Essentials of Anatomy and Physiology

venous return increases, cardiac muscle fibers are
stretched, and the ventricles pump more forcefully
(Starling’s law). Thus, cardiac output and blood pres-
sure increase. This is what happens during exercise,
when a higher blood pressure is needed. When exer-
cise ends and venous return decreases, the heart pumps
less forcefully, which helps return blood pressure to a
normal resting level.
The second intrinsic mechanism involves the kid-
neys. When blood flow through the kidneys decreases,
the process of filtration decreases and less urine is
formed. This decrease in urinary output preserves
blood volume so that it does not decrease further.
Following severe hemorrhage or any other type of
dehydration, this is very important to maintain blood
pressure.
The kidneys are also involved in the renin-

angiotensin mechanism. When blood pressure

decreases, the kidneys secrete the enzyme renin,

which initiates a series of reactions that result in the

formation of angiotensin II. These reactions are

described in Table 13–3 and depicted in Fig. 13–12.

Angiotensin II causes vasoconstriction and stimulates

secretion of aldosterone by the adrenal cortex, both of

which will increase blood pressure.

NERVOUS MECHANISMS

The medulla and the autonomic nervous system are

directly involved in the regulation of blood pressure.

The first of these nervous mechanisms concerns the

heart; this was described previously, so we will not

review it here but refer you to Chapter 12 and Fig.

12–7, as well as Fig. 13–13.

The Vascular System 311

Skeletal muscle

Blood distribution (mL/min)

Resting cardiac output

5,000 mL/min

Exercise cardiac output

15,000 mL/min

Heart Brain Skin GI tract Kidneys Rest of body

Resting

215 mL

% of

total

% of

total

Exercise

1,035 mL 645 mL 430 mL 1,205 mL 950 mL 515 mL

645 mL

10,710 mL 645 mL

1,635 mL

510 mL

510 mL

330 mL

4% 21% 13% 9% 24% 19% 10%

4.5% 71% 4.5% 11% 3.5% 3.5% 2%

Figure 13–11. Blood flow through various organs when the body is at rest and during

exercise. For each organ, the percentage of the total blood flow is given.

QUESTION:During exercise, which organs have the greatest increase in blood flow?

Which organs have the greatest decrease?