Hormones are chemical messengers, which regulate and control a number of activities of the body. Hormones are produced by structures called endocrine glands. These are ductless glands, which pour their secretion directly into blood. The study of endocrine glands and hormonal control is called endocrinology.
A hormone is a regulatory chemical. It has the following properties:
(a) It travels in the blood.
(b) It has its effect at a site different from the site where it is made, called the target, hence the term messenger.
(c) It fits precisely into receptor molecules in the target, like a key in a lock. It is, therefore, specific for a particular target.
(d) It is a small soluble organic molecule.
(e) It is effective in low concentration.
Types of Hormones
The two main types of hormone are:
(1) Steroid hormones which are lipids made from cholesterol, e.g. estrogen, testosterone, cortisone.
(2) Non-steroid hormones are all synthesized from amino acids and there are three main classes of these substances:
(a) The amine hormones are modified versions of single amino acids e.g., thyroxin, epinephrine and norepinephrine.
(b) The peptide hormones are short chains of amino acid as few as three, antidiuretic hormones or vasopressin, oxytocin, thyrotropin.
(c) The protein hormones are made of polypeptide having as many as 200 amino acids e.g. insulin and glucagon.
Some of the endocrine glands are exclusively endocrine in function and some have both endocrine and exocrine function. The figure shows the locations of most of the human endocrine glands.
The following endocrine glands are present in man
(4) Adrenal Gland
(6) Glandular Epithelium
(9) Pituitary Gland
The thyroid gland is a large gland located in the neck where it attaches to the trachea below the larynx. The thyroid gland has two lobes one on each side of the junction between the larynx and trachea. A bridge of thyroid tissue called isthmus connects the two lobes. The thyroid gland is made of spherical thyroid cells filled with stored thyroid hormone.
The hormone produced by thyroid called thyroxin (thyroxin) which has two forms. Thyroxin is usually secreted as T4 (tetraiodothyronine) which contains four iodine atoms, but eventually this form is converted to T3 (triiodothyronine), the active form of the hormone. The two hormones act essentially the same way. Another hormone secreted by thyroid, is called calcitonin.
The thyroid is active continuously but produces higher levels of secretions during period of rapid growth and sexual maturation and in stress situations such as cold and hunger. Thyroxin (T4 and T3) increases the metabolic rate. It does not have one target organ; instead, it stimulates all organs of body to metabolize at a faster rate. More glucose is broken down with the release of heat and more ATF is generated. They also act in conjunctions with somatotropin (growth hormone secreted by anterior pituitary) in bringing about growth and act directly on brain cells causing them to differentiate (in amphibians, they affect the process of metamorphosis. If secretion of thyroid is deficient, the tadpole of frog does not metamorphose to develop into frog, but instead grows into a large size tadpole.
It is a general term for the condition in which too little thyroxin is produced. It can cause the following:
If there is a great deficiency of thyroxin in early childhood, a disease called cretinism occurs. The child develops into a midget called cretin. Cretinism results in mental retardation and irregular development of bones and muscles. The skin is dry, eyelids are puffy, hair is brittle and the shoulders sag. There is also failure to develop sexuality. Ordinarily cretinism cannot be cured but early diagnosis and treatment with thyroxin may arrest the disease before the nervous system is damaged.
The occurrence of hypothyroidism in the adult produces the condition known as Myxedema (myxoedema). It is characterized by lethargy, weight gain, loss of hair, slower pulse rate, lower body temperature and thickness and puffiness of hand and skin.
The disease goiter is the enlargement of the thyroid gland itself. Goiter cause an increase in the size of the entire throat and neck. The thyroid has to work harder to produce a sufficient amount of thyroxin. Thus, it enlarges. It is common in mountainous areas due to lack of iodine in soil and water.
It is also known as
Graves' diseases. It is a condition in
which too much thyroxin is produced.
Symptoms: These include nervousness, irritability, increased heart rate and blood pressure, weakness, weight loss, rapid use of oxygen at rest and bulging eyes (exophthalmic goiter) causes in part by increased fluid behind eyes. This can lead to cardiac failure if prolonged.
Cause: The cause of Graves' disease is the production of an abnormal body protein, which continuously stimulates the thyroid to excessive secretions.
Treatment: Drug therapy that inhibits thyroxin production and the administration of radioactive iodine has successfully replaced surgery as treatment for hyperthyroidism.
High calcium ions concentration in the blood causes stimulation of the synthesis and release of calcitonin, low level of calcium ions suppress its manufacture, it helps to regulate the blood calcium level in the blood and opposes the action of parathyroid hormone. Calcitonin lowers blood calcium (Ca++) by increasing the buildup of bone. Excess or deficiency leads to a disturbance of calcium metabolism with its associated effects on nerve, skeleton, muscles, blood etc.
Located on the thyroid gland are two pairs of very small structures called the parathyroid glands? They secrete the hormone parathormone. Low levels of blood calcium ions stimulate the parathyroid directly to increase parathormone production whereas high levels of calcium ions suppress its release. It controls balance of calcium ions and phosphate in body.
Deficiency: Too much Ca++ ions move from the blood to the bones, the muscles and nerves do not receive enough calcium ions. As a result, they become very sensitive to stimuli. Spasm (contractions and relaxations of muscles) and even death may occur, in case of severe deficiency.
Overproduction: It causes Ca++ ions to leave the bones to the blood and causes weakening of the skeletal system similar to rickets. Nerve and muscle do not respond well to stimuli. Over activity also causes massive formation of kidney stone. Both conditions may be fatal.
Removal: Small as the parathyroid glands are, their removal invariably causes death.
Pancreas is composed of two types of tissue. Exocrine tissue produces and secretes digestive juices that go by way of ducts to the small intestine. Endocrine tissue called pancreatic Islets (island) of Langerhans named after Paul Langerhans who discovered them. These cells are under the control of pituitary hormones STH (Somatotropin hormone) and ACTH (adrenocorticotrophic hormone) and responds directly to the level of blood glucose. Islets of Langerhans secrete two hormones Insulin and glucagon. Both are proteins. Insulin is secreted by the alpha cells, which are large in number, and glucagon is secreted by cells, which are less in number.
Insulin is a small protein composed of 51 amino acids. Insulin is secreted when the level of blood sugar rises, such as right after a meal. The most important affect of insulin is to facilitate glucose transport across cell membrane. Insulin depresses blood glucose level in various ways
(a) Increases the rate of conversion of glucose to glycogen, called glycogenosis, which takes place mainly in the liver and muscle.
(b) Increase in the rate of uptake of glucose by cells.
(c) Increase in the use of glucose rather than other substance.
(d) Increase in the conversion of glucose into lipids and proteins.
(e) Decrease production of glucose.
A deficiency in insulin production leads to the metabolic disease known as diabetes mellitus. The symptoms include:
(1) Sugar in Urine.
(2) Frequent Copious Urination.
(3) Abnormal Thirst.
(4) Rapid Weight Loss
(5) General Weakness
(6) Drowsiness and Fatigue.
If excess of insulin is produced, the utilization of sugar is too great and its level falls in the blood that upsets nerve and muscle functioning.
It is a protein composed of 29 amino acids. Its role is to increase blood glucose level. Its main target is the liver. Glucagon stimulates the conversion of glycogen to glucose (glucogenesis). It also stimulates the breakdown of proteins and fats to glucose and conversion of lactic acid to glucose (gluconeogenesis). Glucose abnormalities seem rare as endocrine disorders. Tumors on beta cells will cause excess glucagon secretions and consequent high blood glucose level.
Each of the two adrenal glands rests on a kidney. Each adrenal gland is composed of an inner portion called the medulla and an outer portion the cortex.
The hypothalamus exerts control over the activity of both portions of the adrenal glands. The hypothalamus, by means of ACTH (adrenocorticotrophic hormone) releasing hormone controls the anterior pituitary's secretion of ACTH, which in turn, stimulates the adrenal cortex.
The two major types of hormones produced by the adrenal cortex arc glucocorticoids, which help to regulate the blood glucose level, and mineralocorticoids, which help to regulate the level of minerals in the blood.
Epinephrine (adrenaline) and norepinephrine are produced by the adrenal medulla. Epinephrine and norepinephrine are involved in the body's immediate response to stress. They bring about all the bodily changes that occur when an individual reacts to an emergency. Both the hormones contribute to the short-term stress response. Both hormones stimulate liver cells to release glucose, thus making more fuel for cellular energy. They also prepare the body for action by increasing the blood pressure, the breathing rate, and the metabolic rate.
In addition, epinephrine and norepinephrine change blood flow patterns making some organs more active and other less so e.g. epinephrine dilates blood vessels in the brain and skeletal muscles, thus increasing alertness and the muscles ability to react to stress. At the same time, epinephrine and norepinephrine constrict blood vessels elsewhere, thereby reducing activities that are not immediately involved in the stress response, such as digestion. The short-term stress response is rapid and usually subsides shortly after we first encounter stress.
The Following Hormones Are Being Discussed:
It is the glucocorticoid that promotes the hydrolysis of muscle protein to amino acid and the liver converts these amino acids to glucose. Cortisol also favors metabolism of fatty acids rather than carbohydrates, It also counteracts the Inflammatory response that leads to the pain and the swelling of joints In arthritis etc.
It is both glucocorticoid and a mineralocorticoid. It Increases blood glucose levels and regulate mineral Ion balance.
It is a mineralocorticoid. It promotes renal absorption of sodium (Na+) and renal excretion of potassium (K+).
When there is less production of cortical hormone e.g. destruction of the adrenal cortex, such as occurs in Addison's disease, will lead to general metabolic disturbance, in particular, weakness of muscle action and loss of salts. Stress situation, such as cold, which would normally be overcome, lead to collapse and death. When too much cortical hormone is produced such as in Cushing's Disease, symptoms are an excessive protein breakdown resulting muscular and bone weakness. The high blood sugar disturbs the metabolism as in diabetes.
Adrenal cortex secretes a small amount of male sex hormones (androgen) and a small amount of female sex hormones in both male and female.
Many parts of the gut function as endocrine tissue. The important hormones produced are:
(a) Gastrin: It is produced by the mucosa of the pyloric region of the stomach. It stimulates the production of gastric juice rich in hydrochloric acid. It is produced under the influence of protein food in the stomach after it is partially digested.
(b) Secretin: It is produced by the mucosa of duodenum, when acidic food touches its lining. It stimulates production of pancreatic juice by the pancreas and also affects the rate of bile production in the liver, and inhibits secretion of gastric juice.
(c) Cholecytokinin (CCK): It is produced by mucosa of duodenum. The stimulus for secretion is fatty food and protein in the duodenum. CCK stimulates increased secretion of pancreatic juice rich in enzymes.
The gonads are the ovaries in the females and testes in the males.
Ovary: Ovaries are located in the abdominal cavity. The ovaries secrete female sex hormones (a) estrogen (oestrogen) and (b) progesterone.
This hormone is secreted by ripening follicles whose development has been initiated by FSH from the anterior pituitary gland. Estrogen performs the following functions
(1) Estrogen secreted at the time of puberty is responsible for secondary sex characteristics in females. The secondary sex characteristics include a more rounded appearance in female than males because of a greater accumulation of fats beneath the skin. Also the pelvic girdles extend more in females than the males, resulting in females having a larger pelvic cavity and wider hips. Estrogen stimulates enlargement of various accessory organs, including (internal vagina, uterus, uterine tubes, and ovaries) and external reproductive structures.
(2) Estrogen is necessary for egg maturation.
(3) At a point during estrous or menstrual cycle exert a positive feedback, which results in a sharp rise in LH output by the pituitary.
(4) Estrogen aids in healing and repair of uterine wall after menstruation.
(5) Under influence of estrogen, some of the cells of uterine wall become glandular and start secreting proteinaceous secretions which are taken up by the embryo during early stages of development.
Deficiency: It leads in the young to failure to mature sexually and sterile in the adult.
This hormone is produced by the ruptured follicles in response to LH (luteinizing hormone) from the pituitary.
The functions performed by progesterone are:
(1) It inhibits further FSH secretion from the pituitary, thus preventing any more follicles from ripening.
(2) It causes further thickening and vascularization of the uterus wall and other areas of the female body preparing it for maintaining state of pregnancy.
(3) Progesterone suppresses ovulation, so it is a major constituent of birth control pill.
(4) Progesterone affects the mammary glands and help to regulate the secretion of gonadotropin from the anterior pituitary gland.
The testes are located in the scrotum. The testes consist of many coiled seminiferous tubules, where sperms develop. The regions between the tubules consist of interstitial cells, which produce gonad hormones called testosterone and 17-beta hydroxytestosterone.
The male sex hormone testosterone has many functions
(1) It is essential for the normal development and functioning of the sex organs in males.
(2) It is necessary for the maturation of sperms.
(3) In the fetus, it initiates the development of the sex organs.
(4) Testosterone brings about and maintains the secondary sex characteristics in males that develop at the time of puberty. Testosterone causes growth of beard, axillary hair, and pubic hair. It prompts the larynx and the vocal cords to enlarge, causing the voice to change.
(5) Testosterone also stimulates oil and sweat glands in the skin.
The thymus is a lobular gland that lies inside the thorax on the ventral side. This organ reaches its largest size and is most active during childhood. Thymus produces various hormones thymosin. Certain lymphocytes that originate in the bone marrow and then pass through the thymus are transformed into T lymphocytes.
It is located deep between the cerebral hemisphere, and is attached to hypothalamus, produces hormone called melatonin, primarily at night Melatonin is involved in a daily cycle called a circadian rhythm.
Hypothalamus and Pituitary Gland
The hypothalamus is a portion of the brain that regulates the internal environment. For example, it helps to control heart rate, body temperature, and water balance, as well as the glandular secretion of the pituitary gland. The pituitary, a small gland about 1 cm in diameter, is connected to the hypothalamus by a stalk like structure. The pituitary has two portions the posterior pituitary and anterior pituitary.
|Hypothalamus and Pituitary Gland|
The axon endings in the posterior pituitary store antidiuretic hormone (ADH, sometimes called vasopressin) and oxytocin.
ADH promotes the re-absorption of water from the collecting ducts within the kidney. Increase levels cause increase water re-absorption in distal part of kidney. Inability to produce ADH causes Diabetes Insipidus (watery urine) in which a person produces copious (plentiful, overflowing) amounts of urine with a resultant loss of ions from the blood and great thirst.
Oxytocin is the other hormone that is made in the hypothalamus and stored in the posterior pituitary. Its release is stimulated by distention of cervix, decrease in progesterone level in blood, and neural stimuli during parturition (childbirth) and suckling. Oxytocin causes the smooth muscles of the uterus to contract and is used to artificially induce labor. It also stimulates the release of milk from the mammary glands when the baby is nursing.
Anterior Pituitary - The Master Gland
It is called master gland because it produces tropic hormones, which stimulate other endocrine glands to release their hormone.
The anterior pituitary produces at least six different types of hormones, each by a distinct cell type.
|Anterior Pituitary - The Master Gland|
It is also known as growth hormone (GH). Somatotropin releasing factor (SRF) is secreted from hypothalamus throughout the life. It promotes cell division, protein synthesis, and bone growth. Evidence suggests that the effects on cartilage and bone may actually be due to hormone called somatomedins, which are released by the liver in response to GH, when growth has mostly ceased after adolescence, the hormone continues to promote protein synthesis throughout the body.
During childhood if too little GH is produced, the individual becomes a pituitary dwarf, and if too much GH is secreted, a person can become a giant.
If there is overproduction of growth hormone in adult, a condition called acromegaly results. Some cartilage and bone becomes thickened. The feet, hands, and face particularly the chin, nose, and eyebrow ridges become very large.
Thyroid Stimulating Hormone (TSH)
The hormone produced by thyroid is thyroxin. The levels of thyroxin in the blood control the release of thyrotrophin releasing factor from hypothalamus. If level of thyroxin is low, there is more production of TSH and vice versa. TSH is secreted throughout life, and reaches high levels during the period of rapid growth and development, TSH acts directly on the cells of thyroid gland. It increases the number of cells and secretory activity of the thyroid gland.
Adrenocorticotrophic Hormone (ACTH)
It stimulates the release of corticotrophin releasing factor from hypothalamus. This is controlled by steroid level in the blood and by direct nervous stimulation of the hypothalamus because of stress, e.g. cold, heat, pain, fright, infections.
These are follicle stimulating hormone (FSH), luteinizing hormone (LH, also called interstitial cell stimulating hormone, ICSH in the male), protecting (sometimes inappropriately called luteotrophic hormone, (LTH). FSH and LH/ICH share a common hypothalamus releasing factor. The FSH acts upon an ovary to stimulate maturation of a follicle and during its development, the follicular cells produce increasing amounts of estrogen and some progesterone. In males, FSH stimulates development of the germinal epithelium of the testes and sperm production.
Prolactin (PRL) is produced in quantity only after childbirth. It causes mammary glands in the breasts to develop and produce milk. It also plays a role in carbohydrate and fat metabolism. Prolactin is continuously produced from the pituitary and is inhibited by protecting inhibiting factor (PIH) from the hypothalamus.
LH works with FSH to stimulate estrogen secretion and rupture of mature follicles to release ovum. It also causes the luteinization (turning yellow) of the rupture follicles. Under the influence of LH, the follicular cells enlarge to form a temporary glandular structure called Corpus Luteum.
ICHS in the male stimulates the interstitial cells of the testes (singular, testis) to secrete testosterone.
It secretes Melanocyte Stimulating Hormone (MSH). The concentration of this hormone in human is very low. MSH causes inhibition of secretion controlled by hypothalamus. External light governs its secretion. More secretion in pregnancy stimulates melanocytes in skin to produce brown pigment, melanin, which darkens the skin. Excess MSH is secreted in Addison's disease (It is the peculiar bronzing of the skin. It is due to adrenal cortex hypo-secretion), and its one of the symptoms is darkening of the skin.
The general principle is that the product of a series of reactions controls its own production by turning off the pathway when it reaches a certain level. This is comparable with a thermostat where the product, heat, switches off its own production when a certain temperature is reached. If there is too little of the product, its production is switched on again.
Commonly, the control of hormonal secretions involves a negative feed back system. An endocrine gland is sensitive either to the concentration of a substance it regulates or the concentration of a product from a process it controls. Whenever this concentration reaches a certain level, the endocrine gland is inhibited (a negative effect) and its secretory activity decreases. Then, as the concentration of the gland's hormone drops, the concentration of the regulated substance drops also, and inhibition of the gland ceases. When the gland is no longer inhibited, it begins to secrete its hormone again. Because of such negative feedback system the concentration of some hormones remain relatively stable, although they may fluctuate slightly within normal range. Example of negative feed back are secretion of thyroxin, cortisol, sex hormones etc.
Example: control of thyroxin release.
1. Neurosecretory cells of the hypothalamus are stimulated by stress or low body temperature and release hormones (TSH).
2. TRH (Thyroid releasing hormone) stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH)
3. a) The level of TSH exerts feedback control over the hypothalamus to release thyroxin (thyroxin),
b) The level of thyroxin exerts feedback control over the anterior pituitary.
c) The level of thyroxin exerts feed back control over the hypothalamus.
Thyroxin causes increase in the metabolic activity of the body, generating ATP and heat. The higher body temperature and higher thyroxin levels in the blood inhibits production of thyroxin. In this way, thyroxin controls its own secretion.