6.8. Endocrine system

Introduction

Activities within the human body are regulated by two systems, the nervous system and endocrine system. Although both systems control body functions, their methods differ.

The nervous system sends its messengers, called impulses, to specific cells, generally muscle or gland cells. The nervous system acts quickly. Its messages travel rapidly and can change instantly. The response is immediate.

The endocrine system uses chemical messengers. They are widely dispersed to every cell throughout the body. However, only specific target cell, equipped with receptors, respond to the messages. The endocrine system generally does not act as quickly as the nervous system. Its messages travel more slowly, but the effect generated by those messages last longer than those from the nervous system.

The Endocrine Glands

The body contains many glands. Glands are cells, groups of cells, or organs that produce and secrete substances. Exocrine glands, such as sweat glands and digestive glands, secrete their products through tubes, or ducts. Endocrine glands, often called ductless glands, release their products directly into the bloodstream. Endocrine glands produce powerful chemicals called hormones, which help regulate the activities of body tissues and organs. Each hormone acts on a specific tissue or organ: that tissue or organ is the hormone’s target.

The Thyroid

The thyroid gland, located on the trachea, secrets thyroxine. Thyroxine controls metabolic activities, including the production of proteins and ATP. Because thyroxine influences protein production, it affects the growth rate of children. This hormone is also necessary for the proper development of the nervous system.

Iodine is necessary for the production of thyroxine. A person needs 1 mg of iodine each week. Eating a moderate amount of iodized salt usually meets that need. Insufficient iodine may cause the thyroid gland to enlarge, a condition called goiter. Frequently a person with goiter also suffers from hypothyroidism, a lack of thyroxine. The result is low metabolic rate. In adults the symptoms are low body temperature, sluggishness, weight gain, and excess fluid in the body. In infants hypothyroidism may cause cretinism. The effects of cretinism include mental retardation and abnormal bone growth. Hyperthyroidism, or an excess of thyroxine, causes a higher-than-normal metabolic rate. The symptoms of hyperthyroidism include weight loss, muscle weakness, excessive sweating, increased heartbeat rate and blood pressure, nervousness, and bulging eyes.

The Parathyroids

On the back of the thryroid gland are four tiny parathyroid glands. They secrete PTH (parathyroid hormone), which regulates the levels of calcium ions and phosphate ions in the blood. These minerals are necessary for proper bone development and for normal functioning of muscles and nerve cells. Too little calcium can make nerve cells so unstable that they send impulses without being stimulated. The result is uncontrollable muscle contractions. If muscles remain contracted, a person may die because breathing stops. The calcium level sometimes is too high. Nerves and muscles then fail to respond to stimuli. Reflexes are slow, and muscle contractions are weak.

The Adrenals

An adrenal gland is located on top of each kidney. Each gland functions as two separate endocrine glands. The inner part of the adrenal gland, called the adrenal medulla, secretes epinephrine and norepinephrine. These hormones produce the same effects as the sympathetic nervous system. They thus help the body respond to stress. For example, they increase blood pressure and heartbeat and breathing rates, dilate the pupils, and inhibit digestion. They also increase metabolism, sometimes as much as 100%.

The outer layer of the adrenal gland is the adrenal cortex, which secretes more than 50 hormones. All belong to a group called corticoids. Among the corticoids are aldosterone: hydrocortisone, also called cortisol; and also androgens. Aldosterone affects water and salt balance by controlling the reabsorption of sodium and potassium ions in the kidneys. Hydrocortisone controls the breakdown of proteins and fats into glucose, inhibits glucose uptake by cells, and aids in healing. Androgens are sex hormones. They regulate development of secondary sex characteristics. A lack of corticoids may result in Addison’s disease. The symptoms of this disease include low blood pressure, darkened skin, dehydration, a low level of sugar and sodium ions in the blood, and a high blood level of potassium ions. A victim will die within a few days if not treated with corticoids. Oversecretion of corticoids may result in Cushing’s disease, characterized by high blood pressure, fat deposits in the face and back, and accumulation of tissue fluids. Excessive secretion of androgens may result in early sexual development in males and excessive hair and a deep voice in females.

The Pancreas

The pancreases is an exocrine gland that produces digestive enzymes. However, it also has special cells called the islets of Langerhans that function as an endocrine gland. They secrete insulin and glucagons. Insulin is a hormone that lowers the level of glucose in the blood. It does so by stimulating the uptake of glucose by body cells and the formation of excess glucose into glycogen in the liver and muscles. Glucagon triggers the break-down of glycogen to glucose when the body needs more energy.

In the absence of insulin, glucose cannot enter body cells. As a result, the cells use their own proteins and fat for energy. The level of glucose in the blood then becomes abnormally high. This condition, called diabetes mellitus, is the third major cause of death in the US. Without proper treatment it can lead to heart disease, strike, kidney failure, severe nerve damage, or blindness. Diabetes may also result in infections so severe that limb amputation is necessary.

The two chief forms of diabetes are Type 2, or non-insulin-dependent, diabetes. In Type 1 diabetes, the islets of Langerhans produce too little or no insulin. Some researchers suspect a virus may be involved in Type 1 diabetes. Type 1 usually first appears in people under 20 years of age and can be controlled by strict diet and daily injections of insulin. Approximately 85%of all diabetics suffer from Type 2 diabetes. Type 2 generally first appears in people over 40 years of age. These diabetics may have normal or even high level of insulin, but their bodies cannot use the hormone. The causes of Type 2 diabetes are believed to be a storage of insulin receptors on body cells or a breakdown of the immune system, which causes the body to become insulin-resistant. Heredity also appears to be factor in both types of diabetes. Type 2 diabetes can generally be controlled through diet.

Excessive levels of insulin in the blood lead to hypoglycemia, a condition in which the level of glucose in the blood brain cells need a constant supply of glucose, as victim may lose consciousness due to the lack of glucose. A diet high in protein and low in carbohydrates can help control hypoglycemia.

The Gonads

Gonads, the gamete-producing organs of the reproductive system, also produce and secrete hormones. The female gonads secrete estrogens that influence the development of female secondary sex characteristics. Among these are wider hips, enlarged breasts, and rounded body contours. The male gonads produce androgens that stimulate development of the male secondary sex characteristics. These include sex hormones play roles in reproduction.

The Pituitary

The pituitary gland, located at the base of the brain, is about the size and shape of a kidney bean. It has two major sections, the anterior lobe and the posterior lobe.

The anterior lobe produces at least six hormones. Four are tropic hormones – that is, hormones that affect the secretions of other glands. Two tropic hormones, FSH (follicle stimulating hormone) and LH (luteinizing hormone), act on the gonads. The other two tropic hormones are TSH (thyroid stimulating hormone) and ACTH (adrenocorticotropic hormone). TSH stimulates the thyroid gland to secrete thyroxine, and ACTH affects the adrenal cortex. The anterior lobe also secretes somatotropin, or growth hormone (GH). Stomatotropin has many effects on metabolism. It stimulates bone and muscle growth and helps control the use of glucose and fatty acids for energy. Prolactin, another hormone of the anterior lobe, stimulates the mammary glands to produce milk after the birth of a child.

The posterior lobe of the pituitary does not produce any hormones, but it stores two hormones produced by the hypothalamus. They are antidiuretic hormone (ADH) and oxytocin. ADH, also called vasopressin, keeps the blood volume constant by controlling reabsorption of water in the kidneys. Oxytocin stimulates the contraction of uterine muscles during childbirth and the release of milk from the breasts after childbirth. Prolactin and oxytocin have no known function in males.

Most disorders associated with the pituitary gland involve somatotropin. An excess during childhood results in gigantism, or excessive growth, One victim grew to 2.7 m. An excess during adulthood results in acromegaly, in which the hands, feet and skull increase in size. Too little somatotropin during childhood results in dwarfism, characterized by a short body but otherwise normal proportions and normal mental and sexual development.

The Hypothalamus

The hypothalamus, which is a part of the brain, may be considered the master switchboard of the endocrine system. It links the endocrine system with the nervous system. The nervous system feeds information from the entire body into the hypothalamus. Based on that information, the hypothalamus then sends signals in the form of tropic hormones to stimulate o inhibit hormone secretion by the pituitary gland. At least nine such hormones have been identified. The hormones that stimulate secretion are called releasing hormones. Releasing hormones trigger secretion of TSH, GH, LH, FSH, ACTH, and Prolactin. Hormones that slow down secretion are inhibiting hormones. The hypothalamus secretes inhibitors for GH, TSH, and Prolactin. It also produces ADH and oxytocin and signals their release from the posterior pituitary.

Table 6.1

Glands: hormones and functions

Gland	Hormone	Target	Functions
Pituitary, anterior gland	Growth hormone (GH, somatotropin)	All cells	Maintains protein production, releases fats and glucose
	Thyroid-stimulating Hormone (TSH)	Thyroid gland	Stimulates production and secretion of thyroxine
	Adrenocorticotropic Hormone (ACTH)	Adrenal cortex	Stimulates production and secretion of corticoids
	Follicle-stimulating hormone (FSH)	Gonads	Plays a role in female monthly cycle, the production of female sex hormones and men gametes
	Luteinizing hormone (LH)	Gonads	Plays a role in female monthly cycle, stimulates production of sex hormones
	Prolactin	Mammary glands	Stimulates growth of gland and production of milk
Hypothalamus	Releasing hormones	Pituitary	Stimulates release of GH, TSH, LH, FSH, ACTH, and Prolactin
	Inhibiting hormones	Pituitary	Inhibits release of GH and Prolactin
	Oxytocin	Uterus, mammary glands	Stimulates muscle contracting during childbirth, milk release
	Antidiuretic hormone (ADH, Vasopressin)	Kidneys	Controls water reabsorption
Thyroid	Thyroxine	All body cells	Stimulates metabolic rate
Parathyroid	Parathyroid hormone (PTH)	Bone	Controls level of calcium ions and potassium ions
Adrenal cortex	Aldosterone	Kidneys	Controls reabsorption of sodium, stimulates excretion of potassium
	Hydrocortisone (cortisol)	Liver, various cells	Inhibits glucose uptake, aids healing, reduces inflammation
	Androgen	Male gonads	Stimulates development of male secondary sex characteristics
Adrenal medulla	Epinephrine, Norepinephrine	Various cells	Controls stress reactions:increases heart and breathing rates, raises blood pressure and glucose level, inhibits digestion
Pancreas (islets of Langerhans)	Insulin, Glucagon	Liver muscle, Liver	Stimulates glucose uptake Triggers breakdown of glycogen into glucose
Female gonads (ovaries)	Progesterone, estrogen	Female sex organs	Controls female secondary sex characteristic development, Female sexual functions
Male gonads (testes)	Testosterone	Male sex organs	Controls development of male gametes and male secondary sex characteristics

Endocrine System Regulation

The endocrine system and the nervous system together control other body systems. However, the endocrine system also controls itself.

Feedback

The endocrine system controls itself through a process called negative feedback. This process is similar to the way a thermostat regulates a household furnace. When the temperature falls below the thermostat setting, the furnace switches on and begins producing heat. When the temperature reaches the thermostat setting, the furnace switches off. Similarly, the level of a hormone in the blood turns its own production off and on.

Negative feedback controls the thyroxine level in the blood. The hypothalamus plays the role of the thermostat. The hypothalamus has cells that detect the presence of thyroxine in the blood. When the thyroxine level is low, the hypothalamus secretes a releasing hormone that stimulates the pituitary to secrete TSH. TSH causes the thyroid to secrete thyroxine. When the thyroxine level returns to normal, the hypothalamus stops secreting the releasing hormone. As a result of this feedback mechanism, the pituitary stops secreting TSH, and the thyroid slows down secretion of thyroxine.

How Hormones Act

There are two types of hormones: steroids, which are fatlike organic compounds, and protein hormones. Sex hormones and corticoids are steroids. All others are protein hormones.

Steroids and protein hormones produce their effect differently. A steroid passes through the target cell membrane. It combines with a receptor molecule and moves into the cell nucleus. There it helps determine the manufacture of specific proteins. Protein hormones affect their target cells through a two-step procedure called a “two-messenger” system. The first messenger, the hormone, combines with the receptor on the target cell membrane. This combination activates an enzyme on the membrane’s inside wall. The enzyme helps change ATP into cyclic adenosine monophosphate, or cyclic AMP. Cyclic AMP triggers enzymes that bring about changes initiated by the original hormone. Thus, cyclic AMP is called the second messenger.