Renal Physiology 2

Question 1

How much sodium is absorbed in the PCT?

Answer 1

65%

Question 2

How much sodium is absorbed in the descending and ascending part of Loop of Henle?

Answer 2

25%

Question 3

How much sodium is absorbed in the DCT?

Answer 3

8%

Question 4

How much sodium is absorbed in the collecting tubules?

Answer 4

2%

Question 5

What is primary active transport?

Answer 5

• primary mechanism driving transport in the PCT
• Na/K ATPase pump exists only on basolateral membrane of tubular cell
• pumps sodium out of the tubular cell into the interstitial fluid against its concentration gradient
• this sets up a gradient for sodium and it moves down this back into the tubular cell and releases energy for 2ndry active transport

Question 6

What is secondary active transport?

Answer 6

Movement of 2 substances at the same time using a protein carrier.

Question 7

What is the sodium concentration in the filtrate vs the tubular cells?

Answer 7

Filtrate - 140 mmol/l

Tubular cells - 10-20 mmol/l (due to Na/K ATPase pump created gradient)

Question 8

How permeable is the descending limb of the Loop of Henle to sodium?

Answer 8

The descending limb is poorly permeable to sodium.

Question 9

How permeable is the thin ascending limb of the LoH to Sodium?

Answer 9

Some sodium is absorbed from the thin ascending limb (this is impermeable to H2O so the osmolarity of the tubular fluid decreases as it ascends).

Sodium diffuses from the tubule lumen into the medullary interstitum down it’s concentration gradient by simple diffusion.

Question 10

How permeable is the thick ascending limb of the LoH to sodium?

Answer 10

• thick ascending limb is impermeable to water and sodium
• BUT pumps are present and there is active reabsorption of sodium, potassium and chloride via secondary active transport
• Na/K ATPase pumps 3 Na+ out of the tubular cell and 2 K+ in
• Na+ moves from the tubular lumen down the gradient produced by the pump
• energy produced by the pump allows co-transport of other substances
  
  • potassium and chloride ions cotransported
  • hydrogen ions counter transported

Question 11

What happens in the distal convoluted tubule?

Answer 11

The Na/K ATPase pump again provides the gradient here.

Sodium moves into tubular cells via a specific sodium channel.

AND

Cotransport with chloride ions

Question 12

What happens to sodium in the collecting ducts?

Answer 12

Reabsorbed in cortical collecting duct under aldosterone influence.

Question 13

How much water is reabsorbed by the PCT?

Answer 13

70%.

Solute movement out of the tubular cells into the interstitial fluid causes a rise in interstitial fluid osmolality and fall in tubular fluid osmolality.

Water reabsorption therefore occurs by osmosis via transcellular and paracellular paths driven by osmotic gradients.

Net movement is water into the interstitium then from here, flow into peritubular capillaries.

Question 14

What is osmolality?

Answer 14

The measure of the osmoles of solute per kg of solvent (osmol/kg).

(If the concentration of solutes is very low, osmolarity and osmolality are equivalent)

Question 15

What is osmolarity?

Answer 15

The measure of solute concentration, osmoles of solute per litre of solution (osm/L)

Measures the number of moles of solute particles per unit volume of solution

Question 16

How is water reabsorbed at the LoH?

Answer 16

The descending limb is highly water permeable. Water diffuses into the interstitium - which has a high osmolality at this point.

The ascending limb of the LoH is impermeable to water, meaning water is trapped in the tubular lumen, and tubular fluid delivered to the DCT is hypotonic.

Question 17

What happens to water in the DCT?

Answer 17

• 15% of H2O reabsorption
• the DCT and collecting ducts are impermeable to water, except in the presence of ADH
• ADH increases their permeability to allow water reabsorption
• ADH inserts aquaporins into the luminal membrane
• water then diffuses into the cortical interstitium and the tubular fluid becomes more concentrated

Question 18

What happens in the collecting ducts?

Answer 18

• the medullary collecting ducts become permeable to water in the presence of ADH
• the medullary interstitium has a high osmolality and therefore water is reabsorbed here
• this means that tubular fluid becomes more concentrated and urine osmolalities are high

Question 19

How much potassium is reabsorbed in the PCT?

Answer 19

90%

Urinary potassium concentration is determined by secretion of potassium in the DCT/collecting ducts.

It is actively transported into tubular cells by basolateral Na pumps

It is passively secreted via channels and K+/Cl- co-transport

Question 20

Why is aldosterone important in potassium secretion into the DCT?

Answer 20

It determines potassium secretion into the DCT as it increases

• sodium pump activity
• luminal membrane K+ permeability

Question 21

What is the renal tubular maximum for transport of glucose?

Answer 21

380 mg/min or 21 mmol/min

Question 22

How does glucose reabsorption happen?

Answer 22

Co-transport with sodium into the tubular cell (using the gradient produced by the Na+/K+ ATPase pump). Glucose then diffuses into the peritubular interstitium.

Question 23

What is the maximum plasma glucose concentration before the renal threshold for transport is reached?

Answer 23

11 mmol/l

Question 24

Where does bicarbonate reabsorption occur?

Answer 24

80% in the PCT. A further 10-15% in the LoH.

It is couples directly with hydrogen ion secretion.

This is essential as loss of bicarb would result in massive pH changes as large amounts are filtered.

Question 25

What is ammonia?

Answer 25

NH3 - part of the buffer system.

When H+ ions are secreted in excess of bicarbonate filtered into tubular fluid, only a small proportion of the excess H+ can be excreted in the ionic form in the urine.

H+ must be combined with ammonia (or phosphate/urate/citrate) to be excreted.

Question 26

What happens to proteins at the glomerular membrane?

Answer 26

In health, larger molecules do not pass through the glomerular membrane. Any protein molecules that do are taken up by endocytosis at the luminal membrane

Question 27

Where is morphine secreted in the kidney?

Answer 27

It’s one of the cations secreted by the PCT