lecture 12 - renal physiology 2 Flashcards
process in the formation of urine?
filtration
reabsorption
secretion
what is the total amount excreted?
amount filtered - amount reabsorbed + amount secreted
specialised epithelial cells of the nephron
whole length is lined by specialised epithelial cells
specialisation changes along the tube
thin limb - flat epithelial cells
• don’t have many mitochondria or organelles as aren’t active
• allow reabsorption of water - passive process
collecting ducts - different cell types
what is reabsorption?
movement of solutes/fluid out of filtrate and into capillaries, via epithelial transport mechanisms
from the lumen, across epithelial cells, into ECF into peritubular capillary
different mechanisms dependent on the solute being moved
epithelial transport mechanisms
route taken by solute depends on their electrochemical gradient and permeability of epithelial junctions
can either be moved by: • epithelial transcellular transport • paracellular transport pathway • passive transport • active transport
epithelial transcellular transport
substances cross apical and basolateral membranes of the tubule epithelial cells
paracellular transport pathway
substances pass through the cell-cell junction between 2 adjacent cells
passive transport
diffusion
leak channels
paracellular transport
active transport
membrane channels transporters co-transporters pumps carriers
proximal convoluted tubule
structure specialised for:
• reabsorption
• secretion
microvilli on apical surface maximise SA available for reabsorption
ER, Golgi, lysosomes and vacuoles all for synthesis of membrane proteins
interdigitations of the basolateral membrane shorten distance to mitochondria - active transport
Na+ reabsorption at PCT
conc of Na+ is higher in filtrate than the cells so passive movement of Na+ into the cells
Na+ is the main solute reabsorbed
electrochemical gradient changes due to Na+ moving, so Cl- moves to equalise it and water move to maintain osmotic gradient
what are the 3 main mechanisms for Na+ transport into the cell
sodium-potassium pump
sodium-glucose transporter
sodium-proton pump
water reabsorption at PCT
paracellular route via osmosis
glucose reabsorption at PCT
co-transport at apical membrane
carrier at basolateral membrane
urate reabsorption at PCT
organic anion transporters
paracellular route
passively transcellular route
low molecule weight proteins / amino acid reabsorption at PCT
endocytosis at apical membrane
breakdown in lysosome
release of amino acid
what is renal threshold?
plasma concentration of substrate at transport maximum
diabetes mellitus
excessive glucose concentration saturates number of carriers and excess glucose appears in urine
some will pass into the nephron and out in the urine as the threshold is exceeded
secretion at PCT
transport of molecules from peritubular capillaries into tubule - active process
how much reabsorption occurs in the PCT?
around 66%
tubular fluid leaving PCT is isometric with plasma = ~300mOsm
how does osmolarity change through the nephron?
isosmotic fluid leaving PCT becomes more concentrated in the defending limb
removal of solute in the thick ascending lim creates a hypo osmotic fluid
permeability to water and solutes in DCT and collecting duct is regulated by hormones
final urine osmolarity depends on reabsorption in the collecting duct
formation of urine
- descending limb permeable to water and impermeable to solutes
- NaCl transport from ascending limb into interstitial
- thick ascending limb impermeable to water
- collecting duct relatively impermeable to water and permeable to urea
- dilute urine produced
formation of concentrated urine
increase water reabsorption
ADH makes collecting duct permeable to water
countercurrent systems maintain osmotic gradient in the medullary interstitium
properties of countercurrent exchange systems
- 2 flows in opposite directions
- vessels anatomically close
- passive transfer of molecules from 1 vessel to another
- water reabsorbed from collecting duct results in concentrated urine
what is a countercurrent multiplier system?
countercurrent exchange enhanced by active transport of solutes
eg. loop of henle and vasa recta
whats different peritubular capillaries and vasa recta?
peritubular capillaries are around cortical nephrons
vasa recta are around juxtamedullary nephrons
what happens in the descending loop of the limb?
water reabsorption
increased filtrate osmolarity
what happens in the ascending loop of the limb?
active solute reabsorption
decreased filtrate osmolarity
what happens in the descending limb of the vasa recta?
water reabsorption
solute uptake
increased blood osmolarity
what happens in the ascending limb of the vasa recta?
water reabsorption
decreased blood osmolarity
how is NaCl actively reabsorbed by the thick ascending limb?
sodium and potassium cotransported with chlorine
sodium/potassium pump used
no paracellular transport
how is pH tightly regulated?
buffers
respiratory adjustment - CO2
renal adjustment
buffers used to regulate pH
cellular proteins
haemoglobin
HPO4 2-
HCO3-
renal adjustment to regulate pH
directly by excreting or reabsorbing H+
indirectly by excreting or reabsorbing HCO3-
acidosis
alkalosis
what is acidosis
alpha (type A) intercalated cells in collecting duct excrete H+ and reabsorb HCO3-
what is alkalosis
beta (type B) interacted cells in the collecting duct excrete HCO3- and reabsorb H+
what happens in acidosis?
high H+ concentration
presence of large amounts of carbonic anhydrase
produces H+ & HCO3-
hydrogen ATPase pumps H+ into the collecting duct to be excreted
HCO3- go back into the extracellular fluid to combine with H+ to prevent pH changing
what happens in alkalosis?
low H+ concentration
similar process to acidosis - channels are flipped around
HCO3- exchange on the apical membrane & HCO3- excreted
H+ actively moved into ECF to increase H+ concentration to increase pH
