Endocrine System - Notes, Biology | General Awareness - Bank Exams PDF Download

Endocrine System

The endocrine system is formed by endocrine (ductless) glands and the hormones they secrete. These glands release chemical messengers called hormones directly into the bloodstream in response to signals from the nervous system or feedback from other hormones. The blood carries hormones to target organs and tissues where they regulate growth, metabolism, reproduction and homeostasis.

• Endocrine glands are ductless and secrete hormones directly into the blood.
• Hormones act as chemical signals and produce specific responses in target cells bearing appropriate receptors.
• The brain (especially the hypothalamus and pituitary) provides central control for many endocrine functions.

Endocrine System

Hypothalamus

The hypothalamus is a region of the forebrain that links the nervous system to the endocrine system. It secretes neurohormones (releasing and inhibiting factors) that regulate secretion from the pituitary gland and thereby control many endocrine functions.

• The hypothalamus produces neurohormones that stimulate or inhibit the anterior pituitary and also synthesises hormones (oxytocin and antidiuretic hormone) that are stored in and released from the posterior pituitary.
• It helps maintain homeostasis by regulating temperature, hunger, thirst and circadian rhythms-often described as acting like the body's thermostat.

Pituitary Gland

The pituitary gland (often called the "master gland") is a small endocrine gland located at the base of the brain. It has two major parts with distinct functions: the anterior pituitary (adenohypophysis) and the posterior pituitary (neurohypophysis).

• The pituitary is the smallest endocrine gland but controls many other endocrine glands through its hormones.
• Hormones secreted by the pituitary:
• Somatotrophic Hormone (Growth Hormone, GH). Controls general growth of the body. Hypersecretion in children causes gigantism; hypersecretion in adults causes acromegaly. Hyposecretion in children causes dwarfism.
• Gonadotrophic Hormones (GTH). These include follicle-stimulating hormone (FSH) and luteinising hormone (LH); they stimulate the gonads (ovaries and testes) to produce sex cells and sex hormones.
• Lactogenic Hormone (Prolactin). Initiates and maintains milk production in nursing females.
• Thyrotrophic Hormone (TSH). Stimulates the thyroid gland to synthesise and release thyroid hormones.
• Adrenocorticotrophic Hormone (ACTH). Stimulates the adrenal cortex to produce steroid hormones.
• Diabetogenic (Metabolic) Hormone. A term used historically for hormones (notably GH) that influence carbohydrate, fat and protein metabolism and can raise blood glucose.
• Oxytocin (Pitocin). Stimulates smooth muscle contraction in the uterus during childbirth and enhances milk ejection from the mammary glands.
• Vasopressin (Antidiuretic Hormone, ADH). Regulates water reabsorption in kidney tubules. Hypoactivity of ADH causes diabetes insipidus, a condition with excretion of large volumes of dilute urine. ADH also causes constriction of small arteries, increasing blood pressure.

Thyroid Gland

The thyroid gland is the largest endocrine gland located in the neck, anterior to the trachea and below the larynx. It plays a central role in regulating basal metabolic rate and development.

• The thyroid controls basal metabolic rate (BMR), which is the minimum energy required to maintain vital functions at rest; for an average adult this is about 1600 Kcal/day.
• Major hormones from the thyroid:
• Thyroxine (T4). Regulates physical, mental and sexual development and overall metabolic rate. Hypoactivity in children causes cretinism. Hypoactivity in adults causes myxoedema (sluggishness, mental slowness and low body temperature). Insufficient dietary iodine (required for thyroxine synthesis) can lead to simple goitre. Hyperactivity increases metabolic activity and may produce nervousness and irritability.
• Calcitonin. Secreted by parafollicular cells (C cells) of the thyroid; helps lower blood calcium levels by inhibiting bone resorption.

Pancreas

The pancreas has both exocrine (digestive enzyme secretion) and endocrine functions. The endocrine portion is organised in clusters known as the islets of Langerhans.

• The islets contain distinct cell types that secrete hormones: beta cells produce insulin, alpha cells produce glucagon, and delta (gamma) cells produce somatostatin.
• Insulin lowers blood glucose by promoting uptake and utilisation of glucose in cells and storage as glycogen; hyposecretion or inactivity of insulin leads to diabetes mellitus.
• Glucagon raises blood glucose by stimulating glycogenolysis and gluconeogenesis in the liver.
• Somatostatin (growth hormone-inhibiting hormone) regulates and inhibits secretion of insulin and glucagon and moderates digestive processes.

Adrenal Glands

There are two adrenal glands, one on top of each kidney. Each adrenal gland has two distinct regions: an outer cortex and an inner medulla. The adrenal gland is sometimes called the "4S gland" referring to sugar metabolism, salt retention, sex hormones and source of energy.

• Adrenal medulla. Secretes catecholamines such as adrenaline (epinephrine) and noradrenaline (nor-epinephrine). These hormones mobilise energy reserves (increase blood glucose), raise heart rate and respiration, stimulate sweating and other changes associated with the sympathetic "fight or flight" response. The medulla is often referred to as the "3F gland" (fear, fight, flight). Noradrenaline is secreted in smaller proportion (about 20%) and has actions broadly similar to adrenaline with some differences in vascular effects.
• Adrenal cortex. Produces steroid hormones grouped into:
  • Mineralocorticoids (e.g., aldosterone). Maintain electrolyte balance by promoting sodium and chloride retention and potassium excretion; influence body water distribution. Hyposecretion causes Addison's disease with excessive loss of Na+ and Cl- and raised blood K+; hypersecretion causes sodium and water retention.
  • Glucocorticoids (e.g., cortisol). Affect carbohydrate, protein and fat metabolism; help maintain blood glucose during fasting and modulate stress responses and inflammation. Hyposecretion can lower blood glucose and reduce body temperature; hypersecretion may lead to Cushing's syndrome, characterised by abnormal fat redistribution, muscle wasting and high blood glucose.

Gonads (Sex Glands)

The gonads-testes in males and ovaries in females-produce gametes and secrete sex hormones that control development of primary and secondary sexual characteristics and reproduction.

Testes

• Interstitial (Leydig) cells of the testes secrete testosterone, which is responsible for male secondary sexual characteristics such as facial and body hair, deepening of the voice, increased muscle mass, libido and stimulation of sperm formation.
• Deficiency of testosterone can lead to underdevelopment of secondary sexual characters and sterility; severe deficiency is historically referred to as eunuchoidism.

Ovaries

• Ovarian follicles and the corpus luteum secrete female sex hormones.
• Oestrogens. Produced by developing follicles; control female secondary sexual characters, including breast development, broadening of the pelvis, growth of the uterus and vagina, pubic hair and the onset of menstrual cycles.
• Progesterone. Secreted primarily by the corpus luteum after ovulation; essential for preparation and maintenance of the uterine lining for implantation and for maintaining pregnancy in early stages. Progesterone also contributes to breast development for lactation.
• Relaxin. Secreted by the corpus luteum (and later by the placenta) near the end of gestation; it relaxes uterine musculature and pelvic ligaments to facilitate childbirth.

Control of Hormone Secretion

Hormone secretion is controlled by neural input, chemical changes in the blood and feedback from other hormones. The most common regulatory mechanism is negative feedback, where an increase in the level or effect of a hormone inhibits further secretion of that hormone or of the tropic hormone that stimulates it.

• The hypothalamic-pituitary axis is a central regulatory pathway: hypothalamic releasing and inhibiting hormones regulate pituitary secretion; pituitary hormones stimulate peripheral endocrine glands; hormones from peripheral glands feed back to the pituitary and hypothalamus.
• Examples: High thyroid hormone levels feedback to reduce TSH release; high cortisol suppresses ACTH release.

Common Endocrine Disorders

• Diabetes mellitus - due to deficiency or lack of action of insulin; characterised by hyperglycaemia, glycosuria and altered metabolism of fats and proteins.
• Diabetes insipidus - due to deficiency of ADH or renal response to ADH; characterised by excretion of large volumes of dilute urine.
• Goitre - enlargement of the thyroid, often due to iodine deficiency impairing thyroxine synthesis.
• Cretinism - severe hypothyroidism in children causing stunted physical and mental development.
• Myxoedema - hypothyroidism in adults with sluggishness and lowered body temperature.
• Acromegaly and gigantism - results of excess growth hormone in adults (acromegaly) and children (gigantism).
• Addison's disease - adrenal cortical insufficiency causing salt loss and weakness.
• Cushing's syndrome - results from excess glucocorticoids causing abnormal fat distribution and high blood glucose.

Clinical and Practical Relevance

• Understanding hormone function is essential for diagnosis and treatment of endocrine diseases using hormone assays, hormone replacement therapies, drugs that modify hormone action and, where necessary, surgical interventions.
• Hormonal balance affects growth, development, metabolism, fertility and stress responses-areas relevant to public health and clinical practice.
• Basic knowledge of endocrine function is important for professionals in allied fields (medical diagnostics, biomedical engineering, pharmacology) and for general awareness of health and disease.

Summary

The endocrine system is a network of ductless glands that secrete hormones into the bloodstream to regulate body functions. Major glands include the hypothalamus and pituitary (central regulators), the thyroid, pancreas, adrenals and gonads. Hormones act through specific receptors and are controlled mainly by negative feedback. Disorders arise from hormone excess, deficiency or resistance and have wide-ranging effects on health.