Cardiovascular Physiology Lecture 6: A look at blood vessels
Learning Objectives
1. Explain how pressures change as blood moves from the aorta back to the vena cava.
Describe where the largest drop in blood pressure occurs and why.
2. Discuss how blood pressure is measured and how mean arterial pressure is calculated.
3. Compare the pressures in the pulmonary and systemic circuit and the reasons for the
differences.
4. Outline the basic structural components of blood vessels, including the importance of
smooth muscle, elastin, and the role of the endothelium.
5. Describe atherosclerosis, how it develops, and what risk factors increase its severity.
6. Compare arteries and arterioles including their similarities and differences.
7. List the factors affecting resistance to flow and be able to predict blood flow alterations
when these factors change.
Much of the focus for cardiovascular physiology up to this point has been on the heart. In
these remaining lectures, we will shift our attention to the blood vessels, look at some of
their general properties, and what affects blood flow through them.
The function of blood vessels
Recall from earlier lectures that the circulatory system is responsible for delivering nutrients,
oxygen, hormones, and other substances to all of our tissues, as well as removing waste
products, such as carbon dioxide. Therefore, the distribution of blood to our various organs
and tissues must meet their metabolic demands. But that isn’t the only role that vessels play.
Our vasculature contributes to heat regulation, participates in inflammatory responses, and
is even essential in the clotting process during injury.
You have learned that blood flows from arteries to arterioles to capillaries to venules and
finally to veins before returning to the heart. As you will see, the structure of each of these
vessels is quite different. It is important to realize that their unique structures are what gives
the vessels their individual properties. It might be helpful to go back and review the
distribution of blood volume and the general functions of an artery, vein, and capillary
covered in cardiovascular lecture 1 before class.
Pressure influences blood flow
The reason blood leaves the heart, flows through arteries, and eventually ends up in veins
before returning to the heart, is because of a pressure gradient. Blood must move from a
high pressure to a low pressure as you learned in the cardiac cycle. Therefore, the highest
pressure in the systemic circuit is found in the aorta, while the lowest pressure is found in
the vena cava, which returns blood to the right side of the heart. In the pulmonary circuit, the
same holds true. The pulmonary arteries experience the largest pressure and the pulmonary
veins have a very low pressure before returning blood to the left side of the heart.
,Measuring blood pressure
You have likely had a physician measure your blood pressure before. Or perhaps you have
done this on your own using a portable cuff or at a local pharmacy. A sphygmomanometer
is used for such recordings. Two numbers will be determined during these measurements:
the systolic blood pressure, which is the force generated on the walls of arteries during
contraction of the ventricles, and diastolic pressure, the force that exists when the ventricles
are relaxed. A typical blood pressure is 120/80 (systolic over diastolic).
The idea behind the sphygmomanometer is that an inflatable cuff is placed around the upper
arm to initially stop blood flow all together, and then slowly deflated until blood resumes
normal flow. Your doctor will listen with a stethoscope to the sounds in the artery below the
cuff. When no blood is flowing, the artery is silent. Once the pressure in the cuff matches
that of your systolic pressure, blood is able to begin to flow through this constricted vessel.
This blood flow is “noisy” because blood moving quickly through a constricted vessel is
turbulent (see below). The sounds that your doctor is listening for are called Korotkoff
sounds. Once the cuff deflates to your diastolic pressure, the vessel is no longer constricted
and blood flows in an ordered, “smooth” fashion in what we refer to as laminar flow, which
is silent. Thus, the pressure where turbulent flow is first heard is your systolic pressure and
your diastolic pressure is when the sound disappears.
Taken from: cvphysiology.com/Hemodynamics/HOO7
You will also hear the term mean arterial pressure (MAP) during this course. While you might
imagine that this should be just the average between your systolic and diastolic blood
pressure, which would be approximately 100 mmHg, it actually isn’t. This is because our
heart spends more time in diastole than it does in systole, and therefore we have to account
for that. Mean arterial pressure is represented mathematically using the following formula:
MAP = diastolic pressure + 1/3 (systolic – diastolic pressure)
Pulmonary versus systemic circuit: Pressures
You have learned that two circuits exist in the cardiovascular system: systemic and
pulmonary. We will spend almost all of our remaining lectures focused on the systemic
circuits where pressures are much higher. This is partly because blood has to be pumped
uphill (think head or lower extremities), something we don’t see for pulmonary circulation.
There is also a lot of redistribution of blood in the systemic circulation due to changing
metabolic demands of tissues and so a higher pressure is required to overcome the higher
resistance in this circuit. We will look at the resistance to flow more in the next lecture.
, Blood Vessels: General Properties
As mentioned earlier, each type of blood vessel has special structural properties that differ
between vessels. The components found in each vessel may include elastic fibres, fibrous
connective tissue, and smooth muscle. All blood vessels contain endothelial cells that line
the interior of the vessel and are in contact with the blood flowing through it.
All blood vessels, except capillaries, contain three layers in their walls:
i) tunica adventitia: the outermost layer, composed mostly of fibrous connective tissue
ii) tunica media: the middle layer, consists of smooth muscle, collagen, and elastin
iii) tunica intima: the innermost layer, composed of endothelial cells
Endothelial cells have a much more important role than simply lining our blood vessels.
These cells are fascinating since they can sense the force of blood flowing through those
1
. .
2
vessels and can both react to substances in the blood as well as create substances that
will change the diameter of our vessels. Nitric oxide (NO) is one of these substances that
E
the endothelial cells will produce. NO will cause relaxation of smooth muscle found in the
g tunica media of vessels, such as arterioles, which will dramatically increase blood flow to
the capillaries of that tissue. We will look at more details in lecture. summe
o
Along with these three layers, veins also contain& valves to ensure blood flows in one
direction - back to the heart. Arteries do not contain valves.
Capillaries, on the other hand, are composed entirely of a single layer of endothelial cells
and a basement membrane. Their thin walls permit the movement of substances into and
out of the blood. These are considered our exchange vessels since this is where nutrients,
wastes, gases, and other substances are exchanged between our tissue cells and our
blood.
g
The amount of connective tissue, smooth muscle tissue, and elastin varies in arteries,
arterioles, venules, and veins, as well as in the larger vessels like the aorta and vena
cava. This is very important because the structure of the vessel wall determines the
function of each vessel. The structure also contributes to the pressure and volume
characteristics throughout circulation. In this lecture, we will look at the structural features
of the different blood vessels. For example, we will discuss the relative amount of elastin,
smooth muscle, and fibrous connective tissue. Please do not memorize these numbers. We
are more interested that you understand trends in their composition and their relative wall
thickness compared to their diameter.
Aorta and Large Arteries
Notice that the pressure in the aorta and large arteries is quite high. As you learned in the
cardiac cycle, it normally fluctuates between 120 mmHg and 80 mmHg, so we say the
pressure is pulsatile. The walls of these vessels contain a relatively large amount of elastin.
The vessels are stretched during systole and recoil during diastole to allow blood to continue
Learning Objectives
1. Explain how pressures change as blood moves from the aorta back to the vena cava.
Describe where the largest drop in blood pressure occurs and why.
2. Discuss how blood pressure is measured and how mean arterial pressure is calculated.
3. Compare the pressures in the pulmonary and systemic circuit and the reasons for the
differences.
4. Outline the basic structural components of blood vessels, including the importance of
smooth muscle, elastin, and the role of the endothelium.
5. Describe atherosclerosis, how it develops, and what risk factors increase its severity.
6. Compare arteries and arterioles including their similarities and differences.
7. List the factors affecting resistance to flow and be able to predict blood flow alterations
when these factors change.
Much of the focus for cardiovascular physiology up to this point has been on the heart. In
these remaining lectures, we will shift our attention to the blood vessels, look at some of
their general properties, and what affects blood flow through them.
The function of blood vessels
Recall from earlier lectures that the circulatory system is responsible for delivering nutrients,
oxygen, hormones, and other substances to all of our tissues, as well as removing waste
products, such as carbon dioxide. Therefore, the distribution of blood to our various organs
and tissues must meet their metabolic demands. But that isn’t the only role that vessels play.
Our vasculature contributes to heat regulation, participates in inflammatory responses, and
is even essential in the clotting process during injury.
You have learned that blood flows from arteries to arterioles to capillaries to venules and
finally to veins before returning to the heart. As you will see, the structure of each of these
vessels is quite different. It is important to realize that their unique structures are what gives
the vessels their individual properties. It might be helpful to go back and review the
distribution of blood volume and the general functions of an artery, vein, and capillary
covered in cardiovascular lecture 1 before class.
Pressure influences blood flow
The reason blood leaves the heart, flows through arteries, and eventually ends up in veins
before returning to the heart, is because of a pressure gradient. Blood must move from a
high pressure to a low pressure as you learned in the cardiac cycle. Therefore, the highest
pressure in the systemic circuit is found in the aorta, while the lowest pressure is found in
the vena cava, which returns blood to the right side of the heart. In the pulmonary circuit, the
same holds true. The pulmonary arteries experience the largest pressure and the pulmonary
veins have a very low pressure before returning blood to the left side of the heart.
,Measuring blood pressure
You have likely had a physician measure your blood pressure before. Or perhaps you have
done this on your own using a portable cuff or at a local pharmacy. A sphygmomanometer
is used for such recordings. Two numbers will be determined during these measurements:
the systolic blood pressure, which is the force generated on the walls of arteries during
contraction of the ventricles, and diastolic pressure, the force that exists when the ventricles
are relaxed. A typical blood pressure is 120/80 (systolic over diastolic).
The idea behind the sphygmomanometer is that an inflatable cuff is placed around the upper
arm to initially stop blood flow all together, and then slowly deflated until blood resumes
normal flow. Your doctor will listen with a stethoscope to the sounds in the artery below the
cuff. When no blood is flowing, the artery is silent. Once the pressure in the cuff matches
that of your systolic pressure, blood is able to begin to flow through this constricted vessel.
This blood flow is “noisy” because blood moving quickly through a constricted vessel is
turbulent (see below). The sounds that your doctor is listening for are called Korotkoff
sounds. Once the cuff deflates to your diastolic pressure, the vessel is no longer constricted
and blood flows in an ordered, “smooth” fashion in what we refer to as laminar flow, which
is silent. Thus, the pressure where turbulent flow is first heard is your systolic pressure and
your diastolic pressure is when the sound disappears.
Taken from: cvphysiology.com/Hemodynamics/HOO7
You will also hear the term mean arterial pressure (MAP) during this course. While you might
imagine that this should be just the average between your systolic and diastolic blood
pressure, which would be approximately 100 mmHg, it actually isn’t. This is because our
heart spends more time in diastole than it does in systole, and therefore we have to account
for that. Mean arterial pressure is represented mathematically using the following formula:
MAP = diastolic pressure + 1/3 (systolic – diastolic pressure)
Pulmonary versus systemic circuit: Pressures
You have learned that two circuits exist in the cardiovascular system: systemic and
pulmonary. We will spend almost all of our remaining lectures focused on the systemic
circuits where pressures are much higher. This is partly because blood has to be pumped
uphill (think head or lower extremities), something we don’t see for pulmonary circulation.
There is also a lot of redistribution of blood in the systemic circulation due to changing
metabolic demands of tissues and so a higher pressure is required to overcome the higher
resistance in this circuit. We will look at the resistance to flow more in the next lecture.
, Blood Vessels: General Properties
As mentioned earlier, each type of blood vessel has special structural properties that differ
between vessels. The components found in each vessel may include elastic fibres, fibrous
connective tissue, and smooth muscle. All blood vessels contain endothelial cells that line
the interior of the vessel and are in contact with the blood flowing through it.
All blood vessels, except capillaries, contain three layers in their walls:
i) tunica adventitia: the outermost layer, composed mostly of fibrous connective tissue
ii) tunica media: the middle layer, consists of smooth muscle, collagen, and elastin
iii) tunica intima: the innermost layer, composed of endothelial cells
Endothelial cells have a much more important role than simply lining our blood vessels.
These cells are fascinating since they can sense the force of blood flowing through those
1
. .
2
vessels and can both react to substances in the blood as well as create substances that
will change the diameter of our vessels. Nitric oxide (NO) is one of these substances that
E
the endothelial cells will produce. NO will cause relaxation of smooth muscle found in the
g tunica media of vessels, such as arterioles, which will dramatically increase blood flow to
the capillaries of that tissue. We will look at more details in lecture. summe
o
Along with these three layers, veins also contain& valves to ensure blood flows in one
direction - back to the heart. Arteries do not contain valves.
Capillaries, on the other hand, are composed entirely of a single layer of endothelial cells
and a basement membrane. Their thin walls permit the movement of substances into and
out of the blood. These are considered our exchange vessels since this is where nutrients,
wastes, gases, and other substances are exchanged between our tissue cells and our
blood.
g
The amount of connective tissue, smooth muscle tissue, and elastin varies in arteries,
arterioles, venules, and veins, as well as in the larger vessels like the aorta and vena
cava. This is very important because the structure of the vessel wall determines the
function of each vessel. The structure also contributes to the pressure and volume
characteristics throughout circulation. In this lecture, we will look at the structural features
of the different blood vessels. For example, we will discuss the relative amount of elastin,
smooth muscle, and fibrous connective tissue. Please do not memorize these numbers. We
are more interested that you understand trends in their composition and their relative wall
thickness compared to their diameter.
Aorta and Large Arteries
Notice that the pressure in the aorta and large arteries is quite high. As you learned in the
cardiac cycle, it normally fluctuates between 120 mmHg and 80 mmHg, so we say the
pressure is pulsatile. The walls of these vessels contain a relatively large amount of elastin.
The vessels are stretched during systole and recoil during diastole to allow blood to continue
