Expansion
Cardiac output rises, resistance falls, and flow is redistributed to muscle
Expansion
Rises
- Cardiac output, four to fivefold (up to sevenfold in athletes)
- Heart rate, the main contributor at higher workloads
- Stroke volume, which plateaus at around 40 to 50 per cent of maximum oxygen uptake
- Systolic blood pressure and pulse pressure
- Oxygen extraction, with arteriovenous oxygen difference widening threefold
- Venous return, via the muscle and respiratory pumps
Falls or unchanged
- Systemic vascular resistance falls, through metabolic vasodilatation in muscle
- Diastolic pressure changes little
- Mean arterial pressure rises only modestly
Redistribution: muscle blood flow rises from about 20 to over 80 per cent of cardiac output, while splanchnic and renal flow fall. Coronary flow rises in proportion to cardiac work, and cerebral flow is maintained.
Control combines central command from the motor cortex, the exercise pressor reflex from muscle mechanoreceptors and metaboreceptors, and baroreceptor resetting to a higher operating point.