If the temperature of the body begins to rise, the flow of blood into vessels in the skin increases. The heat from cells deep in the body is thus carried to the skin, from where it passes out of the body. If the temperature of the body begins to drop, the flow of blood to the skin is restricted. This action enables the body to retain as much heat as possible.
The circulatory system also carries hormones. These chemical substances affect or control the activities of various organs and tissues. Hormones are produced by the endocrine glands, including the thyroid, pituitary, adrenal, and sex glands. These glands release their hormones into the bloodstream.