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Chapter 46 - Intravascular Fluid and Electrolyte Physiology

Alan D. Kaye

Ian J. Kucera

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Water is the major component of all fluid compartments within the body. Total-body water represents approximately 60% of the body's total weight in an average adult. In a 70-kg man, total-body water is about 600 mL/kg, or 40 L. The relative percentage of water varies significantly with age, gender, and adiposity. For example, muscle contains 75% water, whereas adipose tissue contains only 10% water. Water content of the fetus is high initially but decreases progressively during late gestation and the first 3 to 5 years of life.

Total-body water is separated into two basic components, the intracellular and extracellular compartments. The remainder of total-body water is found outside the cells in the extracellular compartment.

The intracellular and extracellular compartments are separated by water-permeable cell membranes. In the adult, the intracellular fluid volume averages 400 to 450 mL/kg (about 30 L), and the extracellular volume averages 150 to 200 mL/kg (about 14 L). Extracellular fluid (ECF) volume is greater in young and in male individuals than in elderly or in female persons ( Table 46-1 ). The blood volume is 60 to 65 mL/kg and is distributed as 15% in the arterial and 85% in the venous system. The main metabolic reactions of the blood occur inside the red blood cells (RBCs) and within the intracellular fluid compartment.

The major components of the extracellular compartment are the plasma volume (30 to 35 mL/kg) and the interstitial fluid (120 to 165 mL/kg). Other constituents of the ECF are transcellular fluids, including pleural fluid, peritoneal fluid, aqueous humor, sweat, urine, lymph, and cerebrospinal fluid.

Plasma deserves special consideration because of its importance in clinical practice. Plasma is the noncellular component of blood and is in continuous equilibrium with the interstitial fluid. A major difference of plasma compared with interstitial fluid is the much higher concentration of proteins. This higher concentration of protein results in plasma having an osmotic pressure 20 mm Hg greater than the interstitial and ECFs. This gradient helps maintain the intravascular volume.

The ECF compartment contains high concentrations of sodium, chloride, and bicarbonate. Permeability to ions and proteins varies in each organ, with the brain having the least and the liver having the most permeability.

Control of body water and its composition is multifactorial and involves atrial natriuretic peptide, vasopressin (i.e. antidiuretic hormone [ADH]), aldosterone (e.g., renin, angiotensin), parathyroid hormone, calcitonin, prostaglandins, dopaminergic receptors, α-adrenergic receptors, the thirst mechanism, and intrinsic renal properties.

Water balance represents the difference between water intake and water loss. The kidneys are the major regulators of water output. Approximately 60% of the daily water loss is excreted in the urine. At high ambient temperatures or with significant exercise, the amount of water lost by sweating increases and may reflect most of the total daily loss of water.

Heavy exercise can increase the loss of water through sweating up to 50 times the normal rate. Increased ventilation amplifies the insensible loss of water through the respiratory tract ( Table 46-2 ). Under these conditions, renal water loss decreases to compensate for the increased sweating and insensible water losses.^[1]