Water Balance Section 7.6 Page 353
Water balance It is important to regulate the amount of water in the body. If water intake increases, urine output increases. If water is lost (due to exercise or decreased water intake), urine output decreases.
Water balance • Maintaining water balance involves interaction between the endocrine and nervous systems. • Important hormones involved: • antidiuretic hormone (ADH) • aldosterone • angiotensin
Factors related to water balance • Osmotic pressure (solute concentration) • Force driving direction and amount of osmosis • Blood pressure and blood volume • Pressure exerted by blood on blood vessel walls • High blood volumes are associated with higher blood pressures
Antidiuretic hormone (ADH) • High osmotic pressure in the blood = High solute concentration • Occurs when water intake decreases, or water is lost • Causes water to leave cells and enter the bloodstream • ADH is released when osmotic pressure is too high • Produced in hypothalamus • Stored and released by pituitary gland
Effects of ADH • Causes increase in reabsorption of water from nephrons • Makes upper part of distal tubule and collecting duct permeable to water. • High [NaCl] in intercellular space pulls water out. • Water diffuses into bloodstream, and reduces osmotic pressure by diluting solutes.
Aldosterone and Angiotensin Respond to changes in blood pressure and blood volume. • Aldosterone • Produced by adrenal glands • Acts on nephrons • Increases Na+reabsorption • Strengthens osmotic gradient • Increases water reabsorption • Angiotensin • Causes blood vessels to constrict (become smaller in diameter) • Stimulates release of aldosterone from adrenal glands
semi-new topic... Kidneys and the pH balance of blood
Kidneys and pH balance of blood • Kidneys’ main functions are excretion of wastes and regulation of water balance. • The kidneys also play a role in maintaining the pH of the blood.
Normal blood pH: 7.3 to 7.5 – very narrow range • Variation can have detrimental effects on cellular functioning • Acid/base balance is maintained by a “buffer system”
Chemistry 101 • Recall: • Acids are substances that donate H+ ions in solution. • Bases are substances that accept H + ions in solution. • The concentration of H + in a solution determines its pH. • An acid-base buffering system consists of a mixture of a weak acid and its conjugate base • Such a system allows resistance to pH changes when small amounts of H + and OH - are added
The bicarbonate buffer system • An important biological buffering system • The bicarbonate ion (HCO3-) acts as a base • It can accept excess H+ ions in order to
The bicarbonate buffer system • Excess H+ in the blood is buffered by bicarbonate ions: H++ HCO3- ↔ H2CO3↔ H2O + CO2 • In order for this system to work, a reliable supply of bicarbonate is required • This is where the kidney comes into play… bicarbonate carbonic acid carbon dioxideis exhaled
The kidneys restore the supply of bicarbonate • Reverses the buffering reaction to produce bicarbonate ion • CO2 is actively transported from the peritubular capillaries into the cells lining the nephron • Here, CO2 joins with H2O: H2O + CO2 ↔ H2CO3↔ H++HCO3- bicarbonate • diffuses back into the blood to restore the buffer recombines with either phosphate or ammonia in the nephron, and is excreted in urine
Homework • Page 356 #3-7, 10