Blood plasma levels of potassium are the most important factor regulating potassium secretion.Potassium balance is chiefly regulated by renal mechanisms, which control the amount of potassium secreted into the filtrate.Potassium is critical to the maintenance of the membrane potential of neurons and muscle cells, and is a buffer that compensates for shifts of hydrogen ions in or out of the cell.Glucocorticoids enhance tubular reabsorption of sodium, but increase glomerular filtration. ![]() Estrogens are chemically similar to aldosterone, and enhance reabsorption of salt by the renal tubules.Atrial natriuretic peptide reduces blood pressure and blood volume by inhibiting release of ADH, renin, and aldosterone, and directly causing vasodilation.Cardiovascular baroreceptors monitor blood volume so that blood pressure remains stable.Angiotensin II, produced by the renin-angiotensin mechanism, causes the adrenal cortex to release aldosterone, and also directly causes kidney tubules to increase Na+ retention as part of a mechanism regulating systemic blood pressure.oThe most important trigger for the release of aldosterone is the renin-angiotensin mechanism, initiated in response to sympathetic stimulation, decrease in filtrate osmolality, or decreased blood pressure. When aldosterone secretion is high, nearly all the filtered sodium is reabsorbed in the distal convoluted tubule and the collecting duct.In addition, some individuals have died after having been forced or enticed to drink excessive amounts of water (e.g., fraternity hazings and water-drinking contests). Convulsions, coma, or death may result in severe cases. The person may experience headaches, nausea, or both. Brain cells become impaired as they swell with excess fluid. One of the consequences of extreme hypotonic hydration is cerebral edema. ![]() Fluid moves from blood plasma into the interstitial fluid, and then into the cells (figure 25.3a). The blood plasma then becomes hypotonic to the other fluid compartments. Both Na+ and water are lost during sweating, and drinking water replaces only the water, but not the solutes. An example would be an amateur athlete who runs a marathon, and drinks excessive amounts of plain water instead of using an electrolyte-enhanced solution. It can result from ADH hypersecretion, but it is generally caused from drinking a large amount of plain water following excessive sweating. Hypotonic hydration is also called water intoxication, or positive water balance. This accounts for the general differences in the percentage of body fluid that are noted between females and males of the same age after puberty. Adipose connective tissue is approximately 20% water, whereas skeletal muscle tissue is approximately 75% water. ![]() The percentage of fluid in the body at each age depends upon the ratio of adipose connective tissue and skeletal muscle tissue because of the difference in water content of these tissues. Relative amounts of adipose connective tissue to skeletal muscle tissue. Children and young and middle-aged adults are usually somewhere in between these two extremes, with a general trend of decreasing percentage of body fluid seen with increasing age. In contrast, elderly individuals have the lowest percentage of fluid at 45%. Infants have the highest percentage of fluid, at approximately 75% fluid by weight. The fluid percentage depends upon two variables: the age of an individual, and the relative amounts of adipose connective tissue and skeletal muscle tissue: Age.
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