The Endocrine System

    The endocrine system is the system that controls the hormones. The hormones are substances that are released in one place in our body and act on other cells in our body. There are three modes of action of substances released by the cells: 1) the autocrine action, where the substance acts on the cell that has released it, 2) the paracrine action, where the substance acts on neighboring cells, and 3) the endocrine action, where the substance acts on distant cells. In this section we will discuss only the endocrine action of the hormones. And, not all the hormones will be discussed, there are many more hormones which have been discovered in the recent decades. For example, some are secreted by the stomach and intestine, and participate to the regulation of hunger and satiety.

    The figure on the left depicts several components of the endocrine system. Just click on the figure for more details.

    In the brain, there is the pineal gland, also called the epiphysis, which secretes melatonin early in the night. That is the sleep hormone.

    Then there are the hypothalamus, a part of the brain itself, and the pituitary gland located at the base of the brain that are responsible for the control of many hormones. This is a very complex system that controls other glands.

    In the neck, you can find the thyroid and parathyroid glands, which secrete thyroid hormone and calcitonin, respectively. The thyroid hormone affects every cell in the body, mainly to adjust their metabolism. The calcitonin is involved in calcium metabolism and bone growth.

    In the thorax you find the thymus which participate to the immune system.

    In the abdominal cavity, the adrenal glands, located just above the kidneys, secrete two different hormones. The center of the gland secretes adrenaline, mainly involved in the control of blood pressure. And, the periphery of the gland secretes the cortisone which is involved in some anti-inflammatory reactions. Together they participate to stress responses.

    In the abdomen, there is also the pancreas that secretes insulin and glucagon. These two hormones are involved in the control of glucose (sugar) metabolism.

    Finally, there are the ovaries in women, and the testis in men, which produce the major sex hormones: estrogen, progesterone and testosterone.