Endocrine system is collection of glands which secrete chemical messages known as hormones. Such signals are passed by blood to arrive at target organ that has cells containing suitable receptor. Exocrine glands (not part of endocrine system) secrete products which are passed outside body. Salivary glands, Sweat glands, and digestive glands are instances of exocrine glands.
Hormones are secretions from specific cells or glands in body which are carried by blood. Their effect is generated in one or more particular parts only. Many hormones are secreted by special glands known as endocrine glands meaning secrete internally. These are also known as ductless glands due to their secretions are poured directly in blood and not through ducts. Definite hormones are produced by other glands or body parts also, for instance, stomach and duodenum.
Nature and functions of hormones:
Hormone Secretors - Endocrine Glands:
In humans there are more than dozen tissues and organs which produce hormones. There are two categories;
Exclusively endocrine: pituitary, thyroid, parathyroid, thymus and adrenals.
Partially endocrine: Gastric, pancreas and duodenal epithelium, gonads (testis in males and ovary in females) and placenta in females
Pituitary gland (also known as hypothesis) is small projection (about size of pea) that hangs from base of mid-brain. It is linked to hypothalamus of brain by pituitary stalk. Hypothalamus, though part of brain, also secretes few hormones one of which is somatostatin that inhibits secretion of growth hormone from anterior pituitary.
Pituitary controls most other endocrine glands. It contains 2 distinct parts: anterior pituitary and posterior pituitary.
Thyroid is bilobed structure located in front region of neck. It secretes 2 hormones thyroxine and calcitonin. Thyroxine controls basal metabolism that is rate of cellular oxidation resulting in heat production. It manages growth and development, body temperature, mental development, ossification of bones, etc.
Undersecretion of thyroxine (hypothyroidism) generates 3 conditions; simple goitre. Swelling of thyroid visible as the puffiness in neck. It occurred becauses of iodine deficiency in food as iodine is required for production of thyroid hormones.
Cretinism: Poor body growth (dwarfism) and mental retardation.
Myxoedema: Enlargement of face and hands, general lethargy.
Oversecretion of thyroxine (hyperthyroidism) generates exophthalmic goitre. This form causes marked increase in metabolic rate, shortness of breath, rapid heartbeat, and eyes protrude out together with goitre in neck.
Calcitonin: It controls calcium and phosphate levels in blood. If calcium level in blood is high more calcitonin is secreted and calcium ions are moved from blood to bones making them harder. Reverse occurs when calcium level in blood is low making bones soft.
Parathyroids: These are 2 small pairs of glands entirely or partially embedded in thyroid gland. Their secretion parathormone elevates blood calcium level by stimulating release of calcium from bones.
Thymus: It is situated at base of neck. It generates some hormones engaged in maturation of T- lymphocytes. It starts to atrophy after puberty.
Adrenals: The adrenals are pair of glands located like caps one above every kidney. Every adrenal comprises of 2 parts: the central medulla and peripheral cortex.
Adrenal medulla secretes adrenaline that:
Adrenal cortex secretes 2 categories of hormones: glucocorticoids and Mineralocorticoids.
Gonads (testis and ovary):
Testes in males have 2 types of cells: sperm-producing germinal cells and hormone-producing interstitial cells. Hormones produced are known as androgens and commonest one among them is testosterone. Testosterone stimulates development of male characters during which body at puberty begins growing facial hair, and their voice cracks and deepens.
Ovaries in females generate two types of hormones - estrogen and progesterone.
Estrogen is secreted from follicles of ovary and stimulates growth of breasts and fat deposition on hip in mature woman. Estrogen creates wall of uterus for receiving fertilized egg.
Progesterone is secreted by corpus luteum (follicle left after release of ovum). It leads to final changes in uterus for retention and development of foetus in pregnancy.
Placenta of pregnant woman generates certain hormones. One such hormone is human chorionic gonadotropin (HCG) that maintains activity of corpus luteum in secreting progesterone constantly.
Hormones from stomach and intestine:
(i) Gastrin is hormone secreted by mucus membrane of pyloric end of stomach. It stimulates gastric glands to secrete gastric juice.
(ii) Secretin is hormone secreted by inner lining of duodenum. It stimulates production of pancreatic juice whereas hormone cholecystokinin stimulates release of bile from gall bladder.
Feedback Mechanism (Control of Hormonal Secretion):
Amount of hormone released by endocrine gland is determined by body's requirement for particular hormone at any given time. Product of target tissue exerts the effect on respective endocrine gland. This result may be positive (secrete more) or negative (secrete no more or slow down).
Feedback mechanism of thyroid activity Hypothalamus releases hormone TSH-RH (TSH- releasing hormone) that instructs anterior pituitary to release TSH (thyroid stimulating hormone). TSH stimulates thyroid to release thyroxine. If level of thyroxine in blood increases, pituitary stops release of TSH. If level of thyroxine turns into still higher, then inhibition of release of thyroxine happens not only at level of pituitary but also at level of hypophysis.
Pheromones - Chemical Messengers at Social Level:
Pheromones are secretions given out by individual in environment that causes specific response in other members of same species. Few examples of pheromones are given below:
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