Video Transcript
In this video, we will learn how to
describe the different circulatory pathways within the human body. We will learn how the pulmonary
pathway gets rid of carbon dioxide and takes up oxygen, how the systemic pathway
brings oxygen to the body cells and takes up waste products from them, and how the
hepatic portal system deals with waste products and possible toxic molecules.
The human body is made up of 11
different organ systems, each with a specific function. For example, the nervous system
coordinates our behavior and transmits signals between different parts of the
body. The digestive system is responsible
for taking in nutrients, allowing the body to function, grow, and repair itself. In today’s video, we will be
focusing on the circulatory system, which is made up of the heart and the blood
vessels. The organ systems do not act in
isolation but rather interact with each other in complex ways. For example, the circulatory system
is constantly interacting with the lungs, as we will soon see.
The heart is the most central and
crucial organ of the circulatory system. It is always beating, whether we’re
sleeping, studying, or performing some sort of exercise. But what function is the heart
performing as it beats? The heart beats in order to pump
blood to every cell in the body, bringing them oxygen and nutrients. This pumping action also enables
the heart to carry away carbon dioxide and other waste products. We can see that while the heart is
the central organ, it acts together with an intricate network of tubes called blood
vessels, which carry blood to and from the various parts of the body. This process is called
circulation. As with other organ systems, the
circulatory system is organized in a way that allows it to perform its functions
efficiently.
Now, let’s take a look at each of
the circulatory components to understand how their structures determine their
function, beginning with the main organ, the heart. The heart is composed of both
muscular and nervous tissue. It sits between the two lungs,
tilted slightly to the left. If you clench your fist, this is
approximately how large your heart is. Its structure consists of four
chambers: two upper chambers known as the atria, or singular atrium, and two lower
chambers known as the ventricles. The right and left atria receive
blood that is entering the heart. The blood flows into the right and
left ventricles, which pump it out of the heart into the rest of the body. Atria are enclosed by thin muscular
walls, while the ventricles are enclosed by thicker muscular walls, which help
support their function of pumping the blood out of the heart.
Before we begin learning more about
the heart, it is important to address the fact that the labels for the right side of
the heart are on the left side of the diagram and the labels for the left side of
the heart are on the right side of the diagram. This is because when we study the
heart, we look at it as if it were the heart of a person facing us. Therefore, this person’s left side
corresponds to the left side of the heart in the diagram and their right side
corresponds to the right side of the heart in the diagram.
The openings between the atria and
the ventricles are guarded by structures called valves, which are flaps of tissue
attached to the atria muscles by strings of connective tissue. If you’ve ever heard someone say
that a sad movie or commercial pulled at their heartstrings, these are the strings
they’re referring to. When the valves are closed, as
shown in the diagram, they prevent blood from flowing in the wrong direction, that
is, from ventricle to atrium. The opening between the right
atrium and the right ventricle is guarded by the tricuspid valve. The prefix tri- means three, which
tells us that there are three flaps of tissue composing the tricuspid valve. The bicuspid valve lies between the
left atrium and left ventricle. The prefix bi- means two, which
tells us that this valve is composed of two flaps of tissue.
The right and left atria are
separated by a thin muscular wall called the interatrial septum. In this diagram, the interatrial
septum is mostly hidden by these large blood vessels. The right and left ventricles are
separated by a thick muscular wall called the interventricular septum. The separation between the left and
right sides of the heart is crucial to the role played by each chamber, as we’ll
soon discover.
There are three main types of blood
vessels in the body. Arteries are responsible for
carrying blood away from the heart to the other tissues and organs of the body,
while veins carry blood to the heart away from the other tissues and organs. Arteries and veins branch into
smaller blood vessels called arterioles and venules. The tiny thin blood vessels that
connect arterioles and venules in a continuous closed circuit are called
capillaries. The walls of the capillaries are so
thin that oxygen and nutrients can pass through them to be taken up by the
cells. Similarly, waste products such as
carbon dioxide can be released by the cells and taken up through the walls of the
capillaries.
Homeothermic or warm-blooded
animals, such as birds and mammals, have high oxygen and nutrient needs. This is because temperature
regulation and strenuous or fast movements, such as flying, require large amounts of
energy. Let’s take a closer look at how the
efficient organization of the circulatory system enables homeothermic animals to
meet these high oxygen and nutrient demands.
When blood is pumped from the left
side of the heart to the other organs of the body, it is rich in oxygen that these
organs need. On the other hand, when blood is
pumped away from these organs and back to the heart, it is rich in carbon dioxide
and other waste products. But how does the right side of the
heart dispose of these waste products? And how does blood leaving the left
side of the heart have so much oxygen?
As mentioned previously, in birds
and mammals, the left and right sides of the heart are separated. This means that when oxygen-poor
blood rich in waste product, represented in blue, enters the right side of the
heart, it cannot flow directly to the left side of the heart. Instead, when the heart beats, this
blood is pumped out of the right side of the heart and into the pulmonary
circuit. The word “pulmonary” describes
anything relating to the lungs, which helps us remember that in the pulmonary
circulation pathway, blood is transported between the lungs and the heart. When the blood arrives at the
lungs, it undergoes a process called oxygenation before returning to the heart.
Oxygenation occurs in the alveoli
of the lungs, which are tiny air sacs where gas exchange occurs. The alveoli are surrounded by a
tight network of capillaries. The blood coming from the right
side of the heart releases carbon dioxide into the alveoli and takes up oxygen. The oxygen-enriched blood,
represented in red, is then transported back to the heart, this time entering on the
left side.
We can now see that the separation
of the heart prevents oxygen-rich blood coming from the lungs from mixing with
carbon-dioxide-rich blood. This enables the delivery of oxygen
to the other organs of the body via the systemic circuit. The term “systemic” is used to
describe things relating to the body, which helps us remember that the systemic
circulation pathway is the transport of blood between the heart and the rest of the
body. In this pathway, oxygen-rich blood
is pumped out of the left side of the heart. It is transported to capillaries
that surround body cells, tissues, and organs. In the capillaries, oxygen and
nutrients are released and carbon dioxide and other waste products are taken up. The blood is then returned to the
right side of the heart. Taken together, the pulmonary
circuit and systemic circuit form a double circulation pathway.
Now, let’s dive deeper into the
specific chambers and blood vessels of each pathway, starting with the pulmonary
circuit. When carbon-dioxide-rich blood that
is lacking in oxygen enters the right side of the heart, it moves through the vena
cava, which are the two large veins responsible for transporting blood from the
organs back to the right atrium. The superior vena cava delivers
blood from the upper body, while the inferior vena cava delivers blood from the
lower body. As the blood enters the right
atrium, it contracts, pushing blood into the right ventricle. The right ventricle muscles then
contract, pulling the tricuspid valve into a closed position and pumping the blood
out through the pulmonary artery.
The pulmonary artery is responsible
for delivering blood to the lungs, where it will undergo oxygenation in the
capillaries of the alveoli. After it has been enriched with
oxygen, the blood will be transported back to the left atrium through vessels called
pulmonary veins. In summary, the flow of blood
through the pulmonary circuit begins at the right atrium, moves to the right
ventricle, is pumped out through the pulmonary artery, travels to the lungs where it
picks up oxygen, and moves through the pulmonary veins to the left atrium.
Now that we have a good
understanding of the pulmonary circuit, let’s take a look at the systemic
circuit. It begins with oxygen-rich blood
entering the left atrium. The muscles of the left atrium
contract, pushing the blood into the left ventricle. Then, just as on the right side,
the left ventricle muscles contract, pulling the bicuspid valve into a closed
position and pumping the blood out through the largest blood vessel in the body, the
aorta. The aorta is the main artery that
carries blood away from the heart, delivering oxygen and picking up carbon dioxide
from every cell in the body, which eventually merge into the vena cava.
Now, let’s summarize the flow of
blood through the systemic circuit. It begins when oxygen-rich blood
arrives from the lungs and flows into the left atrium. The blood fills the left
ventricle. The blood is pumped out through the
aorta and is transported throughout the body, where it delivers oxygen and nutrients
and picks up carbon dioxide and other waste products. It flows back to the heart through
the vena cava and arrives at the right atrium. It is important to note that even
though we have diagrammed them separately, the systemic circuit and pulmonary
circuit form a closed loop. And the movement of blood occurs
through them simultaneously because the left and right ventricle muscles relax and
contract at the same time.
In addition to the pulmonary and
systemic circulation pathways, the heart plays an important role in the hepatic
portal system. You may already know that the word
“hepatic” is used to describe things relating to the liver, which helps us remember
that the hepatic portal system is a group of veins that transport blood from the
digestive system to the liver. Unlike the blood from the other
organs of the body, the blood being transported away from the organs of the
digestive system is not collected by the vena cava right away. Instead, this blood is first taken
up by the hepatic portal system through another vein called the hepatic portal
vein. The hepatic portal vein collects
blood from the capillaries of the digestive system, including the stomach,
intestines, gall bladder, spleen, and pancreas. The blood is then delivered to the
capillaries of the liver.
The liver is the largest internal
organ of the human body, and it performs some extremely important functions. It eliminates toxic molecules,
produces bile to break down fats, and stores and releases glucose according to the
body’s needs. In fact, the liver is involved in
over 200 functions in the body. The importance of the liver is
highlighted by the fact that in humans, it is the only organ capable of
regeneration. So, if someone loses a portion of
their liver, for example, due to liver cancer, the lost portion will gradually grow
back and return to normal function.
Because it performs so many
critical functions, the liver requires a substantial supply of nutrients. The hepatic portal system serves
this purpose by directing nutrient-rich blood from the digestive system straight to
the liver. However, blood arriving from the
digestive system may also contain toxic molecules derived from substances like
ethanol, certain medications, or contaminated food. Therefore, a second function of the
hepatic portal system is to ensure that these molecules are transported in the
bloodstream to the liver to be detoxified. This crucial step in the
circulatory system eliminates toxic substances from the body before the blood
reenters the heart. Once the blood has been processed
by the capillaries of the liver, it flows through the hepatic veins to join up with
the vena cava and be returned to the heart.
Now that we’ve learned how the
circulatory system transports needed oxygen and nutrients and gets rid of waste
products like carbon dioxide and possible ingested toxic molecules, let’s test our
knowledge with a practice question.
Fill in the blank. In the systemic circulatory system,
the blood being transported from the heart to the body is blank. (A) Oxygenated, (B)
deoxygenated.
To answer this question, let’s
first review the main circulatory pathways. In the pulmonary pathway, blood is
transported between the heart and the lungs. In the systemic pathway, blood is
transported between the heart and the rest of the body. In the hepatic portal pathway,
blood is transported from the digestive system to the liver for detoxification
before being returned to the heart. The question is asking us about the
systemic circulatory pathway. So let’s turn our focus to that to
find the correct answer.
The systemic pathway begins when
blood that has been enriched with oxygen in the lungs arrives at the left
atrium. The atrial muscles contract,
pushing the blood into the left ventricle. The muscles of the left ventricle
then contract, pulling the bicuspid valve into a closed position and pumping blood
out through the aorta. The aorta is the largest blood
vessel in the body, although it is not shown to scale in the diagram. It is the main artery that carries
blood away from the heart to the other organs and tissues of the body. And this part of the pathway is
exactly what the question is asking us about.
So we have shown that in the
systemic pathway, blood being transported from the heart to the body is rich in
oxygen, or oxygenated. Therefore, the correct statement is
in the systemic circulatory system, the blood being transported from the heart to
the body is oxygenated.
Now, let’s review and summarize
what we’ve learned about circulatory pathways. The human heart has four
chambers. The upper chambers are atria,
receive blood returning to the heart from the lungs and the body, while the lower
chambers are ventricles, pump blood out of the heart. The three types of blood vessels in
the body are arteries, veins, and capillaries. Arteries carry blood away from the
heart, while veins bring blood back to the heart. Capillaries connect arteries and
veins, forming a network around every cell in the body to exchange gases, nutrients,
and waste products.
Humans and other homeothermic
animals have a double circulatory system. In the pulmonary circuit, blood is
transported between the heart and the lungs, which are the site of oxygenation. In the systemic circuit, blood is
transported between the heart and the organs and tissues of the rest of the body to
deliver oxygen and nutrients and pick up carbon dioxide and waste products. The major blood vessels involved in
the pulmonary circuit are the pulmonary artery and the pulmonary veins. And the major blood vessels of the
systemic circuit are the aorta and the vena cava. The hepatic portal system directs
blood from the digestive system directly to the liver, where it undergoes
detoxification before being returned to the heart.
