Blood Volume

Fluid in the extracellular environment of the body is distributed between the blood and the interstitial fluid compartments by filtration and osmotic forces acting across the walls of capillaries.The function of the kidneys influences blood volume because urine is derived from blood plasma.The hormones ADH and aldosterone act on the kidneys to help regulate the blood volume.

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 compartment. 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 tissue fluid and the blood plasma is determined by a balance between opposing forces acting at the capillaries. Blood pressure, for example, promotes the formation of tissue fluid from plasma, whereas osmotic forces draw water from the tissues into the vascular system. The total volume of intracellular and extracellular 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 tissue fluid thus help to regulate blood volume and, by this means, help to regulate cardiac output and blood flow.

Fox: Human Physiology, I 14. Cardiac Output, Blood I Text I © The McGraw-Hill

Eighth Edition Flow, and Blood Pressure Companies, 2003

Cardiac Output, Blood Flow, and Blood Pressure 413

Cell membrane

Cardiac Output, Blood Flow, and Blood Pressure 413

Cell membrane

Human Water Intake

Water excretion per 24 hrs

Kidneys 0.6-1.5 L

Feces

Water intake per 24 hrs (drink + food) 1.5-2.5 L H2O

Water excretion per 24 hrs

Kidneys 0.6-1.5 L

Water intake per 24 hrs (drink + food) 1.5-2.5 L H2O

Feces

■ 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.

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