Redistribution of blood (vascular shunting vasoconstriction, vasodilation).

The idea

Vascular shunting redirects blood flow during exercise. Chemoreceptors and baroreceptors detect changes in blood gases and pressure, and the vasomotor centre in the medulla oblongata adjusts sympathetic outflow. Arterioles supplying working muscles vasodilate, widening their lumen so local blood flow rises and more oxygen reaches active fibres. Arterioles supplying non-essential areas such as the gut and kidneys vasoconstrict, narrowing their lumen so blood is diverted to where demand is greatest. Pre-capillary sphincters also open or close to fine-tune flow through capillary beds. The result is a redistribution of cardiac output: a greater proportion goes to skeletal muscle, the heart and skin, while the proportion to abdominal organs falls. Skin flow rises to help dissipate heat. This is not a change in total cardiac output alone but a change in its distribution.

What earns the marks

• Vascular shunting is the redistribution of blood flow to areas of greatest demand during exercise.
• Vasodilation widens arterioles supplying working muscles, increasing local blood flow and oxygen delivery.
• Vasoconstriction narrows arterioles supplying non-essential organs such as the gut and kidneys, diverting blood away.
• The vasomotor centre in the medulla oblongata controls sympathetic outflow to arterioles.
• Pre-capillary sphincters open or close to direct blood through specific capillary beds.
• Skin blood flow increases to aid heat loss, while the proportion of cardiac output to skeletal muscle rises.

Common mistakes and how to put them right

• Thinking vasoconstriction occurs in working muscles.

  Put it right by stating that working muscles vasodilate and non-essential organs vasoconstrict.

• Confusing vasodilation with increased heart rate.

  Put it right by explaining that vasodilation changes vessel diameter and local flow, not the rate of contraction.

• Believing blood is created or destroyed during shunting.

  Put it right by stating that the same cardiac output is redistributed, not increased in total by shunting alone.

• Ignoring the role of the vasomotor centre.

  Put it right by naming the medulla oblongata as the control centre for sympathetic outflow.

Exam tips

• Link each vessel change to its purpose: more oxygen to muscles, heat loss at the skin, blood diverted from the gut.
• Use precise terms such as arterioles, lumen and vasomotor centre rather than vague words like tubes or brain.