Lecture 18 : Physiology of the Renal System III : Tubular Reabsorption

Question 1

Proximal Tubule

Answer 1

• First 60% called proximal convoluted tubule
• Second 40% is proximal straight tubule
• Both present in renal cortex
• SA enhanced by microvilli

Proximal Convoluted :
- Major site of reabsorption
- 65% of Na, Cl-, Water
- 100% of glucose and amino acids
- HCO3-
- Secretes H+, organic acids, drugs (creatinine and penicillin)
- highly active and rich in mitochondria

• Ions and molecules move via ion channels, cotransporters and pumps
• Movement of Na+ out increases osmotic pressure outside tubule and so water is drawn out of tubule
• Movement of Na+ down electrochemical gradient is utilised by cotransporting glucose and amino acids against their concentration gradient BY RELEASING ENERGY
• Uses Na+ / K+ ATPase pump to move Na+ out of cells on basolateral membrane to maintain this gradient
• Water moves paracellularly (hydrostatic and osmotic forces) and transcellularly, via AQP1

Proximal straight :
- Continues reabsorption of Na+, Cl-, water and urea
- Similar structures to convoluted
- Except extends into outer medulla, while convoluted remains in renal cortex

Question 2

Fates of molecules along proximal tubule

Answer 2

Question 3

Glucose in proximal tubule

Answer 3

• 90% transported by SGLT2 (sodium glucose cotransporter 2)
• SGLT2 has low affinity but HIGH CAPACITY
• Rest by SGLT1, high affinity but LOW CAPACITY
• Basolaterally, glucose transported by GLUT1 and 2

Tubular maximum transport for glucose is 380mg / min

Question 4

SGLT2 Inhibition

Answer 4

• Treatment for diabetes
• Canagliflozin
• Dapagliflozin
• Blood glucose drops

Question 5

CL- movement in proximal tubule

Answer 5

• Active and passive
• Main active movement is via antiporters for other anions like HCO3- or HCOO-
• Cl- concentration increases across proximal tubule, because other molecules are reabsorbed first, and water follows those solutes by osmosis, and because water leaves, Cl- concentration in tubule increases, so a Cl- concentration gradient forms and Cl- is reabsorbed later in tubule via PASSIVE PARACELLULAR movement down concentration graidnet

Question 6

Active secretion in proximal tubule

Answer 6

Question 7

Loop of Henle

Answer 7

2 distinct components :

1. Descending limb
2. Thick ascending limb

Thick ascending limb creates high osmotic effect in medulla, causing water to diffuse out of descending limb, concentrating urine and retaining water.

It does this by using Na+ and K+ an Cl- cotransporters to move ions out of filtrate

Descending limb permeable to water so water leaves simply by osmosis.

Question 8

Furosemide

Answer 8

Question 9

Thiazides

Answer 9

Question 10

Spironolactone

Answer 10