Page 1058 - Fundamentals of anatomy physiology

Chapter 27 Fluid, Electrolyte, and Acid–Base Balance 1045

Figure 27–2 Cations and Anions in Body Fluids. Notice the differences in cation and anion concentra-

tions in the various body fluid compartments.

200 INTRACELLULAR FLUID PLASMA INTERSTITIAL FLUID KEY
Cations
Na+ HCO3–
Cl– Na+
K+
150
Ca2+
Milliequivalents per liter (mEq/L) HCO3– HCO3–
Mg2+
HPO42–
Anions
K+ HCO3–
100
Cl–
Na+ Cl– Na+ Cl– HPO42–
SO42–
SO42– Organic

50 Proteins K+ HPO42– K+ HSPOO4422–– acid
Anions Ca2+ Org. acid Cations Anions Proteins
Mg2+ Cations Proteins
0 Cations
Anions

enable cells to maintain internal environments with a composi- Physiologists and clinicians pay particular attention to
tion that differs from their surroundings. The principal ions in ionic distributions across membranes and to the electrolyte
the ECF are sodium, chloride, and bicarbonate (HCO3−). The composition of body fluids. The Appendix at the end of the
ICF contains an abundance of potassium, magnesium, and book reports normal values in the units most often used in
phosphate ions (HPO42−), plus large amounts of negatively clinical reports.
charged proteins. Figure 27–2 compares the ICF with the two
major subdivisions of the ECF (plasma and interstitial fluid). Basic Concepts in the Regulation of Fluids and
Electrolytes
If the plasma membrane were freely permeable, diffusion 27
would continue until these ions were evenly distributed across Four basic principles are key to understanding fluid balance
the membrane. But it does not, because plasma membranes are
selectively permeable: Ions can enter or leave the cell only by and electrolyte balance:
specific membrane channels. Also, carrier mechanisms move
specific ions into or out of the cell. 1. All the homeostatic mechanisms that monitor and adjust the
composition of body fluids respond to changes in the ECF,
Despite the differences in the concentration of specific not in the ICF. Receptors monitoring the composition of
substances, the osmotic concentrations of the ICF and ECF are two key components of the ECF—plasma and cerebrospi-
identical. Osmosis eliminates minor differences in concentra- nal fluid—detect significant changes in their composition
tion almost at once, because most plasma membranes are freely or volume and trigger appropriate neural and endocrine
permeable to water. (The only noteworthy exceptions are the responses. This arrangement makes functional sense, be-
apical surfaces of epithelial cells along the ascending limb of cause a change in one ECF component will spread rapidly
the nephron loop, the distal convoluted tubule, and the col- throughout the extracellular compartment and affect all
lecting system.) The regulation of fluid balance and electrolyte the body’s cells. In contrast, the ICF is contained within tril-
balance are tightly intertwined because changes in solute con- lions of individual cells that are physically and chemically
centrations lead to immediate changes in water distribution. isolated from one another by their plasma membranes. For