Secretion and reabsorption

Question 1

Explain the flow of transepithelial transport

Answer 1

Lumen –> epithelial cell - interstitial fluid –> capillary wall –> lumen
along the tube micro villi face in the inside surface

Question 2

what are the two activities that the proximal tubule carries out

Answer 2

1. reabsorption of some filtered solutes
   2, secretion of other solutes that can’t pass through the filtration membrane

A majority of this reabsorption is facilitated by Na+ coupled co-transport.
The Na+ coupled co-transport is facilitated by the active Na+/K+ pump by ATP - this maintains the gradient allowing the Na+ coupled co-transport to fx

with these processes the hydrostatic forces going out the blood vessels is decreased, which means the colloid pressure of the blood vessels which have remained the same now have the end result of osmotic movement into the blood vessel –> therefore this means that reabsorption of material in the blood vessels will occur.

Question 3

Explain the blood supply of the nephrons

Answer 3

The efferent arterial of the glomerulus becomes the vasta recta and the peritubular capillaries

Vasta recta = assists with maintaining the concentration gradient

Peritubular capillary = an important role in reabsorption

Question 4

Explain Na+ reabsorption the four steps

Answer 4

1. Na + enters the endothelial cell via secondary active transport (carrier proteins in the membrane allow either symporter (glucose/amino acids) or antiporter (H+)

2, endothelial cell –> interstitial fluid = done via active Na+/K+ pumps

3. diffusion through intersititum to capillary wall = passive
4. passive into capillary

This process is energy heavy, as 80% of O2 used in the kidney is for this fx

Question 5

Location of Na+ absorption

Answer 5

the location of Na+ absorption is important because it is a major determinant of where water is also absorbed.

70% of Na+. absorbed in the PCT
loop of henle and DCT and collection duct are between 20 to 10 %

Question 6

Explain H20 and Cl- reabsorption

Answer 6

both passively
H20 via osmotic gradient is the force for movement
CI- electrical gradient established by Na+ transport is the force

Question 7

explain glucose reabsorption

Answer 7

1. Co transported by Na+ secondary active transport from the lumen to proximal cell
   2, facilitated transport from proximal cell to interstitial fluid
   3.passive diffusion through interstitial space to capillary wall
2. passive diffusion into the capillary

Question 8

explain the glucose reabsorption maximum

Answer 8

as glucose is reabsorbed via co-transport, when these are all occupied, a maximum rate of globe reabsorption is reached

this is termed the TM = tubular maximum

in healthy animals all glucose and amino acids are reabsorbed in proximal tubule

Question 9

Explain Amino acid reabsorption

Answer 9

In a healthy animal 100% of amino acids are reabsorbed in the proximal tubule by saturable co-transporters (recognising neutral, acidic, basic amino acids)

Question 10

explain reabsorption by solvent drag

Answer 10

via the paracellular pathway between the tubule,

solvents are dragged along by the flow of water

Question 11

explain the role of the vasta recta

Answer 11

forms a counter current flow with the loop of henle
fx - removes the water Na+, CI- and K+ via the counter current flow, this means that there interstitial fluid does not reach equilibrium, by dilution with water entering in abundance from the descending loop of hene

Stx - capillaries are permeable to both NaCl and water to allow the above fx

Question 12

explain the secretions

Answer 12

organic ions and exogenous drugs

As these are often protein bound they usually are not part of the glomerular filtrate

clinical applications
= drugs which are secreted here are effective against things occurring in this region