Homeostasis: A Framework for Human Physiology
5
To sum up, the human body can be viewed as a complex
society of differentiated cells that combine structurally and
functionally to carry out the functions essential to the survival
of the entire organism. The individual cells constitute the
basic units of this society, and almost all of these cells individ-
ually exhibit the fundamental activities common to all forms
of life, such as metabolism and replication.
There is a paradox in this analysis, however. Why are the
functions of the organ systems essential to the survival of the
body when each cell seems capable of performing its own fun-
damental activities? As described in the next section, the resolu-
tion of this paradox is found in the isolation of most of the cells
of the body from the external environment, and in the existence
of a reasonably stable internal environment. The
internal envi-
ronment
of the body refers to the fl
uids that surround cells and
exist in the blood. These fl
uid compartments and one other—
that which exists inside cells—are described next.
Body Fluid Compartments
Water is present within and around the cells of the body, and
within all the blood vessels. Collectively, the fl uid present in
blood and in the spaces surrounding cells is called
extracel-
lular fl
uid.
Of this, only about 20–25 percent is in the fl
uid
portion of blood, the
plasma,
in which the various blood cells
are suspended. The remaining 75–80 percent of the extracel-
lular fl uid, which lies around and between cells, is known as
the
interstitial fl
uid.
As the blood fl ows through the smallest of blood ves-
sels in all parts of the body, the plasma exchanges oxygen,
nutrients, wastes, and other metabolic products with the
interstitial fl
uid. Because of these exchanges, concentrations
of dissolved substances are virtually identical in the plasma
and interstitial fl uid, except for protein concentration. With
this major exception—higher protein concentration in plasma
than in interstitial fl uid—the entire extracellular fl uid may be
considered to have a homogeneous composition. In contrast,
the composition of the extracellular fl uid is very different
from that of the
intracellular fl
uid,
the fl uid inside the cells.
Maintaining differences in fl uid composition across the cell
membrane is an important way in which cells regulate their
own activity. For example, intracellular fl uid contains many
different proteins that are important in regulating cellular
events such as growth and metabolism. These proteins must
be retained within the intracellular fl uid, and are not required
in the other fl
uid compartments.
In essence, the fl
uids in the body are enclosed in com-
partments.
Figure 1–2
summarizes the volumes of the body
fl uid compartments in terms of water, because water is by far
the major component of the fl uids. Water accounts for about
55–60 percent of normal body weight in an adult male, and
slightly less in a female. (Females generally have more body fat
than do males, and fat has a low water content.) Two-thirds
of the water is intracellular fl uid. The remaining one-third is
extracellular. As described previously, 75–80 percent of this
extracellular fl
uid is interstitial fl uid, and 20–25 percent is
plasma.
Compartmentalization is an important general principle
in physiology. Compartmentalization is achieved by barriers
between the compartments. The properties of the barriers
determine which substances can move between compartments.
These movements, in turn, account for the differences in com-
position of the different compartments. In the case of the
body fl uid compartments, plasma membranes that surround
each cell separate the intracellular fl uid from the extracellular
fl uid. Chapter 4 describes the properties of plasma membranes
and how they account for the profound differences between
intracellular and extracellular fl uid. In contrast, the two com-
ponents of extracellular fl uid—the interstitial fl uid and the
blood plasma—are separated by the cellular wall of the small-
est blood vessels, the capillaries. Chapter 12 discusses how this
barrier normally keeps 75–80 percent of the extracellular fl
uid
Capillary
Plasma 3 L
Intracellular fluid
28 L
Interstitial fluid
11 L
(a)
Figure 1–2
Fluid compartments of the body. Volumes are for an average 70-kg (154-lb) person. (a) The bidirectional arrows indicate that fl
uid can move
between any two adjacent compartments. Total body water is about 42 L, which makes up about 55–60 percent of body weight. (b) The
approximate percentage of total body water normally found in each compartment.
(b)
Percent of total body water
Plasma
Interstitial
fluid
Intracellular
fluid
70
60
50
40
30
20
10
(7%)
(26%)
(67%)
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