Osmoregulation

• The medulla contains the loop of henle and part of the collecting duct
• The cortex contains the proximal and distal convoluted tubule, collecting duct and the bowmans capsule
• A unit inside the kidney is known as the nephron
• The glomerulus is an arrangement of capillaries where water, glucose, amino acids, urea and ions are absorbed into the filtrate
• Red blood cells and large proteins cannot diffuse into the bowmans capsule as they are too large
• The afferent arteriole enters the glomerulus, it is wider than the efferent arteriole to provide high hydrostatic pressure
• The hydrostatic pressure in the glomerulus forces large molecules such as glucose and amino acids out of capillary pores and through the basement membrane
• The capillary pores are known as fenestrated endothelium
• Podocytes line the bowmans capsule, they have filtration slits to allow filtrate diffusion
• The basement membrane is the true filter in the bowmans capsule and is mostly made of collagen
• Animals excrete urea whilst fish excrete ammonia and birds excrete uric acid
• Ultrafiltration is the diffusion of substances into the nephron filtrate
• Sodium and chloride ions diffuse into the filtrate at the glomerulus
• Cells in the proximal convoluted tubule have many mitochondria for active transport during cotransport
• Proximal convoluted cells have microvilli to increase surface area for reabsorption
• All glucose and amino acids are reabsorbed in the proximal convoluted tubule, some ions and water is absorbed
• Proximal convoluted tubule cells are close to capillaries to maintain concentration gradients and have many carrier proteins
• The loop of henle is U shaped and involved in reabsorption of water and ions
• In the descending limb of the loop of henle, water diffuses into the interstitial fluid down a water potential gradient via osmosis
• The ascending limb is impermeable to water, sodium and chloride ions are actively transported into the interstitial fluid, this lowers the water potential, allowing osmosis in the descending limb
• The counter current multiplier effect in the loop of henle means that the water potential and concentration gradient is maintained throughout the loop of henle
• In the bottom of the ascending limb of the loop of henle, facilitated diffusion of ions occurs before active transport takes over due to high concentration gradients
• In the distal convoluted tubule, H+ ions are involved in active transport of sodium and chloride ions into the blood, water also diffuses by osmosis into the blood
• As the filtrate reaches the collecting duct, urea, water and ions remain, some water is reabsorbed in osmoregulation, the remainder will travel through the ureter to the bladder
• Osmoreceptors in the hypothalamus will shrink when dehydrated due to osmosis down a water potential gradient out of the cell
• When osmoreceptors shrink, they send a higher frequency of action potentials to the posterior pituitary gland, this causes release of ADH into the bloodstream
• The absorption of H+ ions in the distal convoluted tubule is involved in control of blood pH
• ADH will travel in the bloodstream to cells in the distal convoluted tubule and the collecting duct
• A higher concentration of ADH binding to receptors on collecting duct cells will cause an enzyme chain reaction to cause vesicles containing aquaporins to fuse to the cell membrane, increasing osmosis of water from the duct into the bloodstream
• A lower concentration of ADH binding to receptors on collection duct cells will lead to the reformation of aquaporin vesicles from the membrane, reducing permeability to water
• More ADH released = more reabsorption of water leading to more concentrated urine
• Less ADH released = less water reabsorbed leading to less concentrated urine
• Osmolarity can be interchanged for water potential
• More ADH leads to a lower volume of urine and higher concentration of urine
• Less ADH leads to higher volumes of urine and lower concentrations of urine
• ADH binding acts as a second messenger model, ADH binds to receptors and activates conversion of ATP to cyclic AMP by adenyl cyclase
• ADH binding activates an enzyme chain reaction causing fusion of aquaporin vesicles