2008A09 The skin, the kidneys and the carotid bodies are examples of where
specific organ blood flow is far in excess of that organ’s metabolic requirement.
For each example, explain what the physiological role of the high organ blood flow is,
why this high flow might be an advantage to the person
and a brief description of the mechanism involved.

Skin

Kidneys

Carotid bodies

Absolute blood flow

(mL/min)

500 (can ↑30x, ↓10x)

1100

0.3

Relative blood flow

(mL/min/100g)

12.5

420 average

500 cortex

20 medulla

2000

Absolute VO₂

(mL/min)

12

18

Minimal

Relative VO₂

(mL/min/100g)

0.3

6

Minimal

EO₂

0.125

0.2 cortex

0.8 medulla

Minimal

Why: purpose and explanation

How: mechanism

Skin

(1)Thermostasis: control of blood flow and sweating

·   Preserves enzyme rate and function

·   ↓T: too slow, dysfunction

·   ↑T: too fast, ↑MR, denaturing

·   Preserves pH hence protein function

(2)Blood reservoir

Hypothalamic cutaneous SNS output

Response to heat:

·   ↓SNS

·   Dilatation of AV anastomoses and precapillary sphincters

·   ↓Resistance -> ↑skin blood flow and volume

·   ↑Radiation, convection, conduction, sweat production

Response to cold: opposite

Renal cortex

High flow:

·   Ensure high GFR

·   Fluid, electrolyte, acid-base regulation

·   Selective reabsorption of water and solute

Low resistance:

·   Short, wide renal arteries

·   Branches ++

·   Glomerular fenestrae -> filtration

Renal medulla

Low flow:

·   Prevents washout of interstitium

·   High efferent arteriolar tone

Carotid bodies

·   O₂ supply only by dissolved O₂

·   Allows PaO₂ responsiveness

·   Note unaffected by Hb-bound O₂ hence anaemia, CO poisoning

·   Small size, short diffusion distance

·   Location in large vessels (aortic and carotid)