Renal- Transport Processes

Question 1

Where is the bulk of reabsorption done?

And what makes this possible?

Answer 1

• Proximal tubule
• first segment of PT has lots of mitochondria to facilitate active transport
• the later segments (S2 and S3) have deeper brush border to increase the surface area

Question 2

Na driven transport

Answer 2

• FL of Na is high and 99% of it gets reabsorbed back into blood
• Na/K ATPase pumps allow the Na to be absorbed into the cells by managing the Na gradient within the cell

Question 3

Proximal convoluted tubule

S1, S2

Answer 3

• Active Na cotransport brings glucose, amino acids, phosphate, and organic acids back into the cell.
• Na/H antiporters allow H secretion into the proximal renal tubular fluid while the Na is being reabsorbed

Question 4

Proximal straight tubule

S3

Answer 4

• Na/H antiporters continue to reabsorb Na and secrete H into the tubular fluid
• reabsorbtion of Na causes fluid to follow which creates electrochemical gradient that facilitates Cl reabsorption
  
  • Cl concentration increases along the PT segments as water is reabsorbed
  • Cl^- going into the cell causes other anions to leave the cell back into the filtrate (HCO3-, SO4-, OH-, oxalate)

Question 5

Thin descending tube of Henle

Answer 5

• this segment is impermeable to Na and most other solutes but permeable to H2O
• The interstitial osmotic gradient brings water back into the cell through aquaporins
  
  • this causes the tubular fluid remaining to be more concentrated

Question 6

Thick ascending limb of Henle

Answer 6

• this segment is impermeable to water
• special Na⁺-K⁺-2Cl^- cotransporters allow for reabsorption of electrolytes
  
  • after the electrolytes get reabsorbed, the tubular fluid entering the distal tubule is more dilute.
  • these cotransporters are the where loop diuretics target.
• There is a backleak of K⁺ out of the cells into the tubule lumen, creating a + difference compared to interstitial fluid
  
  • this allows movement of cations out of the tubular lumen
• There are Na/H antiporters that reabsorb Na and secrete H into the tubule

Question 7

Distal Tubule

Answer 7

• early DT has Na⁺-Cl^- cotransporters (can be inhibited by thiazide diuretics)
• has Na/K channels that are increased by aldosterone
  
  • resulting in greater Na and water reabsorption and K excretion

Question 8

Collecting Duct

Answer 8

• also has Na/K channels affected by aldosterone

Question 9

transport maximum

Answer 9

• usually in reference to glucose
• Normally very high transport maximum and all of the FL of glucose is reabsorbed
• If the plasma level of glucose is high, the FL will be more than the Na/glucose transporters can bring back in, causing urine in the glucose.

Question 10

HCO₃^-

How much filtered/absorbed?

Answer 10

freely filtered and 100% reabsorbed

Question 11

How is HCO₃^- reabsorbed?

Answer 11

• reabsorbed indirectly

1. in tubular lumen, filtered HCO₃^- and secreted H⁺ form Carbonic acid, carbonic anhydrase (CA) breaks Carbonic acid into CO₂ and H₂O (catalyzed by brush border and CA)
2. CO₂ and H₂O easily diffuse into the cell
3. In the cell, the CO₂ and H₂O are converted back into carbonic acid(H₂CO₃) (by intracellular CA)
4. Carbonic acid easily breaks up into HCO₃^-
5. HCO₃^- is transported out of the cell via HCO₃^-/Cl^- exchangers or Na/HCO3 cotransporters

Question 12

What happens to K in the nephron?

Answer 12

1. Proximal tubule- K reabsorption happends between cells, not into the cells. (70% of K reabsorption)
2. Thick ascending Limb on Henle- transporters use the Na and Cl gradients to facilitate transport of K (20% of K reabsorption)
3. Late distal tubules- an increase in aldosterone will increase number of Na/K ATPases which will put K back into the tubule. (to keep Na in?)
4. Collecting ducts- an increase in aldosterone will cause K to go out through K channels. An H⁺/K⁺ atpase facilitates K reabsorption in exchange for H secretion into tubular fluid

***normally a net secretion of K into the urine unless there is a dietary K depletion.

Question 13

Potassium handling during high K intake

Answer 13

• High plasma K stimulates secretion of aldosterone which stimulates secretion of K from the Distal tubule and collecting duct into the tubular fluid.

Question 14

How does aldosterone respond to increased plasma K concentrations?

Answer 14

High K causes secretion of aldosterone, which causes Na absorption and K to be secreted into UF at the late distal tubule and the collecting ducts.

Question 15

What do the H⁺/K⁺ ATPases do in the collecting ducts?

Answer 15

maintain system acid/base balance by buffering and excreting the H⁺ into the tubular fluid where it is buffered and excreted

The K enters the cells.

Question 16

60% of Ca²⁺ is freely filtered at the glomeruli and 99% of that is reabsorbed. How and where is Ca²⁺ reabsorbed?

Answer 16

1. Proximal tubule- 70% of Ca is reabsorbed between cells (paracellular) via solvent drag caused by the movement of Na.
2. Thick ascending limb of Henle- paracellular with Na again. 20%
3. Distal tubule- in response to decrease in plasma Ca, PTH is released which increases Ca channels and Na/Ca exchangers and Ca ATPase pumps which transport Ca out of cell and into interstitium.

Question 17

What is the 24 hour urine volume for an adult?

Answer 17

800-2,000 ml

Question 18

How many nephrons does a normal adult have?

Answer 18

• Total: 2.5 million
• Cortical: 2.1 million
• Juxtamedullary: 0.4 million

Question 19

What are mesangial cells?

Answer 19

• Produce matrix that holds the whole thing together (glomerulus)
• Secretes matrix proteins
• phagocytosis
• Regulate GFR
  
  • can contract- decreases filtration surface area
  • can relax- increases filtration surface area (inc GFR)

Question 20

What is the transport maximum of Glucose?

What is the renal threshold?

Answer 20

375 mg/min

300 mg/dL