Endocrine System

V

Parathyroid Glands

The parathyroids are found near or embedded in the thyroid gland. The parathyroid hormone regulates blood levels of calcium and phosphorus and stimulates bone resorption.

VI

Ovaries

The ovaries are the female reproductive organs, or gonads. They are paired, almond-shaped bodies situated on either side of the uterus. The ovarian follicles produce the ova, or eggs, and also secrete a group of hormones called oestrogens, which are necessary for the development of the reproductive organs and of such secondary sexual characteristics as the distribution of fat, widening of the pelvis, breast growth, and pubic and axillary hair.

Progesterone has as its principal function the maintenance of pregnancy, which it accomplishes primarily by its effects on the lining of the uterus. Progesterone also acts in conjunction with oestrogens in promoting the growth and elasticity of the vagina. The ovaries also elaborate a hormone called relaxin, which acts to relax the cervix during childbirth, thus making delivery easier.

VII

Testes

The male gonads, the testes, are paired, ellipsoid bodies suspended in the scrotum. The Leydig cells of the testes produce one or more male hormones, called androgens. The most important of these is testosterone, which stimulates the development of secondary sex characteristics, influences the growth of the prostate and seminal vesicles, and promotes secretory activity of these structures. The testes also contain cells that produce sperm. See Reproductive System.

VIII

Pancreas

The bulk of the pancreas consists of exocrine tissue that releases digestive juices into the duodenum. Distributed throughout this tissue are clusters of endocrine cells called the islets of Langerhans, which secrete insulin and another hormone, glucagon. Insulin affects carbohydrate, protein, and fat metabolism, increasing the rate of sugar utilization and promoting the formation of protein and storage of fat. Glucagon temporarily raises blood-sugar levels, apparently by releasing glucose from the liver.

IX

Placenta

The placenta, an organ formed during pregnancy from the membrane surrounding the foetus and the uterine lining, assumes certain endocrine functions of the pituitary gland and the ovaries that are important in the maintenance of pregnancy. It secretes the hormone called chorionic gonadotrophin, a substance which is found in the urine during pregnancy and constitutes the basis for pregnancy tests. The placenta produces the progesterone and oestrogens; a protein hormone with some of the characteristics of growth hormone, and lactogenic hormones (placental lactogen).