Function of each part of Nephron

Question 1

Proximal Tubule

1. What is being reabsorbed
2. Is there a particular important channel

Answer 1

1. 85% of filtered sodium Bicarbonate is reabsorbed.

40% of filtered NaCl and 60% of H2O

Glucose, amino acids, and other organic solutes are preferentially reabsorbed via specific transport systems in the early proximal tubule.

2. NHE3: Na+/H+ exchanger ( Na+ going into cell, H+ coming out).
   - H+ binds to Bicarvonate= carbonic acid (CA) = with Carbonic anhydrase converts back to H2O and CO2 both of which pass passively into cell at which point CA coverts them back to H2CO3 –> bicarbonate. Bicarbonate is transferred into lumen.

Na+ K+ ATPase is on basal side. Sensitive to digitalis.

Question 2

What is the Organic acid secretory system and where is it located?

Same for Organic Base secretory system

Why are these systems important

Answer 2

For uric acid, NSAIDs, antibiotics etc. excretion.

Located in middle third of the proximal tubule

Organic base secretory system ( creatinine, procainamide, choline etc.) are localized in the early and middle segments of the proximal tubule.

These systems are important for delivery of diuretics to their active sites at the luminal aspects of tubule segments along the entire nephron. ( drugs are secreted into tubular fluids)

So important for elimination and delivery!

Question 3

Thin Descending Limb

1. what is reabsorbed?
2. what drug affects this area

Answer 3

1. Water is reabsorbed by osmotic forces ( glucose, urea)

2. Mannitol or glucose in lumen will interfere with water extraction.

Question 4

Thick Ascending Limb

1. what is reabsorbed
2. important channel and function?

Answer 4

1. 35% of filtered sodium is reabsorbed and is impermeable to water
2. NKCC2 Na+/K+/ 2Cl- cotransporter
   - accumulation of intracellular potassium
   - accumulation of excess K+ leads to back diffusion of K+ into Tubular lumen : + electrical potential of lumen.

+ electrical potential (due to excess K+) provides driving force for reabsorption of Mg2+ and Ca2+.

Question 5

Distal Convoluted Tubule

1. what is reabsorbed, what is secreted? What is not easily permeable?
2. Types of cells:

Answer 5

<10% NaCl reabsorption
relatively impermeable to H2O
Major site of K+ secretion

Principal cells : ADH-sensitive
- driving force for Na+ exceed K+ secretion = negative potenital that drives Cl- reabsorption (paracellularly)

Intercalated cells: Alpha: secretes H+; Beta: secretes HCO3-

Question 6

What causes an enhanced secretion of K+ in the Distal Tubule and collecting duct?

Answer 6

Secretion is enhanced if Na+ delivery to distal tubule is increased (use of thiazides or other diuretics)

Since HCO3- cannot be reabsorbed as readily as Cl-, the lumen-negative potential increases and K+ is enhanced

This mechanism along with aldosterone secretion (due to volume depeltion) is basis for most diuretic-induced K+ wasting.

Question 7

Aldosterone

1. Where does it affect reabsorption and of what?
2. does it have any other effects on other molecules?

Answer 7

Affects ENAC channel ( Na+ channel) on prinicpal cell – enchances Na+ reabsorption ( apical side)

On basolateral side of principal cell enchancs Na+ K+ ATPase – reabsorption of Na+ and secretion of K+
* SECRETION OF K+

In Intercalated cell, it has the effect of enhancing H+ secretion
* SECRETES H+