HUMAN BIOLOGY

130 Chapter 7

structure and Functions of blood vessels

cycle is stored in the “bulge”; the elastic recoil of the artery

then forces that stored blood onward during diastole,

when heart chambers are relaxed. In addition to having

stretchable walls, arteries also have large diameters. For

this reason, they present little resistance to blood flow, so

blood pressure in large arteries is quite stable (Figure 7.14).

arterioles are control points for blood flow

Arteries branch into narrower arterioles, which have a wall

built of rings of smooth muscle over a single layer of elastic

fibers (Figure 7.13B). Being built this way, arterioles dilate

(enlarge in diameter) when the smooth muscle relaxes,

and they constrict (shrink in diameter) when the smooth

muscle contracts. Arterioles offer more resistance to blood

flow than other vessels do. As the blood flow slows, it can

be controlled in ways that adjust how much of the total

volume goes to different body regions. For example, you

may feel sleepy after a large meal in part because control

signals divert blood away from your brain and into vessels

serving your digestive system.

Capillaries are specialized for diffusion

Your body has about 2 miles of arteries and veins but a

whopping 62,000 miles of capillaries. These tiny vessels

often interlace in capillary beds, and their structure

allows substances to readily diffuse between blood and

tissue fluid. Specifically, a capillary has the thinnest wall

of any blood vessel—a single layer of flat endothelium

(Figure 7.13C). As you might guess, the body’s capacity for

maintaining homeostasis depends heavily on the diffusion

of gases (oxygen and carbon dioxide), nutrients, and

wastes that occurs across the walls of capillaries.

Blood can’t move fast in capillaries. However, because

there are so many capillaries and capillary beds, they

present less total resistance to flow than do the arterioles

leading into them, so overall blood pressure drops more

slowly in them.

n There are differences in how different kinds of blood

vessels manage blood flow and blood pressure.

n Links to Epithelium 4.1, Connective tissues 4.2

arteries are large, strong blood pipelines

The wall of an artery has several tissue layers (Figure

7.13A). The outer layer is mainly collagen, which anchors

the vessel to the tissue it runs through. A thick middle layer

of smooth muscle is sandwiched between thinner layers

containing elastin. The innermost layer is a thin sheet of

endothelium. Together these layers form a thick, muscular,

and elastic wall. In a large artery the wall bulges slightly

under the pressure surge caused when a ventricle contracts.

In arteries near the body surface, as in the wrist, you can

feel the surges as your pulse.

The bulging of artery walls helps keep blood flowing

on through the system. How? For a moment, some of the

blood pumped during the systole phase of each cardiac

Figure 7.13 Animated! The structure of a blood vessel

matches its function.

Figure 7.14 Blood pressure changes as blood flows through

different parts of the cardiovascular system.

A Artery

C Capillary

B Arteriole

D Venule

E Vein

connective

tissue coat

smooth

muscle endothelium

elastic tissueelastic tissue

endothelium

smooth muscle rings

over elastic tissue endothelium

connective

tissue coat

smooth

muscle endothelium

connective

tissue coat

smooth muscle,

elastic fibers endothelium

valve

120

80

40

0

Blood pr

essur

e (mm Hg)

(systolic)

(diastolic)

arteries capillaries

venules

veins

arterioles

7. 6

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