The endocrine system consists of specialised cells and glands that secrete hormones into the blood stream. Hormones are proteins or steroids that serve to send chemical messages around the body. The quantity of hormones secreted can be controlled by;
Specific glands function to secrete specific hormones. The main endocrine organs are as follows and will be described individually;
Endocrine organs are seen in their natural colour, any physically associated organs are shown in blue.
The pea-sized pineal body lies on the base of the brain in the mid line, behind the third ventricle. It is stimulated by the optic nerve and secretes the hormone melatonin in response to darkness.
Melatonin promotes sleep and affects reproductive functions by depressing the activity of the gonads. Additionally, it affects thyroid and adrenal cortex functions. Because melatonin production is affected by the amount of light to which a person is exposed, it is tied to circadian rhythm (24 hour clock), annual cycles, and biological clock functions.
The pituitary gland is a small, pea sized gland that lies at the base of the brain. It is suspended from the optic chiasm by a thin stalk, the infundibulum and sits within the pituitary fossa of the sphenoid bone.
The pituitary gland has anterior and posterior lobes that are functionally different. The anterior lobe functions solely as an endocrine organ, whilst the posterior lobe also has nervous functions and is intrinsically connected to the hypothalamus of the brain.
The anterior lobe produces 6 hormones. You should be aware of these hormones and their effect on the body.
The posterior lobe of the pituitary gland releases 2 hormones, which are actually produced in the hypothalamus.
The thyroid gland sits in front of the trachea at the level of C7 - T1. It consists of right and left lobes, which sit on either side and are connected by a strip called the isthmus.
The thyroid releases two hormones, thyroxine and tri-iodothyronine. Both hormones need iodine and thyroxine to function.
The 4 rice-sized parathyroid glands, two superior and two inferior, lie on the posterior surface of the lobes of the thyroid gland, although they are functionally distinct.
The parathyroid glands constantly monitor calcium levels of the blood. When these levels are low, they produce a parathyroid hormone, which triggers calcium to be leached from the bones and deposited in the blood. When calcium levels return to normal, they stop producing the hormone.
The thymus lies in the superior mediastinum above the heart and behind the sternum. It continues to grows up to the 5th year, and from puberty (age 14) decreases in size, until in the elderly it is just represented by a few fibres.
The thymus plays a large role in the lymphatic system. As white blood cells pass through the thymus, they are transformed into T cells. T cells function to identify and destroy infected cells. The thymus secretes hormones called thymosins which stimulate the development and differentiation of T cells. They also play a role in regulating the immune system by stimulating other kinds of immune cells.
The suprarenal glands (also known as adrenal glands) are small yellow lobular glands that lie superior and medial to the kidneys. They lie behind the peritoneum and are surrounded by fat. The adrenal glands consist of an inner medulla and an outer cortex, which are functionally separate.
Each of the secretions of the suprarenal glands are described in the table below;
The pancreas is a yellow, lobular gland that lies in the curve of the duodenum and stretches to the left horizontally as far as the spleen. As well as having an endocrine role it also has a digestive role as it secretes digestive enzymes into the duodenum via the pancreatic duct.
The endocrine role of the pancreas is played by specialised cells, the islets of Langerhans, which are scattered throughout the substance of the pancreas. These cells produce either glucagon (alpha cells) or insulin (beta cells), which enter directly into the bloodstream to control the blood sugar levels.
A condition in which the pancreas no longer produces enough insulin (Type 1) or when cells stop responding to the insulin that is produced, so that glucose in the blood cannot be absorbed into the cells of the body (Type 2).
Symptoms include frequent urination, tiredness, excessive thirst, and hunger. The treatment includes changes in diet, oral medications, and in some cases, daily injections of insulin.
The testes secrete several kinds of steroid hormones known as androgens. They release these chemicals in response to the luteinizing hormone, released by the anterior pituitary gland.
Androgens are the male sex hormones, one of which is testosterone, and are responsible of the development of secondary sexual characteristics such as the deepening of the voice, growth of facial hair, and muscle development. The follicle-stimulating hormone that is released by the anterior pituitary gland controls sperm production.
The ovaries secrete oestrogen and progesterone, the female sex hormones. As in the male, they release these chemicals in response to the luteinizing hormone.
They control ovulation and menstruation and are discussed in more detail in the female reproductive system chapter. They are also responsible for the development of the secondary sexual characteristics such as development of the breasts and broadening of the pelvis.
In both male and female sex hormones are responsible for the increased activity of sweat glands and sebaceous glands, and growth of pubic and armpit hair.