Extracellular fluid compartments

The ability of certain drugs to increase both fluid and electrolyte loss has led to their use in the clinical management of fluid and electrolyte disorders, for example, edema. Regardless of the cause of the syndrome associated with edema, the common factor is almost invariably an increased retention of Na. The aim of diuretic therapy is to enhance Na+ excretion, thereby promoting negative Na" balance. This net Na" (and fluid) loss leads to contraction of the overexpanded extracellular fluid compartment. 

Sodium is the major cation of extracellular fluid. Because it represents approximately 90% of the 154mmol of inorganic cations per liter of plasma, Na is responsible for almost one half the osmotic strength of plasma. It therefore plays a central role in maintaining the normal distribution of water and the osmotic pressure in the extracellular fluid compartment. The normal daily diet contains 8 to 15 g (130 to 260 mmol) of NaCl, which is nearly completely absorbed from the gastrointestinal tract. The body requires only 1 to 2 mmol/day, and the excess is excreted by the kidneys, which are the ultimate regulators of the amount of Na" (and thus water) in the body. 

Hypercalcemia is commonly encountered in clinical prac-results when the flux of calcium into the extracellular fluid compartment from the skeleton, intestine, or Iddney is greater than the efflux. For example, when excessive resorption of bone mineral occurs in malignancy, hy-percalciuria develops. When the capacity of the kidney to excrete filtered calcium is exceeded, hypercalcemia develops. Hypercalcemia can be caused by increased intestinal absorption (vitamin D intoxication), increased renal retention (thiazide diuretics), increased skeletal resorption (immobilization), or a combination of mechanisms (primary hyperparathyroidism). 

The cytotoxic hypoxia of acute CN intoxication affects the energy-dependent processes controlling cellular ionic homeostasis and the ionic disequilibrium normally maintained between the intracellular and extracellular fluid compartments (Maduh et al., 1993). In isolated cell preparations, the cellular ionic disruption results in marked cellular acidosis and accumulation of cytosolic Ca++ (Bondy and Komu-lainen, 1988 Li and White, 1977 Nieminen et al., 1988). This may result in disturbances of Ca++-activated lipolytic enzyme activity, peroxidation of membrane phospholipids, changes in transmitter release and metabolism and effects on other Ca++-modulating cell signaling systems. Johnson etal. (1986) found that CN significantly... 

The major body constituent is water. An average person, weighing 70 kg. contains about 42 litres of water in total. The intracellular fluid compartment (ICF) is the volume of lluid inside the cells (28 I), and the extracellular fluid compartment (ECF) is that volume of fluid which lies outside cells (14 I). The ECF can be further subdivided into plasma (3.5 I) and interstitial lluid (lO.. I). 

Another use for mannitol and urea is in the treatment of dialysis disequilibrium syndrome. Too rapid removal of solutes from the extracellular fluid by hemo- or peritoneal dialysis reduces the osmolality of the extracellular fluid. Consequently, water moves from the extracellular compartment into the intracellular compartment, causing hypotension and CNS symptoms (i.e., headache, nausea, muscle cramps, restlessness, CNS depression, and convulsions). Osmotic diuretics increase the osmolality of the extracellular fluid compartment and thereby shift water back into the extracellular compartment. 

The parameter k represents the sum of the fractional rate constants for the clearance of tracer or tracee from the tissue extracellular fluid compartment (5). In the case of oxygen, k can be represented as... 

Figure 3j5. A. The movements of hydrogen and potassium ions between intra- and extracellular fluid compartments produced by a fall in hydrogen ion concentration of the extracellular fluid. B. Similarly for a fall in potassium ion concentration of the extracellular fluid. C. Movements of ions between the renal tubular fluid and the renal interstitial fluid sodium, chloride and bicarbonate are reabsorbed, hydrogen and potassium ions are secreted.

The electrometric intracellular [Cl ] of 18.7 .1.3 mM, while it accounts for only 2/3 of the total Cl content of proximal tubule cells, is still significantly greater than that expected from a simple passive distribution of this ion between the intracellular fluid and the two extracellular fluid compartments (luminal and peritubular). Therefore, chloride must be actively transported across the luminal membrane by an anionic pump or a neutral NaCl pump. This constitutes the first or luminal step in transcellular chloride reabsorptive transport. In the second or, peritubular step. Cl could passively accompany the actively and electrogenically extruded Na" as well as be a component of a peritubular electroneutral NaCl active transport process. 

Still the electrometrically determined intracellular [Cl ] of 18.7 mM is too high and too far from an equilibrium distribution to be consistent with passive transport in the three-compartment system. Thus intracellular chloride is at a higher electrochemical potential than either luminal or peritubular fluid chloride. This results in the passive efflux of intracellular Cl across the luminal and peritubular cell membranes. Since the tubule cell has greater [Cl ] than that expected from a simple passive distribution (5-7 mM) between intracellular and the two extracellular fluid compartments. Cl must be actively transported into the cell. 

The fluid found between the tissue cells. It is separated from blood plasma by the capillary wall which acts as a semipermeable membrane allowing the passage of water and small molecules but not the larger molecules such as proteins. Together with blood plasma, it constitutes the extracellular fluid compartment. 

It is interesting to use some of the kidney parameters cited to calculate the time required to clear the body of toxins, resulting, for example, from a drug overdose. It is assumed that the toxin is rapidly distributed in the extracellular fluid "compartment" (which includes plasma) of 10-L volume, and that none is metabolized or reabsorbed during its passage through the kidney. A toxin mass balance over the compartment then yields ... 

A second use of ESI is in the measurement of inorganic sulfate in blood and urine. Sulfate is extruded actively from cells and resides virtually exclusively in the extracellular fluid compartment. Sulfur has four stable isotopes, S, " S and S (95.02%, 0.75%, 4.21% and 0.02%, respectively. 

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