When a muscle contracts, it can cut off its own blood supply, so you cannot make muscles contract continuously with high forces for long periods, such as lifting very heavy weights, but muscles can maintain alternating contractions and relaxations, as in walking or running, for a very long time. The contraction of the left ventricle cuts off its own blood supply during systole, similarly to skeletal muscle, and some implications of this were discussed in an earlier handout. Some of the venous drainage from the coronary circulation goes into the chambers of the left side of the heart, which has some consequences for the amount of oxygen in arterial blood that will be covered in the course on respiration. Sympathetic stimulation of the heart increases the coronary blood flow. There are no sympathetic cholinergic vasodilator fibres going to the coronary vessels and it is unclear how much of the vasodilatation is due to stimulation of receptors and how much of the increased flow is produced by the sympathetics increasing the activity of the heart, with this increased activity dilating the coronary vessels.
Hypoxia is a more potent vasodilator than carbon dioxide in the coronary circulation.
The cutaneous circulation shows several adaptations for its role in regulating the body temperature. The metabolic rate of the skin is low and it does not produce much heat, so its temperature is less than 37deg. The skin contains arterio-venous anastomoses which are vessels going from arterioles to venules that have circularly arranged smooth muscle, under sympathetic vasoconstrictor control, in their wall. In extreme cold, there may be a vasodilatation in the skin, called the cold vasodilatation. This has a beneficial effect because when the temperature of the skin goes below 0 C the tissues may freeze and the ice crystals disrupt cells, killing them, so producing frost-bite.