Urinary System Regulation PDF

S2 L4 Urinary System Regulation of K+, Ca+2, PO4-3, and Mg+2 Explained by: Ahmed Basim Edited by: Nada Mohammad Ali Topics : 1- Regulation of Potassium (K+) Excretion. 2- Regulation of Calcium Excretion. 3- Control of Renal Phosphate Excretion. 4...

S2 L4 Urinary System Regulation of K+, Ca+2, PO4-3, and Mg+2 Explained by: Ahmed Basim Edited by: Nada Mohammad Ali Topics : 1- Regulation of Potassium (K+) Excretion. 2- Regulation of Calcium Excretion. 3- Control of Renal Phosphate Excretion. 4- Control of Renal Magnesium Excretion. 5- Integration of Renal Mechanisms for control of Extracellular fluid. 6- Transport of urine from the kidney through the ureters and into the bladder. 1- Regulation of Potassium (K+) Excretion Renal potassium excretion is determined by the sum of three processes: 1- the rate of potassium filtration (GFR multiplied by the plasma potassium concentration). 2- the rate of potassium reabsorption by the tubules. 3- the rate of potassium secretion by the tubules. About 65 percent of the filtered potassium is reabsorbed in the proximal tubule Another 25 to 30 percent of the filtered potassium is reabsorbed in the loop of Henle, especially in the thick ascending part where potassium is actively co-transported along with sodium and chloride. 1- Regulation of Potassium (K+) Excretion Reabsorbed of K+ by intercalated cells in the late distal and collecting tubules, this by a H+- K+ ATPase transport mechanism located in the luminal membrane. This transporter reabsorbs potassium in exchange for hydrogen ions secreted into the tubular lumen, and the potassium then diffuses through the basolateral membrane of the cell into the blood. This transporter is necessary to allow potassium reabsorption during extracellular fluid potassium depletion, 1- Regulation of Potassium (K+) Excretion but under normal conditions it plays only a small role in controlling potassium excretion. Control of Potassium Secretion by Principal Cells The primary factors that control potassium secretion by the principal cells of the late distal and cortical collecting tubules are : 1- the activity of the Na+-K+ ATPase pump. 2- the electrochemical gradient(downhill), passively for potassium secretion from the blood to the tubular lumen. 1- Regulation of Potassium (K+) Excretion 3- the permeability of the luminal membrane for potassium (has channels that specifically permeable to K+ ions). These three factors determinants of potassium secretion by the principal cells are in turn regulated by the most important factors that stimulate potassium secretion by the principal cells include : 1- increased extracellular fluid potassium concentration. 2-increased aldosterone. 3- increased tubular flow rate, sodium intake → aldosterone secretion→ potassium secretion and excretion. 1- Regulation of Potassium (K+) Excretion The high distal tubular flow rate that occurs with a high sodium intake tends to increase potassium secretion. Therefore, these two effects of high sodium intake, counterbalance each other, so that there is little change in K+ excretion and viceversa. While the Acute acidosis is one factor that inhibit potassium secretion; Acute increases in H+ concentration of the ECF (acidosis) reduce potassium secretion (by reducing the activity of the Na+ - K+ ATPase pump) & viceversa. 1- Regulation of Potassium (K+) Excretion There is an increase in urinary K+ excretion due to inhibit excretion due to inhibit proximal tubular sodium chloride and water reabsorption. This effect reverses the inhibitory effect of hydrogen ions on the sodium-potassium ATPase pump. Note: Hyperkalemia (↑ K+ conc.) Hypokalemia (↓ K+ conc.) 2- Regulation of Calcium Excretion Normally, about 99 % of the filtered calcium is reabsorbed by the tubules [65% by proximal tubule, 25 to 30 % by loop of Henle & 4 to 9 % by distal and collecting tubules (as sodium)] & about 1 % is excreted. Almost all the calcium in the body (99 %) is stored in the bone. The bone, therefore, acts as a large reservoir for storing calcium and as a source of calcium when extracellular fluid calcium concentration tends to decrease. 2- Regulation of Calcium Excretion The most important regulator of calcium reabsorption at both of these sites is PTH. Thus, PTH regulates plasma calcium concentration through three main effects: 1- by stimulating bone resorption. 2- by stimulating activation of Vit D3, which then increases intestinal reabsorption of calcium. 3- by directly increasing renal tubular calcium reabsorption. Note: Hypercalcemia (↑ Ca+2 conc.) Hypocalcemia (↓ Ca+2 conc.) 3- Control of Renal Phosphate Excretion The proximal tubule normally reabsorbs 75 to 80 percent of the filtered phosphate. The distal tubule reabsorbs about 10 percent of the filtered load, and only very small amounts are reabsorbed in the loop of Henle, collecting tubules, and collecting ducts. Approximately 10 percent of the filtered phosphate is excreted in the urine. Increase plasma PTH, will decrease tubular phosphate reabsorption and more phosphate is excreted. 4- Control of Renal Magnesium Excretion and extracellular magnesium ion concentration The proximal tubule usually reabsorbs only about 25 percent of the filtered magnesium. The primary site of reabsorption is the loop of Henle, where about 65 percent of the filtered load of magnesium is reabsorbed. Only a small amount (usually