Renal Medullary Oxygenation

Current · V5 (2025) → E1.i Historical · V4 (2023) → H1.i 1 exam appearance

2011B Q21

Exam question

Outline the functional anatomy, and the physiological factors, that determine oxygen delivery to the renal medulla.

CICMWrecks answer

Master answer

Lie in the posterior wall of the abdomen – outside peritoneum (retroperitoneal)
Each kidney approx 150g
Medial side of each kidney = hilum
- Renal artery and vein
- Lymphatic supply
- Nerves
- Ureter
Kidney surrounded by tough fibrous capsule
Kidney organized into outer cortex and inner medulla
Nephrons = functional unit of the kidney (1 million / kidney)
Medulla
- Organised into mutliple pyramids with the base at the corticomedullary junction and apices – the papilla – at the hilum draining into the ureters
Renal blood flow
- ~1.25L/min (24% of cardiac output or 500ml/100g tissue/min) via renal artery
- 95% blood flow to cortex, ~5% to Medulla
Renal artery enters through hilum it divides
- → interlobar arteries
- → arcuate arteries
- → interlobular arteries
- → afferent arterioles
- → glomerular capillaries
- → efferent arterioles
- → peritubular capillaries
  - Peritubular capillaries supply blood to the renal tubules
  - Juxtaglomerular capillaries have specialized vasa recta which supply the LoH and is important in renal concentration of urine
- → venous system → interlobular veins → arcuate veins → interlobar veins → renal vein

Organised into mutliple pyramids with the base at the corticomedullary junction and apices – the papilla – at the hilum draining into the ureters
Medullary blood flow: 1~2% of total renal blood flow
- 0.6ml/g/min in inner medulla
- 2.5ml/g/min in outer medullar
- 5ml/g/min in cortex
20~30% nephrons are juxtaglomerular with specialized vasa recta
Blood from post-glomerulus, therefore decreased plasma (filtered) proportion and increased viscosity
- Decreased flow
- Factors that increase filtration fraction decrease medullary flow (e.g. angiotensin II)
All extrinsic factors which affect afferent arterioles (humoral, neural) affect medullary blood flow similar to RBF
Oxygen supply determined by state of oxyHb dissociation curve
- Right shift favours oxygen supply
- Renal medullary pO₂ = 15mmHg (compared with 50mmHg in cortex)
  - Increased O₂ offloading
Renal oxygen consumption
- Increases with sodium resorption (happens mostly in medulla)

Oxygen \; delivery \; = \; Renal \; Blood \; flow \; \times \; CaO_2

CaO₂ is determined by:

C_aO_2 \; = \; ([Hb] \; \times \; S_aO_2 \; \times \; 1.34) + (0.03 \; \times \; p_aO_2)

Renal \; Blood \; Flow \; = \; {{Renal \; arterial \; pressure \; – \; Renal \; venous \; pressure} \over {Renal \; vascular \; resistance}}

{Hagen-Poisuille \; Equation : \;} {{{\Delta pressure} \times {\pi r^4}} \over {{8} \times {viscosity} \times {length}}}

Regulation:
- Renal vascular resistance maintained by interlobular arteries, afferent arterioles and efferent arterioles
- Autoregulated between 80~170mmHg

Neural	Sympathetic	All renal vesicles richly innervated by sympathetic nerves
		Minimal influence on GFR except in severe acute disturbances (severe haemorrhage)
		Decrease blood flow due to vasoconstriction
Hormonal	Adrenaline and noradrenaline	Vasoconstrict → decrease blood flow
	Endothelin	Vasoconstrict → decrease blood flow
	Angiotensin II	Constricts efferent arterioles → decrease blood flow
	Nitric oxide	Vasodilate → increase blood flow
	Prostaglandins	Vasodilate → increase blood flow

Starling resistors
- Increased intra-abdominal pressure will decrease blood flow in starling resistor model
- Increased intra-capsular pressure will decrease renal blood flow

Sakurai 2016

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