6.9 Control of blood water potential Flashcards
How are nephrons structured?
• Bowman’s capsule, PCT, loop of Henle, DCT, collecting duct
• Associated with a network of blood vessels
Ultrafiltration
• High hydrostatic pressure in glomerulus as diameter of afferent arteriole is wider than efferent
• So small substances (eg. Water, glucose, ions, urea) forced out into glomerular filtrate
• Through pores in capillary endothelium , basement membrane and podocytes
• Large proteins remain in blood
Reabsorption of glucose
• Na+ actively transported out epithelial cells into capillary
• Na+ moves by facilitated diffusion from PCT into epithelial down its concentration gradient, bringing a molecule of glucose with it
• Glucose moves into capillary by facilitated diffusion down its concentration gradient
Reabsorption of water
• Capillaries have a lower water potential than PCT so water moves into capillary by osmosis down water potentail gradient
• In the ascending limb Na+ actively transported out, increasing concentration of Na+ in the medulla, lowering water potential
• Water remains as ascending limb is impermeable to water
• In the descending limb water moves out by osmosis then reabsorbed into capillaries
• Water moves out DCT and collecting duct by osmosis down water potential gradient
Why is glucose found in the urine of an untreated diabetic?
• Blood glucose concentration is too high so not all glucose reabsorbed at the PCT
• As glucose carrier proteins are saturated
What is osmoregulation?
Control of water potential in the blood
What happens when there is a decrease in blood water potential?
• Osmoreceptors in the hypothalamus lose water by osmosis
• Stimulating the pituitary gland to secrete more ADH into the blood
• ADH attaches to receptors on the collecting duct / DCT
• Stimulating addition of aquaporins into cell membranes
• So increases permeability of cells to water
• So more water reabsorbed from collecting duct / DCT by osmosis
• So decreases volume and increases concentration of urine produced
