Porth's Essentials of Pathophysiology, 4e

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Integrative Body Functions

U N I T 2

solution , which has the same effective osmolality as the ICF, neither shrink nor swell. An example of an isotonic solution is 0.9% sodium chloride. When cells are placed in a hypotonic solution , which has a lower osmolality than the ICF, they swell as water moves into the cell (Fig. 8-3 B). When they are placed in a hypertonic solution , which has a greater osmolality than the ICF, they shrink as water is pulled out of the cell (Fig. 8-3 C). Compartmental Distribution of Body Fluids Body water, which constitutes about 60% of body weight in the adult, is distributed between the ICF and ECF compartments. 1–4 The fluid in the ICF compartment constitutes approximately 40% of body weight, and that in the ECF approximately 20% of body weight. The fluid in the ECF compartment is further divided into two major subdivisions: the plasma compartment , which constitutes approximately 5% of body weight, and the interstitial compartment , which constitutes approximately 14% of body weight (Fig. 8-4). The fluid in the interstitial compartment acts as a transport vehicle for gases, nutrients, wastes, and other materi- als that move between the vascular compartment and body cells. The interstitial fluid compartment also pro- vides a reservoir from which vascular volume can be maintained during periods of hemorrhage or loss of vascular volume. An interstitial gel, which is a sponge- like material supported by collagen fibers, fills the tissue spaces and aids in even distribution of interstitial fluid. Normally, most of the fluid in the interstitium is in gel form. 2 The interstitial gel, which has a firmer consis- tency than water, opposes the outflow of water from the capillaries, preventing the accumulation of free water in the interstitial spaces.

The osmotic activity that nondiffusible particles exert in pulling water from one side of the semiper- meable membrane to the other is measured by a unit called an osmole . In the clinical setting, osmotic activ- ity usually is expressed in milliosmoles (one thou- sandth of an osmole) per liter. Each nondiffusible particle, large or small, is equally effective in its abil- ity to pull water through a semipermeable membrane. Thus, it is the number , rather than the size , of the non- diffusible particles that determines the osmotic activity of a solution. The osmotic activity of a solution may be expressed in terms of either its osmolarity or osmo- lality. Osmolarity refers to the osmolar concentration in 1 L of solution (mOsm/L H 2 O) and osmolality to the osmolar concentration in 1 kg of water (mOsm/ kg H 2 O). 2 Osmolarity is often used when referring to fluids outside the body and osmolality for describing fluids inside the body. Because 1 L of water weighs 1 kg, the terms osmolarity and osmolality are often used interchangeably. Serum osmolality, which is largely determined by Na + and its attendant anions (Cl – and HCO 3 – ), normally ranges between 275 and 295 mOsm/kg H 2 O. Blood urea nitrogen (BUN) and glucose, which also are osmotically active, usually account for less than 5% of the total osmotic pressure in the ECF compartment. However, this can change, such as when blood glucose levels are elevated in persons with diabetes mellitus or when BUN levels change rapidly in persons with chronic kidney disease. The term tonicity refers to the tension or effect that a solution with impermeable solutes exerts on cell size because of water movement across the cell membrane. 3 Solutions to which body cells are exposed can be classified as isotonic, hypotonic, or hypertonic depending on whether they cause cells to swell or shrink (Fig. 8-3 A). Cells placed in an isotonic

Total body water = 60% body weight

Intracellular water 40% body weight

Extracellular water 20% body weight 14% 5% 1%

300

200

Interstitial 10 liters

28 liters

100

Plasma 3.5 liters

Transcellular 1 liter

Osmolarity - mOsm/L

A Isotonic solution

B Hypotonic solution

C Hypertonic solution

0

FIGURE 8-3. Tonicity: Red cells undergo no change in size in isotonic solutions (A) .They increase in size in hypotonic solutions (B) and decrease in size in hypertonic solutions (C) .

FIGURE 8-4. Approximate size of body compartments in a 70-kg adult.