Cardiovascular system: Venous return.

The idea

Venous return is the volume of blood flowing back to the heart through the veins per minute. It matters because cardiac output depends on it: the heart can only pump out what it receives, so stroke volume and cardiac output rise when venous return increases. Four mechanisms assist return against gravity: the skeletal muscle pump, where contracting muscles squeeze veins and valves keep blood moving one way; the respiratory pump, where pressure changes during breathing draw blood towards the thorax; valves in veins, which prevent backflow; and the smooth muscle and elastic recoil of veins, which maintain tone and help push blood onward. A useful method is to trace blood from the working legs back to the right atrium and name the mechanism acting at each stage.

What earns the marks

• Venous return is the volume of blood returning to the heart per minute.
• The skeletal muscle pump: contracting muscles compress veins, and valves ensure one-way flow.
• The respiratory pump: pressure changes in the thorax and abdomen during breathing assist flow towards the heart.
• Valves in veins prevent the backflow of blood.
• Smooth muscle and elastic recoil in vein walls maintain venous tone and help propel blood.
• Increased venous return raises stroke volume and cardiac output via the Frank-Starling mechanism.

Common mistakes and how to put them right

• Thinking veins have no role in circulation because they are low pressure.

  Put it right: Their valves, muscle pump and respiratory pump are essential to venous return.

• Confusing venous return with cardiac output.

  Put it right: Venous return is blood coming back to the heart, while cardiac output is blood pumped out.

• Believing the skeletal muscle pump works when muscles relax.

  Put it right: It is muscle contraction that compresses the veins.

• Assuming valves push blood.

  Put it right: Valves only prevent backflow; the pressure gradient and pumps provide the movement.

Exam tips

• Name each mechanism and then state exactly how it moves blood towards the heart.
• Use the term 'one-way flow' when describing the role of valves.
• Link venous return to stroke volume and cardiac output to show why it matters during exercise.
• Apply each mechanism to a sporting example, such as the leg muscles during running or breathing during cycling.