Hormones

HORMONES:

The term ‘hormone’ was introduced by Bayliss and Starling.

Hormones are the chemical messengers which coordinate the activities of living organisms.

Characteristics of Hormones:

(i) They are the secretions of endocrine glands.

(ii) They are produced at a place and act on target organs which are mainly away from their source

(iii) They are poured directly into the blood stream.

(iv) They are required in very small quantities.

(v) They are specific in function.

(vi) Chemically they are mainly proteins. Some of them may be amino acids, steroids etc.

(vii) They are harmful if present in less or excess amounts.

(viii)Hormones are immediately destroyed after their action is over.

Feedback Mechanism of hormone secretion

Any control system should have a feedback mechanism to prevent excessive reaction or over­reaction. For example, the hypothalamus produces the releasing hormone for thyroid stimulating hormone (TSH—RH) which in turn stimulates the pituitary to produce thyroid stimulating hormone (TSH).The TSH activates the thyroid gland to secrete thyroxine. If the level of thyroxine in the blood is less than normal, it has a positive feedback effect on the hypothalamus and pituitary to produce more TSH—RH and TSH, respectively. If the level of thyroxine is more than normal in the blood, a negative feedback effect is seen on the hypothalamus and the pituitary so that they produce less of TSH—RH and TSH, respectively. Positive feedback systems are relatively rare in higher vertebrates as they lead to instability.

Thyroid stimulating hormone (TSH) and thyroxine regulate each other’s level just like the requirement—supply kind of situation. Such a system of opposing effects leads to proper control and balance in a system. The two opposing systems work in coordination and help the body to adjust its output accordingly. The ultimate effect of such a feedback system is homeostasis. Metabolic processes in all the living organisms tend to maintain the internal envrionment of the cell as constant as possible. External conditions on the other hand tend to produce changes. To resist the effect of these changes living organsims have a number of elaborate and complex devices and try to keep the internal environment constant.

This tendency towards constancy is called homeostasis. Homeostasis (Gr., homois, unchanging; stasis, standing) can therefore, be defined as the tendency to maintain uniformity or stability in the internal environment of the organism and to maintain the normal composition of the body fluids.

Pheromones

Pheromones (Gk., pherein to carry; hormon, to excite) are chemical substances secreted by organsims that trigger either behavioural or developmental processes when percieved by other members of the same species. The term pheromone was proposed by Karlson and Butenandt in 1959. As they resemble hormones in that they have an effect at some distance from the point of release they were originally called ectohormones.

The pheromones are not true hormones since they are product of exocrine glands, however, release from these glands often is dependent on hormonal stimulation.

Insects like ants, honey bees and some other animals like rabbits, commnicate through chemicals called pheromones. Ants walking behind in a trail is also regulated by pheromones which is secreted by their anal region. Due to the smell of this chemical they move in straight line.

Various Endocrine Glands Present In The Human Body Are:

• Pituitary gland (or Hypophysis)
•  Pineal gland
• Thyroid gland
• Parathyroid
• Thymus gland
• Adrenal gland
• Pancreas
• Ovaries (xi) Tests

Pituitary Gland (or Hypophysis):

The Pituitary glands is an ovoid slightly flattened body of tissued situated at the base of brain. It is red-grey in color attached to the hypothalamus of the brain by a stalk or infundibulum.

It is known as master endocrine gland because it regulates the activity of other endocrine glands. The pituitary gland consists of three lobes-the anterior lobe, the middle lobe and the posterior lobe.

Hormones of anterior pituitary:

Somatotrophin or Growth hormone (GH):

It stimulates growth and development of all tissues by accelerating protein synthesis and cell division, and by retaining calcium in the body. It also enables the cells to take up more amino acid and mobilise fat and makes the liver to release glucose for energy supply.

Adrenocorticotrophic hormone (ACTH):

This regulates the activity of adrenal cortex. It mainly stimulates the adrenal cortex to secrete corticosteroid hormone which defends the human body under stress.

Thyroid stimulating hormone (TSH):

This is also calledThyrotrophic hormone or thyrotropin. This stimulates growth of the thyroid gland and production of thyroid hormones.

Follicle stimulating hormone (FSH):

It stimulates sperm formation in the male and growth of ovarian follicles in the female. In older persons, it helps to maintain sexual activity.

Luteinising hormone (LTH):

In the male it induces the interstitial cells of the testes to produce male sex hormones namelyandrogenssuch astestosterone. This hormone makes the male genital system to become fully grown and functional. In the female, the luteininsing hormone causes ovulation, secretion of female sex hormones -Oestrogenfrom the maturing ovarian follicle, andprogesteroneby the corpus luteum transformed from the empty ovarian follicle. Follicle stimulating hormone and luetinising hormone are together referred to asgonadotrophic hormones or Gonadotropins.

Luteotrophic hormone (LTH):

This is also known as prolactin. This hormone stimulates growth of mammary glands during pregnancy and promotes lactation after delivery. Prolactin level rises during pregnancy and is very high during lactation.

Hormones of middle pituitary:

(i) Melanocyte stimulating hormone (MSH): This is the only hormone secreted by middle pituitary which controls the growth and development of melanocytes responsible for skin colour.

(ii) Hormones of posterior pituitary:

(A) Vasopressin or Antidiuretic hormone (ADH): This causes the re-absorption of water into the blood from the collecting tubules of the kidneys, thereby concentrating the urine and reducing its volume.

(B) Oxytocin: This hormone stimulates uterus contractions at the time of child birth and causes release of milk from mammary glands. It is also known as birth hormone or milk ejecting hormone.

Pineal Gland: It is a small gland reddish-grey in colour, about the size of a pea, attached to the roof of the third ventricle of the brain. It contributes in regulating gonadal development. It controls development & concentration of melanin.

  

Thyroid Gland:

A gland that makes and stores hormones that help regulate the heart rate, blood pressure, body temperature, and the rate at which food is converted into energy.Thyroid hormonesare essential for the function of every cell in the body. They help regulate growth and the rate of chemical reactions (metabolism) in the body. Thyroid hormones also help children grow and develop.

The thyroid gland is located in the lower part of the neck, below theAdam's apple, wrapped around thetrachea(windpipe). It has the shape of a butterfly: two wings (lobes) attached to one another by a middle part.

Two hormones secreted by the thyroid gland are:

(i) Thyroxine: It is the principal hormone secreted by the thyroid gland and its main role is to increase the metabolic rate of the organs and tissues of the whole body. 60% of thyroxine consists of iodine, an element which is essential for the gland to enable it to synthesize its hormone. The basal metabolic rate (B.M.R.) is increased in hyperthyroidism and reduced in hypothyroidism.

(ii) Calcitonin: This hormone lowers the calcium level in two ways:

(A) By inhibiting renal tubular calcium re-absorption.

(B) By inhibiting bone calcium re-absorption.

Hypothyroidism: This results from lack or deficiency of thyroid hormone secretion. It is manifested differently in children as compared with adults. Cretinism affects children and is due to congenital defect of either absence or defect of the gland. In this disease growth is stunted, the features are coarse, frequently the child has a protruding tongue and an enlarged abdomen; the mentality of the child is low and retarded. Myxoedema is the condition caused by thyroid deficiency in adults. It affects women more frequently than men. It is characterized by puffy face, thick skin, dry cough, cold and loss of hair. There is a deposition of mucin and fluid retention in extracellular spaces. BMR is lowered Iodine deficiency causes simple goitre.

 Hyperthyroidism : This results from excessive secretion and over action of thyroid hormones. An excessive amount of thyroxine is poured into the blood and the metabolism of the body is speeded up. The person starts losing weight, has an increased pulse rate, suffers from nervous excitement and there is protrusion of eye balls. These toxic signs and symptoms are responsible for the condition being known as toxic goitre. Other names are thyrotoxicosis, exophthalamicgoitre and grave's disease.

Parathyroid Glands:

The parathyroid glands are usually four small rounded bodies of the size like peas which lie embedded in the dorsal surface of the thyroid gland.

They secrete Parathormone. It is released when there is low blood calcium level.

Parathormone maintains blood calcium and phosphate level by mobilising bone calcium into the blood and by pre preventing loss of calcium in urine.

Removal of glands results in the death of the individual within a week due to disturbances caused in the calcium metabolism, which leads to muscular tremors, cramps and convulsion, a condition called tetany.

Thymus Gland:

Thethymusgland is an organ in the upper chest cavity that processes lymphocytes, a type of white blood cell that fights infections in the body. This organ is part of both the lymphatic system, which makes up a major part of the immune system, and the endocrine system, which includes all glands that produce hormones. Thethymusis most important in children and young adults, when it programs lymphocytes to attack antigens, like viruses. People who do not have this gland, or in whom it does not function correctly, usually have compromised immune systems and difficulty fighting disease.

Adrenal Glands:

These are two small semi-lunar structures lying one each on upper pole of the kidneys. That is why they are also known as supra renal glands. Each gland consists of two structurally and physiologically separate parts known as cortex and medulla. The cortex occupies outer peripheral portion which is yellowish in colour and medulla is inner brownish part. Cortex secretes three different kinds of hormones known as corticosteroids. They are:

(i) Mineralocorticoids : These regulate sodium and potassium balance in the body.

(ii) Glucocorticoids : These derive their name from their influence on carbohydrate metabolism. e.g. Glycogenesis is promoted in liver.

(iii) Sex hormones: Small quantities of sex hormones as androgens and oestrogen are produced by adrenal glands which influence sexual development and growth.

•  Adrenal medulla is important in raising defense mechanisms and supplementing sympathetic actions in the body. It secretes two hormones.

(i) Adrenaline: It is a stress hormone causes increase in systolic blood pressure, dilation of coronary blood vessels, increased sweating and increase in metabolic rate. It brings restlessness, muscle fatigue and anxiety.

(ii) Noradrenaline: It is a general vasoconstrictor, increases both systolic and diastolic pressures. Both of these hormones are helpful in emergency conditions. Thus are called as "fight or flight response".

Pancreas:

The pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as an exocrine gland, secreting pancreatic juice containing digestive enzymes that pass to the small intestine. These enzymes help in the further breakdown of the carbohydrates, protein, and fat in the chyme.

• Insulin is secreted by the beta cells and like other hormones, passes directly into the blood. Insulin is required to convert glucose into glycogen (glycogenesis) and store it in liver. Deficiency of insulin due to defect in islets of Langerhans results in diabetes mellitus, a condition in which blood glucose is high and is passed in the urine.
• The alpha cells of pancreas secrete glucagon, the metabolic effects of which are opposite to those of insulin. It causes the breakdown of liver glycogen, thereby releasing glucose into the blood stream.
• The third hormone somatostatin is secreted by the delta cells of the islets of Langerhans. It is able to inhibit the secretion of many hormones. As it inhibits the release of growth hormone of pituitary gland, it is also known as growth hormone release inhibiting hormone (GHRIH).

Ovaries:

Ovaries secrete three hormone:

(i) Oestrogen: FSH from the anterior pituitary controls the secretion of oestrogen by acting on the Graffian follicles. This hormone effects the development of female secondary sex characters. The oestrogen secretion influences the follicular phase. Its secretion is maximum during ovulation period. Moreover during pregnancy the oestrogen secretion by placenta keeps on increasing till full term.

(ii) Progesterone: It is secreted by corpus Iuteum. This hormone in contrast to estrogen which is produced continuously during the reproductive years is secreted only after ovulation. Progesterone prepares the uterus for receiving the embryo. It prepares inner lining of the uterus .i.e. endometrium to receive the implanting embryo for about a week. If ovum gets fertilized, the corpus luteum continues to play a role in maintaining the pregnancy for the first three months, after which the placenta takes over the role of corpus luteum by secreting progesterone itself. This hormone is essential for the maintenance of pregnancy and is therefore called pregnancy hormone. If pregnancy does not follow ovulation, corpus luteum degenerates and breaks down due to the lack of progesterone.

(iii) Relaxin : This hormone is secreted during later stages of pregnancy and leads to relaxation of muscles of the pelvic area to enable easy child birth and reduce the pressure on the foetus.

(iv) Testes: Testosterone is the main testicular hormone secreted by interstitial cells of the testis. It is mainly concerned with the development and maintenance of male sex characters and enhancing the process of spermatogenesis.