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Chapter 6 Endocrine System NBF Short Notes
Hormones are small, soluble organic molecules that serve as chemical messengers in the body, effective in very low concentrations. Their primary function is to transport a signal from one cell to another, affecting specific target cells that possess unique receptors. This interaction is often described by the "lock and key" model, where the hormone is the "key" and the receptor is the "lock."
Hormones facilitate two main types of communication:
- Gland-to-Gland: A hormone from one endocrine gland stimulates another endocrine gland. For example, the pituitary gland releases thyroid-stimulating hormone (TSH), which then stimulates the thyroid gland.
- Gland-to-Organ: A hormone from an endocrine gland acts on a specific target organ. For example, the pancreas releases insulin, which acts on muscles and the liver.
Hormones are classified by their chemical nature:
- Steroid hormones: Made of steroids; include cortisol, aldosterone, testosterone, estrogen, and progesterone.
- Proteinaceous/Polypeptide hormones: Include somatotrophic, thyrotrophic, and gonadotrophic hormones from the anterior pituitary, as well as insulin.
- Catecholamines: Adrenaline and noradrenaline, secreted by the adrenal medulla.
- Amino acid derivative: Thyroxine, secreted by the thyroid gland.
- Peptide hormones: Include melanocyte-stimulating hormone, oxytocin, vasopressin, adrenocorticotrophic hormone, calcitonin, and parathormone.
The Pituitary Gland
The pituitary gland is located below the hypothalamus and connected to it by the infundibulum. It is divided into three lobes: anterior, posterior, and median.
Anterior Lobe of Pituitary (Master Gland)
The anterior lobe secretes hormones that regulate other endocrine glands. Key hormones include:
- Growth hormone (GH) / Somatotrophic hormone (STH): Regulated by growth hormone-releasing factor (GHRF) and somatostatin. GH affects cell growth, division, and protein synthesis.
- Dwarfism: Caused by a deficiency of GH.
- Gigantism: Caused by over-secretion of GH in childhood.
- Acromegaly: Caused by over-secretion of GH in adults, leading to the enlargement of hands, feet, and jawbones.
- Thyroid-stimulating hormone (TSH): Stimulates the synthesis and release of thyroxine from the thyroid gland.
- Adrenocorticotrophic hormone (ACTH): Stimulates the adrenal cortex to secrete hormones like cortisol and aldosterone.
- Gonadotrophic hormones (FSH, LH, Prolactin): Act on the reproductive system.
Median Lobe of Pituitary
This lobe produces melanocyte-stimulating hormone (MSH), which stimulates melanocyte production and melanin release in the skin.
Posterior Lobe of Pituitary Gland
The posterior pituitary is not a gland and does not produce hormones. It stores and releases two hormones that are synthesized in the hypothalamus.
- Antidiuretic hormone (ADH) / Vasopressin: Released in response to dehydration and low blood pressure. It promotes water reabsorption in the kidneys.
- Disorder: Under-secretion causes diabetes insipidus, leading to excessive production of diluted urine.
- Oxytocin: Released during childbirth and nursing. It causes uterine contractions during labor and promotes milk ejection during lactation. Over-secretion can lead to the rupture of the uterine wall.
The Thyroid Gland
The thyroid gland is located on either side of the trachea and produces Triiodothyronine (T3), Thyroxine (T4), and Calcitonin. The production of T3 and T4 is stimulated by TSH from the anterior pituitary.
Functions of Thyroid Hormones (T3 and T4)
- Promote the basal metabolic rate.
- Enhance glucose catabolism and cholesterol synthesis.
- Promote the development of the nervous system and muscles.
- Aid in growth and skeletal maturation.
- Promote normal gastrointestinal tract motility.
Thyroid Disorders
- Hyperthyroidism: Caused by excessive secretion of T3 and T4, which can lead to Graves' disease.
- Hypothyroidism: Caused by under-secretion of thyroxin (T4).
- In adults, severe hypothyroidism is called myxedema, characterized by a low metabolic rate, coldness, puffy eyes, and an enlarged thyroid gland (goiter). It can be caused by iodine deficiency.
- Congenital hypothyroidism is called cretinism, leading to mental retardation and poor physical growth.
Calcitonin
Produced by the thyroid's C-cells, calcitonin is stimulated by high blood calcium levels. Its functions include:
- Increasing the deposition of calcium into bones.
- Inhibiting calcium absorption by the intestines.
- Decreasing calcium reabsorption by kidney tubules.
Parathyroid Glands: Short Notes
These four small glands are located on the posterior surface of the thyroid gland. Their main hormone is parathormone, which regulates calcium balance in the blood. It is released when blood calcium levels are low and works antagonistically to calcitonin.
Conditions of Abnormal Secretion
- Over-secretion (Hypersecretion): Causes hypercalcemia (elevated blood calcium), which leads to the release of calcium from bones, making them weak and prone to fractures. It can also cause kidney stones.
- Under-secretion (Hyposecretion): Causes hypocalcemia (low blood calcium), which increases the excitability of neurons and can lead to tetany, a condition of prolonged muscle contraction.
The Pancreas
The pancreas has both exocrine and endocrine functions. Its endocrine tissue, organized into Islets of Langerhans, contains two main cell types:
- Alpha (α) cells: Secrete glucagon when blood glucose is low to increase blood sugar levels.
- Beta (β) cells: Secrete insulin when blood glucose is high to lower blood sugar by promoting glucose uptake and conversion to glycogen.
Diabetes Mellitus
This metabolic disease is caused by insufficient insulin secretion, leading to excessively high blood glucose levels in the blood and urine.
The Adrenal Glands
The adrenal glands sit on top of the kidneys and are divided into two parts:
- Medulla (inner portion): Produces epinephrine (adrenaline) and norepinephrine, which are released during stressful situations. They increase metabolic activity, heart rate, and blood pressure.
- Cortex (outer portion): Produces:
- Glucocorticoids (e.g., cortisol): Regulate blood glucose levels.
- Mineralocorticoids (e.g., aldosterone): Regulate mineral levels.
- Androgens (e.g., testosterone): Produced in small amounts.
The Gonads
Gonads are special endocrine glands that produce both hormones and gametes.
Ovaries
The female gonads secrete estrogen and progesterone.
- Estrogen: Responsible for female secondary sex characteristics and uterine wall repair.
- Under-secretion can cause a failure of sexual maturation.
- Over-secretion may lead to fibroids and polycystic ovaries.
- Progesterone: Produced by the corpus luteum and placenta. It causes the uterine wall to thicken for pregnancy, suppresses ovulation, and inhibits FSH secretion.
- Low levels can lead to miscarriage.
Testosterone
Produced by the testes in males, it is secreted by interstitial cells. Testosterone stimulates the maturation of male reproductive organs and the development of secondary sex characteristics. It is essential for sperm production.
- Low levels can cause the development of female characteristics and male sterility.
The Hypothalamus
The hypothalamus, a part of the brain, is the master control center of the endocrine system. It regulates essential body functions like hunger, thirst, and temperature. It contains neurosecretory cells that produce and release two types of hormones:
- Releasing factors: Stimulate the pituitary gland.
- Inhibiting factors: Inhibit the pituitary gland.
It also produces ADH and oxytocin, which are stored and released by the posterior pituitary gland.
Feedback Mechanisms
A feedback mechanism is a type of interaction where a controlling mechanism is regulated by the product of the reactions it controls.
Positive Feedback
An end product speeds up its own production. These responses are not homeostatic and are rare. A classic example is childbirth, where the baby's head pushing on the cervix stimulates the release of more oxytocin, which in turn causes stronger contractions, leading to the release of even more oxytocin.
Negative Feedback
In a negative feedback loop, the end product reverses the direction of change to maintain homeostasis.
- Example: Blood Glucose Control:
- High Blood Glucose: Beta cells release insulin, which makes cells more permeable to glucose and converts glucose to glycogen, lowering blood sugar.
- Low Blood Glucose: Alpha cells secrete glucagon, which converts glycogen back to glucose, raising blood sugar levels.
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