Changes in blood supply to organs with activitySpec D3.3.11
In short
The blood supply to organs changes with activity because arterioles dilate or constrict to redirect blood where it is needed. During vigorous physical activity cardiac output rises and most extra blood goes to skeletal muscles, while flow to the gut and kidneys falls. Blood flow to the brain stays roughly constant during sleep, wakeful rest and exercise.
The heart's output is shared between organs. Smooth muscle in the walls of arterioles contracts (vasoconstriction) or relaxes (vasodilation), so blood can be redistributed to match each organ's needs.
| Organ | Sleep | Wakeful rest | Vigorous physical activity |
|---|---|---|---|
| Skeletal muscles | Low: muscles relaxed and inactive | Moderate share | Greatly increased: by far the largest share, for oxygen and glucose supply and removal of CO₂ and heat |
| Gut | Substantial supply continues for digestion and absorption | Large share, especially after a meal | Reduced by vasoconstriction, so blood is diverted to muscles |
| Kidneys | Large share for filtration | Large share for filtration | Reduced, so less filtration and less urine |
| Brain | Roughly constant | Roughly constant | Roughly constant in volume, so a smaller percentage of the increased cardiac output |
- During vigorous activity, cardiac output rises (faster heart rate and larger stroke volume) and arterioles in active muscles dilate.
- During sleep, cardiac output is at its lowest, and skeletal muscles receive little blood.
- The brain needs a steady supply of oxygen and glucose at all times, so its flow is kept almost constant.
(opens full size in a new tab)Linking question: for what reasons do organisms need to distribute materials and energy? Redistribution of blood matches oxygen and glucose supply to the organs working hardest.
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Written and checked against the IB Biology HL specification · Updated October 2026