Physiology - Endocrine System

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Endocrine System

The endocrine system affects bodily activities by releasing chemical messages, called hormones, into the bloodstream from exocrine and endocrine glands. The function of hormones is to:

• Control the internal environment by regulating its chemical composition and volume
• Respond to environmental changes to help the body cope with emergencies - infection, stress etc
• Help regulate organic metabolism and energy balance
• Contribute to the management of growth and development

Hormones are chemicals that cause certain changes in particular parts of the body. Their effects are slower and more general than nerve action. They can control long-term changes such as rate of growth, rate of activity and sexual maturity.

The endocrine or ductless glands secrete their hormones directly into the blood stream. The hormones are circulated all over the body and reach their target organ via the blood stream. When hormones pass through the liver, they are converted by the kidneys. Tests on such hormonal products in urine can be used to detect pregnancy.

The endocrine system consists of a series of glands that secrete hormones; they are found throughout the body and include the pituitary, thyroid, parathyroids, thymus, supra-renal or adrenal glands, part of the pancreas and parts of the ovaries and testes. Although these glands are separate, it is certain that they are functionally closely related because the health of the body is dependent upon the correctly balanced output from the various glands that form this system

The Pituitary Gland (Hypophysis)

This gland has been described as the leader of the endocrine orchestra. It consists of two lobes, anterior and posterior. The anterior lobe secretes many hormones, including the growth-promoting somatotropic hormone which controls the bones and muscles and in this way determines the overall size of the individual. Over secretion of the hormone in children produces gigantism and under secretion produces dwarfism. The anterior lobe also produces gonadotropic hormones for both male and female gonad activity. Thyrotropic hormones regulate the thyroid and adrenocorticotropic hormones regulate the adrenal cortex. It also produces metabolic hormones.

The posterior lobe produces two hormones - oxytocin and vasopressin. Oxytocin causes the uterine muscles to contract; it also causes the ducts of the mammary glands to contract and, in this way, helps to express the milk that the gland has secreted into the ducts. Vasopressin is an antidiuretic hormone that has a direct effect on the tubules of the kidneys and increases the amount of fluid they absorb so that less urine is excreted. It also contracts blood vessels in the heart and lungs and so raises the blood pressure. It is not certain whether these two hormones are actually manufactured in the posterior lobe or whether they are produced in the hypothalamus and passed down the stalk of the pituitary gland to be stored in the posterior lobe and liberated from there into the circulation.

The Thyroid

The right and left lobes of this gland lie on either side of the trachea united by the isthmus. Average size of each lobe is 4cm long and 2cm across but these sizes may vary considerably. The secretion of this gland is thyroxine and tri-iodothyronine. Thyroxine controls the general metabolism. Both hormones contain iodine but thyronine is more active than thyroxin. Under-secretion of this hormone in children produces cretinism; the children show stunted growth (dwarfism) and fail to develop mentally. Under secretion in adults results in a low metabolic rate. Over secretion in adults gives rise to exophthalmic goitre and the metabolic rate is higher than usual. Such persons may eat well but burn up so much fuel that they remain thin. This is usually accompanied by a rapid pulse rate. This gland, therefore, has a profound influence on both mental and physical activity.

The Parathyroid Glands

There are four of these glands, two on either side lying behind the thyroid. Their secretion is parathormone - the function of which is to raise the blood calcium as well as maintain the balance of calcium and phosphorus in both the blood and bone structures. Under secretion gives rise to a condition known as tetany in which the muscles go into spasm, and over secretion causes calcium to be lost to the blood from the bones giving rise to softened bones, raised blood calcium and a marked depression of the nervous system

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