H1ii: Physiological factors that determine oxygen delivery to the renal medulla

Renal Vascular Anatomy

  • No glomeruli exist in renal medulla
  • Efferent arterioles of deep (juxtaglomerular) nephrons descend down to medulla + form VASA RECTA

Efferent arteriole

↓

Descending vasa recta

↓

Ascending vasa recta

↓

Veins

  • Vasa recta have different structure along their tracts
    • Desc = arteriole like
    • Loop = capillary like
    • Asc = fenestrated endothelium to allow solute exchange
Cortex

Renal Blood Flow

  • RBF = 20% CO

        = 1.2L/min

        = 600mL/kidney/minute

  • But RBF is not evenly distributed
    • 90%
      • Cortex
      • Flow dependent function
      • Glomerular filtration & reabsorption
      • 5mL/g/min
      • PaO2 = 50mmHg
    • 10%
      • Medulla
      • Urine [ ]
      • Outer medulla = 1mL/g/min
      • Inner medulla = 0.2mL/g/min
      • PaO2 8 – 15mmHg (only just meets its E requirements!)
    • ∴even though the medulla is more metabolically active & extremely vulnerable to hypoxic injury, it has less BF which is prioritised to cortex to allow adequate GFR
    • Most tubular O2 consumption is to reabsorb Na+
    • And most tissues ↑BF in response to ↑demand
    • BUT ↑RBF = ↑GFR = ↑Na+ load = ↑workload & O2 requirements
    • BF to medulla is v. close to its O2 requirements & pO2 of medulla much lower than Renal A
    • Key role of medulla Thick Asc. LoH = active transport of NaCl = high E consumption
    • So to concentrate our urine we force medullary hypoxia
    • Medulla tubular cells use glycolysis (minimal E consumption) & a Na/K/2Cl co-transporter to minimise O2 requirements
    • But this constant risk of hypoxia requires internal regulation for protection

Determinants of O2 Delivery To Kidneys

DO2 = CaO2 x CO

O2 delivery = O2 constant of blood x CO

CaO2 = (([Hb] x 1.34 x SpO2) + (0.003 x PO2)

CO = HR x SV

  • ∴factors affecting CaO2 & CO will affect how much O2 is delivered to kidneys

CaO2

[Hb]

Saturation of Hb

pO2 of blood

CO

HR

Preload

Afterload

Contractility

Regulation of Renal Blood Flow

  • Autoregulation
  • Sympathetic nerves
  • Vasoactive hormones
RBF

Autoregulation

  • Definition = the intrinsic ability of an organ to maintain a constant BF despite changes in perfusion pressure → in kidneys, autoregulation RBF b/w MAP 95 – 170 mmHg
  • 2 mechanisms of autoregulation of RBF:
    1. MYOGENIC → responds to ∆ arterial P
    2. TUBULOGLOMERULAR FEEDBACK → responds to ∆ [NaCl] of tubule fluid

Sympathetic Nerves

  • Aff. & Eff. Arterioles innervated by symp n’s
  • Basal symp tone is minimal
  • ↓ECFV = symp. release of NA + Adrenaline → as receptor stimulation of aff. arterioles = ↓GFR/RBF

Vasoactive Hormones

VC

AII

Renin

Endothelin

Adenosine

VD

Prostaglandin

NO

ANP/BNP

Bradykinin

Factors Affecting Blood Flow to Medulla

  • BF to juxtamedullary nephrons is NOT autoregulated (KAM 228pp)
  • ∴influenced by symp stimulation & vasoactive factors
  • Also ↓BGL & high protein meal = ↑medullary BF