PSC1002/L12 Reabsorption

Question 1

Define reabsorption.

Answer 1

Movement of solutes/fluid out of filtrate and into capillaries via epithelial transport mechanisms

Question 2

Describe epithelial transcellular transport.

Answer 2

Substances cross apical and basolateral membranes of the tubule epithelial cells

Question 3

Describe the paracellular transport pathway.

Answer 3

Substances pass through the cell-cell junction between two adjacent cells

Question 4

How is the proximal convoluted tubule (PCT) specialised for its function? (3)

Answer 4

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

Question 5

Describe Na+ reabsorption. (4)

Answer 5

Passively at apical membrane
Ion exchange with other +ve ions
Co-transport with essential solutes
Actively at basolateral membrane

Question 6

Describe water reabsorption.

Answer 6

Paracellular route via osmosis

Question 7

Describe transport of other ions in the PCT. (2)

Answer 7

Paracellular route along electrochemical gradient
Transcellular route (diffusion)

Question 8

Describe transport of glucose at the PCT (apical and basolateral).

Answer 8

Co-transport at apical membrane, carrier at basolateral membrane

Question 9

Describe reabsorption of urate at the PCT. (4)

Answer 9

Organic anion transporters
Paracellular route
Passively transcellular route
Secretion via organic anion transporters

Question 10

Describe reabsorption of low MW proteins/amino acids. (3)

Answer 10

Endocytosis at apical membrane
Breakdown in lysosomes
Release of amino acids

Question 11

Define Tm.

Answer 11

Transport maximum rate i.e. rate of saturation of renal transporter/carrier

Question 12

Define renal threshold.

Answer 12

Plasma concentration of substrate at transport maximum

Question 13

What percentage of reabsorption occurs in the PCT?

Answer 13

66%

Question 14

Describe osmolarity changes through the nephron. (4)

Answer 14

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

Question 15

Describe formation of urine. (4)

Answer 15

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

Question 16

Describe 2 systems in formation of concentrated urine.

Answer 16

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

Question 17

Give 3 properties of countercurrent exchange systems.

Answer 17

2 flows moving in opposite directions
Vessels anatomically very close together
Passive transfer of molecules from one vessel to another

Question 18

What is a countercurrent exchange system?

Answer 18

Countercurrent exchange enhanced by active transport of solutes

Question 19

What is the function of the countercurrent systems in the kidney?

Answer 19

Maintain osmotic gradient for reabsorption of H2O by preventing reduction in osmolarity of medullary interstitium

Question 20

What occurs in the descending limb of loop of Henle? (2)

Answer 20

H2O reabsorption
Increased filtrate osmolarity

Question 21

What occurs in the ascending limb of loop of Henle? (2)

Answer 21

Active solute reabsorption
Decreased filtrate osmolarity

Question 22

What occurs in the descending limb of vasa recta? (3)

Answer 22

H2O reabsorption
Solute uptake
Increased blood osmolarity

Question 23

What occurs in the ascending limb of vasa recta? (2)

Answer 23

H2O reabsorption
Decreased blood osmolarity

Question 24

Give 2 examples of buffers in the kidneys.

Answer 24

HPO4(2-)
HCO3-
Cellular proteins
Haemoglobin
CO2

Question 25

Describe acidosis (a) type A.

Answer 25

Intercalated cells in collecting duct excrete H+, reabsorb HCO3-

Question 26

Describe alkalosis (B) type B.

Answer 26

Intercalated cells in collecting duct excrete HCO3-, reabsorb H+