Chapter 26
Fluid, Electrolyte, and Acid-Base Balance
993
26
Many factors can change ECF and ICF volumes. Because water
moves freely between compartments, however, the osmolalities of
all body fluids are equal (except during the first few minutes a±er
a change occurs in one of the fluids). Increasing the ECF solute
content (mainly the NaCl concentration) causes osmotic and vol-
ume changes in the ICF—namely, a shi± of water out of the cells.
Conversely, decreasing ECF osmolality causes water to move into
the cells. Tus, ECF solute concentration determines ICF volume.
It is important for you to understand the concepts above,
because they underlie all events that control fluid balance in the
body.
Check Your Understanding
1.
Which do you have more of, extracellular or intracellular
fluid? Plasma or interstitial fluid?
2.
What is the major cation in the ECF? In ICF? What are the
intracellular anion counterparts of ECF’s chloride ions?
3.
If you eat salty pretzels without drinking, what happens to
the volume of your extracellular fluid? Explain.
For answers, see Appendix H.
Water Balance
and ECF Osmolality
List the routes by which water enters and leaves the body.
Describe feedback mechanisms that regulate water intake
and hormonal controls of water output in urine.
Explain the importance of obligatory water losses.
Describe possible causes and consequences of dehydration,
hypotonic hydration, and edema.
Notice that sodium and potassium ion concentrations in
ECF and ICF are nearly opposite (Figure 26.2). Te charac-
teristic distribution of these ions on the two sides of cellular
membranes reflects the activity of cellular A²P-dependent
sodium-potassium pumps, which keep intracellular Na
1
con-
centrations low and K
1
concentrations high. Renal mechanisms
can reinforce these ion distributions by secreting K
1
into the
filtrate as Na
1
is reabsorbed from the filtrate.
Electrolytes are the most abundant solutes in body fluids and
determine most of their chemical and physical reactions, but
they do not constitute the
bulk
of dissolved solutes in these flu-
ids. Proteins and some nonelectrolytes (phospholipids, choles-
terol, and triglycerides) found in the ECF are large molecules.
Tey account for about 90% of the mass of dissolved solutes in
plasma, 60% in the IF, and 97% in the ICF.
Fluid Movement Among Compartments
Osmotic and hydrostatic pressures regulate the continuous ex-
change and mixing of body fluids. Although water moves freely
between the compartments along osmotic gradients, solutes are
unequally distributed because of their size, electrical charge, or
dependence on transport proteins. Anything that changes the sol-
ute concentration in any compartment leads to net water flows.
Figure 26.3
summarizes the exchanges of gases, solutes,
and water across the body’s borders and between the three fluid
compartments within the body. In general, substances must pass
through both the plasma and IF to reach the ICF. In the lungs,
gastrointestinal tract, and kidneys, exchanges between the “out-
side world” and the plasma occur almost continuously. Tese
exchanges alter plasma composition and volume, with plasma
serving as the “highway” for delivering substances throughout
the body (see Chapter 17). Compensating adjustments between
the plasma and the other two fluid compartments follow quickly
so that balance is restored.
Let’s review the movement of water and solutes across the
boundaries between these compartments:
Exchanges between plasma and IF occur across capillary
walls.
We described the pressures driving these fluid move-
ments in
Focus on Bulk Flow Across Capillary Walls
(Figure
19.17 on pp. 718–719). ²o recap, the hydrostatic pressure of
blood forces nearly protein-free plasma out of the blood into
the interstitial space. Tis filtered fluid is then almost com-
pletely reabsorbed into the bloodstream in response to the
colloid osmotic pressure of plasma proteins. Under normal
circumstances, lymphatic vessels pick up the small net leak-
age that remains behind in the interstitial space and return it
to the blood.
Exchanges between the IF and ICF occur across plasma
membranes.
As described in Chapter 3, exchanges across the
plasma membrane depend on its permeability properties. As
a general rule, two-way osmotic flow of water is substantial.
But ion fluxes are restricted and, in most cases, ions move
selectively by active transport or through channels. Move-
ments of nutrients, respiratory gases, and wastes are typically
unidirectional. For example, glucose and oxygen move into
the cells and metabolic wastes move out.
Lungs
Interstitial
fluid
Intracellular
fluid in tissue cells
Blood
plasma
O
2
CO
2
H
2
O,
Ions
H
2
O,
Ions
Nitrogenous
wastes
Nutrients
O
2
CO
2
H
2
O
Ions
Nitrogenous
wastes
Nutrients
Kidneys
Gastrointestinal
tract
Figure 26.3
Exchange of gases, nutrients, water, and wastes
between the three fluid compartments of the body.
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