Reabsorption

Question 1

180L/day are filtered through the glomerulus into the renal tubule, however only 1-2L/day are excreted as urine, what happens to the remainder of the filtrate

Answer 1

More than >99% of the plasma entering the kidney return to the systemic regulation though the mechanism of reabsorption

Question 2

What is responsible for reabsorption

Answer 2

The peritubular capillaries

Question 3

How does reabsorption occur

Answer 3

The pressure in the peritubular capillaries is very low because the hydrostatic pressure overcoming the frictional resistance of the efferent arteriolar

The Oncotic pressure is high, due to the peritubualr capillaries having a higher concentration of plasma proteins compared to filtrate

As a consequences , of the high osmotic pressure and low pressure of peritubualr capillaries, it favour reabsorption of filtrate from the lumen to the peritubular capillaries

Question 4

What is reabsorbed at the proximal tubule

Answer 4

NaCl 65-75%

Water 65-75%

Amino acids -100%

Glucose -100%

Urea -50%

K+ -100%

Ca2+ -100%

(percentages in normal physiology)

Question 5

What is reabsorbed at the distal tubule

Answer 5

NaCl 5-20%

Water 5-20%

Question 6

What is the major site of NaCl and water reabsorption

Answer 6

proximal tubule 65- 75%

Question 7

How are many substances (excluding Na+ and H2O) reabsorbed

Answer 7

By a carrier mediated transport system

Question 8

What prevents an excess of reabsorption of substances of filtrate

Answer 8

limited by a
number of carriers

as the carried mediated transport system have a maximum transport capacity (Tm)

Question 9

What is the purpose of the Tm

Answer 9

Tm is set at normal level of plasma concentrations, so if TM is saturated, then the excess substates remains in the urine and is excreted, so plasma regulations is maintained

Question 10

Define renal threshold

Answer 10

The plasma concentration at which saturation of TM occurs

Question 11

Explain the titration curve for glucose

Answer 11

Glucose is freely filtred, so whatever its plasma concentration it will be filtered

The plasma glucose that is reabsorbed is up to 10mmoles/L (= renal threshold for glucose)

If the glucose filtered is above 10mmoles/L, 10mmole/L will be reabsorbed and the remainder will remain in filtrates and be excreted

Question 12

Why is the Tm for glucose higher than the plasma concentration of 5mmoles/L

Answer 12

Ensures that all the valuable nutrient is normally reabsorbed

Tm is set above any possible level of non diabetic glucose concentration

Question 13

Why is the Tm for amino acids so high

Answer 13

So that amino acids are not excreted into urine, as some proteins does get through (0.5%) but want to make sure all is reabsorbed in normal physiology

Question 14

Why is the TM value close to the plasma value for most other substances

Answer 14

So automatically excrete excess

Question 15

How is Na+ reabsorbed from the filtrate

Answer 15

Active transpot

not bt Tm as couldn’t cope with the demand

Question 16

How as a gradient established for Na+ across the tubule wall

Answer 16

There is a high sodium concentration in the intercostal fluid due to the active transport, so leaves a low sodium concentration the tubular cells

The high sodium concentration n the tubule lumen compared to the tubule cell is so passively diffuses in

So the active transport sodium drives the gradient on the other side

Question 17

Where is the sodium pumps located

Answer 17

On the basolateral surface, where there is a high density of mitochondria

Question 18

How is the proximal tubules permeable to Na+

Answer 18

Has a high permeability to Na+ ions than most other membrane in the body due to the enormous surface are offered by the microvilli and large number of Na+ ion channel, so facilitates the diffusion of Na+

Question 19

What is the additional benefit of active transport sodium reabsorption

Answer 19

Due to it being a charged particle, it is key to the reabsorption of other components of the filtrates as creates a concentration gradient

Na+ is also used for carriers mediated transport system

Question 20

What components of the filtrate are reabsorbed via the initiation of sodium concentration gradient

Answer 20

Negative ions such as Cl- diffuse passively

This creates an osmotic force and draws H2O out of the tubules

The removal of H2O concentrates, all the substances left creating concentration gradients

Causing permeable solutes K+ Ca2+ and urea to be reabsorbed by diffusion

Question 21

What does the rate of reabsorption of non actively permeable solutes (K+, Ca2+, urea) depend upon

Answer 21

The amount of H2O removal (which determines the extent of the concentration gradient)

The permeability of the membrane to any particular solute

Question 22

How permeable is the membrane tubule to urea and how much urea is reabsorbed

Answer 22

Only moderately permeable so only about 50% is reabsorbed

Question 23

What substances are impermeable to the tubular membrane

Answer 23

Insulin

mannitol

Question 24

How is Na+ used for a carrier mediated transport system

Answer 24

As substances such as glucose, amino acids share the same carrier molecule as Na+ symport

Question 25

What is the benefit it Na+ used a carrier mediate transport system

Answer 25

The active drive of Na+ is strong enough to co-transport glucose against its concentration gradient

(the transport of glucose needs energy which is provided indirectly by Na+ pump)

Question 26

Drugs are usually lipid soluble, how are they not reabsorbed, when the concentration gradient is set up to the active transport of sodium

Answer 26

As the liver metabolises drugs to make them non polar and reduce there permeability, preventing reabsorption occurring in the kidney, so will remain in tubule and be excreted

Question 27

What stimulates Na+ reabsorption at the distal tubule

Answer 27

Aldosterone