Video Transcript
In this video, we will learn to
relate the structure and function of the different types of blood vessels in the
human circulatory system. First, we’ll learn about the
adaptations present in arteries then in veins then in capillaries. And finally, we’ll review what
we’ve learned.
Blood vessels are organs within the
circulatory system. They’re flexible, hollow tubes that
blood flows through to travel from place to place. Blood vessels consist of many
layers of tissue that surround the opening, called the lumen, that contains the
blood. The inner layer is a smooth lining
made of epithelial cells. The middle layer is made of smooth
muscle tissue and special elastic tissue. This allows the blood vessels to
expand, constrict, and stretch when necessary. The outer layer is made of
connective tissue, and it helps to strengthen the blood vessels and to give them
structure.
Here is a simplified diagram of the
human circulatory system. In the center is the heart, which
pumps the blood through the blood vessels. Blood vessels that carry blood away
from the heart are called arteries. An easy way to remember this is
that the “a” in arteries is the same as the “a” in away. The blood vessels that carry blood
towards the heart are called veins. An easy way to remember this is
that there’s an N in the word vein, and they carry blood in to the heart.
These circles that say lungs and
body actually represent many, many capillaries. Through the capillaries in the
tissues of your body, oxygenated blood delivers oxygen to the cells, which use it
for cellular respiration. And the blood absorbs carbon
dioxide, which is generated by this process as waste. Through the capillaries in our
lungs, deoxygenated blood becomes oxygenated again when it absorbs oxygen that we
breathe in from the air. And carbon dioxide in the blood is
removed when we exhale.
Frequently, oxygenated blood and
the blood vessels that transport it are represented in red, while deoxygenated blood
and the blood vessels that transport it are represented in blue. But let’s be clear. All of the blood in your body is
actually red in color. And the color of your veins and
arteries depends on the composition of those layers of tissue that we described a
little bit earlier. We just use these colors as a
convenient way to indicate the difference in oxygen concentration.
Also worth pointing out here is
that your circulatory system is what’s called a closed system. All of the blood in your body is
contained within your blood vessels at all times, unless, of course, you’ve
sustained an injury. If you’ve ever accidentally fallen
and scraped your knee, you may notice blood oozing from your wound. You’ve broken one or more blood
vessels and blood is leaking from the closed system. Sometimes students get the mistaken
idea that our bodies are sacks filled with blood in the same way that cells are
sacks filled with cytoplasm. But this is not the case.
So, we’ve already mentioned that
arteries carry blood away from the heart and veins carry blood back towards the
heart. These two types of blood vessels
have some other important differences that make them specifically adapted to their
functions. The walls of arteries are thicker
and more muscular in comparison to veins. This is because arteries transport
blood under very high pressure, and the thick walls prevent the blood vessels from
being damaged. In contrast, veins carry blood
under very low pressure, so they tend to have thinner, more fibrous walls. And many veins also possess valves,
which are special structures that keep blood flowing in one direction.
Well, now that we’re familiar with
the features of veins and arteries, what about the capillaries? Within a closed system, fluids
always flow from high to low pressure. And this fact is what moves our
blood from our heart through all of our blood vessels and back again. Blood leaves the heart through the
aorta and from there travels through arteries, which continually branch off,
becoming smaller and more numerous until they split off into a network of extremely
tiny blood vessels, known as the capillary bed. These capillary beds are extremely
numerous. They’re found in almost every
single tissue of your body. And that’s because the capillaries
are what allow materials to pass into and out of your bloodstream.
So, let’s take a closer look at the
structure of the capillary and how it’s adapted to its function. Here we have an enlarged diagram of
a capillary. These are the smallest blood
vessels in the body, but by far the most numerous. Capillaries have a lumen so small
that red blood cells have to pass through in single file. And the walls of capillaries are
only one cell thick. Capillaries allow many important
materials to pass into and out of the bloodstream through their very thin walls. These include gases like oxygen and
carbon dioxide, glucose and other nutrients from our food, hormones from our glands,
waste materials generated by our various cells, and even water.
Capillaries are the most numerous
type of blood vessel because they’re the sites where materials are exchanged between
the blood and the other tissues of your body. Capillary beds exist in close
contact with almost every tissue in your body. Capillaries deliver oxygen and
glucose to all of our cells, which use those materials to carry out cellular
respiration to generate the cellular energy that powers all our life functions. The capillaries also remove the
excess carbon dioxide that’s produced as a result.
In the lungs, the capillaries
deliver the carbon dioxide, which is removed when we exhale. And they also absorb oxygen from
the air we breathe into the bloodstream. In our kidneys, waste and excess
water are filtered out of the bloodstream through the capillaries, and these
materials are removed from our body in the form of urine. In our small intestines, the
capillaries absorb glucose and other nutrients from our food. And in endocrine glands like our
pituitary gland, the capillaries absorb hormones into the bloodstream so that they
could be circulated throughout the body. Our blood does the job of
transporting materials throughout our bodies. And our capillaries are the blood
vessels that allow those materials to enter and leave the bloodstream.
So, now that we’re familiar with
veins, arteries, capillaries, their structures, and their functions, let’s go ahead
and try a practice question.
Complete the table to correctly
compare the structure of the three major blood vessels.
This question provides us with a
partially filled-in table. We’re given the names of the three
types of blood vessels along the top and various features of their structure along
the side. And we’re being asked to fill in
these four blank spaces with the correct information. In order to fill in these lengths,
we’ll review what we know about the structure of the three major types of blood
vessels. And we’ll start by reviewing their
functions, since we know that structure and function are directly related.
Well, these are all types of blood
vessels. So, we know that their primary
function is to carry blood. The function of veins is to carry
blood into the heart. We’re reminded of this because we
can see the word “in” in the word vein. The function of the arteries is to
carry blood away from the heart. And we’re reminded of this because
both artery and away start with “a.” The function of the capillaries is
to carry blood and allow the exchange of materials.
Here we have a diagram that
represents the general route of blood flow through these three types of blood
vessels. Blood from the heart pumps through
the arteries, where eventually it branches off into a network of tiny blood vessels
known as capillaries. Here, materials like oxygen, carbon
dioxide, water, glucose, and many more pass into and out of the bloodstream. The capillaries connect and
eventually join a vein, and the vein carries the blood back to the heart.
In order to answer our question,
I’ve drawn more detailed diagrams of a vein, an artery, and a capillary. And the information we’re looking
for in these diagrams is the size of the lumen or the space that the blood passes
through, the width of the wall of the blood vessel, and whether or not the blood
vessel possesses any valves which are special structures that keep blood flowing in
one direction.
We’ll start by filling in the
missing information for the vein column. We can see that the width of the
wall is thin, and that part is already filled in. The size of the lumen compared to
the other blood vessels is quite large. And we also can see that valves are
present. Because of the size of the lumen of
the vein and their relative distance from the heart, the blood pressure within them
is quite low, which is why some veins possess valves, which keep the blood flowing
in the correct direction and prevent it from flowing backwards.
Next, let’s work on the artery
column. The size of the lumen is relatively
small. And the blood pressure in arteries
is higher than in veins, so they don’t need valves to keep blood flowing in one
direction. Because of the high blood pressure,
arteries also need thick, muscular walls that prevent them from being damaged.
Finally, let’s complete the
capillary column. Capillaries have very thin walls,
only one cell layer thick, which is what allows the easy exchange of materials into
and out of the bloodstream. The size of the lumen is very
small, so small in fact that blood cells pass through in single file. And there are no valves present in
capillaries. The blood within them flows from
the high pressure of the artery to the low pressure of the vein without the need for
these special structures.
And now our table is complete. We filled in that arteries have
thick walls, veins have a large lumen, and that valves are present in veins but not
present in capillaries.
Let’s wrap up our lesson by taking
a moment to review what we’ve learned. In this video, we learned about the
three major types of blood vessels. We learned that veins have features
that make them specially adapted to carry blood towards the heart under relatively
low pressure. We learned that the adaptations of
arteries allow them to carry blood away from the heart under relatively high
pressure. And we learned that capillaries are
specially adapted to allow the exchange of materials into and out of the
bloodstream.
