The endocrine system is the second great coordinating system of the body, complementing the nervous system. While the nervous system transmits fast, point-to-point signals, the endocrine system provides slow, prolonged chemical coordination through hormones. Hormones are non-nutrient chemicals, produced in small quantities by endocrine glands, which are ductless glands that secrete their products directly into the bloodstream. The hormones travel through the blood to their target organs or tissues, where they regulate metabolic activities, growth, development, reproduction and homeostasis.
The endocrine glands are called ductless glands because they lack ducts and release their secretions directly into the blood. There are also exocrine glands, which have ducts and secrete their products onto body surfaces or into cavities (e.g., sweat glands, salivary glands). Some organs, like the pancreas, are mixed glands, having both endocrine and exocrine functions. This chapter describes the major endocrine glands, their hormones, the functions of each hormone, the mechanism of hormone action, and the disorders resulting from hormonal imbalances.
The hypothalamus is located in the forebrain and is the coordinating link between the nervous and endocrine systems. It secretes several hormones. It produces two important hormones, oxytocin and vasopressin (ADH), which are stored and released by the posterior pituitary. The hypothalamus also secretes releasing and inhibiting hormones that regulate the anterior pituitary. For example, GnRH (gonadotropin releasing hormone) stimulates the pituitary to release gonadotropins, and somatostatin inhibits the release of growth hormone.
The pituitary gland is a small, pea-sized gland located in a depression of the sphenoid bone called the sella turcica. It is attached to the hypothalamus by the infundibulum. It is divided into the adenohypophysis (anterior pituitary) and the neurohypophysis (posterior pituitary).
The anterior pituitary secretes six major hormones: - Growth hormone (GH or somatotropin): Promotes the growth of the body tissues, especially bones and muscles. Hyposecretion in childhood causes dwarfism, while hypersecretion causes gigantism; hypersecretion in adults causes acromegaly. - Prolactin (PRL): Regulates the growth of the mammary glands and milk production after childbirth. - Thyroid stimulating hormone (TSH): Stimulates the thyroid gland to synthesise and secrete thyroxine. - Adrenocorticotropic hormone (ACTH): Stimulates the adrenal cortex to secrete cortisol and other glucocorticoids. - Luteinizing hormone (LH) and Follicle stimulating hormone (FSH): The gonadotropins, which regulate the activity of the gonads (ovaries and testes).
The posterior pituitary stores and releases oxytocin (which stimulates the contraction of the uterus during childbirth and milk ejection) and vasopressin or antidiuretic hormone (ADH) (which promotes the reabsorption of water in the kidneys).
The pineal gland is located on the dorsal side of the forebrain. It secretes melatonin, which regulates the 24-hour (diurnal) rhythm of the body, including the sleep-wake cycle. Melatonin also influences the development of gonads in seasonal breeders.
The thyroid gland is composed of two lobes located on either side of the trachea, connected by an isthmus. It secretes three hormones: thyroxine (T4), triiodothyronine (T3) and calcitonin. The thyroid hormones are iodine-containing hormones and are essential for the regulation of the basal metabolic rate (BMR), carbohydrate, protein and fat metabolism, and the growth and development of the body. Iodine is essential for the synthesis of T3 and T4; a deficiency of iodine causes goitre. Hypothyroidism in children causes cretinism (mental and physical retardation), while in adults it causes myxoedema. Hyperthyroidism causes exophthalmic goitre (Graves' disease), characterised by an increased BMR and bulging eyes. Calcitonin lowers the blood calcium level.
The parathyroid glands are four small glands embedded in the thyroid gland. They secrete parathyroid hormone (PTH), which increases the blood calcium level. PTH stimulates the osteoclasts to break down bone and release calcium, and it increases the reabsorption of calcium in the kidneys. Thus, PTH and calcitonin have opposite effects and together regulate the calcium balance of the body.
The thymus is located behind the sternum, between the lungs. It secretes thymosin, which plays a major role in the differentiation of T-lymphocytes (T cells), providing cell-mediated immunity. The thymus is large in children and shrinks after puberty.
The adrenal glands are located on the top of each kidney, and each gland has an outer cortex and an inner medulla.
The adrenal cortex secretes: - Glucocorticoids (e.g., cortisol): Regulate carbohydrate, protein and fat metabolism, and provide resistance to stress. Cortisol is called the anti-inflammatory hormone. Hyposecretion causes Addison's disease, characterised by low blood pressure, fatigue and skin pigmentation. - Mineralocorticoids (e.g., aldosterone): Regulate the balance of sodium, potassium and water in the body. Aldosterone increases the reabsorption of sodium and water. - Androgens and small amounts of oestrogens: Androgens regulate the development of male secondary sexual characteristics.
The adrenal medulla secretes adrenaline (epinephrine) and noradrenaline (norepinephrine), which are together called catecholamines. These are secreted in response to stress and emergency situations (fight or flight response). They increase the heart rate, blood pressure, blood glucose level and the rate of respiration, and they cause the constriction of the skin blood vessels.
The pancreas is a mixed gland, with an exocrine part (secretes digestive enzymes) and an endocrine part. The endocrine part consists of the islets of Langerhans, which contain alpha cells that secrete glucagon and beta cells that secrete insulin. - Glucagon: Raises the blood glucose level by stimulating the conversion of glycogen to glucose (glycogenolysis) and the synthesis of glucose from amino acids. - Insulin: Lowers the blood glucose level by stimulating the uptake of glucose by cells and its conversion to glycogen (glycogenesis). A deficiency of insulin or its receptors causes diabetes mellitus, characterised by hyperglycaemia, glycosuria and polyuria.
Hormones act at very low concentrations (10^-8 to 10^-12 M). Hormones that bind to intracellular receptors (lipid-soluble hormones like steroid hormones and thyroid hormones) enter the cell, bind to receptors, and act on the DNA to regulate gene expression and protein synthesis. Hormones that bind to cell surface receptors (protein, peptide and amino-acid-derived hormones) act through second messengers such as cyclic AMP (cAMP), calcium ions or inositol triphosphate, which trigger the cellular response without entering the cell.
In addition to the classic endocrine glands, the heart, kidney and gastrointestinal tract also secrete hormones. The heart secretes ANF, the kidneys secrete erythropoietin, and the GI tract secretes gastrin, secretin and cholecystokinin. These hormones are important for maintaining blood pressure, red blood cell production and digestion respectively.
| Hormone | Target | Function |
|---|---|---|
| GH | Tissues | Growth |
| Prolactin | Mammary glands | Milk production |
| TSH | Thyroid | Thyroxine secretion |
| ACTH | Adrenal cortex | Cortisol secretion |
| LH, FSH | Gonads | Gamete and hormone production |
| Oxytocin | Uterus, mammary | Contraction, milk ejection |
| ADH | Kidneys | Water reabsorption |
| Region | Hormone | Function |
|---|---|---|
| Cortex | Glucocorticoids (cortisol) | Metabolism, stress resistance |
| Cortex | Mineralocorticoids (aldosterone) | Na+, K+, water balance |
| Cortex | Androgens | Secondary sexual characters |
| Medulla | Adrenaline | Fight or flight |
| Medulla | Noradrenaline | Fight or flight |
| Disorder | Cause |
|---|---|
| Dwarfism | GH hyposecretion in childhood |
| Gigantism | GH hypersecretion in childhood |
| Acromegaly | GH hypersecretion in adults |
| Goitre | Iodine deficiency |
| Cretinism | Hypothyroidism in children |
| Myxoedema | Hypothyroidism in adults |
| Addison's disease | Adrenal cortex hyposecretion |
| Diabetes mellitus | Insulin deficiency |
Chemical coordination and integration complete the picture of how the human body maintains homeostasis, complementing the rapid nervous system with sustained hormonal control. The endocrine system, from the hypothalamus and pituitary through the thyroid, parathyroid, thymus, adrenals, pancreas and gonads, produces a remarkably diverse array of hormones that regulate growth, metabolism, reproduction, stress responses and the internal environment. The precise regulation of blood glucose by insulin and glucagon, of blood calcium by PTH and calcitonin, and of blood pressure by ANF and the renin-angiotensin system exemplifies the exquisite balance of hormonal control. Understanding hormone action and the disorders caused by their imbalances provides the foundation for endocrinology and medicine. With this chapter, the student gains a complete, integrated view of the physiological systems of the human body, from the cellular to the organismal level.