Chapter 27. Fluid, Electrolyte, And Acid-Base Homeostasis

Body fluid refers to body water and its dissolved substances. About two-thirds of the body’s fluid is located in cells and is called intracellular fluid (ICF).
The other third is called extracellular fluid (ECF) and consists of interstitial fluid (between cells) and blood plasma.
Fluid balance means that the various body compartments contain the required amount of water and electrolytes. Osmosis is the primary way in which water moves in and out of body compartments. The concentration of electrolytes in the fluids is a major determinant of fluid balance.

Regulation of Water Gain (Figure 27.3)
The main way to regulate body water balance is by adjusting the volume of water intake. When water loss is greater than water gain, dehydration occurs (Figure 27.3). Dehydration stimulates fluid intake by triggering thirst sensations; one mechanism for stimulating the thirst center in the hypothalamus is the renin-angiotensin II pathway, which responds to decreased blood volume.

Regulation of Water and Solute Loss (Figure 27.4)
Loss of body water or excess solutes depends mainly on regulating how much is lost in the urine. Under normal conditions, fluid output (urine production) is regulated by antidiuretic hormone (ADH), atrial natriuretic peptide (ANP), and aldosterone.
An imbalance between the intracellular and interstitial fluids may result from a change in osmolarity, as a result of a change in the concentration of Na⁺.

Electrolytes serve four general functions in body fluids.

1. They control the osmosis of water between body compartments.
2. They help maintain the acid-base balance required for normal cellular activities.
3. They carry electrical current, which allows production of action potentials and controls secretion of some hormones and neurotransmitters.
4. Several ions are cofactors needed for optimal activity of enzymes.

Concentrations of Electrolytes in Body Fluids (Figure 27.6, Table 27.1)

• Sodium (Na⁺) is the most abundant extracellular cation. It is involved in nerve transmission, muscle contraction, and creates most of the osmotic pressure of extracellular fluid.
• Chloride (Cl^-) is the major extracellular anion. It plays a role in regulating osmotic pressure between compartments and forming HCl in the stomach.
• Potassium (K⁺) is the most abundant cation in intracellular fluid. It is involved in maintaining fluid volume, nerve conduction, muscle contraction, and regulating pH.
• Bicarbonate (HCO₃^-) is a prominent anion in the plasma. It is significant in electrolyte balance and is a major component of the plasma acid-base buffer system.
• Calcium (Ca⁺²), the most abundant ion in the body. It is a structural component of bones and teeth. It also functions in blood coagulation, neurotransmitter release, maintenance of muscle tone, and excitability of nervous and muscle tissue.
• Magnesium (Mg⁺²) is primarily an intracellular cation. It activates several enzyme and is needed for operation of the sodium pump. It is also important in neuromuscular activity, transmission in the central nervous system, and myocardial functioning.

Acid-Base Balance
The overall acid-base balance of the body is maintained by controlling the H⁺ concentration of extracellular fluid.
The normal pH of extracellular fluid is 7.35-7.45. Homeostasis of pH is maintained by buffer systems, exhalation of carbon dioxide, and kidney excretion.

Buffer Systems
Most buffer systems of the body consist of a weak acid and a weak base that function to prevent rapid changes in the pH of a body fluid by changing strong acids and bases into weak acids and bases. Buffers work within fractions of a second.
The important buffer systems include the protein system, the carbonic acid-bicarbonate system, and the phosphate system.

• The protein buffer system is the most abundant buffer in body cells and plasma. The protein hemoglobin in red cells is a good buffer for carbonic acid.
• The carbonic acid-bicarbonate buffer system is an important regulator of blood pH and is based on the bicarbonate ion.
• The phosphate buffer system is an important regulator of pH, both in red blood cells and in the kidney tubular fluids.

Exhalation of Carbon Dioxide (Figure 27.7 and Table 27.2)
The pH of body fluids may be adjusted by a change in the rate and depth of respiration.
An increase in rate and depth of breathing causes more CO₂ to be exhaled, and less CO₂ stays in the blood thus increasing pH.
A decrease in rate and depth of breathing causes less CO₂ to be exhaled, and more CO₂ stays in the blood thus decreasing pH.
The pH of body fluids, in turn, affects the rate of breathing. The kidneys excrete H⁺ and reabsorb HCO₃^- to aid in maintaining pH.

Acid-Base Imbalances (Table 27.3)
The systemic arterial blood has a normal pH range between 7.35-7.45. A change in blood pH that leads to acidosis or alkalosis can be compensated to return pH to normal.

• Acidosis is a blood pH below 7.35. Its principal effect is depression of the central nervous system.
• Alkalosis is a blood pH above 7.45. Its principal effect is overexcitability of the CNS.

Respiratory acidosis and respiratory alkalosis are primary disorders of blood CO₂ concentration.
Metabolic acidosis and metabolic alkalosis are primary disorders of bicarbonate concentration.

• Respiratory acidosis is characterized by an increased blood PCO₂ and decreased pH and is caused by hypoventilation or other causes of reduced gas exchange in the lungs.
• Respiratory alkalosis is characterized by a decreased arterial blood PCO₂ and increased pH and is caused by hyperventilation.
• Metabolic acidosis is characterized by a decreased bicarbonate level and decreased pH, and results from an abnormal increase in acid metabolic products, loss of bicarbonate, or failure of the kidneys to excrete H⁺ ions.
• Metabolic alkalosis is characterized by increased bicarbonate concentration and increased pH, and results from loss of acid (e.g., excessive vomiting) or excessive intake of alkaline drugs.