Anatomy & Physiology - Body Systems
The Endocrine System
The branches of science that will help you understand the body parts and functions are anatomy and physiology. Anatomy deals with the study of the human body (the components, structure and position) and physiology the study of how the body functions.
The 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:
Hormones are chemicals that cause specific 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, the rate of activity and sexual maturity.
The endocrine or ductless glands secrete their hormones directly into the bloodstream. The hormones are circulated all over the body and reach their target organ via the bloodstream. 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 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 right and left lobes of this gland lie on either side of the trachea united by the isthmus. The 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 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. A rapid pulse rate usually accompanies this. 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 marked depression of the nervous system.
The Thymus Gland
This gland lies in the lower part of the neck and attains a maximum length of about 6cm. After puberty, the thymus begins to atrophy so that in the adult-only fibrous remnants are found. Its secretion is thought to act as a brake on the development of sex organs so that as the thymus atrophies, the sex organs develop. Recent research into the activity of this gland reveals that it plays an important part in the body's immune system by producing T lymphocy - the T standing for thymus derived.
The Suprarenal or Adrenal Glands
These are two in number, triangular and yellow. They lay one over each kidney. They are divided like the kidney into two parts, the cortex and the medulla. The cortex is the outer part of the gland and produces several hormones called cortico-steroids. Their function is to control sodium and potassium balance, stimulate the storage of glucose and affect or supplement the production of sex hormones. The medulla or inner layer produces adrenaline, a powerful vasoconstrictor. Adrenaline raises vessels and raised blood sugar by increasing the output of sugar from the liver. The amount of adrenaline secreted is increased considerably by excitement, fear, or anger, which has caused the adrenals sometimes to be referred to as the glands of fright and fight.
The Gonads or Sex Glands
These glands are naturally different in men and women because they serve different, though, in many respects, complementary functions. In the female, the gonads are the ovaries and in the male the testes. Female sex hormones are oestrogen and progesterone. The male sex hormone is testosterone, though each sex produces a small quantity of the opposite hormone. The female hormones are responsible for developing the rounded, feminine figure, breast growth, pubic and auxiliary hair and all the normal manifestations of femininity and reproduction. Male hormone is responsible for voice changes, increased muscle mass, development of hair on the body and face and the usual development of manliness.
The endocrine part of the pancreas consists of clumps of cells called islets of Langerhans that secrete insulin. Insulin regulates the sugar level in the blood and the conversion of sugar into heat and energy. Too little insulin results in a disease known as diabetes mellitus. This disease is divided into one form, juvenile onset, which occurs before the age of 25, and another form that begins in maturity. It is a prevalent disease. It is known that some half million people in the United Kingdom suffer from it sufficiently badly to need treatment but it has been estimated that there are many more people in whom the disease exists at a sub-treatment level.
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