You do not have enough blood to supply every organ at full capacity at once. Sprint after a large meal and your muscles and your gut are competing for the same litres. The body’s solution is not to make more blood — it is to redirect the blood it already has.
📚 What you need to know
The circulatory system supplies cells with oxygen and nutrients and removes the waste products of metabolism.
Different organs need different amounts depending on the body’s activity level.
Blood is diverted by vasodilation and vasoconstriction of the arterioles supplying each capillary bed.
Skeletal muscle: low flow during sleep, higher when awake, hugely increased during exercise.
Gut: flow increases after a meal, and decreases during exercise.
Brain: flow stays relatively constant whatever you are doing, rising slightly during REM sleep.
Kidneys: flow changes little, rising slightly at rest and falling slightly during prolonged exercise.
Why the demand keeps changing
Cells respire all the time, but not at the same rate. A muscle cell contracting rapidly needs far more oxygen and glucose per second than the same cell resting, and it produces far more carbon dioxide and heat that has to be carried away.
During exercise, skeletal muscles need more oxygen and glucose to fuel contraction, so they need an increased blood supply.
After a meal, the digestive system needs more oxygen and glucose to fuel digestion and absorption, so it needs an increased blood supply.
Those two demands can arrive at the same time, which is why the system has to prioritise rather than simply satisfy everything.
How blood is redirected
The control point is the arteriole feeding each capillary bed. Arterioles have muscle in their walls, so they can change diameter.
Vasodilation — the muscle relaxes, the arteriole widens, and more blood flows into that capillary bed.
Vasoconstriction — the muscle contracts, the arteriole narrows, and less blood flows into that capillary bed.
Because the total volume of blood is fixed, constricting the arterioles of one organ makes more available elsewhere. Redirection is always a trade.
After a meal the picture reverses: the gut arterioles dilate and the muscle arterioles constrict. It is the same switch, thrown the other way.
Organ by organ
Skeletal muscles
During sleep the skeletal muscles are relaxed, so blood flow to them is relatively low.
During wakefulness some muscle groups are working just to hold the body upright, so flow increases.
During physical exercise there is a large increase, because many muscle groups are contracting rapidly.
Gut
Soon after a meal, blood flow to the gut increases to support digestion and absorption.
During exercise it decreases, so that more blood can be diverted to the skeletal muscles.
Brain
Blood flow to the brain stays relatively constant regardless of activity level, because it carries out processes that must continue all the time.
Flow increases slightly during the stage of sleep known as REM.
Kidneys
Blood flow does not change significantly with activity level, because filtration has to continue whatever you are doing.
It increases slightly during sleep and rest, and decreases slightly during prolonged exercise.
Total flow rises a great deal during exercise because the heart is pumping harder, so an organ can receive the same volume while receiving a much smaller share of the total. Read the question carefully to see which is being asked about.
Be careful with the words volume and proportion. During exercise the kidneys receive roughly the same litres per minute as at rest, but a far smaller percentage of the total output. Both statements are true, and a data question will happily test whether you can tell them apart.
Organ
During exercise
After a meal
During sleep
Skeletal muscle
Large increase
Little change
Relatively low
Gut
Decreases
Increases
Little change
Brain
Relatively constant
Relatively constant
Slight rise during REM
Kidneys
Slight decrease if prolonged
Little change
Slight increase
The link back to thermoregulation. Vasodilation of the skin arterioles appears in both topics, and it is the same event. During exercise the skin dilates to dump the extra heat from respiring muscles — which is why your face goes red when you run, and why running in a heatwave is so much harder: your skin and your muscles are competing.
Worked examples
WE 1
Explaining the change at the gut
Explain why blood flow to the gut decreases during vigorous exercise. (3 marks)
Point 1: the mechanism
Muscle in the walls of the arterioles supplying the gut contracts, causing vasoconstriction so less blood enters the gut capillary beds.
Point 2: the reason
The skeletal muscles need much more oxygen and glucose for respiration to fuel contraction, so blood is diverted to them.
Point 3: why the gut can spare it
Digestion and absorption can continue at a reduced rate, so the gut is not damaged by the temporary reduction.
Constrict where it can wait, dilate where it cannotthis is also why exercising straight after a big meal can cause stomach cramps.
WE 2
Calculating a share of cardiac output
At rest, a person’s cardiac output is 5.0 dm3 min−1 and their skeletal muscles receive 1.0 dm3 min−1. During heavy exercise, cardiac output rises to 25.0 dm3 min−1 and the muscles receive 18.0 dm3 min−1. Calculate the percentage of cardiac output going to the muscles in each case, and comment on the result. (3 marks)
Step 1: at rest(1.0 ÷ 5.0) × 100 = 20 %Step 2: during exercise(18.0 ÷ 25.0) × 100 = 72 %Step 3: comment
The muscles take a much larger share, and the actual volume has risen 18-fold. Both happen at once: the heart pumps more blood and a greater proportion of it is directed to the muscles by vasodilation.
20 % at rest, 72 % during exerciseshow the working line even when the arithmetic is easy — method marks are given for it.
WE 3
Why the brain is protected
Suggest why blood flow to the brain remains almost unchanged during exercise, while flow to the gut falls sharply. (3 marks)
Point 1: what the brain needs
Brain cells respire continuously and rely almost entirely on glucose and oxygen delivered in the blood, with very little store of their own.
Point 2: the consequence of reducing it
Even a short reduction in supply would reduce ATP production and cause loss of consciousness, so the processes it controls cannot be paused.
Point 3: the contrast
Digestion in the gut can be delayed without harm, so gut arterioles can be constricted while brain flow is maintained.
The brain cannot postpone what it does; the gut can“suggest” again — you are applying the general principle, not reciting a fact.
💡 Exam tips
Always name the vessel: it is the arteriole that dilates or constricts, not the artery or the capillary.
Give a reason as well as a direction. “Flow to the gut decreases” is a description; “because blood is diverted to respiring muscles” is the explanation.
Watch for graphs plotted as percentage of cardiac output rather than volume — the brain looks like it drops when it has not.
The brain and kidneys are the “constant” pair. Learn them as the exceptions.
Link the skin to thermoregulation if a question mentions exercise and temperature together.
⚠ Common mistakes
Saying the body makes more blood during exercise. The volume of blood is fixed. It is redistributed, and pumped faster.
Saying capillaries constrict. They cannot — no muscle in the walls.
Saying brain blood flow increases a lot during exercise. It is remarkably constant.
Confusing volume with proportion when reading data.
Forgetting the skin. Exercise increases skin blood flow too, to lose the extra heat.
Pulling the topic together
Six pages, one idea. Every system in this topic follows the same pattern: something changes, a receptor detects it, a coordination system passes the message on, and an effector produces a response that reverses the change.
Factor
Receptor
Coordination
Effector and response
Blood glucose
α and β cells of the islets of Langerhans
Insulin and glucagon in the blood
Liver and muscle store or release glucose
Body temperature
Skin thermoreceptors and hypothalamus
Nervous system, plus thyroxin
Arterioles, sweat glands, hair muscles, shivering
Blood water content
Osmoreceptors in the hypothalamus
ADH from the posterior pituitary
Collecting duct changes water reabsorbed
Blood supply to organs
Detection of changing activity level
Nervous system
Arterioles dilate or constrict at each organ
If you can fill in that table from memory, you have this topic. When a question describes an unfamiliar system, find those four columns in it and the marks will follow.
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