The gonads produce sex-appropriate gametes and hormones.
- Ovaries – support female sexual characteristics and reproductive functions.
- Testes – support male sexual characteristics and reproductive functions.
The parathyroid glands are tiny glands, stacked in pairs and embedded in the posterior surface on each side of the thyroid glands. Most people have four; but as many as 14% of the population may have five; and a few are known to have six. A fully mature parathyroid gland is less than a quarter of an inch in diameter.
The parathyroid glands contain two types of cells: chief cells and oxyphils (whose contribution is still unknown).
The chief cells are the major source of the hormone PTH (parathyroid hormone), which targets bone, digestive tract, and kidneys regulating calcium concentrations in association with a form of Vitamin D (hydroxycholecalciferol) and calcitonin.
- In bone, PTH enhances bone reabsorption by increasing digestion of the bone matix thus affecting the release of calcium from the bone into the blood stream.
- In the kidneys, PHT increases the excretion of phosphate and the reabsorption of filtered calcium.
- In the intestine, PHT increases the intestinal absorption of calcium.
The pineal gland is a tiny cone-shaped gland located close to the roof of the thalamus in the brain. It contains neurons, glial cells, and secretory cells that synthesize the hormone melatonin.
Melatonin production is at its lowest during daylight hours and highest in the dark of night. Increased secretion at night has been suggested as a cause for SAD (seasonal affective disorder), which is characterized by changes in mood, eating habits, and sleeping patterns.
Melatonin has several functions:
- slows the timing of sexual maturity;
- acts as an antioxidant;
- establishes day and night cycles of activity;
- determines hair and skin color.
The thymus gland is embedded in a mass of connective tissue inside the thoracic cavity just behind the sternum. In a newborn, the gland is relatively enormous, growing slowly with the child until it reaches its maximum size just before puberty. From then on, its size gradually diminishes and, by the age 50, it may only weigh 12 grams (0.4 oz).
The thymus produces several hormones, known collectively as thymosins. Thymosin plays a key role in the development and maintenance of immune defenses by controlling white blood cell maturation. It activates the immune system through lymphocyte stimulation, which enables them to destroy microorganisms that cause disease.
The heart releases a hormone in response to an increase in blood volume. The endocrine cells of the heart are called cardiac muscle cells, found in the walls of the right atrium (the chamber that receives venus blood). If the blood volume becomes too great, these cells stretch excessively, forcing them to release the hormone ANP (atrial natriuretic peptide), which lowers the blood volume and reduces the stretching. (see also Cardiovascular System)