Page 1060 - Fundamentals of anatomy physiology

salt conservation. The resulting diuresis (fluid loss by the kid-
neys) decreases both blood pressure and plasma volume, elimi-
nating the stretching.
The Interplay between Fluid Balance
and Electrolyte Balance
At first glance, it can be very difficult to distinguish between
water balance and electrolyte balance. For example, when you
lose body water, plasma volume decreases and electrolyte con-
centrations increase. Conversely, when you gain or lose excess
electrolytes, you also gain or lose water due to osmosis.
However, it is often useful to consider fluid balance and
electrolyte balance separately because the regulatory mecha-
nisms involved are quite different. This distinction is absolutely
vital in a clinical setting, where problems with fluid balance and
electrolyte balance must be identified and corrected promptly.
Ch e c k p o i n t
3. Name three hormones that play a major role in
adjusting fluid and electrolyte balance in the body.
4. What effect would drinking a half-gallon of distilled
water have on ADH levels?
See the blue Answers tab at the back of the book.
27-3 Hydrostatic and osmotic
pressures regulate the movement of
water and electrolytes to maintain fluid
balance
Learning Outcome Describe the movement of fluid within the ECF,
between the ECF and the ICF, and between the ECF and the environment. in the continuous filtration of fluid from the capillaries into
Water circulates freely within the ECF compartment. At cap-
illary beds throughout the body, hydrostatic pressure forces
water out of plasma and into interstitial spaces. Some of that
water is then reabsorbed along the distal portion of the capil-
lary bed, and the rest enters lymphatic vessels for transport to
the venous circulation. Fluid also moves continuously among
the minor components of the ECF:
1. Water moves back and forth across the mesothelial sur-
faces that line the peritoneal, pleural, and pericardial cavi-
ties, and through the synovial membranes that line joint
capsules. The flow rate is significant. For example, about
7 liters (1.8 gal) of peritoneal fluid are produced and re-
absorbed each day. However, the actual volume present at
any time in the peritoneal cavity is very small—less than
35 mL (1.2 oz).
2. Water also moves between blood and cerebrospinal fluid
(CSF), between the aqueous humor and vitreous humor of
Chapter 27 Fluid, Electrolyte, and Acid–Base Balance 1047
the eye, and between the perilymph and endolymph of the
internal ear. The volumes involved in these water move-
ments are very small, and the volume and composition of
the fluids are closely regulated. For those reasons, we will
largely ignore them in the discussion that follows.
Water movement can also take place between the ECF
and the ICF, but under normal circumstances the two are in
osmotic equilibrium. For this reason, no large-scale circulation
occurs between the two compartments. (A small amount of
water moves from the ICF to the ECF each day, as the result of
mitochondrial water generation. We will consider this topic in
a separate section.)
We cannot easily determine the body’s water content.
However, the concentration of Na1, the most abundant ion
in the ECF, provides useful clues to the state of water balance.
When the body’s water content increases, the Na1 concentra-
tion of the ECF becomes abnormally low. And when the body’s
water content decreases, the Na1 concentration becomes ab-
normally high.
Fluid Movement within the ECF
In our discussion of capillary dynamics in Chapter 21, we con-
sidered the basic principles that determine fluid movement
among the divisions of the ECF. p. 765 Recall that the ex-
change between plasma and interstitial fluid, by far the largest
components of the ECF, is due to the relationship between the
net hydrostatic pressure, which tends to push water out of the
plasma and into the interstitial fluid, and the net colloid os-
motic pressure, which tends to draw water out of the interstitial
fluid and into the plasma. The interaction between these op-
posing forces is diagrammed in Figure 21–11 (p. 766). It results
the interstitial fluid. This volume of fluid is then redistributed:
After passing through the channels of the lymphatic system, the
fluid returns to the venous system. At any moment, interstitial
27fluid and minor fluid compartments contain approximately
80 percent of the ECF volume, and plasma contains the other
20 percent.
Any factor that affects the net hydrostatic pressure or the
net colloid osmotic pressure will alter the distribution of fluid
within the ECF. The movement of abnormal amounts of water
from plasma into interstitial fluid is called edema. Pulmonary
edema, for example, can result from an increase in the blood
pressure in pulmonary capillaries. Generalized edema can re-
sult from a decrease in blood colloid osmotic pressure, as
in advanced starvation, when plasma protein concentrations
decrease. Localized edema can result from damage to capillary
walls (as in bruising), the constriction of regional venous cir-
culation, or a blockage of the lymphatic drainage (as in lymph-
edema, introduced in Chapter 22). p. 812