Physiology 4- proximal tubule and loop of Henle

Question 1

How much of the glomerular fluid is reabsorbed in the proximal tubule?

Answer 1

About 2/3rds

Question 2

What are the two types of transport across the epithelium?

Answer 2

Transcellular and paracellular (across the tight junctions)

Question 3

What are the main types of transport mechanisms involved in transcellular transport?

Answer 3

Primary active transport (ATPases- e.g. pumping three Na+ out of the cell in exchange for two K+)
Secondary active transport (molecule is moved coupled to the conc. gradient of an ion, usually Na)
Facilitated diffusion (speeds up the usual diffusion process- glucose)
Simple diffusion across lipid bilayer (uncharged molecules like oxygen)

Question 4

What mechanism is used to reabsorb sodium?

Answer 4

Na/K ATPase at the basolateral membrane

Question 5

How are glucose and amino acids absorbed at the brush border?

Answer 5

Co-transport with sodium

Question 6

How is H+ pumped out into the filtrate?

Answer 6

Counter-transport with sodium

Question 7

How is glucose moved at the basolateral membrane?

Answer 7

Facilitated diffusion

Question 8

What is the consequence of absorption of sodium, glucose and amino acids in the PCT?

Answer 8

Osmosis-fluid reabsorption

Question 9

What is the transport maximum and renal threshold of glucose? What happens after the renal threshold is exceeded?

Answer 9

Renal threshold- 10-12mmol/l
Transport maximum- 2mmol/min
“Spill-over” of glucose into the blood

Question 10

What does the Tm represent? Why is the Tm finite for a given molecule?

Answer 10

Maximal rates of secretion/reabsorption.

These are carrier-driven pathways and are therefore saturable

Question 11

What can be said about fluid and water reabsorption in the descending and ascending limbs of the loop of Henle?

Answer 11

Ascending- salt reabsorbed, water not

Descending- permeable to water, impermeable to salt

Question 12

How does the countercurrent multiplier ultimately dilute the filtrate?

Answer 12

Fluid is concentrated in the descending limb by the removal of water; salt is removed from this concentrated fluid in the ascending limb, creating a diluted fluid (hypo-osmotic) entering the DCT.

[the salt removed from the ascending loop ENHANCES the removal of water from the descending limb]

Question 13

What is the purpose of the countercurrent multiplier?

Answer 13

To create a concentrated interstitial fluid suitable for the removal of water (according to the bodies needs and ADH secretion) in the collecting duct

Question 14

What is the purpose of the vasa recta?

Answer 14

Ensures that the interstitial solute gradient is not “washed away” by the blood