

The liver normally cannot be felt, as it is shielded by the lower ribs. However, blows to the solar plexus – the soft part of the upper abdomen where the ribcage parts – can injure the liver.

Liver pain may appear as pain in the upper abdomen. It is usually on the left side or in the center, but can sometimes appear on the right or even seem to come from lower in the abdomen.

Doctors will often examine the abdomen if liver abnormalities are suspected, as a swollen liver can often be felt despite the ribcage.

Acetaminophen Toxicity

Some medicinal compounds are considered so safe and effective that they are used in dozens of different medications. Acetaminophen is one of these. The main ingredient in Tylenol, it relieves pain and reduces fever with a very good safety profile – as long as you take it in the correct doses.

Unfortunately, because acetaminophen is so effective, it is found in many, many medications. One of the most common causes of liver damage in the U.S. is people taking two or more of these medications at once, not realizing that they all contain the same ingredient. This can result in an overdose, especially if multiple medications containing acetaminophen are taken over long periods of time.

Acetaminophen is processed by the liver into a toxic product, which can poison the liver. In the proper doses, the liver can eliminate this byproduct without any problem, but when people take more acetaminophen than they should over a period of days or weeks, the product can build up and kill liver tissue.

This is unfortunately common since acetaminophen is included in some medications which are advertised to treat joint pain, fever, heartburn, flu symptoms, cough, menstrual cramps, and more. People often don’t think that these medications are related or that they might contain the same ingredients.

When taking multiple medications on the same day, it is a good idea to check the labels for shared ingredients. Taking a proper dose of acetaminophen is safe and effective – but taking two or three times the proper dose can cause very big problems.

Medications which contain acetaminophen and should not be taken together include:

Actifed	Alka-Seltzer Plus	Anacin
Bitartrate	Butalbital	Cepacol
Contac	Coricidin	Dayquil
Dimetapp	Dristan	Endocet
Excedrin	Fioricet	Feverall
Formula 44	Goody’s Powders	Hycotab
Hydrocet	Liquiprin	Lortab
Midol	MIDRIN	Mucinex
NORCO	Nyquil	Oxycodone
Panadol	Percocet	Phenaphen
Robitussin	ROXICET	Saint Joseph Aspirin-Free
Sedapap	Singlet	Sinutab
Sudafed	Tapanol	Tramadol
Triaminic	Tylenol Brand Products	Tylox
Ultracet	Vanquish	Vicodin
Vicks	Zydone

Quiz

1. Which of the following is NOT a function of the liver?
A. Breaking down toxins into harmless substances
B. Breaking down old red blood cells
C. Metabolizing glucose for the body
D. Storing carbohydrates

Answer to Question #1

C is correct. The liver metabolizes protein when it is needed for fuel – not glucose. All cells are capable of metabolizing glucose for energy, while protein metabolism requires the liver because it produces toxic byproducts.

2. Which of the following is NOT a common cause of liver failure?
A. Chronic, heavy alcohol consumption
B. Viral hepatitis
C. Too much exercise
D. Acetaminophen toxicity

Answer to Question #2

C is correct. A, B, and D are common causes of liver failure.

3. Pain in which of the following areas might originate with the liver?
A. Right upper quadrant of the abdomen
B. Central upper quadrant of the abdomen
C. Right upper quadrant or lower abdomen
D. All of the above

Answer to Question #3

D is correct. While the liver is primarily located in the right and central upper parts of the abdomen, the path of nerves in the abdomen can sometimes cause liver pain to manifest on the left side, or even in the lower abdomen.

References

• Boron, W. F., & Boulpaep, E. L. (2009). Medical physiology: a cellular and molecular approach. Philadelphia, PA: Saunders/Elsevier.


• Common Medicines With Acetaminophen. (n.d.). Retrieved July 11, 2017, from http://www.knowyourdose.org/common-medicines/


• Abdel-Misih, S. R., & Bloomston, M. (2010). Liver Anatomy. Surgical Clinics of North America,90(4), 643-653. doi:10.1016/j.suc.2010.04.017


• Acetaminophen Toxicity. (2017, January 06). Retrieved July 11, 2017, from http://emedicine.medscape.com/article/820200-overview


• Hoffman, M. (n.d.). Picture of the Liver. Retrieved July 11, 2017, from http://www.webmd.com/digestive-disorders/picture-of-the-liver#1


• How does the liver work? (2016, August 22). Retrieved July 11, 2017, from https://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0072577/

Liver

Heart

Heart Definition

The heart is a muscular organ that pumps blood throughout the body. It is located in the middle cavity of the chest, between the lungs. In most people, the heart is located on the left side of the chest, beneath the breastbone.

The heart is composed of smooth muscle. It has four chambers which contract in a specific order, allowing the human heart to pump blood from the body to the lungs and back again with high efficiency. The heart also contains “pacemaker” cells which fire nerve impulses at regular intervals, prompting the heart muscle to contract.

This animation shows the functioning of this extraordinarily complex pump in action. As you read this article, try scrolling back up and seeing if you can spot the chambers, valves, and blood vessels we’re discussing in action:

(click to play animated GIF)

The heart is one of the most vital and delicate organs in the body. If it does not function properly, all other organs – including the brain – begin to die from lack of oxygen within just a few minutes. As of 2009, the most common cause of death in the world was heart disease.

Most heart disease occurs as a result of age or lifestyle. Cholesterol can build up in the arteries as a person gets older, and this is more likely for people who have diets high in saturated fat and cholesterol. Rarely, however, heart disease can also occur due to a virus or bacterium that infects the heart or its protective tissues.

Scientists have had some success replicating the heart’s pumping action with artificial pumps, but these pumps can be rejected by the body, and they break down over time.

The four-chambered heart found in mammals and birds is more efficient than the one, two, or three-chambered hearts found in some other animals. It is thought that warm-blooded animals need highly efficient circulation to satisfy their cells’ high demand for oxygen. This is especially true of humans, whose huge brains require a near-constant supply of oxygen to function!

Function of the Heart

The heart pumps blood through our immense and complicated circulatory systems at high pressure. It is a truly impressive feat of engineering, as it must circulate about five liters of blood through a full 1,000 miles worth of blood vessels each minute! We will talk more about how the heart accomplishes this remarkable task under the “Heart Structure” section below.

The pumping action of the heart allows the movement of many substances between organs in the body, including nutrients, waste products, and hormones and other chemical messengers. However, arguably the most important substance it circulates is oxygen.

Oxygen is required for animal cells to perform cellular respiration. Without oxygen, cells cannot break down food to produce ATP, the cellular currency of energy. Soon, none of their energy-dependent processes can function. Without its energy-dependent processes, a cell dies.

Neural tissues, including the brain, are particularly sensitive to oxygen deprivation. Neural tissues maintain a special cellular chemistry which must be constantly maintained through the consumption of lots and lots of energy. If ATP production stops, neural cells can begin to die within minutes.

For this reason, the body has taken many special measures to protect the heart. It is located below the strongest part of the ribcage and cushioned between the lungs. It is also surrounded by a protective membrane called the pericardium, which is filled with additional cushioning fluid.

Heart Structure

The heart’s unique design allows it to accomplish the incredible task of circulating blood through the human body. Here we will review its essential components, and how and why blood passes through them.

Layers of the Heart Wall

The heart has three layers of tissue, each of which serve a slightly different purpose. These are:

• The Epicardium. The epicardium is also sometimes considered a part of the protective pericardial membrane around the heart. It helps to keep the heart lubricated and protected.


• The Myocardium. The myocardium is the muscle of the heart. You can remember this because the root word “myo” comes from “muscle,” while “cardium” comes from “heart.”
  

  The myocardium is an incredibly strong muscle that makes up most of the heart. It is responsible for pumping blood throughout the body.



• The Endocardium. The endocardium is a thin, protective layer on the inside of the heart. It is made of smooth, slippery endothelial cells, which prevent blood from sticking to the inside of the heart and forming deadly blood clots.

Chambers of the Heart

The heart has four chambers, which are designed to pump blood from the body to the lungs and back again with extremely high efficiency. Here we’ll see what the four chambers are, and how they do their jobs:

• The Right Atrium. The right and left atria are the smaller chambers of the heart, and they have thinner, less muscular walls. This is because they only receive blood from the veins – they don’t have to pump it back out through the whole circulatory system!
  

  The right atrium only has to receive blood from the body’s veins and pump it into the left ventricle, where the real pumping action starts.



• The Right Ventricle. The ventricles are larger chambers with stronger, thicker walls. They are responsible for pumping blood to the organs at high pressures.
  

  There are two ventricles because there are two circuits blood needs to be pumped through – the pulmonary circuit, where blood receives oxygen from the lungs, and the body circuit, where oxygen-filled blood travels to the rest of the body.

  
  

  Maintaining these two separate circuits with two separate ventricles is much more efficient than simply pumping blood to the lungs and allowing it to flow to the rest of the body from there. With two ventricles, the heart can generate twice the force, and deliver oxygen to our cells much faster.

  
  

  The right ventricle is the one attacked to the pulmonary circuit. It pumps blood through the pulmonary artery and to the lungs, where the blood fills with oxygen, at high pressure. The blood then returns to…



• The left atrium receives oxygenated blood from the pulmonary veins. It pumps this blood into the left ventricle, which…


• The left ventricle pumps blood throughout the rest of the body.

After the blood has circulated through the body and the oxygen has been exchange for carbon dioxide waste from the body’s cells, the blood re-enters the right atrium and the process begins again.

In most people, this whole circulatory path only takes about a minute to complete!

Valves of the Heart

You may be wondering how the heart ensures that blood flows in the right direction between these chambers and blood vessels. You may also have heard of “heart valves” referred to in a medical context.

Heart valves are just that – biological valves that only allow blood to flow through the heart in one direction, ensuring that all the blood gets to where it needs to be.

Here is a list of the most important valves in the heart, and an explanation of why they are important:

• The Tricuspid Valve. The tricuspid valve is what is called an “atrioventricular” valve. As you might guess by the name, it ensures that blood only flows from the atrium to the ventricle – not the other way around.
  

  These atrioventricular valves have to stand up to very high pressures to ensure that no blood gets through, as the ventricle contracts quite powerfully to squeeze blood out.

  
  

  The tricuspid valve is the valve that ensures that blood in the right ventricle goes into the pulmonary artery and reaches the lungs, instead of being pushed back into the right atrium.



• The Pulmonary Valve. The pulmonary valve is what is called a semilunar valve. Semilunar valves are found in arteries leaving the heart. Their role is to prevent blood from flowing backwards from the arteries into the heart’s chambers.
  

  This is important because the ventricles “suck” blood in from the atria by expanding after they have expelled blood into the arteries. Without properly functioning semilunar valves, blood can flow back into the ventricle instead of going to the rest of the body. This drastically decreases the efficiency with which the heart can move oxygenated blood through the body.

  
  

  The pulmonary valve lies in between the pulmonary artery and the left ventricle, where it ensures that blood pumped into the pulmonary artery continues to the lungs instead of returning to the heart.



• The Mitral Valve. The mitral valve is the other atrioventricular valve. This one lies between the left atrium and the left ventricle. It prevents blood from flowing back from the ventricle into the atrium, ensuring that that blood is pumped to the rest of the body instead!
  

  The mitral valve lies at the opening of the aorta, which is the largest blood vessel in the body. The aorta is the central artery from which all other arteries fill. It is thicker than a garden hose, extends all the way from our hearts down to our pelvis, where it splits in two to become the femoral artery of each leg.



• The Aortic Valve. As you might have guessed, the aorta needs a semilunar valve just like the pulmonary artery does. The aortic valve prevents blood from flowing backwards from the aorta into the left ventricle as the left ventricle “sucks” in oxygenated blood from the left atrium.

Many people have minor irregularities with these valves, such as mitral valve prolapse, which make their hearts less efficient or more prone to experiencing problems. People with minor valve issues can often lead a normal, healthy life.

However, total failure of any of these valves can be catastrophic for the heart and for blood flow. That’s why people with conditions like mitral valve prolapse are often turned down by the military and other programs that involve conditions which can be very taxing for the heart.

The Sinoatrial Node

The sinoatrial node is another very important part of the heart. It is a group of cells in the wall of the right atrium of the heart – and it is what keeps the heart pumping!

The cells in the sinoatrial node produce small electrical impulses in a regular rhythm. These impulses are what drive the contractions of the four chambers of the heart.

Artificial pacemakers replicate the action of the sinoatrial node by making similar electrical impulses for people whose sinoatrial node isn’t functioning properly. However, healthy people have a natural pacemaker built right into the heart!

Quiz

1. Which of the following is NOT true of the human heart?
A. It has four chambers.
B. It has a built-in “pacemaker” called the sinoatrial node.
C. It is responsible for pumping oxygen-filled blood around the body.
D. It is the only organ in the body you cannot live without.

Answer to Question #1

D is correct. Although the heart is a very important organ, there are many organs you cannot live without!

2. Which of the following is NOT a layer of the heart?
A. The myocardium
B. The endocardium
C. The myometrium
D. The epicardium

Answer to Question #2

C is correct. The myometrium is a part of the uterus – not of the heart. It may help to remember that “cardio” is the Greek word for “heart.” This is also where we get words like “cardiologist” and “cardiovascular system.”

3. Why does the heart need valves?
A. To keep out pathogens and toxins that could damage the heart
B. To ensure that blood goes not flow backward into the wrong chamber of the heart
C. To ensure that the heart does not pump too much blood at once
D. None of the above

Answer to Question #3

B is correct. The valves of the heart make sure that blood goes to its destination, instead of flowing backwards and preventing new blood from getting through!

References

• Moore, K. L., Agur, A. M., & Dalley, A. F. (2018). Clinically oriented anatomy. Philadelphia: Wolters Kluwer.


• Heart. (n.d.). Retrieved July 08, 2017, from http://www.innerbody.com/image/card01.html


• (n.d.). Retrieved July 08, 2017, from https://training.seer.cancer.gov/anatomy/cardiovascular/heart/structure.html


• Monfredi, O., Dobrzynski, H., Mondal, T., Boyett, M. R., & Morris, G. M. (2010). The Anatomy and Physiology of the Sinoatrial Node-A Contemporary Review. Pacing and Clinical Electrophysiology, 33(11), 1392-1406. doi:10.1111/j.1540-8159.2010.02838.x


• Blood Vessels. (2017, May 19). Retrieved July 08, 2017, from https://www.fi.edu/heart/blood-vessels

Heart