15B03: Exam Report

Compare and contrast renal and hepatic blood flow, and their regulation.

54% of candidates passed this question.

It was expected candidates would describe the salient features of the anatomy, distribution and content of blood flow and influences on each circulation. Answers with a clear organisation and context for the normal influences of blood flow on the functioning of each organ system scored highly. Anatomy was often sufficiently covered, but candidates often did not take advantage of that by linking the anatomical features to the functional concepts. Figures should be clearly and accurately labelled to score well. Many answers failed to demonstrate a depth of understanding of key concepts. For example tubuloglomerular feedback, relationship between hepatic arterial and portal venous flows and autoregulation within both those systems was often poorly described.

G4iii / 15B03: Compare and contrast renal and hepatic blood flow, and their regulation.

O2 Consumption

Renal

18mL O­2/min

Hepatic

50mL O2/min

Body O2 Consumption

Renal

7%

Hepatic

20%

Blood Flow

Renal

1200mL/min

Hepatic

1500mL/min

% CO

Renal

25%

Hepatic

30%

Function

Renal

Glomerular filtration

Hepatic

Metabolic activity

Vascular Anatomy

Renal

Renal a.

→ Interlobar a.

→ Arcuate a.

→ Interlobular a.

Hepatic

Dual vascular supply

  • 30% blood from HEPATIC a.
  • 70% blood from PORTAL v.

(both supplying 50:50 of O2)

→ Sinusoids feeding Portal Triad

→ Specialised capillary system feeding hepatocytes

→ Hepatic v

→ IVC

Autoregulation

Renal

Strict autoregulation 90 – 180mmHg (to maintain GFR 125mL/min)

Hepatic

Yes → only hepatic a., but not as tightly as renal autoregulation

BF Regulation

Renal

INTRINSIC

  • \( \begin{cases} \text{Tubuloglomerular feedback} \\ \text{Myogenic mechanism} \end{cases} \)
  • Tubuloglomerular feedback
    • NaCl dependent
    • Macula Densa sense [NaCl]
    • ↓[NaCl] = ↓resistance of aff. arteriole & ↑RENIN release by juxtaglom. cells
      • Myogenic mechanism
    • Pressure dependent
    • ↑arterial P → aff. Arteriole stretched → stimulates smooth m. contraction

EXTRINSIC

  • nerves
    • & eff. Arterioles
    • NA & adrenaline binds α1 receptor → VC → ↓RBF
  • Hormones & endothelial factors
    • AII, PGE, NO, Endothelin, BIC, Adenosine → VC/VD to alter BF
  • High protein meal → ↑glom. cap pressure & ∴↑RBF

Hepatic

INTRINSIC

  • \( \begin{cases} \text{HABR} \\ \text{Metabolic} \\ \text{Autoregulation}\end{cases} \)
  • Hepatic arterial buffer response
    • Hepatic a. & portal v. equilibrate to maintain a constant BF to liver
    • ↑Portal v. BF = ↓Hep a. BF & vice versa
    • Due to ∆[Adenosine]
    • Adenosine = VD produced at a constant rate by metabolism
    • ∴↓Portal v. BF
    • ↑[Adenosine] = ↓artery resistance → ↑Hep a. BF
  • Metabolic
    • ↓PaO2/↑H+ of Portal V = ↑hepatic a. BF
    • ↑osmolarity post meal = ↑Hep a. & portal v. BF
  • Pressure autoregulation
    • Only in hepatic a.
    • ↑arterial P
    • ↑transmural P
      • ↑myogenic tone (VC)
      • ↓BF

EXTRINSIC (hepatic)

  • Neurons
    • NS regulates splanchnic vessels
    • ↓symp tone = ↑ splanchnic reservoir
    • ↑symp tone = 500mL of hepatic blood vol transferred to circulation