Renal molecule movement

Question 1

What is the definition of reabsorption and secretion?

Answer 1

• Reabsorption is the movement of water and solutes from the nephron tubule back into circulation.
• Secretion is the movement of solutes and water from the circulation into the nephron tubule.
• The vasa recta is the network of capillaries that follow the nephron, it allows for reabsorption and secretion.
• Sodium and water play a key role in regulating blood pressure.
• HCO3- and H+ play a key role in acid / base balance.

Question 2

Proximal convoluted tubules

Answer 2

• Reabsorption of Na+, Cl-, K+, glucose, amino acids, uria and bicarbonates.
• Sodium gets reabsorbed together with glucose or amino acids via a symporter, the other products is then taken back up into circulation via the vasa recta.
• An exchanger which moves Na+ into the epithelial cells lining the nephron in exchange for H+, this is an antiporter and the Na+ will then enter the blood via exchange with K+ (ATPase located on the basal membrane).
• Acid / base balance regulated as hydrogen can be secreted while bicarbonates are absorbed.
• Hydrogen ions inside the lumen of the nephron (exchanged for sodium) react with bicarbonate ions to form carbonic acid via carbonic anhydrase.
• Carbonic acid is converted to water and CO2 which is a gas which may diffuse back into cell and back into circulation.
• Within the cell, the same reaction can take place therefore carbonic acid is reformed then dissociated, the produced H+ can be exchanged for sodium once more continuing the cycle.
• Hydrogen can also enter through circulation, bicarbonate (reverse reaction) can leave into circulation if a more alkaline pH is required, this is done via a symporter with sodium.
  -Majority of calcium is reabsorbed here.

Question 3

Loop of Henle

Answer 3

• Water reabsorption occurs in the descending limb and in the ascending limb the reabsorption of Na+, K+ and Cl- occurs.
• In the ascending Loop of Henle there is a tri-symporter located on the apical membrane which bring in these 3 (2 Cl-).
• The K+ and Cl- go through another symporter back into vasa recta. Sodium is exchanged for K+ using the ATPase pump.
• There are passive channels on the apical membrane that allow sodium to enter the cell from lumen and for potassium enter lumen from cell.

Question 4

Distal convoluted tubules

Answer 4

• Reabsorption of Na+, Cl-, K+, Ca2+, Mg+ and HCO3-.
• Sodium and Chloride are absorbed from lumen into the cell via a symporter, Cl- enters blood via passive channel while Na+ is exchanged for K+ using the ATPase pump. K+ can passively go through into lumen.
• Magnesium and Calcium enter circulation via paracellular routes via diffusion.

Question 5

Collecting duct

Answer 5

• Reabsorption of Na+, Cl- and uria (water)
• Sodium is reabsorbed into the cell via an antiporter with H+ and again using the same type of pump into the blood.
• H+ being present in the lumen allows for acid / base balance to occur via bicarbonate interactions.
• Bicarbonate reabsorption into circulation uses an exchanger with chloride which is contrary to the PCT where bicarbonate reabsorption is sodium dependent.
• In the DCT and CD, there are channels which exchange Na+ and K+ which do not require ATP. This is done via the hormone aldosterone (response to low blood pressure). Encourages reabsorption of sodium as it allows water to reenter blood to increase bp.
• The apical membrane of the collecting ducts have channels called aquaporins which allow for reabsorption of water which increases the osmolarity of urine (regulated by ADH).

Question 6

What are the mechanisms for renin release.

Answer 6

1. Decreased blood pressure detected by baroreceptors.
2. Beta-1 androgenic receptors detect changes to blood pressure via the sympathetic nervous system flight or fight response.
3. Macula Densa, chemoreceptors that detect how much NaCl is passing through the distal convoluted tubule, stimulated when NaCl decreases (when it increase the cells are stimulated to stop).

Question 7

How does renin cause an effect?

Answer 7

• Renin is an enzyme secreted from the kidney to increase blood pressure.
• Produced by juxtaglomerular cells and leaves through the efferent arteriole into circulation.