HUMAN BIOLOGY

224 Chapter 12

the Challenge: ever-Changing extracellular Fluid

The fluid inside cells is intracellular f luid. A variety of

gases and other substances move constantly between

intracellular and extracellular fluid. Those exchanges are

crucial for keeping cells functioning smoothly. They can’t

occur properly unless the volume and composition of the

ECF are stable.

Yet the ECF is always changing, because gases, cell prod-

ucts, ions, and other materials enter or leave it. To maintain

stable conditions in the ECF, especially the concentrations

of water and vital ions such as sodium (Na^1 ) and potassium

(K^1 ), there must be mechanisms that remove substances

as they enter the extracellular fluid or add needed ones as

they leave it. The urinary system performs this task. Before

examining how it operates, we’ll now take a general look at

the traffic of substances into and out of extracellular fluid.

the body gains water from food

and metabolic processes

Ordinarily, each day you take in about as much water

as your body loses (Table 12.1). Some of the water is

absorbed from foods and liquids you consume. The rest

is produced during metabolic reactions, including cellular

respiration and condensation reactions.

Thirst influences how much water we take in. When

there is a water deficit in body tissues, the brain “urges” us

to seek out water—for example, from a water fountain or

a cold drink from the refrigerator. We’ll discuss this thirst

mechanism later in the chapter.

the body loses water in urine, sweat,

feces, and by evaporation

Water leaves the body in four ways: excretion in urine,

evaporation from the lungs and skin, sweating, and in

feces. Of these four routes, urinary excretion is the form

of water loss over which the body has the most control.

Urinary excretion eliminates excess water, as well as excess

n    The chemical makeup of body fluid changes constantly as

water and solutes enter and leave it.

n    Links to Water and life 2.5, Condensation reactions 2.8,

Metabolism 3.13, Homeostasis 4.10, Blood 8.1, Nutrient

absorption 11.4

If you are an adult female in good health, by weight

your body is about 50 percent fluid. If you are an adult

male, the ratio is about 60 percent. This fluid is extremely

important both in the composition of body structures and

in nearly all body functions. Chapter 4 introduced the

concept of two “fluid compartments” in the body—one

that is inside cells, and a second that is outside cells. This

concept, summarized in Figure 12.1, is the starting point

for understanding why a “fluid management” system is so

important in body functioning.

the urinary system adjusts fluid

that is outside cells

Section 4.10 explained that tissue fluid fills the spaces

between cells and other components of tissues. Blood,

which is mostly watery plasma, circulates in blood vessels.

As you may remember, tissue fluid, blood plasma, and the

relatively small amounts of other fluids (such as in lymph)

outside cells together make up the body’s extracellular f luid,

or ECF.

Figure 12.1 Body fluid occurs in two compartments—one

inside cells and the other outside cells. Extracellular fluid

consists of blood and tissue (interstitial) fluid. (© Cengage Learning)

Intracellular

Fluid

(30 quarts/28 liters)

Extracellular

Fluid (ECF)

(16 quarts/15 liters)

Human Body Fluids

(46 quarts/43 liters)

interstitial

fluid (tissue)

plasma

lymph, cerebrospinal fluid,

mucus, and other fluids

12.1

© Ferenc Cegledi/Shutterstock.com

Water Gain Water Loss

(milliliters) (milliliters)

Ingested in solids: 850 Urine: 1,500

Ingested as liquids: 1,400 Feces: 200

Metabolically derived: 350 Evaporation: 900

2,600 2,600

Table 12.1 Normal Daily Balance between Water Gain

and Water Loss in Adult Humans

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