6.6 - Controlling blood water potential Flashcards
Define osmoregulation
control of blood WP
Describe ultrafiltration
- blood enters kidney via renal artery, which divides into afferent arteriole and capillary network (glomerulus)
- high hydrostatic pressure of blood flow forces H2O and soluble plasma components out of glomerulus (not large proteins) forming filtrate in Bowman’s capsule
- how: pressure aided by efferent arteriole being narrower than afferent
Describe selective reabsorption
- Na+ ions AT from PCT epithelial cells into blood
- creates Na conc gradient in epithelial cells
- Na+ ions FD from lumen into epithelial cells bringing glucose with it (reabsorbed) via co-transport
- creates glucose conc gradient in epithelial cells
- glucose FD from epithelial cells into capillaries
- increases WP in nephron filtrate (lumen)
- H2O moves by osmosis into blood capillaries
Describe the role of the loop of Henle
- Na+ ions AT out of ascending limb using ATP
- decreases WP of interstitial fluid
- H2O moves out of descending limb by osmosis to area of lower WP + enters capillaries by osmosis
- increases WP of interstitial fluid
- some Na+ ions diffuse out of filtrate at base of ascending limb
Describe what happens in the distal convoluted tube and collecting duct
- H2O moves out of DCT and collecting duct by osmosis
- moving down medulla = ion conc in collecting duct increases, as runs parallel to loop of Henle
- remaining liquid in collecting duct forms urine, which has a low WP as most H2O reabsorbed into blood
How can the permeability of the collecting duct be increased by hormones when there is a decrease in WP?
- osmoreceptors in hypothalamus detect fall in blood WP
- receptor shrinks and MORE ADH released by posterior pituitary gland, secreted into blood
- bind to receptors on collecting duct surface and activates enzyme phosphorylase
- causes vesicles containing aquaporins to embed in cell surface membrane
- increases H2O permeability and urea permeability
- urea leaves collecting duct; more H2O leaves and is reabsorbed into blood
- urine is more concentrated and lower volume
How can the permeability of the collecting duct be decreased by hormones when there is a rise in blood WP?
- osmoreceptors in hypothalamus detect rise in blood WP
- NO ADH released by posterior pituitary gland
- decreases H2O permeability and urea permeability
- less H2O reabsorbed into blood
- urine is more dilute and larger volume
What would a longer nephron do?
- increase multiplier effect
- interstitial fluid around DCT has even higher solute conc
- so more H2O exits filtrate and enters capillaries by osmosis because there is a greater WP gradient
Where is urea produced and what from?
in liver from breakdown of EXCESS amino acids
Describe the structure of the nephron
- outer fibrous capsule (for protection)
- cortex: Bowman’s capsule, convoluted tubes, blood vessels
- medulla: loops of Henle, collecting ducts, blood vessels
- renal pelvis (collects urine into ureter; sphincter muscle)
Where does ultrafiltration occur?
Bowman’s capsule
Where does selective reabsorption occur?
PCT
Describe how the loop of Henle acts as a counter current multiplier
- as filtrate moves down the collecting duct, it loses H2O, decreasing its WP
- but due to the pumping of ions out of the ascending limb, the WP of surrounding tissues in medulla is even lower than in collecting duct
- so allows H2O to continue to move out of filtrate down whole length of collecting duct
How do the ascending and descending limb differ?
ascending limb is impermeable to water, descending limb is permeable
