Human Physiology, 14th edition (2016)

Cardiac Output, Blood Flow, and Blood Pressure 459

Regulation of Blood Volume

by the Kidneys

The formation of urine by the kidneys begins in the same
manner as the formation of interstitial fluid—by filtration of
plasma through capillary pores. These capillaries are known
as glomeruli, and the filtrate they produce enters a system of
tubules that transports and modifies the filtrate (by mecha-
nisms discussed in chapter 17). The total blood volume is only
about 5.5 L, yet the kidneys produce about 180 L/day of blood

Clinical Investigation CLUES

Mark had hypoproteinemia (low plasma protein concen-

tration) as a result of protein leakage through his intes-

tine, and he had edema.

• How does his hypoproteinemia relate to his edema?


• Once his hypoproteinemia is corrected, how will his
  edema be resolved?

CLINICAL APPLICATION

Filariasis is a tropical disease in which bloodsucking insects

such as mosquitos spread a parasitic nematode worm. In

elephantiasis ( fig. 14.10 ), species of these worms take up

residence in the lymphatic system, where their larvae block

the lymphatic drainage. The edema that results can greatly

swell tissues and produce a thickening and cracking of the

skin. This disease is found in about 72 tropical countries,

where over a billion people live and are threatened by infec-

tion. However, there are effective drugs available against the

filariasis parasite, and the Global Program to Eliminate Fila-

riasis, begun in 2000, has successfully treated millions of

people in an effort to eradicate this disease.

Figure 14.10 The severe edema of elephantiasis.

Parasitic larvae that block lymphatic drainage produce tissue

edema and the tremendous enlargement of the limbs and

scrotum in elephantiasis.

Table 14.2 | Causes of Edema

Cause Comments

Increased blood pressure or venous

obstruction

Increases capillary filtration pressure so that more tissue fluid is formed at the arteriolar ends of

capillaries.

Increased tissue protein

concentration

Decreases osmosis of water into the venular ends of capillaries. Usually a localized tissue edema

due to leakage of plasma proteins through capillaries during inflammation and allergic reactions.

Myxedema due to hypothyroidism is also in this category.

Decreased plasma protein

concentration

Decreases osmosis of water into the venular ends of capillaries. May be caused by liver disease (which

can be associated with insufficient plasma protein production), kidney disease (due to leakage of

plasma protein into urine), or protein malnutrition.

Obstruction of lymphatic vessels Infections by filaria roundworms (nematodes) transmitted by a certain species of mosquito block

lymphatic drainage, causing edema and tremendous swelling of the affected areas.

filtrate; thus, most of this filtrate must be returned to the vascu-

lar system and recycled. Only about 1.5 L of urine is excreted

daily; 98% to 99% of the amount filtered is reabsorbed back

into the vascular system.

The volume of urine excreted can be varied by changes in

the reabsorption of filtrate. If 99% of the filtrate is reabsorbed,

for example, 1% must be excreted. Decreasing the reabsorption

by only 1%—from 99% to 98%—would double the volume of

urine excreted (an increase to 2% of the amount filtered). Car-

rying the logic further, a doubling of urine volume from, for

example, 1 to 2 liters, would result in the loss of an additional

liter of blood volume. The percentage of the glomerular filtrate

reabsorbed—and thus the urine volume and blood volume—is

adjusted according to the needs of the body by the action of