The function of the endocrine system is to produce and release hormones directly into the blood stream. The hormones in turn coordinate a number of activities throughout the body. The endocrine system is a symphony of glands and hormones. A gland is an aggregate of cells which produces a hormone. In that glands are structural units that perform a specific function, they are organs in and of themselves.
A hormone is a chemical substance which regulates processes and functions of remote cells or an organ. The process of production and release of hormones is termed secretion. Endocrine glands release their hormones directly into the bloodstream. In contrast exocrine glands locally release the chemical substances they produce into tubular structures called ducts.
Signals from the nervous system and bloodstream control the glands of the endocrine system by stimulating receptors in them. The portion of the brain primarily involved in controlling the endocrine system is the hypothalamus. It is located in the lower front central part of the brain known as the forebrain.
By its regulation of certain functions, the endocrine system helps the body to maintain a state of healthy balance. It does so by making necessary adjustments of normal functional processes. The term for the ability or tendency to maintain that balance is homeostasis. The adjective for normal as it relates to functional processes of the body is physiological.
Many chemical and physical processes within the body convert food and other substances into energy. Others build complex body structure by creating more complex substances from simple ones obtained through the diet. The term for the sum of the energy creating processes and the building processes is metabolism. The term for the energy producing aspect is catabolism. The term for the building aspect is anabolism.
- Thyroid – The thyroid is a butterfly shaped gland at the front base of the neck wrapped around the sides of the trachea. It secretes three hormones. They are:
- Parathyroid – There are four parathyroid glands. They are located in the neck just behind the thyroid gland, two behind each side of the gland. The parathyroid glands produce parathyroid hormone (PTH). PTH regulates calcium in the blood stream and keeps it from dropping too low. It does so by stimulating bone cells (osteoclasts) to release calcium and kidney cells to conserve calcium if levels in the bloodstream drop.
- Adrenals – The adrenal glands are a pair of triangular structures, one on the right and one on the left. They sit just above the kidneys. The outer portion of the gland, the adrenal cortex, produces three classes of hormones. They are as follows:
2. Mineralocorticoids regulate salt and water balance in the body. The hormone of this class the adrenal glands produce is aldosterone. It acts to increase sodium reabsorption and potassium secretion in the distal tubules and collecting ducts of the kidney.
3. Androgens cause the development of male secondary sex characteristics (those which are specific for gender such as body fat, voice pitch and hair distribution, but are not essential for reproduction). The adrenal glands do not produce nearly as much androgens as the testes though.
- Pancreas – The endocrine portion of the pancreas produces insulin which lowers blood sugar by driving it into cells. It also produces glucagon which raises blood sugar by triggering the breakdown of glycogen.
- Gonads – Gonads are the male and female sex hormone producing glands. They are the testes in males and the ovaries in females.
2. The ovaries produce two types of female hormones, estrogen and progesterone. Estrogens regulate the growth and development of female sex organs and secondary sex characteristics. Progesterone is necessary for ovulation, pregnancy and development of the fetus during pregnancy.
- Pituitary – The pituitary is a small pea-sized gland located at the base of the brain. The front portion (anterior pituitary) is the true glandular part. It is under the control of the hypothalamus which stimulates it to release or inhibits it from releasing six important hormones it produces. Those hormones are:
6. Prolactin (PRL) which has many effects on the body, the chief being stimulation of the mammary glands of the breast to produce milk
The back portion of the gland (posterior pituitary) is composed of nervous tissue rather than true glandular tissue. It is an extension of the hypothalamus which secretes two hormones. Those hormones flow into and are stored in the posterior pituitary. They are:
2. Antidiuretic hormone (ADH) which prevents water loss from the body by increasing its reuptake by the kidneys and decreasing blood flow to the sweat glands. Vasopressin and pitressin are alternate names for this hormone. The use of a synthetic form of the hormone is for the treatment of diabetes insipidus.
- Pineal – The pineal is a small gland shaped like a pine cone. It is located just behind the thalamus of the brain. It produces melatonin, a hormone that helps regulate the human sleep-cycle. It is produced primarily only in low light or darkness.
- Thymus – The thymus is a triangular gland that sits behind the sternum (breastbone). It produces a hormone that trains and develops T lymphocytes (white blood cells of the immune system) during fetal development and childhood. The thymus becomes inactive during puberty. Fat tissue slowly replaces it.