Endocrine System

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

The nervous system communicates through electrical signals called nerve impulses, which transmit information quickly and usually have short-lived effects.

In the endocrine system, on the other hand, communication takes place through chemical signals, through substances called hormones. The endocrine system responds more slowly and typically causes longer lasting effects.

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

The endocrine system produces its effects through the secretion of hormones. Hormones are chemical messengers that influence or control the activities of other tissues or organs. Most hormones are carried by the 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, about the size of a pea, also known as the pituitary gland. It has a reddish-gray color, measuring about 12 mm in transverse diameter and 8 mm in anteroposterior diameter and weighing approximately 0.5 to 06 g. 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 covered superiorly by the circular saddle diaphragm of the dura mater. The pituitary is attached to the lower surface of the hypothalamus by a short rod called the infundibulum. It has two parts: an anterior one, the adenohypophysis, and a posterior one, the neurohypophysis. The pituitary gland secretes eight hormones and therefore affects almost every function in the body.

adenohypophysis

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

– Somatotropin (STH), involved in controlling the growth of the body;

– Mamotropin (LTH), which stimulates the growth and secretion of the female breast;

– Adrenocorticotropin (ACTH), which controls the secretion of some cortical hormones from the adrenal gland;

– Thyrotropin (TSH), which stimulates the activity of the thyroid gland;

– Follicle Stimulating Hormone (FSH), which stimulates the growth and secretion of estrogens in the ovarian follicles and spermatogenesis in the testes;

– Interstitial cell hormone (ICSH), which activates androgen secretion through the testis;

– Luteinizing Hormone (LH), which induces the secretion of progesterone by the corpus luteum;

– Melanocyte-stimulating hormone (MSH), which increases skin pigmentation.

neurohypophysis

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

– Vasopressin (ADH), antidiuretic, which controls the absorption of water through the kidney tubules;

– Oxytocin, which promotes contraction of the non-striated muscle of the uterus and breast.

The two neurohypophysis hormones are produced in the hypothalamus and transported inside the infundibulum (pituitary stalk) and stored in the gland until used. Nerve impulses to the hypothalamus stimulate the release of hormones from the neurohypophysis.

Thyroid gland

The thyroid gland is reddish-brown in color, about 25 g and is highly vascularized. It is located in the anteroinferior 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 lobe 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 parafollicular 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:

– Triiodothyronine (T3)

– Tetraiodothyronine (T4 or thyroxine)

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

The parafollicular glands secrete the following hormone:

– Calcitonin, which regulates calcium metabolism, mainly by supplying bone resorption.

parathyroid glands

The parathyroid glands are small ovoid or lentiform, yellowish-brown 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, top and bottom.

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

The parathyroid glands secrete parathyroid hormone (PTH) which is involved in controlling the level and distribution of calcium and phosphorus. PTH acts on three target organs: bones, digestive tract (gut) and kidneys. The overall effect of PTH is to increase plasma calcium levels and decrease plasma phosphate levels.

Adrenal (adrenal) glands

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

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

adrenal cortex

The adrenal cortex, a thin outer (peripheral) layer, shows three cellular zones: the glomerulosa (outermost), fasciculated (wider) and reticulate (innermost) zones. The cortex secretes hormones called steroids.

Zona Glomerulosa: Produces aldosterone (mineralocorticoid), which plays an important role in regulating blood volume and pressure, and in the concentration of blood electrolyte balance. In general, aldosterone retains sodium and water and eliminates potassium.

Zona Fasciculata: Produces hormones that maintain the balance of carbohydrates, proteins and fats (glucocorticoids). The main glucocorticoid is cortisol.

Zona Reticulate: Can 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 a marked effect on carbohydrate metabolism.

2 – Norepinephrine (Norepinephrine), which produces heart acceleration, 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):

– Conversion of glycogen to glucose in the liver;

– Elevation of the metabolic pattern of most cells;

– Dilation of the bronchi.

pancreas

The pancreas is an elongated organ that lies transversely across the upper abdomen, extending from the duodenum to the spleen. 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). The islets are made up of spheroid or ellipsoidal agglomerations of cells, dispersed in the exocrine tissue, together with sparse, often solitary, endocrine cells. The human pancreas can contain over a million islands, usually most numerous in the tail. These islets contain two types of cells: alpha endocrinocytes, which produce glucagon, and beta endocrinocytes, which produce insulin. These two hormones help control blood glucose levels. The effect of insulin is to lower glucose levels while glucagon increases these levels.

Action of Insulin : Lowers glucose levels through two mechanisms:

1) increases the transport of glucose from the blood into the cells;

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

Action of Glucagon: This hormone increases blood glucose in two ways:

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

2) stimulating the conversion of proteins into glucose.

Gonads (Ovaries and Testicles)

The gonads are sex glands, which constitute the ovaries (women) and testes (men). These gonads, in addition to producing 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 the female genital organs and in the expression of female sexual characteristics, and these characteristics develop mainly in response to estrogen. They include:

– Breast development;

– Distribution of fat in the hips, thighs and breasts;

– Distribution of hair in specific areas of the body;

– Maturation of genital organs;

– Closure of the epiphyseal cartilages of the long bones.

Both estrogen and progesterone are controlled by releasing hormones in the hypothalamus, and by gonadotropins from the anterior pituitary.

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

The main hormone secreted by the testes is testosterone, a steroid produced by their interstitial cells. The stimulus for testosterone secretion is luteinizing hormone (LH), from the anterior pituitary.

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

– Growth and development of male genitals;

– Musculoskeletal growth;

– Hair growth and distribution;

– Enlargement of the larynx, accompanied by voice changes.

Testosterone secretion is controlled by releasing hormones produced in the hypothalamus, and by luteinizing hormones from the anterior pituitary.

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 into old age. At birth it weighs about 10 to 15 g, growing until puberty, when it weighs 30 to 40 gm, that is, 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. In early life, it is pinkish-gray, soft and finely lobulated, consisting of two equal pyramidal lobes, 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 superior 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 superior and inferior anterior mediastinum, extending inferiorly to the fourth costal cartilage, with its upper parts tapering towards the neck and sometimes reaching the inferior poles of the thyroid gland.

The thymus has the function of producing several 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 alpha I, and thymosin beta IV.

Thymulin is produced within the thymus and needs the presence of zinc for functional activity (it reacts exclusively with T cells). Thymopoietin enhances several T cell functions. Thymulin and thymopoietin act systematically to provide finely tuned immune regulation of T cells, helping to maintain a balance between the activities of their different subsets. The activities of thymosin alpha I and beta IV are unclear. Thymosins are known to promote lymphocyte maturation within the thymus and also stimulate the development and activity of lymphocytes in carrying out their lymphatic functions throughout the body.

Body (Gland) Pineal

Put on Nervous System: Its 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 superior colliculi. It is inferior to the splenium of the corpus callosum, separated from it by the choroidal meshwork of the third ventricle. The body measures approximately 8 mm in length. Its base is attached by a peduncle that divides into inferior and superior laminae, separated by the pineal recess of the third ventricle. And containing, respectively, the epithalamic and habenula commissures.

The pineal body contains cords and follicles of pinealocytes and neuroglia cells between which many blood vessels and nerves branch. Septa extend into the body from the adjacent pia mater.

The pineal body modifies the activity of the adenohypophysis, neurohypophysis, endocrine pancreas, parathyroids, adrenal cortex and medulla, 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 releasing hormones from the hypothalamus. Melatonin is also believed to be related to the sleep/wake cycle, having a tranquilizing effect. It has been called the “body clock”, controlling most biorhythms.

OTHER HORMONES

Hormones Associated with Specific Organ System

These hormones normally control the activities of a specific organ. For example, hormone-producing cells 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 arachidonic acid. They are produced by various tissues and generally act close to their sites of secretion. They play an important role in the regulation of smooth muscle contraction and the inflammatory response. Prostaglandins are also associated with increased sensitivity of nerve endings to pain.

Summary of Endocrine Glands and Hormones: