25.9 Regulation of fluid volume and composition  (Page 2/9)

When blood volume drops 5–10 percent, causing a decrease in blood pressure, there is a rapid and significant increase in ADH release from the posterior pituitary. Immediate vasoconstriction to increase blood pressure is the result. ADH also causes activation of aquaporin channels in the collecting ducts to affect the recovery of water to help restore vascular volume.

Diuretics and fluid volume

A diuretic    is a compound that increases urine volume. Three familiar drinks contain diuretic compounds: coffee, tea, and alcohol. The caffeine in coffee and tea works by promoting vasodilation in the nephron, which increases GFR. Alcohol increases GFR by inhibiting ADH release from the posterior pituitary, resulting in less water recovery by the collecting duct. In cases of high blood pressure, diuretics may be prescribed to reduce blood volume and, thereby, reduce blood pressure. The most frequently prescribed anti-hypertensive diuretic is hydrochlorothiazide. It inhibits the Na ⁺ / Cl ^– symporter in the DCT and collecting duct. The result is a loss of Na ⁺ with water following passively by osmosis.

Osmotic diuretics promote water loss by osmosis. An example is the indigestible sugar mannitol, which is most often administered to reduce brain swelling after head injury. However, it is not the only sugar that can produce a diuretic effect. In cases of poorly controlled diabetes mellitus, glucose levels exceed the capacity of the tubular glucose symporters, resulting in glucose in the urine. The unrecovered glucose becomes a powerful osmotic diuretic. Classically, in the days before glucose could be detected in the blood and urine, clinicians identified diabetes mellitus by the three Ps: polyuria (diuresis), polydipsia (increased thirst), and polyphagia (increased hunger).

Regulation of extracellular na ⁺

Sodium has a very strong osmotic effect and attracts water. It plays a larger role in the osmolarity of the plasma than any other circulating component of the blood. If there is too much Na ⁺ present, either due to poor control or excess dietary consumption, a series of metabolic problems ensue. There is an increase in total volume of water, which leads to hypertension (high blood pressure). Over a long period, this increases the risk of serious complications such as heart attacks, strokes, and aneurysms. It can also contribute to system-wide edema (swelling).

Mechanisms for regulating Na ⁺ concentration include the renin–angiotensin–aldosterone system and ADH (see [link] ). Aldosterone stimulates the uptake of Na ⁺ on the apical cell membrane of cells in the DCT and collecting ducts, whereas ADH helps to regulate Na ⁺ concentration indirectly by regulating the reabsorption of water.

Regulation of extracellular k ⁺

Potassium is present in a 30-fold greater concentration inside the cell than outside the cell. A generalization can be made that K ⁺ and Na ⁺ concentrations will move in opposite directions. When more Na ⁺ is reabsorbed, more K ⁺ is secreted; when less Na ⁺ is reabsorbed (leading to excretion by the kidney), more K ⁺ is retained. When aldosterone causes a recovery of Na ⁺ in the nephron, a negative electrical gradient is created that promotes the secretion of K ⁺ and Cl ^– into the lumen.