Week 1 - C - Physiology 4(A) - Proximal tubule reabsorption

Question 1

What is the major site of tubular reabsorption in the kidney?

Answer 1

The proximal convoluted tubule

Question 2

What is the glomerular filtration rate value?

Answer 2

125ml/min - equates to roughly 180litres/day

Question 3

What percentage of urea from the tubules is reabsorbed?

Answer 3

Only about 50%

Question 4

Why is tubular reabsorption specific whereas as filtration is relatively non-specific?

Answer 4

This is due to the tubular reabsorption requiring specific transport processes

Question 5

As kidneys filter at 125ml/min and the proximal tubules reabsorb at 80ml/min, what is the volume of blood entering the loop of Henle per minute?

Answer 5

45ml/min enter the loop of Henle

Question 6

What happens to the osmolality of the blood as it travels through the proximal convoluted tubule?

Answer 6

It remains iso-osmotic (omsolarity does not change)

Question 7

Describe the transcellular tubular reabsorption acorss the tubular lumen to the plasma?

Answer 7

Substance travels across tubular lumen and into the tubular epithelial cell Then travels across the cell and into the interstitial fluid before it reaches the plasma membrane of the peritubular capillary

Question 8

What is the difference between transcellular and paracellular?

Answer 8

Paracellular travels in between the cells Transcellular is through the cell

Question 9

There are different types of carrier-mediated membrane transport Primary active transport Secondary active transport Facilitated diffusion Describe each?

Answer 9

Primary active transport requires ATP as energy Secondary active transport is usually coupled to concentration gradient of an ion such as Sodium Facilitate diffusion is a carrier mediated down the concentration gradient - passive

Question 10

Name one example of primary active transport where ATP is required to move ions?

Answer 10

The sodium potassium pump (3sodium out, 2potassium in, 1ATP)

Question 11

What is the class of enzymes that phosphorylates ATP into ADP to provide energy for the sodium potassium pump?

Answer 11

ATPases

Question 12

An energy-dependent Na+-K+ ATPase transport mechanism at the basolateral membrane is essential for Na+ reabsorption What happens here?

Answer 12

ATPase phosphorylates ATP to ADP which leads to 3Na leaving the tubular cell and travelling into the peritubular capillary and 2K entering the tubular cell

Question 13

Where are sodium potassium pumps found?

Answer 13

Exclusively at the basolateral membranes of tubular cells

Question 14

What is the difference between apical and basolateral?

Answer 14

The portion of the cell exposed to the lumen is called its apical surface. The rest of the cell (i.e., its sides and base) make up the basolateral surface. - basolateral is exposed to blood

Question 15

At the apical membrane there is different transporters to bring sodium into the cell Why does there continue to be a concentration gradient where sodium is low inside the tubular cells and therefore is brought in from the lumen?

Answer 15

This is due to the Na+/K+/ATPase pump which continually actively transport sodium out of the cell and into the interstitial fluid

Question 16

Give an example of a sodium transporter at the apical membrane? (either a co transport mechanism or a countertransport mechanism)

Answer 16

Sodium glucose co transporter at the apical membrane cause the entry of sodium and glucose into the cell Sodium hydrogen counter transport where hydorgen leaves the cell into the lumen of the tubule and sodium enters the tubular cell

Question 17

Due to the sodium ions being transported from the tubules to the ECF, what does this cause? (chloride and water)

Answer 17

This causes a chemical gradient leading to paracellular transport of chloride ions This causes passive water absorption down concentration gradient formed by NaCl

Question 18

As NaCl and H20 are absorbed into the plasma in equal quantities, what does this do to the omsolality?

Answer 18

Keeps it iso-osmolar

Question 19

What is the rate of filtration of a substance equal to?

Answer 19

Rate of filtration = Plasma concentration of the substance x glomerular filtration rate

Question 20

In the example of glucose, when it is filtered it is also completely reabsorbed As the plasma concentration of glucose increases, what does this do to the filtration and reabsorption rate?

Answer 20

It will cause both the rate of filtration to increase and the rate of reabsorption to increase

Question 21

What will eventually happen if the plasma concentration of glucose continues to increase?

Answer 21

The rate of reabsorption will plateau This is due to the sodium glucose co transporters only being able to move so many molecules across the membrane per unit of time, the transport processes can be saturated if there is a lot of substrate available

Question 22

When there is hyperglycaemia (diabetes), the glucose co transport system is saturated meaning the glucose reabsorption rate is at its transport maximum Transport maximum is the maximum rate at which the substrate can be reabsorbed What is this value for glucose?

Answer 22

Transport maximum for glucose is 2mmol/min

Renal threshold for glucose is around 10 meaning after this transport maximum is reached and glucosuria may occur

Question 23

To sum up, what transporter is sodium reabsorption driven by?

Answer 23

Driven by the Na+/K+/ATPase transporter

Question 24

Na+ reabsorption also drives Cl- reabsorption through the paracellular pathway What does this cause to water?

Answer 24

This causes the water to be reabsorbed via osmosis