Human Physiology, 14th edition (2016)

Blood, Heart, and Circulation 431

13.6 Blood Vessels

The thick muscle layer of the arteries allows them to trans-

port blood ejected from the heart under high pressure. The

thinner muscle layer of veins allows them to distend when

an increased amount of blood enters them, and their one-

way valves ensure that blood flows back to the heart. Cap-

illaries facilitate the rapid exchange of materials between

the blood and interstitial fluid.

Figure 13.25 The relationship between changes
in intraventricular pressure and the ECG. The QRS
wave (representing depolarization of the ventricles) occurs at
the beginning of systole, whereas the T wave (representing
repolarization of the ventricles) occurs at the beginning of
diastole. The numbered steps at the bottom of the figure
correspond to the numbered steps at the top.

R

P

Q

S

ECG

T

S 1 S 2

Heart

sounds

Q

P

0

20

40

60

80

100

120

0 0.2 0.4

Time (seconds)

1. Intraventricular
   pressure rises
   as ventricles
   contract


2. AV valves
   close


3. Semilunar
   valves close


4. Intraventricular
   pressure falls
   as ventricles
   relax

0.6 0.8

Systole Diastole

Pressure in ventricle (mmHg)

| CHECKPOINT

10a. Describe the electrical activity of the cells of the SA

node and explain how the SA node functions as the

normal pacemaker.

10b. Using a line diagram, illustrate a myocardial action

potential and the time course for myocardial

contraction. Explain how the relationship between

these two events prevents the heart from sustaining

a contraction and how it normally prevents abnormal

rhythms of electrical activity.

11a. Draw an ECG and label the waves. Indicate the

electrical events in the heart that produce these waves.

11b. Draw a figure that shows the relationship between ECG

waves and the heart sounds. Explain this relationship.

11c. Describe the pathway of electrical conduction of the heart,

starting with the SA node. How does damage to the AV

node affect this conduction pathway and the ECG?

LEARNING OUTCOMES

After studying this section, you should be able to:

12. Compare the structure and function of arteries and
    veins, and the significance of the skeletal muscle
    pumps.


13. Describe the structures and functions of different
    types of capillaries.

Blood vessels form a tubular network throughout the body that

permits blood to flow from the heart to all the living cells of the

body and then back to the heart. Blood leaving the heart passes

through vessels of progressively smaller diameters, referred to as

arteries, arterioles, and capillaries. Capillaries are microscopic

vessels that join the arterial flow to the venous flow. Blood return-

ing to the heart from the capillaries passes through vessels of pro-

gressively larger diameters, called venules and veins.

The walls of arteries and veins are composed of three coats,

or “tunics.” The outermost layer is the tunica externa, the mid-

dle layer is the tunica media, and the inner layer is the tunica

interna. The tunica externa is composed of connective tissue,

whereas the tunica media is composed primarily of smooth

muscle. The tunica interna consists of three parts: (1) an inner-

most simple squamous epithelium, the endothelium, which lines

the lumina of all blood vessels; (2) the basement membrane

(a layer of glycoproteins) overlying some connective tissue

fibers; and (3) a layer of elastic fibers, or elastin, forming an

internal elastic lamina.

Although arteries and veins have the same basic structure

( fig. 13.26 ), there are some significant differences between them.

Arteries have more muscle for their diameters than do compara-

bly sized veins. As a result, arteries appear more rounded in cross

section, whereas veins are usually partially collapsed. In addition,

many veins have valves, which are absent in arteries.

Arteries

In the aorta and other large arteries, there are numerous layers

of elastin fibers between the smooth muscle cells of the tunica

media. These large elastic arteries expand when the pressure

of the blood rises as a result of the ventricles’ contraction;

they recoil like a stretched rubber band when the blood pres-

sure falls during relaxation of the ventricles. This elastic recoil

drives the blood during the diastolic phase—the longest phase