Download Chapter 21: Blood Vessels and Circulation

Crowther’s Tenth Martini, Chapter 21
Summer 2015
Chapter 21:
Blood Vessels
and Circulation
In general, circulatory systems have three basic components: fluid to transport materials, one or
more pumps to propel the fluid, and vessels to hold the fluid. Having covered the fluid in
Chapter 19 (Blood) and the main pump in Chapter 20 (The Heart), we now consider the vessels,
as well as the cardiovascular system as an integrated whole, in Chapter 21.
21.0: Outline
21.1: Overview of circulatory system anatomy
 Blood flowing away from the heart proceeds through arteries, arterioles, capillaries,
venules, and veins. Arteries and veins are the fewest in number and are the biggest in
terms of lumen size and wall thickness.
21.2: Forces affecting blood flow: pressure and resistance
 Bulk flow through a blood vessel is directly proportional to the pressure difference
between the start of the vessel and the end of the vessel. Flow is inversely proportional to
the resistance, which in turn depends on vessel length, vessel radius and fluid viscosity.
These influences in blood flow are encapsulated in Poiseuille’s Law of Laminar Flow.
21.3: Filtration, reabsorption, and exchange at the capillaries
 Fluid movement out of and into capillaries depends on hydrostatic pressure (higher in the
capillaries, driving fluid out) and osmotic pressure (higher outside the capillaries, driving
fluid into the capillaries).
 Capillaries exchange nutrients and wastes with tissues via diffusion, which is governed
by Fick’s Law.
21.4: Regulation of blood pressure
 Blood pressure must be maintained at or above a setpoint to ensure adequate flow to the
body’s tissues, especially the brain.
 Baroreceptors located in the carotid arteries and aorta sense changes in blood pressure.
 The sympathetic nervous system responds immediately to falling blood pressure by
increasing the pumping of the heart and constricting the arterioles.
 Longer-term hormonal responses to low blood pressure include the release of antidiuretic
hormone (ADH) by the posterior pituitary, erythropoietin by the kidney, and aldosterone
by the adrenal cortex.
21.5: Recommended review questions
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Crowther’s Tenth Martini, Chapter 21
Summer 2015
21.1: Overview of circulatory system anatomy
The word “cardiovascular” literally means the heart (cardio) plus blood vessels (vascular).
Having looked in detail at the heart in Chapter 20, we will now focus on the vessels to which it is
directly or indirectly connected.
The general arrangement of cardiovascular circuits is shown in 10th Martini Figure 21-17 (A
Schematic Overview of the Pattern of Circulation). Note that the pulmonary circuit, which
carries blood to the lungs, is separate from the systemic circuit, which carries blood everywhere
else. Also note that the systemic circuit includes several parallel branches: to the brain, upper
limbs, kidneys, gastrointestinal organs, gonads, and lower limbs.
As an overview figure, 10th Martini Figure 21-17 simplifies both the number of “branches” of the
circulatory system and the branching pattern within each branch. If you consult 10th Martini
Figure 21-23 (Major Arteries of the Trunk), you will see that there are more than 15 major
arteries off of the aorta. (We shall see a bit later that blood flow can be selectively directed
toward the specific systemic branches that need it most.) In addition, within a given “branch,”
the main artery divides into smaller arteries, which in turn form arterioles, capillaries, venules,
and veins, as shown in CTM Figure 21.1.
CTM Figure 21.1: The
branching of the
circulatory system. A
major artery branches
into several smaller
arteries and arterioles,
which branch into
capillary beds, which
merge into venules
and then veins. Figure
taken from Scott
Freeman et al.,
Biological Science (5th
edition).
As you might expect, the structure of the large arteries and veins is quite different from that of
the tiny capillaries. While all blood vessels are lined with endothelial cells, the walls of arteries
and veins also include considerable amounts of connective tissue (including elastic fibers) and
smooth muscle. Capillaries have only a thin coat of connective tissue surrounding the
endothelial cells; arterioles and venules have walls only slightly thicker than those of capillaries.
Detailed cross-sectional views of the vessels are shown in 10th Martini Figure 21-2 (Histological
Structure of Blood Vessels).
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Crowther’s Tenth Martini, Chapter 21
Summer 2015
21.2: Forces affecting blood flow: pressure and resistance
If you have taken physics, you may remember Ohm’s law, which states that the flow of current
through a conductor (such as a wire) equals the voltage divided by the resistance: I = V/R. A
very similar relationship governs blood flow; blood flow through a vessel (often abbreviated Q)
equals the pressure difference between the beginning and the end of the vessel divided by the
resistance: Q = ∆P/R.
Perhaps the idea of a pressure difference is fairly straightforward: blood flows from an area of
higher pressure to an area of lower pressure. But what does resistance mean in the context of
blood flow? Resistance encompasses factors that slow down the flow of blood, including the
viscosity of the blood, friction between the blood and the surrounding walls, and turbulence
(disruptions to uniform flow caused by high flow speeds and irregularities in vessels). If we
ignore the turbulence issue for now, we can write out an equation for resistance as follows:
Resistance = ((8/ᴨ)*(viscosity)*(length))/(radius)4. This is commonly abbreviated as: R =
((8/ᴨ)*viscosity*L)/r4. Note that the dominator is radius to the 4th power! That is, if the radius
of the vessel doubles, the resistance goes down to 1/16 of what it was before! Therefore even
small changes in vessel radius can cause significant changes in resistance, and thus blood flow.
If we substitute the formula for R into our previous equation of Q = ∆P/R, we get what is known
as Poiseuille’s Law of Laminar Flow, named after the French physician who formulated it: Q =
(∆P* r4)/((8/ᴨ)*viscosity*L). In this version of the relationship, it becomes more obvious that
flow is directly proportional to vessel radius to the 4th power. Using the same example as before,
if the radius doubles, the flow increases by a factor of 16 (assuming everything else stays
constant).
Though 10th Martini is generally equation-phobic and does not formally present Poiseuille’s
Law, Figure 21-7 (Factors Affecting Friction and Vascular Resistance) illustrates some of the
contributing variables.
21.3: Filtration, reabsorption, and exchange at the capillaries
As blood flows through the arteries and arterioles, it eventually gets to the numerous thin-walled
capillaries. Blood flow through the capillaries more or less follows Poiseuille’s Law; however,
movement of materials also occurs through the capillary walls. Movement of fluids – and any
solutes they contain that are small enough to get out – is known as filtration if the net movement
is out of the capillaries and reabsorption if the net movement is into the capillaries. As shown in
CTM Figure 21.2, filtration generally occurs at the upstream end of capillaries and reabsorption
takes place at the downstream end.
The key to CTM Figure 21.2 is that movement of water is a balance of two forces: hydrostatic
pressure (what we normally think of as blood pressure) and osmotic pressure (the attraction of
water toward the region with the highest concentration of solutes). At the upstream end of the
capillary, the blood is under relatively high pressure and forces some of the fluid out of the
capillary. However, by the downstream end of the capillary, the blood has lost much of its
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Crowther’s Tenth Martini, Chapter 21
Summer 2015
remaining hydrostatic pressure (due to continuing resistance along the length of the capillary),
and the higher concentration of solutes inside the capillary draws some water back into it. These
filtration and absorption processes are especially important in the kidney, which has the task of
getting rid of wastes while reabsorbing valuable nutrients such as glucose and amino acids.
CTM Figure 21.2: Filtration out of and reabsorption into capillaries is driven by hydrostatic and
osmotic pressures. Figure taken from Scott Freeman et al., Biological Science (5th edition).
As shown in CTM Figure 21.2, a bit more fluid is typically filtered out of the capillaries than
reabsorbed into them; the excess is taken up by the lymphatic system and ultimately returned to
the veins. However, if there is an imbalance between filtration, reabsorption, and lymphatic
uptake, fluid may accumulate in the interstitial fluid surrounding the capillaries – a condition
known as edema. Edema may result from changes in hydrostatic pressure or osmotic pressure or
both. For example, in people with very high blood pressure, or hypertension, higher-than-normal
amounts of fluid may be driven into the interstitium. Starvation is a very different situation with
a somewhat similar outcome: there is a decline in the levels of plasma proteins (which are too
big to leave the capillaries, and thus provide osmotic pressure drawing water into the capillaries),
so the blood has less osmotic “pull” on the water and more accumulates in the interstitium.
Edema is discussed as a Clinical Note on p. 741 of 10th Martini.
Amidst all of this movement of fluids and solutes in and out of the capillaries, surrounding cells
absorb nutrients (O2, glucose, amino acids, etc.) and release waste products (CO2, lactic acid,
etc.) This exchange of nutrients and wastes between the capillaries and surrounding cells is the
circulatory system’s main purpose, so it’s probably wrong for me to bury it here in this
unremarkable paragraph. Anyway, such exchange occurs by diffusion, and is thus driven by
concentration gradients (NOT the pressure gradients mentioned above for Poiseuille). To help us
understand diffusion better, we have another equation (not explicitly presented by 10th Martini):
Fick’s Law of Diffusion. This law says that diffusion rate equals the concentration gradient
times the surface area over which diffusion occurs times a diffusion constant, all divided by the
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Commented [J1]: This made me chuckle! I imagine it could be
moved to the beginning somehow, but I enjoy how it is stated here
Crowther’s Tenth Martini, Chapter 21
Summer 2015
thickness of the diffusion barrier. More succinctly, diffusion rate = ((C1 – C2)*A*k)/D. If we
are considering the diffusion of a gas, the concentration gradient (C1 – C2) will generally be
reported as a partial pressure gradient (P1 – P2), but do not confuse this partial pressure gradient
with the (hydrostatic) pressure gradient in Poiseuille’s law; these are two very different kinds of
pressure. Also, for completeness, note that the “constant” k is only constant for a given
substance diffusing through a given medium.
Fick’s Law helps us understand the factors that can increase or decrease diffusion rate. For
example, since diffusion rate is proportional to surface area, it makes sense that capillaries are so
heavily branched, providing lots of area across which substances can diffuse. Keeping D small is
also important, and thus helps explain why the capillaries’ walls are so thin. In cases of edema,
mentioned above, accumulation of fluid between the capillaries and nearby cells increases D and
thus slows diffusion rate. This can be a real problem, for example, in the case of pulmonary
edema, when fluid in the lungs impairs the diffusion of oxygen into the blood.
21.4: Regulation of blood pressure
In clinical settings, blood pressure usually refers to blood pressure in the main arteries, which
varies during the cardiac cycle, as shown in 10th Martini Figure 21-9 (Pressures within the
Systemic Circuit). The systolic blood pressure is the peak of arterial pressure generated during
ventricular contraction (systole); the diastolic blood pressure is the lowest arterial pressure that
occurs while the heart is relaxing (diastole). However, blood pressure can be measured all along
the entire circuit, as shown in 10th Martini Figure 21-9. It can be seen that as the blood gets
farther and farther away from the heart, it loses pressure as it encounters resistance. By the time
the blood gets back to the heart, its pressure is close to 0 mm Hg!
Although blood flow outside the heart gets a boost from such actions as deep inhalations that
create negative pressure in the thorax (the so-called “respiratory pump”) and the pumping of
active skeletal muscles, the fact remains that the heart is the main driver of blood flow, so blood
in the arteries must have adequate momentum to traverse the rest of the circulatory system
quickly. Arterial pressure is especially important for maintaining blood flow to the brain, which
usually occurs against gravity. Therefore, keeping arterial blood pressure at or above a minimum
setpoint is a top priority for the body.
Arterial pressure is sensed by baroreceptors in the aorta and carotid arteries. These receptors
are similar to the mechanoreceptors that sense touch and pressure in your skin; the higher the
pressure, the more distorted these receptors become, and the more dramatic the changes in their
membrane potential and neurotransmitter release. If baroreceptors report to the hypothalamus –
negative feedback central – that blood pressure is low, the hypothalamus makes adjustments to
bring blood pressure back up. These adjustments can be divided into immediate responses
governed by the nervous system and longer-term responses governed by hormones.
In the short term, the hypothalamus directs the medulla’s cardiac center (controlling the heart)
and vasomotor center (controlling smooth muscle tone in the blood vessel walls) to increase the
pumping of the heart and constrict the arterioles. These pathways are part of the sympathetic
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Summer 2015
nervous system; their effects on cardiac muscle and smooth muscle cells are caused by the
neurotransmitter norepinephrine (often abbreviated NE).
Thus, the short-term solution to low blood pressure is to make the heart pump harder and to
decrease the size of the containers holding the blood. A longer-term solution is to increase the
amount of blood in the system. This can be achieved through actions of hormones like these:
 Erythropoietin from the kidney stimulates production of new red blood cells in the red
bone marrow.
 Antidiuretic hormone (ADH, or vasopressin) from the posterior pituitary stimulates
retention of water in the kidney, so that it is not lost in the urine.
 Aldosterone from the adrenal cortex promotes salt retention in the kidney, which
osmotically helps the body hang onto its water.
Such regulatory mechanisms come into play in situations such as hemorrhage, where a loss of
blood may lead to dangerous drops in blood pressure. If the sympathetic nervous system does
not maintain blood pressure at the setpoint, the bleeding victim may faint. This seemingly
inconvenient response is actually a useful mechanism for maintaining blood flow to the brain,
since the heart of a passed-out person no longer has to fight gravity in pumping blood to the
brain.
10th Martini covers blood pressure regulation in figures such as Figure 21-12 (Short-Term and
Long-Term Cardiovascular Responses), Figure 21-13 (Baroreceptor Reflexes of the Carotid and
Aortic Sinuses), and Figure 21-16 (Cardiovascular Responses to Blood Loss). On the whole,
though, I find these figures unnecessarily complicated.
21.5: Recommended review questions
If your understanding of this chapter is good, you should be able to answer the following 10th
Martini questions at the end of Chapter 21: review questions #2, 4, 5, 11, 17, 20, 23, and 30.
(Note that these are NOT the Checkpoint questions sprinkled throughout the chapter.)
Explanation
This document is my distillation of a chapter of the textbook Fundamentals of Anatomy &
Physiology, Tenth Edition, by Frederic H. Martini et al. (a.k.a. “the 10th Martini”). While this
textbook is a valuable resource, I believe that it is too dense to be read successfully by many
undergraduate students. I offer “Crowther’s Tenth Martini” so that students who have purchased
the textbook may benefit more fully from it. No copyright infringement is intended.
-- Greg Crowther
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