T3 C

11 c T3

By decreasing osmolality (receptors in the liver and brain) and via hypervolemia (stretch receptors in the right atrium) an excess of water normally inhibits ADH release and thus triggers diuresis (^p. 100). The blood pressure, raised by the hypervolemia, inhibits the renin-angiotensin-aldosterone system. At the same time the release of ANF and possibly also of ouabain is stimulated. The result is natriu-resis which, after some delay, brings about the correction of the plasma volume and osmolal-ity. Excess NaCl increases ADH release via hy-perosmolality and thus leads to antidiuresis and also an adjustment of osmolality.

An excess of water and NaCl (^ A) occurs, for example, when fluid with greater osmolality than that of urine is ingested (e.g., shipwrecked people drinking sea-water). The renal excretion of water and NaCl is also reduced in impaired renal function (GFR i). Uncontrolled infusion of isotonic NaCl solution can then lead to an excess of NaCl and water, while infusion of isotonic glucose solution results in an excess of water that remains in the body after glucose has been metabolized. Even when kidney function is intact, there will be an excess of water or NaCl if the release of mineralocorti-coids or ADH is inappropriately increased (e.g., by hormone-producing tumors, ^ p. 260, 266). If the filtration balance in the peripheral vasculature is tipped, edemas occur at the expense of plasma volume (^ p. 234). This results in a decreased plasma volume, which stops the release of natriuretic factors (Atrial natriuretic factor, ouabain) and stimulates that of ADH, renin, angiotensin, and aldoste-rone. The renal retention of NaCl then leads to the correction of plasma volume, and thus to an increase in extracellular volume.

A lack of water and NaCl (^ B)canbe the result of external fluid loss as is the case, for example, with excessive sweating (fever, heat), in diarrhea, blood loss, burns or salt-losing kidney (^p. 108). Renal water loss can occur in ADH deficiency (central diabetes insipidus; ^ p. 260) and in lack of responsiveness of the kidney to ADH (renal diabetes insipidus; ^ p. 100). Even when the external balance is kept, dangerous "internal losses" can occur, such as a shift of plasma volume into the in testinal lumen (in ileus; ^ p. 156), into the abdominal cavity (ascites; ^ p. 170) or in the periphery (edema; ^ p. 234).

An excess of water (hyperhydration) necessarily leads to the enlargement of one body compartment (^ C). If there is NaCl excess at the same time (isotonic or hypertonic hyper-hydration), the extracellular space is increased. In hypertonic hyperhydration the extracellular space is increased, partly by osmotic withdrawal of water from the cells. If the NaCl content is normal or reduced (hypotonic hyperhydration), it is mainly the intracellular space that is enlarged.

In lack of water (dehydration) the extracellular space is reduced, especially when there is a simultaneous lack of NaCl (isotonic or hypotonic dehydration). In isolated lack of water the intracellular space is reduced (hypertonic dehydration), while it is increased in isolated lack of NaCl (hypotonic dehydration).

Any reduction in extracellular space is especially dangerous because of the decrease in plasma volume (hypovolemia). Signs of this are reduced central venous pressure, tachycardia, and a tendency to faint. If there is a drop in blood pressure, renal function is impaired and the release of ADH and aldosterone leads to oliguria (danger of urolithiasis). Conversely, an enlargement of extracellular volume leads to a rise in blood pressure when a part of the volume remains in the intravascular space (^ p. 114). On the other hand, the dilution of intravascular proteins promotes filtration in the peripheral capillaries and edema formation (^ p. 234) and, in the worst case, pulmonary edema ( ^ p. 80).

If the intracellular volume is enlarged, there is a particular danger that cerebral edemas will develop^ p. 358). Reduction in theintra-cellular volume also leads mainly to disorders of the central nervous system that can progress to loss of consciousness and even death.

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