Despite the different compositions of ICF and ECF, the total solute concentration (osmolality) of these two fluid compartments is normally the same. ICF and ECF are in osmotic equilibrium because of the high water permeability of cell membranes, which does not permit an osmolality difference to be sustained. If the osmolality changes in one compartment, water moves to restore a new osmotic equilibrium (see Chapter 2).
The volumes of ICF and ECF depend primarily on the volume of water present in these compartments. But the latter depends on the amount of solute present and the osmolality. This fact follows from the definition of the term concentration: concentration = amount/volume,- hence, volume = amount/concentration. The main osmotically active solute in cells is K+,- therefore, a loss of cell K.+ will cause cells to lose water and shrink (see Chapter 2). The main osmotically active solute in the ECF is Na+,- therefore, a gain or loss of Na+ from the body will cause the ECF volume to swell or shrink, respectively.
The distribution of water between intracellular and extracellular compartments changes in a variety of circumstances. Figure 24.2 provides some examples. Total body water is divided into the two major compartments, ICF and ECF. The y-axis represents total solute concentration and the x-axis the volume,- the area of a box (concentration times volume) gives the amount of solute present in a compartment. Note that the height of the boxes is always equal, since osmotic equilibrium (equal osmolalities) is achieved.
In the normal situation (shown in Figure 24.2A), two thirds (28 L for a 70-kg man) of total body water is in the ICF, and one third (14 L) is in the ECF. The osmolality of both fluids is 285 mOsm/kg H2O. Hence, the cell compartment contains 7,980 mOsm and the ECF contains 3,990 mOsm.
In Figure 24.2B, 2.0 L of pure water were added to the ECF (e.g., by drinking water). Plasma osmolality is lowered, and water moves into the cell compartment along the osmotic gradient. The entry of water into the cells causes them to swell, and intracellular osmolality falls until a new equilibrium (solid lines) is achieved. Since 2 L of water were added to an original total body water volume of 42 L, the new total body water volume is 44 L. No solute was added, so the new osmolality at equilibrium is (7,980 + 3,990 mOsm)/44 kg = 272 mOsm/kg H2O. The volume of the ICF at equilibrium, calculated by solving the equation, 272 mOsm/kg H2O X volume = 7,980 mOsm, is 29.3 L The volume of the ECF at equilibrium is 14.7 L. From these calculations, we conclude that two thirds of the added water ends up in the cell compartment and one third stays in the ECF. This description of events is artificial because, in reality, the kidneys would excrete the added water over the course of a few hours, minimizing the fall in plasma osmo-lality and cell swelling.
In Figure 24.2C, 2.0 L of isotonic saline (0.9% NaCl solution) were added to the ECF. Isotonic saline is isosmotic to plasma or ECF and, by definition, causes no change in cell volume. Therefore, all of the isotonic saline is retained in the ECF and there is no change in osmolality.
Figure 24.2D shows the effect of infusing intravenously 1.0 L of a 5% NaCl solution (osmolality about 1,580 mOsm/kg H2O). All the salt stays in the ECF. The cells are exposed to a hypertonic environment, and water leaves the cells. Solutes left behind in the cells become more concentrated as water leaves. A new equilibrium will be established, with the final osmolality higher than normal but equal inside and outside the cells. The final osmolality can be calculated from the amount of solute present (7,980 + 3,990 + 1,580 mOsm) divided by the final volume (28 + 14 + 1 L), it is equal to 315 mOsm/kg H2O. The final volume of the ICF equals 7,980 mOsm divided by 315 mOsm/kg H2O or 25.3 L, which is 2.7 L less than the initial volume. The final
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.