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Body water compartments and physiological fluids

Body water compartments and physiological fluids
Osmosis is the passive movement of water across a semi-permeable membrane from regions of low solute concentration to those of higher solute concentration. Biological membranes are semi-permeable in that they usually allow the free movement of water but restrict the movement of solutes. The creation of osmotic gradients is the primary method for the movement of water in biological systems. This is why the osmotic potential (osmolality) of body fluids is closely regulated by a number of homeostatic mechanisms (Chapter 35).

A fluid at the same osmotic potential as plasma is said to be isotonic; one at higher potential (i.e. more concentrated solutes) is hypertonic and one at lower potential is hypotonic. The osmotic potential depends on the number of osmotically active particles (molecules) per litre, irrespective of their identity. It is expressed in terms of osmoles, where 1 osmole equals 1 mole of particles, as osmolarity (osmol/L), or osmolality (osmol/kg H2O).The latter is preferred by physiologists as it is independent of temperature, though in physiological fluids the values are very similar. The osmolality of plasma is 290 mosmol/kg H2O, mostly due to dissolved ions and small molecules (e.g. glucose and urea). These diffuse easily across capillaries, and the crystalloid osmotic pressure they exert is therefore the same either side of the capillary wall. Proteins do not easily pass through capillary walls, and are responsible for the oncotic (or colloidal osmotic) pressure. This is much smaller than crystalloid osmotic pressure, but is critical for fluid transfer across capillary walls because it differs between plasma and interstitial fluid (Chapter 23). Oncotic pressure is expressed in terms of pressure, and in plasma is normally 25 mmHg. Maintenance of plasma osmolality is vital for regulation of blood volume (Chapter 22). Drinking fluids of differing osmotic potentials has distinct effects on the distribution of water between cells and extracellular fluid (Fig. 2b).

Body water compartments and physiological fluids

Body water compartments
Water is the solvent in which almost all biological reactions take place (the other being membrane lipid), and so it is fitting that it accounts for some 50–70% of the body mass (i.e. about 40 L in a 70-kg person). The nature of biological membranes means that water moves freely within the body, but the materials dissolved in it do not. There are two major ‘fluid compartments’: the water within cells (intracellular fluid, ICF), which accounts for about 65% of the body total, and the water outside cells (extracellular fluid, ECF). These compartments are separated by the plasma membranes of the cells, and differ mark- edly in terms of the concentrations of the ions that are dissolved in them (Fig. 2a; Chapter 4). Approximately 65% of the ECF comprises the tissue fluid found between cells (interstitial fluid, ISF), and the rest is made up of the liquid component of blood (plasma). The barrier between these two fluids consists of the walls of the capillaries (Fig. 2a; Chapter 23).

Intracellular versus extracellular fluid
Many critical biological events, including all bioelectrical signals (Chapter 5), depend on maintaining the composition of physiological fluids within narrow limits. Figure 2a shows the concentrations of ions in the three main fluid compartments. It should be noted that, within any one compartment, there must be electrical neutrality, i.e. the total number of positive charges must equal the total number of negative charges. The most important difference between ICF and ECF lies in the relative concentrations of cations. The K+ ion concentration is much higher inside the cell than in ECF, while the opposite is true for the Na+ ion concentration. Ca2+ and Cl ion concentrations are also higher in ECF. The question arises as to how these differences come about, and how they are maintained. Ion channel proteins allow the cell to determine the flow of ions across its own membrane (Chapter 4). In most circumstances, relatively few channels are open so that the leakage of ions is low. There is, however, always a steady movement of ions across the membrane, with Na+ and K+ following their concentration gradients into and out of the cell, respectively. Uncorrected, the leak would eventually lead to the equalization of the compositions of the two compartments, effectively eliminating all bioelectrical signalling (Chapter 5). This is prevented by the activity of the Na -K ATPase, or Na+ pump (Chapter 3). Of the other ions, most Ca2+ in the cell is transported actively either out of the cell or into the endoplasmic reticulum and mitochondria, leaving very low levels of free Ca2+ in ICF. Cl  ions are differentially distributed across the membrane by virtue of their negative charge. Intracellular proteins are negatively charged at physiological pH. These and other large anions that cannot cross the plasma membrane (e.g. phosphate, PO 3) are trapped within the cell and account for most of the anion content of ICF. Cl ions, which can diffuse across the membrane through channels, are forced out of the cell by the charge on the fixed anions. The electrical force driving Cl− ions out of the cell is balanced by the chemical gradient driving them back in, a situation known as the Gibbs–Donnan equilibrium. Variations in the large anion content of cells mean that the concentration of Cl− ions in ICF can vary by a factor of 10 between cell types, being as high as 30 mm in cardiac myocytes, although lower values (around 5 mm) are more common.

Interstitial fluid versus plasma
The main difference between these fluids is that plasma contains more protein than does ISF (Fig. 2a). The plasma proteins (Chapter 8) are the only constituents of plasma that do not cross into ISF, although they are allowed to escape from capillaries in very specific circumstances (Chapter 10). The presence of impermeant proteins in the plasma exerts an osmotic force relative to ISF (plasma oncotic pressure; see above) that almost balances the hydrostatic pressure imposed on the plasma by the action of the heart, which tends to force water out of the capillaries, so that there is a small net water movement out of the plasma into the interstitial space. The leakage is absorbed by the lymphatic system (Chapter 23). Transcellular fluid is the name given to fluids that do not contribute to any of the main compartments, but which are derived from them. It includes cerebrospinal fluid and exocrine secretions, particularly gastrointestinal secretions (Chapters 37–41), and has a collective volume of approximately 2 L.