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Chapter: 11th 12th standard bio zoology Human Body higher secondary school

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Thyroid gland and thyroxine

Thyroid gland consists of a pair of lobes, which lie one on either side of larynx in the neck region. The paired lobes are joined by a narrow ante-rior bridge of glandular tissue called Isthmus of the thyroid. The lobes of thyroid in turn are divided into many lobules. The lobules consist of follicles.

Thyroid gland and thyroxine

 

Thyroid gland consists of a pair of lobes, which lie one on either side of larynx in the neck region. The paired lobes are joined by a narrow ante-rior bridge of glandular tissue called Isthmus of the thyroid. The lobes of thyroid in turn are divided into many lobules. The lobules consist of follicles. The follicles are called acini (acinus - singular). Each acinus is lined with glandular cubical epithelial cells. The cavity of acinus is filled with a gelatinous material called colloid, which contains the thyroxine. The hormone thyroxine is an iodinated hormone. It contains 65% iodine. The amino acid residue in thyroxine is tyrosine.


Functions of thyroid:

 

Thyroxine stimulates normal growth and development, especially the skeletal and nervous systems. It controls the rate of cellular oxidation and increases the basal metabolic rate. The basal metabolic rate (BMR) is defined as the amount of heat produced in the body in a given time, in complete state of physical and mental rest at 20oC room temperature.

 

Actions of thyroxine:

 

(i) This hormone is very essential for the development of nervous system particularly at the time of birth and during the first year, (ii) This hormone increases the metabolism of all tissues except brain, gonads and accessory sex organs, lymph nodes, spleen and lungs, (iii) The most important function is to increase the absorption of glucose from the small intestine. (iv) This hormone reduces serum cholesterol level, (v) It promotes protein anabolism, and helps in growth, (vi) It increases heart beat rate, force of contraction and pulse pressure, (vii) Presence of optimum level of thyroxine in the blood maintains efficient muscle functions and (viii) The optimum level of thyroxine in the blood is also necessary for normal gonadal function.


Hypothyroidism:

 

The physiological effect due to deficiency of thyroid hormone is referred to as hypothyroidism. It is manifested by iodine deficiency and simple goiter, cretinism and myxoedema. If the dietary intake of iodine becomes inadequate (below 10 micro grams per day) the synthesis of thyroxine is impaired. As a result, the thyroxine level falls in circulation and secretion of TSH increases, causing the hypertrophy of thyroid gland as a consequence. The thyroid enlarges to enormous proportions. This is called simplegoiter. This condition is also called endemic goiter. It is caused due to lack of iodine in the soils of different regions of the world.

 

Cretinism:

 

Cretinism is found in children who are deficient of thyroxine hormone from the time of birth. The characteristics of cretinism are, retardation of mental growth to extreme degree, dwarf stature, protruding tongue and

 

abdomen, low basal metabolic rate, subnormal body temperature, retardation in skeletal growth and arrest of pubertal sexual maturity etc.

 

Myxoedema:

 

Myxoedema in adults, is a syndrome with the following characteristics viz., low BMR, dry, coarse, scaly skin, puffy and bloated face, coarse and sparse hair, hoarse voice, slow speech, slow thought processes, poor memory, etc. Other symptoms are muscular weakness and fatigue, low blood pressure, anaemia with increased serum cholesterol, etc.

 

Hyperthyroidism or thyrotoxicosis (Grave's disease or exophthalmic goiter):

 

The hyper function of thyroid gland results in Grave's disease. Grave's disease is characterized by increased BMR with increased pulmonary ventilation, protrusion of eye balls from the sockets (exophthalmas), increased heart beat rate, nervousness, emotional instability, weight loss, increased blood glucose and decreased serum cholesterol, derangement of sexual function etc.

 

Parathyroid gland

 

In man the parathyroid glands are small oval yellow-brown bodies found attached to the posterior surface of the thyroid gland. The parathyroid glands secrete two hormones namely, 1. Parathormone and 2. Calcitonin.

 

1. Parathormone:

 

Purified parathormone is a simple polypeptide chain. It has short duration of biological activity. The half-life of the hormone is of about 20-30 minutes only.

 

Physiological effects of Parathormone:

 

Parathormone manifests its regulatory effects at three different loci in the body namely the skeleton, kidneys and the gastro intestinal tract. In skeleton, the hormone directly acts upon the bone tissue to stimulate the activity of osteoclast cells (bone destroying cells). Under the influence of this hormone calcium is released from the bone matrix into the circulation. As a result plasma calcium level increases. Thus it helps in the skeletal remodelling.

In kidney, parathormone induces a marked increase in phosphate excretion. In the gastro intestinal tract, parathormone stimulates the absorption of calcium and phosphate from the gut by enhancing the vitamin D synthesis. As a result, more phosphate and calcium are transported into the blood stream. Its other physiological effects include its inhibitory action over the osteoblasts or bone forming cells, bicarbonate reabsorption and reduction of urine pH, etc.

 

2. Calcitonin:

 

It is a calcium-lowering hormone secreted by the parafollicular cells of the parathyroid gland. It is a protein. Its physiological effects are antagonistic to that of parathormone. It inhibits bone resorption. In kidney, it inhibits the reabsorption of calcium, phosphorus, sodium, potassium, magnesium and other ions. It decreases gastric HCl secretion. It also decreases the insulin and glucagon secretion.

 

Hyperparathyroidism

 

It is a condition where there is an increased amount of parathyroid hormone in circulation. Excess secretion of parathormone brings about demineralization of the bones. The protein matrix of the bone is also absorbed. These changes result in bone cyst and the elevation of the calcium level in the blood. The latter causes calcification of kidneys, arteries, stomach and lungs.

 

Hypoparathyroidism

 

Removal of parathyroids causes the blood calcium levels to fall and results in tetany. Tetany is characterized by low serum calcium (Hypocalcemea), reduced urinary excretion of calcium and phosphate, neuromuscular hyperexcitability, spasms of muscles and cramps etc.

 

Pancreas

 

The endocrine part of the pancreas consists of specialized groups of cells known as Islets of Langerhans. These cells synthesize, store and secrete two hormones namely insulin and glucagon. There are two kinds of cells namely, alpha and beta cells. The alpha cells produce glucagon while the beta cells secrete insulin. In addition to alpha and beta cells another type of cells called delta cells are present in human pancreas. According to some investigators the delta cells represent the transitional forms of the two cell types alpha and beta.

 

Insulin:

 

Insulin is a protein hormone or a polypeptide hormone with 51 amino acid residues. Human insulin has a molecular weight of 5,734 daltons. It consists of two chains A and B, which are linked together by disulphide bridges formed between two cystine residues.

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