[16.6] role of the nephron in osmoregulation Flashcards
describe the stages the nephron carries out in its role of osmoregulation (4)
- formation of glomerular filtrate by ultrafiltration
- reabsorption of glucose and water by the PCT
- maintenance of a gradient of sodium ions in the medulla by the LoH
- reabsorption of water by the DCT and collecting ducts
how is glomerular filtrate formed by ultrafiltration?
- walls of glomerular capillaries are made up of endothelial cells with pores between them
- afferent arteriole has a greater diameter than efferent arteriole
- build up of hydrostatic pressure in glomerulus
- water, glucose and mineral ions are squeenzed out of the capillary to form glomerular filtrate
what molecules can leave blood in the glomerulus? (5)
- glucose
- AAs
- ions
- urea
- water
what molecules cannot leave blood in the glomerulus as they are too large? (3)
- plasma proteins
- RBC
- WBC
what happens to glucose and AAs after they pass into bowman’s capsule?
reabsorbed back into the bloodstream
what happens to ions and water after they pass into bowman’s capsule?
- lose some but are mostly reabsorbed into the bloodstream
- excess remain in the nephron
what happens to urea after it passes into bowman’s capsule?
- remain in nephron
- want all of it to be removed from the body
what resists the movement of glomerular filtrate out of the glomerulus? (5)
- capillary endothelial cells
- connective tissue and endothelial cells of the blood capillary
- epithelial cells of BC
- hydrostatic pressure of the fluid in BC space
- low water potential of blood in glomerulus
what reduces the barrier to the flow of filtrate, but allowing filtrate to leave the glomeruluar capillaries?
- podocytes - have spaces which filtrate can pass between
- spaces between endothelium of glomerular capillaries - fluid passes between, rather than through, these cells
what are the adaptations of the endothelial cells of the PCT to help it reabsorb substances into the blood? (3)
- microvilli - large SA to reabsorb substances from the filtrate
- infoldings at their bases - large SA to transfer reabsorbed substances into blood capillaries
- high density of mitochrondria - provide ATP for active transport
describe the process of selective reabsorption (6)
- Na+ ions are actively transported out of the basal side of the PCT cells into the blood capillaries via the Na/K pump
- this creates a Na+ ion concentration gradient in the cell
- Na+ enter cells passively by FD from the nephron lumen via co-transporter proteins
- co-transporter proteins also bring glucose, AAs, and Cl- ions from the lumen into the PCT
- molecules which have been co-transported into PCT cells diffuse into the blood
- therefore, glucose, water and other useful molecules are reabsorbed
what is water reabsorped in the PCT? (3)
- removal of Na+ ions from the filtrate in the nephron creates a ψ gradient ie. makes ψ in PCT cells more negative than lumen
- water moves from the lumen into the PCT cell, then into the blood capillaary by osmosis due to the ψ gradient
- 85% of water reabsorption happens in the PCT
how is 15% of water reabsorped after the PCT?
from the collecting duct due to the functioning of the loop of henle
what does the LoH do?
- reabsorbs water from the collecting duct
- concentrates urine so it has a lower ψ than the blood
describe the differences between the 2 regions of the LoH
- DL = narrow, AL = wider
- DL = thin walls, AL = thick walls
- DL = walls are highly permeable to water, AL = walls are impermeable to water
how does the LoH act as a counter-current multiplier?
- Na+ ions are actively transported out of the AL using energy from ATP hydrolysis
- lower ψ in interstitial region
- thick AL walls are impermeable so very little water leaves by osmosis
- DL walls are permeable so water passes into interstitial space my osmosis
- water enters blood capillaries in this region by osmosis and is carried away
- filtrate continues to lose water in this way so lowest ψ is at apex
- at AL base, Na+ ions diffuse of out the filtrate
- as filtrate moves up the AL, Na+ ions are also actively transported out (point 1)
- filtrate develops a progressively higher ψ
what happens in the collecting duct?
- collecting duct is permeable to water
- as filtrate moves down it, water passes out by osmosis
- this water passes by osmosis through specific channel proteins into blood vessels and is carried away
- as water passes out of the filtrate, its ψ is lowered
- however, ψ is also lowered in the interstitial space, so water continues to move out by osmosis down the whole length of the collecting duct
what does the counter-current multiplier ensure?
- there is always a water potential gradient drawing water out of the tubule
- filtrate in the collecting duct with a low ψ meets inteerstitial fluid that has an even lower ψ
how is the DCT well adapted for its function?
- cells that make up the walls of the DCT have microvilli
- many mitochondria allow them to reabsorb material rapidly from the filtrate by active transport
what is the role of the DCT?
- to make final adjustments to water and salts that are absorbed
- to control the pH of the blood by selecting which ions to reabsorb
