Chemical Control and Coordination Handwritten Notes
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Endocrine Glands
Endocrine glands are ductless glands. Their secretions are referred to as hormones. Hormones have been defined as non-nutrient chemicals which are produced in small amounts and are intracellular messengers. Hormones only stimulate metabolic reactions, they do not take part in such reactions.
Human Endocrine System
The location of the endocrine glands is in different parts of the human body. Different endocrine glands are the pituitary, pineal, thyroid, parathyroid, thymus, adrenal, and gonads. The kidney, liver, gastrointestinal tract also produces some hormones.
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The Hypothalamus
The location of the hypothalamus is at the base of the forebrain. The group of secretory cells present in the hypothalamus is known as nuclei that produce hormones. This regulates the synthesis and secretion of pituitary hormones are regulated by the hormones secreted by the hypothalamus. The hormones produced from the hypothalamus are of two types-
(i) the releasing hormones
(ii) the inhibiting hormones.
For example, the gonadotropin releasing hormone stimulates the release of gonadotropins from the pituitary. These hormones reach the pituitary gland with the assistance of the portal circulatory system.
The Pituitary Gland
The location of the pituitary gland is in the cavity known as sella turcica which is attached to the hypothalamus via a stalk. It has two divisions as adenohypophysis or anterior pituitary and neurohypophysis or posterior pituitary. Hormones such as adrenocorticotropic hormone, growth hormone, thyroid-stimulating hormone melanocyte-stimulating hormone, prolactin, follicle-stimulating hormone, and luteinizing hormone are secreted by the adenohypophysis. The release of Vasopressin and oxytocin are by neurohypophysis.
The Pineal Gland
The location of the pineal gland is on the dorsal side of the forebrain. The pineal gland secretes the melatonin hormone. The melatonin hormone regulates 24 hours rhythm in the body like body temperature, sleep-wake cycle etc. Along with the sleep-wake cycle, melatonin also controls:
(i) metabolism
(ii) pigmentation
(iii) menstrual cycle
(iv) defense capability
Thyroid Gland
Thyroid glands are located on either side of and anterior to the trachea. An isthmus connects the bilobed gland. The lobules of the thyroid gland are made up of follicles and thyroid follicles bound together by connective tissue are called stromal tissues. Thyroid follicles are composed of follicular cells that produce two hormones- tetraiodothyronine or thyroxine (T4) and triiodothyronine (T3). A vital element required for thyroid hormone synthesis is Iodine. Deficiency of iodine will lead to hypothyroidism, called Goitre. During pregnancy, hypothyroidism affects the growth of the baby, its mental condition also gets affected. There will be a low intelligence quotient, abnormal skin, deaf. For adult women, there will be an irregular menstrual cycle in hypothyroidism. Another condition is when there is an increase in the secretion of thyroid hormone, it will be called hyperthyroidism. A form of hyperthyroidism is Exophthalmic goiter which is an enlargement of the thyroid gland, protrusion of the eyeballs, increased basal metabolic rate, and weight loss also referred to as Graves’ disease.
Parathyroid Gland
Normally, there are four parathyroid glands in the human being, one superior and one inferior located immediately behind each upper and lower pole of the thyroid. A peptide hormone is secreted by parathyroid called parathyroid hormone. It has the following effects:
(i) Maintenance of proper calcium and phosphate level in blood.
(ii) Role in bone formation.
(iii) Excretion of calcium and phosphate.
(iv) Effect on intestinal absorption of calcium, phosphate, and vitamin D.
Thymus
The location of the thymus is in between the lungs behind the sternum. An important role is played by them in the development of the immune system. The hormone secreted by this gland is called thymosin. It participates in cell-mediated immunity. It also participates in the production of antibodies.
Adrenal gland
It is a pair of glands located in the anterior part of the kidney. It is composed of two types of tissues- the outer adrenal cortex and the inner medulla. The adrenal medulla produces two hormones known as adrenaline and noradrenaline. Adrenaline is also known as an emergency hormone as it is secreted at the time of fear, fight, or flight. Adrenaline dilates the pupil, increases alertness, sweating, etc.
The 3 layers of adrenal cortex are-
(i) zona reticularis (inner layer),
(ii) zona fasciculata (middle layer), and
(iii) zona glomerulosa (outer layer).
Hormones of the adrenal cortex are commonly known as corticoids. Corticoids which participate in carbohydrate metabolism are referred to as glucocorticoids. They stimulate proteolysis, lipolysis, and gluconeogenesis. The glucocorticoid includes cortisol, cortisone in which cortisol accounts for 95% of glucocorticoid activity which helps in anti-inflammatory reactions and suppresses the immune system.
Corticoids that help in regulating the level of water and electrolyte balance are referred to as mineralocorticoids. Aldosterone is the main hormone of mineralocorticoids. It helps in the reabsorption of sodium ions and water and the excretion of potassium and phosphate ions.
Pancreas
The pancreas is considered to be a dual gland, i.e., it is endocrine as well as exocrine in function. The endocrine part of the pancreas consists of islets of Langerhans that are mainly made up of two types of cells- alpha cells and beta cells. Alpha cells secrete hormones known as glucagon whereas beta cells secrete insulin. Glucagon maintains normal blood glucose. It increases blood glucose levels. It also promotes gluconeogenesis. So, glucagon is a hyperglycemic hormone.
Insulin also regulates glucose levels in our bodies. It enhances cellular glucose uptake thus decreases blood glucose levels. So, it is known as a hypoglycemic hormone. Prolonged hyperglycemia causes diabetes mellitus.
Testes
The location of testes is in the scrotal sacs present outside the abdominal cavity. They are the primary sex organ as well as endocrine in function. The testis includes seminiferous tubules and interstitial cells. Interstitial cells or Leydig cells secrete hormones known as testosterone. This hormone regulates spermatogenesis, the development of beards and mustaches, as well as the maturation of male accessory sex organs.
Ovary
The paired ovaries lie in the lower pelvic region of the abdominal cavity. It is one of the primary sex organs. Ovaries are mainly concerned with the production of ova or eggs and they also secrete two hormones, known as estrogen and progesterone.
Estrogen Helps in:
(i) development of accessory sex organs.
(ii) development of mammary glands.
(iii) menstrual cycle.
(iv) ovulation.
(v) water balance, by increasing water content and thickness of the skin.
Progesterone is responsible for changes during pregnancy hence, called pregnancy hormones.
Progesterone Helps in:
(i) enhancing the blood supply of the wall of the uterus.
(ii) placentation.
(iii) development of breast during pregnancy.
(iv) helps in premenstrual changes in the uterus.
(v) also helps in the promotion of retention of water and sodium salts by affecting kidney functions.
Hormones of Kidney, Gastrointestinal Tract, and Kidney
Heart wall secretes a hormone referred to as atrial natriuretic factor (ANF) which decreases the blood pressure. ANF is released when blood pressure increases.
The hormone secreted by kidney cells is called erythropoietin that helps in the promotion of RBC formation.
Gastrin, Secretin, Cholecystokinin, and Gastric inhibitory peptides are secreted by the gastrointestinal tract.
Mechanism of Hormone Action
The function of different hormones is to control and regulate activity levels of the target tissue. The binding of hormones to hormone-receptor produces their effects on target tissues. The hormone-receptor complex formed will result in certain biochemical changes within the target tissue. There will be the interaction of hormones with the membrane receptors as well as intracellular receptors. Steroid receptors are generally intracellular in nature whereas peptide hormones generally bind to membrane receptors.
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