Filtration and Clearance

Question 1

Define renal clearance.

Answer 1

• Renal Clearance
  
  • Defined as the rate of urinary excretion of a substance, relative to its plasma concentration

Question 2

How is GFR related to MAP?

Answer 2

• Blood flow is maintained in the case of changing MAP through myogenic responses and tubuloglomerular feedback, which maintains a nearly constant GFR when MAP is between 80 and 180 mmHg.

Question 3

Explain the process of tubuloglomerular feedback

Answer 3

• GFR increases => Flow through tubule increases
• Flow through macula densa increases
• Paracrine diffuses from macula densa to afferent arteriole
• Afferent arteriole constricts
  
  • Resistance in afferent arteriole increases
  • Hydrostatic pressure in glomerulus decreases => GFR decreases

Question 4

Recall the RAAS System.

Answer 4

• Angiotensin II causes constriction preferentially to the efferent arteriole – increase GFR
  
  • Also causes increase reabsorption of sodium – activating channels in the PCT (sodium hydrogen exchanger)
• Aldosterone causes increase reabsorption of sodium – epithelial sodium channel in the collecting duct

Question 5

How does SNS affect the filtration/clearance process?

Answer 5

It causes the release of noradrenaline which constricts afferent arteriole as well as increasing reabsorption of sodium in the PCT. This overall leads to a decrease in GFR and increase in blood pressure.

Question 6

Hows do ANP affect the filtration/clearance process?

Answer 6

• Atrial Natriuretic Peptide - hormone secreted by the heart => secretion of sodium by the kidney
  
  • In response to the increase in heart blood volume (stretch receptor)
  • Dilates afferent arteriole – increase GFR
  • Reabsorbs less sodium in collecting duct => water and chloride passively diffuse as well