tubular absorption/secretion

Question 1

What are the general modes of transport?

Answer 1

1) Primary active (Na/K ATPase)
2) Secondary active (cotransporters-glucose)
3) Diffusion (CO2)
4) Facilitated Diffusion (aquaporins)
5) Pinocytosis (proteins)
6) Solvent Drag (osmotic action sucks particles through cell pores)

Question 2

What’s the difference between paracellular and transcellular?

Answer 2

Reabsorption and secretion occur BETWEEN renal tubular cells (paracellular) vs occurring ACROSS the tubular cells (transcellular)

Question 3

Amino acids need Na+ cotransporters to get around in the kidney. If there are more amino acids than there are Na+ cotransporters, then the process cannot go any faster. This is an example of _______________.

Answer 3

Saturation kinetics

Question 4

Saturation kinetics are at work when we see glucose in the urine of a diabetic patient. Explain.

Answer 4

Glucose relies on cotransporter Na+. There are enough sodium transporters to filter about 200mg/dL of glucose. If there is more glucose than 200mg/dL it begins to accumulate because the sodium transporter is the rate limiting step.

Question 5

If we were talking about any substance, you could say that:

The amount filtered - _______________ = the amount reabsorbed.

Answer 5

amount excreted

Question 6

What’s happening with Urea?

Answer 6

Urea is being secreted into the descending limb from the vasa recta. As it continues down the nephron and gets to the collecting duct, it is very concentrated (because all the water has been reabsorbed). The collecting duct is permeable to urea and has a large concentrated supply, so it goes back into the blood supply. The blood supply it goes into happens to be the vasa recta. The vasa recta secretes the urea back into the descending limb where it gets more and more concentrated until it reaches the collecting duct, and around and around it is recycled.

Question 7

What is the primary role of urea?

Answer 7

To concentrate the urine.