The Endocrine System


The endocrine system consists of several sets of cell structures, known as glands, that regulate many functions in the body. Endocrine glands produce hormones. Hormones are chemicals secreted into the bloodstream that, like neurotransmitters, have an effect of behavior and mental processes.


The hypothalamus is a tiny cluster of brain cells, just above the pituitary gland, which transmits messages from the body to the brain. It effectively uses the pituitary gland to link the nervous system to the endocrine system

Parathyroid Glands

The parathyroid glands are four small oval bodies located on either side of, and on, the dorsal aspect of the thyroid gland. These glands control the level of calcium in the blood. Calcium is important, not only for bones and teeth, but also for nerve functioning, muscle contractions, blood clotting and glandular secretion. If we don't have enough calcium for these functions, the body will take it from the bones, causing them to weaken and increasing the risk of fracture. Calcium deficiency may also cause twitching, spasms, convulsions, and even death, while too much calcium can lead to a weakening of muscle tone and kidney stones.

Thyroid Gland

The thyroid gland is shaped like a butterfly and usually weighs less than one ounce. The thyroid cartilage covers the larynx and produces the prominence on the neck known as the Adam's apple. The thyroid gland controls the rate at which the body produces energy from nutrients. If the body does not get enough iodine, the thyroid gland cannot produce a proper amount of hormones for this conversion process. The result can be a goiter, an enlargement of the thyroid gland. In some parts of the world, iodine is so scarce that most of the population has goiters. The parathyroid glands are four small oval bodies located on either side of and on the dorsal aspect of the thyroid gland. These glands control the level of calcium in the blood. The thyroid gland secretes hormones that regulate energy, and emotional balance may rely upon its normal functioning. When the rate of production is excessive, the results can be weight loss, nervousness, or even emotional disturbances.

Pineal Gland

The pineal gland is an important endocrine gland. It is a small, oval structure descending from the roof of the diencephalon, a section of the brain that relays sensory information between the brain's different regions. Although it's very tiny-only about six millimeters long- the pineal gland produces several important hormones. The most significant of these is melatonin, a hormone which regulates the circadian rhythm, or sleep cycle. In some of the lower vertebrates this gland grows into an eyelike structure; in others, although it isn't a fully developed eye, it is still able to act as a light receptor. Because of this, the pineal gland is also known as the third eye. Along with secreting melatonin, the pineal gland also regulates other endocrine functions and converts signals from the nervous system into endocrine signals. Melatonin production can contribute to a person feeling awake or becoming sleepy.

Pituitary Gland

The pituitary gland is a pea-sized gland located in the center of the skull inferior to the hypothalamus of the brain and the posterior to the bridge of the nose. It is an important link between the nervous and endocrine systems and releases many hormones which affect growth, sexual development, metabolism and human reproduction. The pituitary gland, also known as the hypophysis, is connected to the hypothalamus of the brain by a tiny infundibulum. It sits within a small cavity in the sphenoid bone of the skull known as the hyophyseal fossa. Thus the sphenoid bone surrounds and protects the delicate pituitary gland from damage by external forces. While the pituitary gland has previously been considered to be a single structure in the body, further study of its structure has revealed that in fact it is made of two structurally and functionally distinct regions.

Thymus Gland (Upper Torso)

The thymus gland, despite containing glandular tissue and producing several hormones, is much more closely associated with the immune system than with the endocrine system. The thymus serves a vital role in the training and development of T-lymphocytes or T cells, an extremely important type of white blood cell. T cells defend the body from potentially deadly pathogens such as bacteria, viruses, and fungi. The thymus is a soft, roughly triangular organ located in the mediastinum of the thoracic cavity anterior and superior to the heart and posterior to the sternum. It has two distinct but identical lobes that are each surrounded by a tough, fibrous capsule. Within each lobe is a superficial region of tissue called the cortex and a histologically distinct deep region called the medulla. Epithelial tissues and lymphatic tissues containing dendritic cells and macrophages make up the majority of both regions of the thymus.


The ovaries, a pair of tiny glands in the female pelvic cavity, are the most important organs of the female reproductive system. Their importance is derived from their role in producing both the female sex hormones that control reproduction and the female gametes that are fertilized to form embryos. Each ovary is a small glandular organ about the shape and size of an almond. The ovaries are located on opposite sides of the uterus in the pelvic cavity and are attached to the uterus by the ovarian ligament. The open ends of the fallopian tubes rest just beyond the lateral surface of the ovaries to transport ova, or egg cells, to the uterus. The ovaries play two central roles in the female reproductive system by acting as both glands and gonads. Acting as glands, the ovaries produce several female sex hormones including estrogens and progesterone. Estrogen controls the development of the mammary glands and uterus during puberty and stimulates the development of the uterine lining during the menstrual cycle. Progesterone acts on the uterus during pregnancy to allow the embryo to implant and develop in the womb.


The pancreas is a glandular organ in the upper abdomen, but really it serves as two glands in one: a digestive exocrine gland and a hormone-producing endocrine gland. Functioning as an exocrine gland, the pancreas excretes enzymes to break down the proteins, lipids, carbohydrates, and nucleic acids in food. Functioning as an endocrine gland, the pancreas secretes the hormones insulin and glucagon to control blood sugar levels throughout the day. Both of these diverse functions are vital to the body’s survival. he pancreas is a narrow, 6-inch long gland that lies posterior and inferior to the stomach on the left side of the abdominal cavity. The pancreas extends laterally and superiorly across the abdomen from the curve of the duodenum to the spleen. The head of the pancreas, which connects to the duodenum, is the widest and most medial region of the organ. Extending laterally toward the left, the pancreas narrows slightly to form the body of the pancreas. The tail of the pancreas extends from the body as a narrow, tapered region on the left side of the abdominal cavity near the spleen. The pancreas is classified as a heterocrine gland because it contains both endocrine and exocrine glandular tissue. The exocrine tissue makes up about 99% of the pancreas by weight while endocrine tissue makes up the other 1%. The endocrine tissue is arranged into many small masses known as acini. Acini are small raspberry-like clusters of exocrine cells that surround tiny ducts. The exocrine cells in the acini produce digestive enzymes that are secreted from the cells and enter the ducts. The ducts of many acini connect to form larger and larger ducts until the products of many acini run into the large pancreatic duct.

Suprarenal Glands

The supraneal, or adrenal, glands are a pair of glands that secrete hormones directly into the bloodstream. Each gland can be divided into two distinct organs. The outer region, the adrenal cortex, secretes hormones which have important effects on the way in which energy is stored and food is used, on chemicals in the blood, and on characteristics such as hairiness and body shape. The smaller, inner region - the adrenal medulla - is part of the sympathetic nervous system and is the body's first line of defense and response to physical and emotional stresses. The adrenal glands are shaped like the French Emperor Napoleon's hat and, just as Napoleon's three-cornered hat sat on his head, so each gland is perched on each of the kidneys. These glands are about one to two inches in length; they weigh only a fraction of an ounce each yet are among the most productive of all of the body's glands, secreting more than three dozen hormones. The adrenal cortex takes instruction from the pituitary glands and have important effects on physical characteristics, development and growth. The adrenal gland has two parts. The cortex, or outer, yellow layer, takes its instructions from the pituitary hormone ACTH. The hormones secreted here are called steroids and have three main types: those which control the balance of sodium and potassium in the body; those which raise the level of sugar in the blood; and sex hormones. The inner, reddish brown layer of the adrenal gland (the adrenal medulla) makes two types of hormones; this part of the adrenal gland takes its instruction from the nervous system, producing chemicals which react to fear and anger and are sometimes called fight or flight hormones.


The testes commonly known as the testicles, are a pair of ovoid glandular organs that are central to the function of the male reproductive system. The testes are responsible for the production of sperm cells and the male sex hormone testosterone. The testes produce as many as 12 trillion sperm in a male's lifetime, about 400 million of which are released in a single ejaculation. The testes are connected to the vital organs of the ventral body cavity via the spermatic cords. Nerves, blood vessels, and lymphatic vessels travel through the spermatic cords to support the testes. The vas deferens also passes through the spermatic cord carrying sperm out of the testes toward the prostate and urethra. The cremaster muscle wraps around the exterior of the spermatic cord to lift the testes closer to the body or permit them to descend. The testes are wrapped by the tunica vaginalis, an extension of the peritoneum of the abdomen, and the tunica albuginea, a tough, protective sheath of dense irregular connective tissue. Each testis is divided by invaginations of the tunica albuginea that divide it into several hundred small segments called lobules. Each lobule contains several tightly coiled tubes called seminiferous tubules. Each sperm produced by the testes takes about seventy-two days to mature and its maturity is overseen by a complex interaction of hormones. The scrotum has a built-in thermostat that keeps the testes and sperm at the correct temperature. It may be surprising that the testes should lie in such a vulnerable place outside the body, but it is too hot for them inside. Spermatogenesis requires a temperature that is three to five degrees Fahrenheit below body temperature. If it becomes too cool on the outside, the cremaster muscle will contract to bring the testes closer the body for warmth.


The uterus, also commonly known as the womb, is a hollow muscular organ of the female reproductive system that is responsible for the development of the embryo and fetus during pregnancy. An incredibly distensible organ, the uterus can expand during pregnancy from around the size of a closed fist to become large enough to hold a full term baby. It is also an incredibly strong organ, able to contract forcefully to propel a full term baby out of the body during childbirth. The uterus is approximately the shape and size of a pear and sits in an inverted position within the pelvic cavity of the torso. It is located along the body’s midline posterior to the urinary bladder and anterior to the rectum. The narrow inferior region of the uterus, known as the cervix, connects the uterus to the vagina below it and acts as a sphincter muscle to control the flow of material into and out of the uterus. The body (or corpus) of the uterus is the wider region of the uterus superior to the cervix. The body is an open and hollow region where the fertilized egg, or zygote, implants itself and develops during pregnancy. The walls of the body are much thicker than those of the cervix as they provide for the protection and support of the developing fetus and contain the muscles that propel the fetus out of the mother’s body during childbirth.