Endocrine System

ENDOCRINE SYSTEM

The nervous system and endocrine glands are the two main mechanisms of communication and coordination of the human body. They regulate almost all organ systems. Although the nervous system and endocrine system work closely together, they have several differences.

The nervous system communicates through electrical signals called nerve impulses, which transmit information quickly and generally perform short-term effects.

In the endocrine system, by contrast, communication is by chemical signals, through substances called hormones. The endocrine system responds more slowly and usually causes longer lasting effects.

The endocrine system is made up of endocrine glands that produce hormones and are widely distributed throughout the body. Endocrine glands are ductless glands, that is, they secrete hormones directly into the capillaries (blood).

The endocrine system produces its effects through hormone secretion. Hormones are chemical messengers that influence or control the activities of other tissues or organs. Most hormones are carried by blood to other parts of the body, exerting effects on more distant tissues.

The main endocrine glands are:

Hypophysis

The pituitary gland is a small gland, an ovoid body, similar in size to a pea, also known as a pituitary gland. It has a reddish-gray color, measuring about 12 mm in transverse diameter and 8 mm in anteroposterior diameter and weighing approximately 500 mg. The pituitary is located below the hypothalamus, posterior to the optic chiasm, in a saddle-shaped depression of the sphenoid bone called the pituitary fossa. It is topped by the circular saddle diaphragm of the dura mater. The pituitary is fixed to the inferior surface of the hypothalamus by a short stem called the infundibulum. It has two parts: one anterior, the adenohypophysis, and another posterior, the neurohypophysis. The pituitary secretes eight hormones and thus affects almost all body functions.

Adenohypophysis

The anterior part of the pituitary, the adenohypophysis, is composed of glandular epithelial tissue and is highly vascular and consists of epithelial cells of varying size and shape, arranged in cords or irregular follicles. Synthesizes and releases at least eight important hormones:

& #8211; Somatotropin (STH), involved in controlling body growth;

& #8211; Mamotropin (LTH), which stimulates the growth and secretion of the female breast;

& #8211; Adrenocorticotropin (ACTH), which controls the secretion of some cortical hormones of the adrenal gland;

& #8211; Thyrotropin (TSH), which stimulates thyroid gland activity;

& #8211; Follicle stimulating hormone (FSH), which stimulates estrogen growth and secretion in the ovarian follicles and spermatogenesis in the testes;

& #8211; Interstitial Cell Hormone (ICSH), which activates androgen secretion through the testis;

& #8211; Luteinizing Hormone (LH), which induces progesterone secretion by the corpus luteum;

& #8211; Melanocyte stimulating hormone (MSH), which increases skin pigmentation.

Neurohypophysis

The posterior pituitary lobe is a descending descent of the floor of the diencephalon. The posterior portion of the pituitary is composed of nervous tissue and is therefore called the neurohypophysis. Synthesizes two hormones:

& #8211; Vasidressin (ADH), antidiuretic, which controls the absorption of water through the renal tubules;

& #8211; Oxytocin, which promotes contraction of the unstriated muscle of the uterus and breast.

The two neurohypophysis hormones are produced in the hypothalamus and transported within the infundibulum (pituitary stem) and stored in the gland until use. Nerve impulses to the hypothalamus stimulate the release of neurohypophysis hormones.

Thyroid gland

The thyroid gland has a brownish-red hue, about 25 g and is highly vascularized. It is located in the antero-inferior region of the neck, anterolaterally to the trachea and just below the larynx, at the level between the fifth cervical vertebra and the first thoracic vertebra. The thyroid has two lobes (right and left) that are connected to each other by a central part called the isthmus of the thyroid gland. Each wolf is approximately 5 cm long. The gland is surrounded by a connective tissue capsule and contains two types of cells: the follicular cells, located in the thyroid follicles, and the parafolicular cells, located between the follicles.

Thyroid Follicle: The thyroid gland is made up of many secretory units called follicles. Follicular cells secrete and store two thyroid hormones:

& #8211; Triiodothyronine (T3)

& #8211; Tetraiodothyronine (T4 or Thyroxine)

Of the two thyroid hormones, T3 is probably the primary stimulator of the cell's metabolic rate, with very powerful and immediate action, while T4 is powerful but less rapid.

The parafolicular glands secrete the following hormone:

& #8211; Calcitonin, which regulates calcium metabolism, mainly supplying bone resorption.

Parathyroid Glands

Parathyroid glands are small, yellowish-brown ovoid or lentiform structures weighing about 30 mg and usually lying between the margins of the posterior lobe of the thyroid gland and its capsule. There are usually two on each side, upper and lower.

Each parathyroid gland has a thin connective tissue capsule with intraglandular septa, but lacking lobes.

Parathyroid glands secrete the parathyroid hormone (PTH) which is related to the control of calcium and phosphorus level and distribution. PTH acts on three target organs: bones, digestive tract (intestines) and kidneys. The overall effect of PTH is increased plasma calcium levels and decreased plasma phosphate levels.

Adrenal Glands (Adrenal)

The adrenal glands are small yellowish bodies, flattened anteroposteriorly, they are situated anterior superior to each upper extremity of the kidney. Surrounded by connective tissue containing a lot of perinephric fat, they are surrounded by the renal fascia but separated from the kidneys by fibrous tissue. Each measures approximately 50 mm vertically, 30 mm transversely and 10 mm in anteroposterior dimension, weighing about 5 g.

A sectioned adrenal gland reveals a yellow outer cortex forming the main mass and a thin dark red medulla forming about 10% of the gland. The medulla is completely enveloped by the cortex except its hilum.

Adrenal cortex

The adrenal cortex, a thin outer (peripheral) layer, shows three cell zones: the glomerulous (outer), fasciculated (broader) and reticulated (innermost) zones. The cortex secretes hormones called steroids.

Glomerular Zone: They produce aldosterone (mineralocorticoid), which plays an important role in regulating blood volume and pressure, and in concentrating the electrolyte balance of the blood. In general, aldosterone retains sodium and water and eliminates potassium.

Fasciculated Zone: Produce hormones that maintain the balance of carbohydrates, proteins and fats (glucocorticoids). The main glucocorticoid is cortisol.

Reticulated Zone: May produce sex hormones (progesterone, estrogens and androgens).

The cortex is essential for life; Complete removal is lethal without replacement therapy. It also exerts considerable control over lymphocytes and lymphatic tissue.

Adrenal medulla

The adrenal medulla, the inner part of the gland, is considered an extension of the sympathetic part of the autonomic nervous system. It consists of groups and columns of chromaffin cells separated by large venous sinusoids. Small groups of neurons occur in the spinal cord.

The adrenal medulla secretes two hormones:

1 - Epinephrine (Adrenaline), which has marked effect on carbohydrate metabolism.

2 - Norepinephrine (Noradrenaline), which produces accelerated heart vasoconstriction and high blood pressure.

These hormones are classified as amines and because they are in the chemical group called catechol, they are called catecholamines. These hormones are produced in emergency and stress situations, producing the following effects (in addition to those described above):

& #8211; Conversion of glycogen to glucose in the liver;

& #8211; Elevated metabolic pattern of most cells;

& #8211; Dilatation of the bronchi.

Pancreas

The pancreas is an elongated organ that lies transversely in the upper abdomen, extending from the duodenum to the spleen. The detailed anatomy of the pancreas is described in Digestive system.

The pancreas secretes two hormones: insulin and glucagon. The cells that produce these hormones are called pancreatic islets (Langerhans). Islets are made up of spheroid or ellipsoid cell clusters scattered in the exocrine tissue, along with sparse, often solitary endocrine cells. The human pancreas can contain more than one million islands, usually more numerous in the tail. These islets have two cell types: alpha endocrinocytes, which produce glucagon, and beta endocrinics, which produce insulin. These two hormones help control blood glucose levels. The effect of insulin is to lower glucose levels while glucagon increases those levels.

Insulin Action: Decreases glucose levels through two mechanisms:

1) increases blood glucose transport into cells;

2) Stimulates cells to burn glucose as fuel. Insulin is the only hormone that lowers blood glucose.

Glucagon Action: This hormone raises blood glucose in two ways:

1) stimulating the conversion of glycogen to glucose in the liver;

2) stimulating the conversion of proteins to glucose.

Gonads (Ovaries and Testicles)

The gonads are sexual glands, which make up the ovaries (women) and testicles (men). These gonads, besides producing the gametes (eggs and sperm), also secrete hormones, which will be described below.

Ovaries: There are two ovaries located on either side of the pelvic cavity. Its detailed anatomy is described in Female Genital System.

The ovaries produce two female sex hormones: estrogen and progesterone. These hormones participate in the development and functioning of female genital organs and the expression of female sexual characteristics, and these characteristics develop mainly in response to estrogen. They include:

& #8211; Breast development;

& #8211; Fat distribution in the hips, thighs and breasts;

& #8211; Hair distribution in specific areas of the body;

& #8211; Maturation of genitals;

& #8211; Closure of epiphysial cartilage of long bones.

Both estrogen and progesterone are controlled by release hormones in the hypothalamus and adenohypophysis gonadotropins.

Testicles: They are located inside the scrotum. Its detailed anatomy is described in Genital System Male.

The main hormone secreted by the testicles is testosterone, a steroid produced by its interstitial cells. The stimulus for testosterone secretion is the luteinizing hormone (LH) from the adenohypophysis.

Testosterone aids in sperm maturation and is responsible for male sexual characteristics such as:

& #8211; Growth and development of male genital organs;

& #8211; Musculoskeletal growth;

& #8211; Hair growth and distribution;

& #8211; Enlargement of the larynx, accompanied by voice changes.

Testosterone secretion is controlled by release hormones produced in the hypothalamus and by luteinizing hormones in the pituitary gland.

thymus

The thymus has certain hormonal and lymphatic secretory functions (producing T lymphocytes). It varies in size and activity, depending on age, disease, and physiological state, but remains active even in old age. At birth it weighs about 10 to 15 g, growing to puberty, when it weighs 30 to 40 gm, ie it is much larger in children than in adults, and after puberty, the gland involutes, or becomes smaller, being replaced by connective to adipose tissue. Early in life, it is rosy gray, soft and finely lobulated, consisting of two equal pyramidal wolves, joined by loose connective tissue. After middle age, the thymus becomes yellowish due to its gradual replacement by adipose tissue.

The thymus is located in the upper part of the thoracic cavity, posterior to the sternum and the four upper costal cartilages, inferior to the thyroid gland. And anterior to the pericardium, aortic arch and its branches. More precisely, the thymus is located in the upper and lower anterior mediastinum, extending inferiorly to the fourth costal cartilage, with its upper parts tapering toward the neck and sometimes reaching the lower poles of the thyroid gland.

The thymus has the function of producing various substances (including hormones) that regulate lymphocyte production, differentiation and activities in the thymus. These substances include four chemically well-distributed major polypeptides: thymulin, thymopoietin, thymosin alfa I and thymosin beta IV.

Thymulin is produced within the thymus and needs the presence of zinc for functional activity (reacts exclusively with T cells). Thymopoietin enhances various T-cell functions. Thymulin and thymopoietin act systematically to give perfectly adjusted immune regulation of T cells, helping to maintain the balance between the activities of their different subsets. The activities of thymosin alpha I and beta IV are not very clear. Thymosins are known to promote lymphocyte maturation within the thymus and also stimulate lymphocyte development and activity in the performance of their lymphatic functions throughout the body.

 Body (Pineal Gland)

Put on nervous system: Your anatomy is described in Endocrine System.

The pineal body or epiphysis of the brain is a small, reddish gray piriform organ that occupies a depression between the upper colliculi. It is inferior to the splenium of the corpus callosum, separated from it by the choroidal screen of the third ventricle. The body measures approximately 8 mm in length. Its base is secured by a peduncle that divides into lower and upper laminae, separated by the pineal recess of the third ventricle. And containing, respectively, the epitalamic and habituate commissures.

The pineal body contains pinealocyte cords and follicles, and neuroglia cells between which many blood vessels and nerves branch off. Septum extends to the body from the adjacent pia mater.

The pineal body modifies the activity of the adenohypophysis, neurohypophysis, endocrine pancreas, parathyroid glands, cortex and medulla of the adrenal gland and gonads. Pineal secretions can reach their target cells via cerebrospinal fluid or through the bloodstream.

The pineal gland secretes melatonin, a hormone that alters the reproductive cycle by influencing the secretion of hypothalamus-releasing hormones. Melatonin is also believed to be related to the sleep / wake cycle, having a tranquilizing effect. It has been called the body's biological clock, controlling most biorhythms.

OTHER HORMONES

Specific Organic System Associated Hormones

These hormones usually control the activities of a specific organ. For example, hormone-producing cells present in the digestive tract secrete cholecystokinin, gastrin, and secretin. These hormones help regulate digestion. The kidneys secrete erythropoietin, which helps regulate the production of red blood cells.

Prostaglandins

Prostaglandins are chemicals (hormones) derived from fatty acids and aracdonic acid. They are produced by various tissues and usually act near their secretion sites. They play an important role in regulating smooth muscle contraction and inflammatory response. Prostaglandins are also associated with increased sensitivity of nerve endings to pain.

Summary of Endocrine Glands and Hormones: