Human Physiology, 14th edition (2016)

Cardiac Output, Blood Flow, and Blood Pressure 463

decreases resistance and increases flow, whereas vasocon-

striction increases resistance and decreases flow. These

changes occur in response to various regulatory mechanisms.

Figure 14.13 Negative feedback correction of increased venous return. Stimulation of stretch receptors in the left
atrium causes secretion of atrial natriuretic peptide (ANP), which increases urinary excretion of salt and water. At the same time,
stimulation of these stretch receptors leads to decreased antidiuretic hormone (ADH) secretion. Because ADH stimulates the kidneys to
reabsorb (retain) water, a fall in ADH works together with the increased ANP to increase urine volume. This maintains homeostasis by
lowering blood volume and venous return.

Negative feedback

Water immersion

or

increased blood volume

Kidneys

Vagus nerves

Venous return

Urine volume

Blood volume

Atrial natriuretic

peptide (ANP)

NaCl

and H 2 O

excretion

H 2 O

reabsorption

Antidiuretic

hormone (ADH)

−

Stretch of

left atrium

Brain

Posterior pituitary

Sensor

Integrating center

Effector

| CHECKPOINT

4a. Describe the composition of interstitial fluid. Using a

flow diagram, explain how interstitial fluid is formed

and how it is returned to the vascular system.

4b. Define the term edema and describe four different

mechanisms that can produce this condition.

5a. Describe the effects of dehydration on blood and

urine volumes. What cause-and-effect mechanism is

involved?

5b. Explain why salt deprivation causes increased salt

and water retention by the kidneys.

5c. Describe the actions of atrial natriuretic peptide and

explain their significance.

LEARNING OUTCOMES

After studying this section, you should be able to:

6. Describe the factors that affect blood flow through
   vessels.


7. Describe the intrinsic and extrinsic regulation of
   peripheral resistance.

14.3 VASCULAR RESISTANCE

TO BLOOD FLOW

The rate of blood flow to an organ is related to the resis-

tance to flow in the small arteries and arterioles. Vasodilation

The amount of blood that the heart pumps per minute is equal

to the rate of venous return, and thus is equal to the rate of

blood flow through the entire circulation. The cardiac output

is about 5 to 6 L per minute, depending upon body size and

other factors. This total cardiac output is distributed unequally

to the different organs because of unequal resistances to blood

flow through the organs. The distribution of the cardiac output

to various organs at rest, in terms of percentages and rates of

blood flow, is provided in table 14.3.