L12 Renal System

Question 1

What is the equation for the amount of urine excreted?

Answer 1

Amount filtered - amount reabsorbed + amount secreted

Question 2

What is the role of reabsoprtion?

Answer 2

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

Question 3

What is epithelial transcellular transport?

Answer 3

1) epithelial transcellular transport
- substances cross apical and base lateral membranes of the tubule epithelial cells

Question 4

What is the paracellular transport pathway?

Answer 4

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

Question 5

How does the solute choose what epithelial transport route to take?

Answer 5

Depends on
- electrochemical gradient
- permeability of epithelial junctions

Question 6

In the PCT, how are microvilli specialised for function?

Answer 6

Microvilli on the apical surface maximise surface area available for reabsorption

Question 7

What is responsible for the synthesis of proteins in the PCT?

Answer 7

ER, Golgi apparatus, lysosomes and vacuoles are responsible for the synthesis of membrane proteins

Question 8

What is the role of interdigitations of the basolateral membrane?

Answer 8

They shorten the distance to the mitochondria which is beneficial in active transport

Question 9

Outline the sodium ion reabsorption at the PCT

Answer 9

Happens passively at the apical membrane - down an electrochemical gradient

Ion exchange with other positively charged ions

Co-transport with essential solutes e.g. glucose, amino acid actively at the basolateral membrane

Question 10

How is H2O reabsorbed at the PCT?

Answer 10

Though a paracellular route via osmosis

Question 11

How is glucose transported at the PCT?

Answer 11

Co-transport at the apical membrane
A carrier at the basolateral membrane

Question 12

How is urate reabsorped at the PCT?

Answer 12

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

Question 13

How are low molecular weight proteins/amino acids reabsorbed at the PCT?

Answer 13

Endocytosis at the apical membrane
Breakdown in lysosomes
Release of amino acids

Question 14

Define diabetes mellitus

Answer 14

Excessive glucose concentration saturates the number of carriers and excess glucose appears in urine

Question 15

What happens during secretion at the PCT?

Answer 15

Transfer of molecules from the peritubular capillaries into the tubule - active process

Transport of organic anions and cations

Question 16

Why does osmolarity change throughout the nephron?

Answer 16

1) isometric fluid leaving the PCT becomes more concentrated in the descending limb
2) removal of solute in the thick ascending limb creates hypo osmotic fluid
3) permeability to water and solutes in the distal tubule and collecting duct is regulated by hormones
4) final urine osmolarity depends on reabsorption in the collecting duct

Question 17

Outline the permeability of the ascending and descending limb

Answer 17

Descending
- permeable to water
- impermeable to solutes

Ascending
- impermeable to water

Question 18

Outline the permeability of the collecting duct

Answer 18

Permeable to urea
Impermeable to H2O

Question 19

How is concentrated urine formed?

Answer 19

H2O reabsorption needs to be increased

1) anti-diuretic hormone makes collecting duct permeable to H2O - H2O is passively reabsorbed driven by the osmotic gradient in the medullary interstitium

2) countercurrent systems maintain osmotic gradient in the medullary interstitium

Question 20

What are the properties of the countercurrent exchange system?

Answer 20

• two flows moving in opposite directions
• vessels anatomically very close together
• passive transfer of molecules from one vessel to another to another

Question 21

What is the countercurrent multiplier system?

Answer 21

The countercurrent exchange is enhanced by active transport of solutes
E.g. loop of henle and vasa recta

Question 22

How and why is the osmotic gradient maintained?

Answer 22

Maintained for the reabsorption of H20
Done by preventing the reduction is osmolarity of medullary interstitium

Question 23

What role do the vasa recta and the collecting duct play in countercurrent exchange?

Answer 23

The H2O is reabsorbed from the collecting duct
H2O is removed from the medulla by the ascending vasa recta
Results in concentrated urine (high osmolarity)

Question 24

What happens when H2O is not reabsorbed from the collecting duct?

Answer 24

Due to the absence of vasopressin

Results in dilute urine - low osmolarity

Question 25

How is renal adjustment carried out?

Answer 25

Directly by excreting or reabsorbing H+

Indirectly by excreting or reabsorbing HCO3-

Question 26

What is acidosis?

Answer 26

Alpha intercalated cells in the collecting duct excrete H+
Reabsorb HCO3-

Question 27

What is alkalosis?

Answer 27

Beta intercalated cells in the collecting duct excrete HCO3-
Reabsorb H+

Question 28

What happens at different place during countercurrent multiplier exchange?

Answer 28

Descending limb of loop
- H2O re absorption
- increased filtrate osmolarity

Ascending Iimb of loop
- active solute re absorption
- decreased filtrate osmolarity

Descending limb of vasa recta
- H2O reabsorption
- solute uptake
- increased blood osmolarity

Ascending limb of vasa recta
- H2O reabsorption
- decreased blood osmolarity