16-3 Maintaining Water-Salt Balance The kidneys maintain the water-salt balance of the blood within normal limits. By doing so, they also maintain blood volume and blood pressure. Most of the water and salt (NaCl) present in the filtrate is reabsorbed across the wall of the proximal convoluted tubule. Approximately 65-70% of Na+ ions are reabsorbed at the Proximal Convoluted tubule. Approximately 25 -30% of Na + is reabsorbed along the Ascending Limb of the Loop Of Henle The remaining 0 -10% of Na + ions are regulated at the Distal Convoluted Tubule, based on what hormone levels are signalling these cells. 65% 25% 0-10%
16-4 Reabsorption of Water Salt passively diffuses out of the lower portion of the ascending limb of the loop; the upper thick portion actively extrudes salt into the tissue of the outer renal medulla. Water is reabsorbed by osmosis from all parts of the tubule. The ascending limb of loop of the nephron establishes an osmotic gradient that draws water from the descending limb of the nephron and the collecting duct.
16-5 The permeability of the collecting duct is under the control of antidiuretic hormone (ADH). Remember the H = Affect on H 2 0 Diuresis is an increase in urine flow and antidiuresis is a decrease. When ADH is present, more water is reabsorbed, blood volume and blood pressure rise, and there is a decreased amount of urine. If there is insufficient water intake, the posterior pituitary releases ADH, causing more water to be reabsorbed with a decreased urine output.
16-6 ADH is actually made in the Hypothalamus and then sent to the Posterior Pituitary for storage and release.
16-8 Reabsorption of Salt Kidneys regulate salt balance by controlling excretion and reabsorption of ions. Two hormones, Aldosterone and Atrial Natriuretic Hormone (ANH), control the kidneys’ reabsorption of sodium (Na). ALDOSTERONE = Affect on Na+ reabsorption. Works on the cells of the Distal Convoluted Tubule to Increase their ability to actively transport Na+ ions back into the blood. This regulates the last 0-10% Na+ ion reabsorption. More water will follow – Solutes SUCK ! Blood Volume will go up so to will Blood Pressure. Aldosterone is produced by the Adrenal Glands.
16-9 Juxtaglomerular Apparatus and RENIN When the juxtaglomerular (Juxtaposition is the act or placement of two things near each other) apparatus detects low blood volume/blood pressure, it secretes RENIN that eventually results in the adrenal cortex releasing aldosterone that restores blood volume and pressure through reabsorption of sodium ions.
16-11 A.N.H. = Atrial Natriuretic Hormone – Sensors in the atria can detect when Blood Pressure is getting dangerously high. Then A.N.H. is secreted into blood. This A.N.H. will inhibit the effect of Aldosterone on the nephron cells, so less Na+ is actively reabsorbed. As a result, more Na+ will stay in filtrate (urine) causing more H 2 O to stay in urine, so you pee more and blood volume is reduced and Blood Pressure goes down. “Natriuresis” takes place = Peeing Out More Sodium, water will follow.
16-12 Reabsorption of salt increases blood volume and pressure because more water is also reabsorbed. ANH is secreted by the atria of the heart when cardiac cells are stretched by increased blood volume. ANH inhibits secretion of renin; the resulting excretion of sodium also causes excretion of water and blood volume drops.
16-13 Diuretics Diuretics are chemicals that lower blood pressure by increasing urine output. Alcohol inhibits secretion of ADH; dehydration after drinking may contribute to the effects of a hangover. Caffeine increases the glomerular filtration rate and decreases tubular reabsorption of sodium. Diuretic drugs inhibit active transport of Na + so a decrease in water reabsorption follows. A.D.H Alcohol inhibits the secretion of ADH from the Pituitary Gland. Reduced ADH will cause collecting duct cells to become less permeable to water.
Maintaining Acid-Base Balance Kidneys rid the body of acidic and basic substances. If the blood is acidic, hydrogen ions (H + ) are excreted and bicarbonate ions (HCO 3 - ) are reabsorbed. If the blood is basic, H + are not excreted and HCO 3 - are not reabsorbed. Breathing also ties up H + when carbon dioxide is exhaled.
16-15 Acid-base balance When would you want to increase HCO 3 - ion reabsorption into blood? When would you want to decrease H+ Excretion during Tubular Secretion?