Kidney Functions and regulation

STIMULATING Blood Production

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. 0-10% Approximately 65-70% of Na+ ions are reabsorbed at the Proximal Convoluted tubule. 65% Approximately 25 -30% of Na + is reabsorbed along the Ascending Limb of the Loop Of Henle 25% The remaining 0 -10% of Na + ions are regulated at the Distal Convoluted Tubule, based on what hormone levels are signalling these cells.

Solutes SUCK ! 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. Solutes SUCK !

The permeability of the collecting duct is under the control of antidiuretic hormone (ADH). Remember the H = Affect on H20 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.

Reabsorption of water Salt (NaCl) diffuses and is actively transported out of the ascending limb of the loop of the nephron into the renal medulla; also, urea is believed to leak from the collecting duct and to enter the tissues of the renal medulla. This creates a hypertonic environment, which draws water out of the descending limb and the collecting duct. This water is returned to the cardiovascular system. (The thick black line means the ascending limb is impermeable to water.)

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. Renin is an enzyme that changes angiotensinogen from the liver into angiotensin I. Later, angiotensin I is converted to angiotension II, a powerful vasoconstrictor that also stimulates the adrenal cortex to release aldosterone.

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. This drawing shows that the afferent arteriole and the distal convoluted tubule usually lie next to each other. The juxtaglomerular apparatus occurs where they touch. The juxtaglomerular apparatus secretes renin, a substance that leads to the release of aldosterone by the adrenal cortex. Reabsorption of sodium ions and then water now occurs. Therefore, blood volume and blood pressure increase.

Effects Of Renin – Brings up BP

A.N.H. = Atrial Natriuretic Hormone 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 H2O 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.

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.

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 (HCO3-) are reabsorbed. If the blood is basic, H+ are not excreted and HCO3- are not reabsorbed. Breathing also ties up H+ when carbon dioxide is exhaled. Hydrogen ions combine with ammonia (NH3), thus ammonia provides a means for buffering hydrogen ions in urine.

Acid-base balance When would you want to increase HCO3- ion reabsorption into blood? When would you want to decrease H+ Excretion during Tubular Secretion? In the kidneys, bicarbonate ions (HCO3-) are reabsorbed and hydrogen ions (H+) are excreted as needed to maintain the pH of the blood. Excess hydrogen ions are buffered, for example, by ammonia (NH3), which is produced in tubule cells by the deamination of amino acids.