Balancing Act • Water is vital to life. It is required for both chemical reactions and the excretion of waste. • Your body adjusts the amount of urine you produce to coincide with the amount of water used by the body. • The hormone that regulates water balance is called antidiuretic hormone or ADH.
Regulating ADH – Part I • ADH primarily acts on the kidneys, causing them to increase the rate of water reabsorption. • The osmoreceptors of the hypothalamus (brain) detects the osmotic pressure of the blood. • When water is lost – the solute concentration (dissolved stuff) in the blood increases and the osmotic pressure increases. It’s like the blood turning from water to concrete. • More water needs to move into the bloodstream in order to balance out the solutes – osmosis.
Regulating ADH – Part II • When water intake is decreased or water loss is increased (by sweating, for example) – cells of hypothalamus shrink and tell pituitary to release ADH. • ADH travels to kidneys and signals the reabsorption of more water. More water back into blood and less into urine. • Shrinking hypothalamus cells also initiate the sensation of thirst. You drink and more water moves into the system to replenish the low concentration in the blood. • As hypothalamus cells swell again, the hypothalamus sends a message to the pituitary and the release of ADH is lessened.
Kidneys & Blood Pressure • Kidneys also help balance blood pressure through the use of a hormone called aldosterone. • Aldosterone increases the reabsorption of sodium ions (Na+) from the nephron, which in turn attracts more water to follow. • The water will follow the salt and try to balance it out. Put salt in blood and water comes after it and blood pressure goes back up.
Blood Pressure – Detailed • Glomerulus has low blood pressure detector. When blood pressure is low – it releases renin. • Renin causes angiotensinogen to become angiotensin. • Angiotensin causes release of aldosterone from the adrenal glands; this acts to bump up the blood pressure through reabsorption of water. • Angiotensin also constricts (squeezes) blood vessels to help increase pressure.
pH Balance • The kidneys help maintain pH balance in the blood by using a buffer system that includes the following… H2O + CO2 H2CO3 HCO3- + H+ • The carbonic acid (H2CO3) can lower blood pH while the bicarbonate (HCO3-) can raise blood pH.