Syllabus coverage
- Explain venous return mechanisms, limited to the skeletal muscle pump, valves, the respiratory pump, gravity, smooth muscle / venous tone and cardiac suction
- Explain the distribution of cardiac output at rest, during exercise and during recovery, limited to the vascular shunt mechanism
- Explain the role of the autonomic nervous system, including the sympathetic nervous system and the parasympathetic nervous system, in the distribution of cardiac output
- Explain the role of receptors in the distribution of cardiac output
- Explain the role of the vasomotor control centre in the distribution of cardiac output
- Explain the role of the arterioles and precapillary sphincters in the distribution of cardiac output
- Explain the roles of vasoconstriction and vasodilation in the distribution of cardiac output
Part of the AS Level PE 8386 syllabus 8386.
The five vessels and what each is built for
Every vessel type has a structure matched to its job. Learn the pair together: thick elastic walls belong with high pressure, and walls one cell thick belong with diffusion.
Blood leaves the heart through arteries, passes into arterioles, is exchanged in capillaries, is collected by venules and is returned to the heart by veins.
| Vessel | Structure | Function |
|---|---|---|
| Arteries | Thick, muscular and elastic walls with a narrow lumen | Carry blood away from the heart at high pressure; elastic tissue stretches with each heartbeat and recoils to smooth out the flow |
| Arterioles | Small branches of the arteries with a high proportion of smooth muscle in their walls | Control the volume of blood flowing into each capillary bed; the main site where vasoconstriction and vasodilation happen |
| Capillaries | Walls one cell thick, forming a large total surface area; contain a precapillary sphincter at their entrance | Site of gaseous exchange between the blood and the tissues — oxygen diffuses out and carbon dioxide diffuses in |
| Venules | Thin-walled vessels collecting blood from the capillary beds | Collect deoxygenated blood from the tissues and merge into veins |
| Veins | Wide lumen, thin walls and valves along their length | Carry blood back to the heart at low pressure; the wide lumen and thin walls allow them to act as a blood reservoir |
Venous return mechanisms
Venous return is the volume of blood returned to the heart through the veins. Blood in the veins is at low pressure, so six mechanisms working together move it back to the right atrium. Learn all six; questions ask you to name and explain them.
The skeletal muscle pump and the respiratory pump are the two that increase most during exercise, which is why venous return, and therefore stroke volume, rises as intensity rises.
| Mechanism | How it works |
|---|---|
| Skeletal muscle pump | Rhythmic contraction of the skeletal muscles surrounding the deep veins squeezes the veins, pushing blood towards the heart between contractions |
| Valves | One-way pocket valves inside the veins open towards the heart and close behind the moving blood, preventing backflow, especially in the legs where blood must travel against gravity |
| Respiratory pump | During inspiration the diaphragm moves down and the pressure in the chest falls while pressure in the abdomen rises, so the pressure difference squeezes blood out of the abdominal veins and into the chest and heart |
| Gravity | Blood returning from the head and upper body is helped downwards towards the heart by gravity; this mechanism works less well in vessels that have to carry blood upwards |
| Smooth muscle / venous tone | The smooth muscle in the vein walls maintains a degree of constriction, so the veins do not simply balloon with blood but stay pressurised and push blood onwards |
| Cardiac suction | As the heart relaxes during diastole a pressure drop inside the atria draws blood in from the vena cava |
The vascular shunt mechanism: what it is and who controls it
The vascular shunt mechanism is the redistribution of cardiac output away from tissues that need little blood and towards the tissues that need most, such as the working muscles and the skin.
The vasomotor control centre sits in the medulla oblongata, alongside the cardiac and respiratory control centres. It receives impulses from receptors and then sends impulses along the sympathetic nervous system to the arterioles and the precapillary sphincters, altering the diameter of the vessels and so the amount of blood flowing into each region.
- Baroreceptors detect changes in blood pressure and allow the vasomotor control centre to maintain it as the shunt alters the size of different vessel networks
- Chemoreceptors detect changes in carbon dioxide, oxygen and pH, giving the control centre information about which tissues need more blood
- Mechanoreceptors detect movement and stretch in muscles and joints, telling the control centre when muscles have started to work
- Thermoreceptors detect changes in temperature, which influences how much blood is sent to the skin
- The sympathetic nervous system carries the impulses that cause vasoconstriction and vasodilation
- The parasympathetic nervous system works to restore the resting distribution of blood during recovery
Vasoconstriction and vasodilation: the arterioles and precapillary sphincters
Vasoconstriction is a decrease in the diameter of the arterioles, caused by contraction of the smooth muscle in their walls. It reduces blood flow to that area. Vasodilation is an increase in the diameter, caused by relaxation of that smooth muscle, and it increases blood flow to the area.
The precapillary sphincters are rings of smooth muscle at the entrance to each capillary bed. When a sphincter constricts, blood bypasses that capillary bed altogether and flows along the network towards a more active region. When it dilates, the capillary bed fills with blood and exchange speeds up. This is why the vascular shunt can be so selective: the body does not simply divert whole arteries, it opens and closes individual capillary networks.
Receptors in the muscles report that oxygen demand has risen, the vasomotor control centre responds through the sympathetic nervous system, and the local vessels shift the balance towards vasodilation in the working muscle and vasoconstriction elsewhere.
| Vasoconstriction | Vasodilation |
|---|---|
| Diameter of the arterioles decreases | Diameter of the arterioles increases |
| Smooth muscle in the vessel wall contracts | Smooth muscle in the vessel wall relaxes |
| Blood flow to that area falls | Blood flow to that area rises |
| Occurs in the digestive system, kidneys and inactive muscle during exercise | Occurs in the working muscles, at the skin when temperature rises, and in the lungs during exercise |
Distribution of cardiac output at rest, during exercise and during recovery
At rest, cardiac output is spread across the body: the digestive system, kidneys, brain and muscles all receive a share, because none of them is working at high intensity. Blood is evenly distributed and the precapillary sphincters of most capillary beds are open.
During exercise, cardiac output rises and the vascular shunt redirects it. The working muscles receive a far larger share, blood flow to the skin increases so heat can be lost by radiation and convection, and the digestive system is shut down to almost nothing because digestion is not needed while performing. This is why eating a large meal immediately before intense exercise causes cramp and discomfort.
During recovery, the shunt reverses. Blood flow to the working muscles falls as their oxygen demand falls, flow to the skin stays high for a time so that heat can continue to be lost, and flow to the digestive system returns as the parasympathetic nervous system takes over again.
| Region | At rest | During exercise | During recovery |
|---|---|---|---|
| Working muscles | Receives a moderate share of the cardiac output | Receives a much larger share; arterioles and precapillary sphincters dilate and oxygen delivery increases to match demand | Flow falls back towards resting levels as oxygen demand falls |
| Skin | Small share, enough to maintain normal temperature regulation | Share increases so that heat produced by the muscles can be lost to the environment | Stays high for a time, then returns to resting level |
| Digestive system | Receives a good share as digestion takes place | Flow is cut to a minimum by vasoconstriction, so blood can be redirected to the muscles | Flow returns as digestion resumes and the parasympathetic system dominates again |
Putting the shunt together with heart rate and cardiac output
The vascular shunt only works because cardiac output has already increased. Heart rate and stroke volume rise to lift cardiac output from around 5 l/min at rest to 20–40 l/min at maximal intensity; the shunt then decides where that extra blood goes.
Learn the sequence as one chain: more intense exercise → chemoreceptors and mechanoreceptors detect the change → vasomotor control centre → sympathetic stimulation → vasodilation in working muscles and skin, vasoconstriction in the digestive system → blood flow redirected to where the oxygen demand is.
Key terms you must be able to define
- Arteries
- Vessels with thick, muscular and elastic walls that carry blood away from the heart at high pressure.
- Arterioles
- Small branches of the arteries with a high proportion of smooth muscle, which control the flow of blood into the capillary beds.
- Capillaries
- Very fine vessels with walls one cell thick that form the site of gaseous exchange between the blood and the tissues.
- Venules
- Thin-walled vessels that collect deoxygenated blood from the capillary beds and merge into veins.
- Veins
- Wide-lumen, thin-walled vessels with valves that carry blood back to the heart at low pressure and act as a blood reservoir.
- Venous return
- The volume of blood returned to the heart through the veins.
- Skeletal muscle pump
- The squeezing of the deep veins by contracting skeletal muscles, which pushes blood towards the heart.
- Respiratory pump
- The pressure changes in the chest and abdomen during breathing that squeeze blood out of the abdominal veins and into the chest and heart.
- Venous tone
- The maintained contraction of the smooth muscle in vein walls, which keeps the veins pressurised and helps move blood onwards.
- Cardiac suction
- The pressure drop in the atria during diastole that draws blood in from the vena cava.
- Vascular shunt mechanism
- The redistribution of cardiac output away from tissues that need little blood towards those that need most, such as the working muscles and the skin.
- Vasomotor control centre
- The region of the medulla oblongata that regulates the diameter of the arterioles and precapillary sphincters through the sympathetic nervous system.
- Vasoconstriction
- A decrease in the diameter of the arterioles caused by contraction of the smooth muscle in their walls, which reduces blood flow to that area.
- Vasodilation
- An increase in the diameter of the arterioles caused by relaxation of the smooth muscle in their walls, which increases blood flow to that area.
- Precapillary sphincters
- Rings of smooth muscle at the entrance to the capillary beds that can shut off or open up the blood supply to individual capillary networks.
Applied to your sport
Cambridge requires you to apply this knowledge to physical activities. Use these as models, then write your own for the sport you play.
After sprinting forward, a winger's leg muscles need large volumes of oxygenated blood, so the arterioles and precapillary sphincters in the legs dilate while those supplying the digestive system constrict. Blood flow to the legs can increase many times over while digestion is effectively paused.
The cool water and the heat produced by the working muscles both affect blood flow to the skin. Thermoreceptors tell the vasomotor control centre how much blood to send to the skin, so the swimmer can lose heat without diverting too much blood away from the arms.
In the second half, a prop who stands still in a ruck does not need the same muscle blood flow as a winger who has just run forty metres. The vascular shunt is selective, opening capillary beds in the active muscles and constricting the inactive ones.
A 5000 m runner must keep walking during recovery rather than stopping dead, because the skeletal muscle pump in the legs keeps venous return high and prevents blood pooling in the veins, which would reduce stroke volume and cause dizziness.
As breathing deepens, the respiratory pump helps more blood out of the abdominal veins and back into the chest, adding to the skeletal muscle pump and raising venous return. Both mechanisms increase venous return, which by Starling's law raises stroke volume.
Flashcards
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Exam-style questions
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Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Mark scheme — tick each point you actually made
Exam technique for this topic
- For any question about redirecting blood during exercise, use the full chain: receptor → vasomotor control centre → sympathetic nervous system → arterioles and precapillary sphincters → vasoconstriction or vasodilation → effect on blood flow.
- Venous return questions are worth writing out as a numbered list of mechanisms. Name each mechanism and explain how it moves blood, because each correct mechanism is usually a separate mark.
- When naming a vessel, always attach its structural feature to its function. "Capillaries are one cell thick so gases only have a short distance to diffuse" scores; "capillaries are thin" does not.
- Keep the three control centres separate in your answers. Writing "the cardiac control centre controls blood vessel diameter" is a common and expensive slip.
- If a question mentions the skin, it is usually testing temperature regulation as well as the shunt: more blood to the skin means more heat lost.
Why students lose marks here
Where this fits in AS Level PE 8386
This page is one topic of Cambridge AS Level PE 8386. AS Level PE 8386 is the course home: every area and every topic in Cambridge's own order, the papers you sit and what each one tests, and the exam-practice tools. The full topic list, with each topic linked to its revision page, is on the AS Level PE 8386 syllabus page.