Human Physiology, 14th edition (2016)

456 Chapter 14

14.2 Blood Volume

Fluid in the extracellular environment of the body is distrib-

uted between the blood and the interstitial fluid compart-

ments by forces acting across the walls of capillaries. The

kidneys influence blood volume because urine is derived

from blood plasma, and the hormones ADH and aldoste-

rone act on the kidneys to help regulate the blood volume.

Figure 14.8 The distribution of body water between the intracellular and extracellular compartments. The
extracellular compartment includes the blood plasma and the interstitial (tissue) fluid.

Water intake

per 24 hrs

(drink + food)

1.5–2.5 L H 2 O

Interstitial

fluid

volume

11–13 L

Blood

plasma

volume

3.0–3.5 L

Capillary wall

Extracellular

14–16.5 L

Intracellular

27–30 L

Skin (sweat glands)

0.2–1.0 L

Lungs

0.3–0.4 L

Kidneys

0.6–1.5 L

Feces

0.1–0.2 L H 2 O

Cytoplasm

Cell

membrane

GI tract

Water

excretion

per 24 hrs

| CHECKPOINT

1. Describe the effects of sympathoadrenal and
   parasympathetic nerve activity on the cardiac rate
   and stroke volume.
   2a. Describe the factors that regulate the venous return,
   and explain how the venous return is related to the
   end-diastolic volume and the stroke volume.
   2b. Describe how the stroke volume is intrinsically
   regulated by the end-diastolic volume and explain
   the significance of this relationship.
   2c. Define the terms preload and afterload and explain
   how they affect the cardiac output.


2. Use the Frank-Starling law of the heart to explain
   how an increase in venous return can result in an
   increase in stroke volume and cardiac output.

LEARNING OUTCOMES

After studying this section, you should be able to:

4. Explain the forces that act in capillaries and how
   edema can be produced.


5. Explain how the kidneys regulate blood volume, and
   the hormonal regulation of this process.

Blood volume represents one part, or compartment, of the

total body water. Approximately two-thirds of the total body

water is contained within cells—in the intracellular com-

partment. The remaining one-third is in the extracellular

compartment. This extracellular fluid is normally distributed

so that about 80% is contained in the tissues—as tissue, or

interstitial, fluid —with the blood plasma accounting for the

remaining 20% ( fig. 14.8 ).

The distribution of water between the interstitial fluid and

the blood plasma is determined by a balance between oppos-

ing forces acting at the capillaries. Blood pressure, for exam-

ple, promotes the formation of interstitial fluid from plasma,

whereas osmotic forces draw water from the tissues into the

vascular system. The total volume of intracellular and extra-

cellular fluid is normally maintained constant by a balance

between water loss and water gain. Mechanisms that affect

drinking, urine volume, and the distribution of water between

plasma and interstitial fluid thus help to regulate blood volume

and, by this means, help to regulate cardiac output and blood

flow.