eLFH - Renal Physiology Part 2

Question 1

Percentage of sodium reabsorption at each division of the renal tubule

Answer 1

Proximal convoluted tubule - 65%
Descending and ascending parts of loop of Henle - 25%
Distal convoluted tubule - 8%
Collecting tubules - 2%

Question 2

Where does primary active transport occur in the renal tubule

Answer 2

Throughout the nephron

Question 3

Sodium reabsorption in proximal convoluted tubule

Answer 3

Na+ concentration in filtrate is high ~ 140 mmol/L
Na+ concentration in tubular cells is low ~10 to 20 mmol/L due to gradient produced by Na+/K+ ATPase

Gradient favours transport of Na+ from filtrate to tubular cells and contributes to driving force for secondary active transport

Question 4

Sodium reabsorption in Loop of Henle

Answer 4

Thin descending limb is poorly permeable to Na+

Some Na+ reabsorbed from thin ascending limb

Most Na+ reabsorption in loop of Henle occurs in thick ascending limb

Question 5

Mechanism for Na+ reabsorption in Thin ascending limb of Loop of Henle

Answer 5

Thin ascending limb is permeable to Na+ but impermeable to water

Na+ diffuses from tubule lumen to medullary interstitium down concentration gradient

Osmolarity of tubular fluid decreases as it ascends

Question 6

Mechanism for Na+ reabsorption in Thick ascending limb of Loop of Henle

Answer 6

Thick ascending limb is impermeable to both Na+ and water

Na+/K+ ATPase creates concentration gradient for secondary active transport

Therefore reabsorption of Na+, K+ and Cl- occurs (K+ and Cl- co-transported)

H+ ions are counter transported

Question 7

Carrier proteins in Thick ascending limb of loop of Henle

Answer 7

Na+ / K+ / Cl- co-transporter (from tubule luminal fluid into tubular cell)

Na+ / H+ counter transporter (Na+ from tubule fluid into tubular cell, and H+ from cell to tubular fluid

Question 8

Sodium reabsorption in Distal convoluted tubule

Answer 8

Na+/K+ ATPase again generates Na+ gradient

Na+ reabsorbed via specific sodium channel
AND
Via co-transport with chloride ions

Question 9

Sodium reabsorption in the collecting ducts

Answer 9

Na+ reabsorption occurs in cortical collecting duct under influence of aldosterone

Question 10

Percentage of water reabsorption occurring in Proximal convoluted tubule

Answer 10

70%

Question 11

Mechanism for water reabsorption in proximal convoluted tubule

Answer 11

Solute movement out of tubular cells generates osmotic gradient from tubular fluid to interstitial fluid

Osmosis occurs via transcellular and paracellular paths

Question 12

Water reabsorption in Loop of Henle

Answer 12

Descending limb highly water permeable

Ascending limb impermeable to water so tubular fluid delivered to distal convoluted tubule is hypotonic

Question 13

Percentage of water reabsorption occurring in Descending limb of Loop of Henle

Answer 13

15%

Question 14

Percentage of water reabsorption occurring in Distal convoluted tubule and collecting ducts

Answer 14

DCT - 0 to 15%

Collecting ducts - 5%

Question 15

Mechanism of water reabsorption in distal convoluted tubule and collecting ducts

Answer 15

Dependent on ADH

DCT and collecting ducts are impermeable to water except in presence of ADH

ADH results in aquaporin insertion into luminal membrane

Water then diffuses into cortical interstitium and tubular fluid becomes more concentrated

Question 16

Location of reabsorption of Potassium

Answer 16

Proximal convoluting tubule - 60%

Ascending limb of loop of Henle - 30%

Question 17

Determinate of urinary potassium concentration

Answer 17

Secretion of K+ in distal convoluted tubule or collecting ducts by principle cells

Question 18

Mechanism of K+ secretion into urine

Answer 18

Active transport of K+ into tubular cells by basolateral membrane Na+/K+ ATPase

Passive secretion via channels and K+/Cl- co-transport

Question 19

Factors which increase K+ secretion into urine

Answer 19

Aldosterone increases K+ secretion into distal convoluting tubule by:
- Increased Na+/K+ ATPase activity
- Increased luminal membrane permeability to K+

Question 20

Why does glucose appear in urine in diabetes

Answer 20

Renal tubular transport of glucose has a maximum threshold

In health this is not reached but in diabetes plasma glucose concentration is raised so renal threshold is reached

Question 21

Renal threshold of plasma glucose concentration and tubular transport maximum

Answer 21

Renal threshold: 11 mmol/L

Tubular transport maximum: 21 mmol/L/min (aka 380 mg/min)

Question 22

Mechanism of glucose reabsorption in renal tubule

Answer 22

Co-transport with sodium into the tubular cell
Glucose then diffuses into peritubular interstitium

Question 23

Graph illustrating glucose filtration, reabsorption and excretion as plasma glucose concentration increases

Answer 23

Question 24

Location of Glucose reabsorption in renal tubule

Answer 24

Proximal convoluted tubule

Question 25

Location of bicarbonate reabsorption in the renal tubule

Answer 25

Proximal convoluting tubule - 80%

Thick ascending limb of loop of Henle - 10 to 15%

Question 26

Mechanism of Bicarbonate reabsorption in the renal tubule

Answer 26

Question 27

Importance of ammonia reabsorption

Answer 27

Part of ammonia buffer system

Role in acid base balance when H+ ions secreted in excess of the bicarbonate filtered into tubular fluid

Question 28

Other buffer systems (all of which are much less important than the ammonia buffer system)

Answer 28

Phosphate buffer system

Urate and citrate systems

Question 29

Mechanism of Ammonia reabsorption

Answer 29

Question 30

Location of Morphine cation secretion into the renal tubule

Answer 30

Proximal convoluting tubule