LECTURE 3 (Kidney metabolism)

Question 1

What is the function of the Early Proximal Convoluted Tubule (PCT)?

Answer 1

• Reabsorbs all glucose and amino acids and most HCO3-, Na+, Cl-, PO43-, K+, H2O and uric acid
• Isotonic absorption
• Generates and secretes NH3 which enables kidney to secrete more H+

Question 2

What is the function of the Thin descending loop of Henle?

Answer 2

• Passively reabsorbs H2O via medullary hypertonicity (impermeable to Na+)
• Concentrating segment -> makes urine hypertonic

Question 3

What is the function of the Thick ascending loop of Henle?

Answer 3

• Reabsorbs Na, K and Cl
• Indirectly induces paracellular reabsorption of Mg and Ca through +ve lumen potential generated by K backleak
• Impermeable to H2O
• 10-20% reabsorbed

Question 4

What is the function of the Early descending convoluted tubule?

Answer 4

• Reabsorbs Na and Cl
• Impermeable to H2O
• Makes urine fully dilute (hypotonic)
• 5-10% Na reabsorbed

Question 5

What is the function of the Collecting tubule?

Answer 5

• Reabsorbs Na+ in exchange for secreting K+ and H+ (regulated by aldosterone)
• Aldosterone
  [in PRINCIPAL CELLS: increased apical K+ conductance, Sodium potassium pump, Epithelial Na+ channel activity and K+ secretion
  in INTERCALATED CELLS: increased H ATPase activity, H+ secretion and HCO3/Cl exchanger activity]
• ADH
  [acts at V2 receptor -> insertion of aquaporin H2O channels on apical side]
• 3-5% Na reabsorbed

Question 6

What are the two types of acids produced via metabolism?

Answer 6

• Volatile
• Non-volatile

Question 7

Describe the Volatile acids produced via metabolism

Answer 7

• CO2
• Combines with water to form carbonic acid and H+
• Eliminated by lungs (not kidneys)

Question 8

Describe the Non-volatile acids produced via metabolism

Answer 8

• Not from CO2
• Derived from amino acids, fatty acids and nucleic acids
• Acid buffered by bicarbonate (no change pH)
• Bicarbonate must be replenished by kidneys

Question 9

How does the kidney regulate acid-base balance?

Answer 9

• Reabsorbs or generates bicarbonates
• Excrete H+

Question 10

Why are other buffers required other than bicarbonate?

Answer 10

High H+ (low pH) damages the nephron -> Urine needs buffers to soak up H+ and protect nephron from low pH -> All bicarbonate is reabsorbed by the time urine reaches COLLECTING DUCT -> Other buffers needed (TITRATABLE ACIDS & AMMONIA)

Question 11

What are the properties of Titratable acids?

Answer 11

• Urinary substances that absorb H+
• Measured by titration method
• Mostly phosphate
• Exists in multiple states (HPO4, H2PO4)

Question 12

What is the difference between HPO4 and H2PO4 (titratable acids)?

Answer 12

HPO4 = monohydrogenate filtered by glomegrulus

H2PO4 = dehydrogenate form excreted in urine -> excretion of extra H+

Question 13

What happens when there is a lack of dietary intake of phosphate?

Answer 13

• Supply of ammonia (NH3) is adaptable
• More NH3 generated by kidneys when increased H+
• Synthesised from glutamine

Question 14

What are the properties of Erythropoietin?

Answer 14

• Released by interstitial cells in PERITUBULAR CAPILLARY BED in response to HYPOXIA
• Stimulates RBC proliferation in bone marrow
• Administered for anemia secondary to chronic kidney disease
• Increase risk of HTN

Question 15

What do Proximal Convoluted Tubule (PCT) cells convert?

Answer 15

25-OH vitamin D3 (CALCIDOL) to 1,25-OH vitamin D3 (CALCITRIOL, active form)

Question 16

Which hormone do PCT cells secrete?

Answer 16

Dopamine