![]() ![]() Increasing carbon dioxide levels and decreasing body fluid pH stimulate neurons in the respiratory centre of the brain and cause the rate and depth of ventilation to increase. Respiratory system response rapidly to a change in pH and helps bring the pH of the body fluids back toward normal. Buffers tend to keep the H+ concentration, and thus the pH, within narrow range of values because of the major buffers in the body fluids. The buffers in the body fluids contain salts of either weak acids or weak bases that combine with H+ when H+ increases in those fluids, or release H+ when H+ decreases in those fluids. Buffers are chemicals that resist a change in the pH of a solution when either acids or bases are added to the solution. The pH of the body fluids is controlled by three factors: buffers, the respiratory system, and the kidneys. The concentration of H+ in the body fluids is reported as the pH. Calcitonin reduces the rate at which bone is broken down and decreases the release of Ca2+ from bone. An elevated blood Ca2+ concentration causes the thyroid gland to secrete calcitonin, and a low blood Ca2+ concentration inhibits calcitonin secretion. Calcitonin reduces the blood Ca2+ concentration when it is too high. Calcitonin is secreted by the thyroid gland. PTH affects the intestinal uptake of Ca2+ because PTH increases the rate of vitamin D production in the body. Some vitamin D is consumes in food, and the blood produces the rest. Vitamin D increases Ca2+ concentration in the blood by increasing the rate of Ca2+ absorption by the intestine. PTH also increases the rate of Ca2+ reabsorption from kidney nephrons. PTH causes osteoclasts to degrade bone and release Ca2+ into the body fluids. An higher Ca2+ concentration inhibits the secretion of PTH and a reduced Ca2+ concentration stimulates the secretion of PTH. The rate of PTH secretion is regulated by the extracellular Ca2+ concentration. Parathyroid hormone, secreted by the parathyroid glands, increases extracellular Ca2+ concentrations. Increased extracellular concentrations of Ca2+ inhibit action potentials in nerve and muscle cells, causing a reduction of excitability and either muscle weakness or paralysis. Increased extracellular Ca2+ concentrations make cell membranes less permeable to Na+ therefore making them less electrically excitable. ![]() Decreased extracellular concentrations of Ca2+ cause spontaneous action potentials in nerve and muscle cells, resulting in hyper excitability and muscle tetany. For example, decreased extracellular Ca2+ concentration make cell membranes more permeable to Na+, thus making them more electrically excitable. Increases and decreases in the extracellular concentration of Ca2+ have dramatic effects on the electrical properties of excitable tissues. The extracellular concentration of Ca2+, like that of other ions, is maintained with a narrow range. One percent or less remains as urine, when ADH is present. By the time the filtrate reaches the tip of the renal pyramid, an additional 19% of the filtrate is reabsorbed. If ADH is present, water moves by osmosis form the less concentration filtrate into the more concentrated interstitial fluid. The distal convolutes tubule and collecting duct are permeable to water if ADH is present. By the times the filtrate reaches the cortex of the kidney, the concentration is approximately 100mOsm/L, which is less concentrated than the interstitial fluid of the cortex. The volume of the filtrate doesn't change as it passes through the ascending limb, but the concentration is greatly reduced. Na+ are actively transported, and K+ and Cl- are co-transported from the filtrate of the thick segment into the interstitial fluid. ![]() The ascending limb of the loop of Henle is not permeable to water. By the time the filtrate reaches the tip of the renal pyramid, the concentration of the filtrate is equal to the concentration of the interstitial fluid. Because the wall of the descending limb is permeable to water, water moves by osmosis from the tubule into the more concentrated interstitial fluid. The descending limb passes through the concentrated interstitial fluid of the medulla. About 15% of the filtrate volume is reabsorbed in this segment of the descending limb of the loop of Henle. Water moves by osmosis because the cells of the tubule wall are permeable to water. In the proximal convolutes tubule, solute molecules move by active transport and co-transport form the lumen of the tubule into the interstitial fluid. About 180L of filtrate enters the nephron each day of that volume, 65% is reabsorbed in the proximal convolutes tubule. ![]()
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