Renal physiology
Functions
Fluid and electrolyte balance.
Excretion of waste products of metabolism or drugs.
Acid/base.
Ca+ and vit D homeostasis.
Endocrine – renin, EPO, PGs, kinins.
Glomerulus
Fluid and electrolytes move from capillaries to Bowman’s capsule driven by hydrostatic pressure. Limited by size and charge.
Glomerular pressure MAP (90). Bowman’s capsule pressure 15. Plasma oncotic pressure 25 so filtration pressure 50.
GFR 125mls/min or 180L/day.
GFR = urine conc (a) X urine flow (mls/min) / plasma conc (a)
(a) can be inulin or radiolabelled EDTA
(Cl = Rel / plasma conc)
Creatinine freely filtered so reflects GFR.
Plasma creatinine shows a rectangular hyperbolic relationship with creatinine clearance so GFR must be ↓ 50% before creatinine starts to rise.
GFR ↓ 1% / year after age 30.
Blood flow
25% CO = 1250mls/min = 625mls/min plasma flow.
75% to cortex; 25% to medulla.
RBF determined by renal perfusion pressure and renal vascular resistance.
Constant at MAP 65 -180. ↓↓ <60. Note that investigations such as renal doppler have shown that renal blood flow is not significantly increased above a MAP of 65 even in patients with hypertension. In my opinion it is a myth that increasing MAP in hypertensive patients increases urine output - if you have seen it in a few instances it was probably just coincidence.
Oxygen consumption
AV difference small as large blood supply but vulnerable to hypoxia because:
Hypovolaemia → symp stim → renal vasoconstriction (to divert blood to brain and heart).
Hypovolaemia → Na+ and water retention → ↑ O2 consumption.
Medulla has poor blood supply.
80% O2 consumption to drive Na/K pump.
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