PSC1002/L12 Reabsorption Flashcards
Define reabsorption.
Movement of solutes/fluid out of filtrate and into capillaries via epithelial transport mechanisms
Describe epithelial transcellular transport.
Substances cross apical and basolateral membranes of the tubule epithelial cells
Describe the paracellular transport pathway.
Substances pass through the cell-cell junction between two adjacent cells
How is the proximal convoluted tubule (PCT) specialised for its function? (3)
Microvilli on apical surface maximise surface area available for reabsorption
ER, Golgi, lysosomes, vacuoles all for synthesis of membrane proteins
Interdigitations of basolateral membrane shorten distance to mitochondria - active transport
Describe Na+ reabsorption. (4)
Passively at apical membrane
Ion exchange with other +ve ions
Co-transport with essential solutes
Actively at basolateral membrane
Describe water reabsorption.
Paracellular route via osmosis
Describe transport of other ions in the PCT. (2)
Paracellular route along electrochemical gradient
Transcellular route (diffusion)
Describe transport of glucose at the PCT (apical and basolateral).
Co-transport at apical membrane, carrier at basolateral membrane
Describe reabsorption of urate at the PCT. (4)
Organic anion transporters
Paracellular route
Passively transcellular route
Secretion via organic anion transporters
Describe reabsorption of low MW proteins/amino acids. (3)
Endocytosis at apical membrane
Breakdown in lysosomes
Release of amino acids
Define Tm.
Transport maximum rate i.e. rate of saturation of renal transporter/carrier
Define renal threshold.
Plasma concentration of substrate at transport maximum
What percentage of reabsorption occurs in the PCT?
66%
Describe osmolarity changes through the nephron. (4)
Isosmotic fluid leaving PCT becomes more progressively more concentrated in descending limb
Removal of solute in thick ascending limb creates hypoosmotic fluid
Permeability to water and solutes in DT and CT regulated by hormones
Final urine osmolarity depends on reabsorption in CT
Describe formation of urine. (4)
Descending limb permeable to water & impermeable to solutes
NaCl transport from ascending limb into interstitium
Thick ascending limb of loop impermeable to water
Collecting duct relatively impermeable to H2O, permeable to urea
Describe 2 systems in formation of concentrated urine.
ADH makes collecting duct permeable to H2O
H2O reabsorbed passively driven by osmotic gradient in medullary interstitium
Countercurrent systems maintain osmotic gradient in medullary interstitium
Give 3 properties of countercurrent exchange systems.
2 flows moving in opposite directions
Vessels anatomically very close together
Passive transfer of molecules from one vessel to another
What is a countercurrent exchange system?
Countercurrent exchange enhanced by active transport of solutes
What is the function of the countercurrent systems in the kidney?
Maintain osmotic gradient for reabsorption of H2O by preventing reduction in osmolarity of medullary interstitium
What occurs in the descending limb of loop of Henle? (2)
H2O reabsorption
Increased filtrate osmolarity
What occurs in the ascending limb of loop of Henle? (2)
Active solute reabsorption
Decreased filtrate osmolarity
What occurs in the descending limb of vasa recta? (3)
H2O reabsorption
Solute uptake
Increased blood osmolarity
What occurs in the ascending limb of vasa recta? (2)
H2O reabsorption
Decreased blood osmolarity
Give 2 examples of buffers in the kidneys.
HPO4(2-)
HCO3-
Cellular proteins
Haemoglobin
CO2
Describe acidosis (a) type A.
Intercalated cells in collecting duct excrete H+, reabsorb HCO3-
Describe alkalosis (B) type B.
Intercalated cells in collecting duct excrete HCO3-, reabsorb H+