Skeletal muscle circulation
The blood flow to the skeletal muscle is not low in rest, when it accounts for 15% of the cardiac output. However, during exercise the blood flow may increase 15 – 25 fold.
During exercise the sympathetic nervous system will be activated. This has multiple effects:
- The heart will be stimulated, so that cardiac output increases
- The splanchnic blood flow will be reduced
- Veins will vasoconstrict, which increases venous return and therefore cardiac output
The sympathetic activation doesn’t directly increase skeletal muscle blood flow during exercise. However, as exercise goes on the muscles will use more and more oxygen and nutrients, producing more and more CO2 and metabolites like H+, K+, adenosine and lactate. Increased CO2, decreased O2 and metabolites are vasodilators, so when these compounds accumulate in the muscle the arteries and arterioles to the muscle will vasodilate, which increases blood flow.
During exercise catecholamines like norepinephrine and epinephrine will be released from the medulla of the adrenal glands. These catecholamines will bind to beta receptors on skeletal muscle arteries and cause them to vasodilate.
The major function of the circulation in the skin is to regulate body temperature. When it is warm outside the blood flow to the skin will increase. This increases the amount of heat which is lost from the body. When it’s cold outside the skin will vasoconstrict, to decrease the amount of heat which is lost. Depending on the outside temperature the blood flow to the skin can vary from 1% of the cardiac output to 30%.
This regulation is mediated by the sympathetic nervous system. Arteries in the skin are heavily innervated by sympathetic nerves. When a person is cold their sympathetic nervous system is activated, which causes vasoconstriction in the skin. When it’s warm the sympathetic nervous system is deactivated, which releases the vasoconstriction.
31. Splanchnic circulation
33. Nervous control of the heart