Question

Describe why and how cardiac output changes with changes in exercise intensity. This should include why and how overall blood flow rates through the systemic circuit change with changes in exercise intensity. This should also include how and why there is preferential blood flow to certain organ systems during exercise.

Answer 1

During exercise there is extra demand of oxygen for the skeletal muscles, so the need to transport oxygen to the working muscles means heart has to work harder, as a reult heart rate increases and more blood is pumped around the body as a result stroke volume, heart rate and cardiac output also changes. Increased flow occurs due to local arteriole vasodilation. In both skeletal muscles and cardiac muscles vasodilation is mediated by local metabolic factors and in the skin it is caheived by decrease in the firing of sympathetic neurons supplying skin vessels.

These alterations are coordinated by the sympathetic nervous system, which directs increased sympathetic outflow to the heart, resulting in increased cardiac output, and evokes baroreflex-mediated vasoconstriction in peripheral organs like , kidneys, small and large intestines, and non-exercising skeletal muscles to redistribute their blood flow to contracting skeletal muscles and maintain blood pressure.

Vasodilation of arteriole in skeletal muscles and heart muscles and skin causes a decrease in peripheral resistance, this decrease is partialy offset by vasoconstriction of arterioles in other organs but the vasodilation in muscle arteriole is not compensated and the net result is marked decrease in total peripheral resistance to blood flow.

Because exercise increases the skeletal muscle activity, the venous return, the volume of blood returning to the heart by veins increases, and by Starling law, increased venous return causes greater diastolic filling of the heart because of which cardiac muscle stretch increases, leading to more force of contraction, all resulting in increased cardiac output.