HOMEOSTATIC MECHANISMS 1. Endocrine System a. Posterior pituitary secretes ADH, which regulates the water reabsorption by the kidneys. b. Adrenal cortex secretes aldosterone which promotes sodium retention and potassium excretion, thereby causing an increase in plasma volume. c. Parathyroid regulates levels of calcium in the ECF. 2. Lymphatic System – assists to return excessive protein and fluids that escape into the tissue back to the plasma volume. 3. GI System – is the primary means of fluid intake. 4. Cardiovascular System – maintains BP to insure adequate renal perfusion. ELECTROLYTE BALANCE The main source of electrolyte is FOOD. Electrolytes are normal constituents of our mixed diet. SODIUM BALANCE Na controls 90–95% of the effective osmotic pressure of the ECF. Regulation of the EC Sodium Concentration a. ADH Feedback Control System An increase in osmolarity (excess Na) stimulates osmoreceptor located in the hypothalamus to cause release of ADH by the posterior pituitary gland. ADH increases permeability of the collecting tubules to reabsorb water. Water causes dilution of Na and the ECF. Conversely, when there is less Na, less ADH is formed and excess water is lost, thus, concentrating body fluids to normal. b. Thirst Mechanisms When a person is thirsty, he drinks large amount of water. In approximately 30 minutes later, his urine output increases to 8 times the normal. This happens until osmolarity of ECF returns to normal. The basic stimulus for exciting the thirst center is INTRACELLULAR DEHYDRATION. Others would include dryness of the mouth and low cardiac output as a result of blood loss. When a person feels thirsty, he craves for water, which would help maintain balance between extracellular and intracellular compartments. c. Aldosterone This acts on the kidney tubules to reabsorb the Na ions and returns them to the blood. Aldosterone, however, has a very little effect in Na concentration. POTASSIUM BALANCE a. Aldosterone Feedback System An increase in K concentration causes an increase in aldosterone concentration, which results in K excretion by the kidneys. As K is excreted, ECF K concentration returns to normal. b. Hydrogen Ion Concentration When K moves out of the cell, sodium and hydrogen ions move into the cell. When extracellular concentration of Na and hydrogen ions increases, they are carried into the cells osmotically and K is moved out of the cell. CALCIUM BALANCE a. Regulation of Calcium Concentration (Role of Parathyroid Gland) An increase in parathyroid (parahormone) causes an increase in absorption from renal tubules. When the bone has accumulated enough Ca, parahormone is reduced. This would decrease in Ca absorption by the intestines and kidneys. Conversely, when the bone becomes depleted with Ca salts, parahormone secretion is increased to allow maintenance of normal plasma concentration. This results in an increased Ca absorption by the intestines and kidneys. A large amount of Ca is excreted via the intestines. Only 1% is excreted in the urine and this depends upon the concentration of Ca in the ECFs. PHOSPHATE BALANCE The renal tubules reabsorb all the phosphates in the glomerular filtrate when less amounts of PO4 are present in the ECF. When there is excess, it is excreted in the urine. Parathyroid hormone causes rapid loss of phosphates in the urine. This results from diminished absorption of PO4 ions from the renal tubules. The PO4 ions are largely lost through the kidneys. Only a small amount is lost through the intestines. MAGNESIUM BALANCE Magnesium is abundant in the IC compartment. It activates the enzyme systems needed to produce energy by the breakdown of ATP to ADP. It activates PO4 which catalyze essential chemical reactions of the liver and bone. The kidneys in relation to plasma concentration maintain EC concentration of Mg. As plasma Mg concentration increases, renal excretion of Mg increases and vice-versa. Mg is poorly excreted in the intestines and because of this, it is used as a laxative. CHLORIDE BALANCE This is mainly an EC electrolyte. However, it easily diffuses into the IC compartment. Chloride intake in the body is usually in combination with Na. Therefore, regulation of Cl is directly related to Na regulation. NaCl is lost through the skin by sweating. Loss of chloride also occurs during GI disturbances as in vomiting. The kidneys also excrete Cl the same way it excrete Na. Chloride regulates osmotic pressure between fluid compartments.
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