Endocrine system

• Endocrine system

  One of the body's two communication systems (along with the nervous system) that regulates body functions
• Endocrine system

  • Responds more slowly (minutes) to a stimulus by producing and secreting chemical messengers called hormones into the bloodstream to alter the activities of target cells

  
• Nervous system

  • Responds quickly (milliseconds) to a stimulus by generating and conducting electrical signals along neurons to alter the activities of an effector (muscle or gland)
• Hormone production and secretion

  1. Hormones are produced and secreted by endocrine glands in response to a stimulus

  2. Stimuli that trigger endocrine glands include: hormonal, humoral, and neural

  
• Steroid hormones


  Made from cholesterol, lipid-soluble, bind to intracellular receptors
• Amino acid-based hormones


  Vary in size, lipid insoluble, bind to plasma membrane receptors
• Hormone actions


  • Activate/deactivate enzymes, change plasma membrane permeability, stimulate protein synthesis, induce cell secretory activity, stimulate cell division
• Hormones alter target cell activity to control metabolic reactions, facilitate transportation, regulate fluid/electrolyte balance, blood pressure, reproductive functions, development and growth, and maintain homeostasis
• Hypothalamus-posterior pituitary


  1. Hypothalamus produces hormones (oxytocin, ADH)

  2. Hypothalamic-hypophyseal tract transports hormones to posterior pituitary

  3. Posterior pituitary stores and secretes hormones into bloodstream when stimulated

  
• Antidiuretic hormone (ADH)


  • Secreted by posterior pituitary in response to increased blood Na+ levels and/or decreased blood volume/pressure
  • Targets kidneys to increase water reabsorption, dilute blood, and restore normal blood volume/pressure

  
• Oxytocin


  • Secreted by posterior pituitary in response to uterine stretching during labour and infant suckling
  • Targets uterus to stimulate contractions and mammary glands to stimulate milk ejection

  
• Conditions like dehydration or haemorrhage increase ADH secretion, while hyposecretion of ADH results in diabetes insipidus
• Oxytocin


  Targets the uterus to stimulate smooth muscle contractions during labour and the mammary glands to stimulate the ejection of milk during breastfeeding
• Oxytocin secretion regulation


  1. Increased stretching of the cervix during childbirth or the suckling action of the infant after birth stimulates the posterior pituitary to secrete more oxytocin

  2. This in turn enhances the uterine contractions which move the baby through the birth canal or the ejection of milk from the mammary glands

  
• Suckling promotes the secretion of oxytocin


  Placing the infant at the mother's breast after childbirth helps expel the placenta and return the uterus to its smaller size
• Dehydration or haemorrhage


  Decrease blood volume and therefore increase the secretion of ADH
• Hyposecretion of ADH


  Results in diabetes insipidus, an endocrine disorder caused by a blow to the head that damages the hypothalamus or posterior pituitary. Symptoms include excessive urination (polyuria) and extreme thirst (polydipsia)
• If a patient is unconscious following a head injury, their urine output must be monitored carefully
• Hypothalamic neurons


  Produce and secrete releasing and inhibiting hormones that regulate (stimulate or inhibit) the secretion of anterior pituitary hormones

  
• Hypothalamic-Pituitary-Thyroid axis (HPT axis)


  1. When stimulated, hypothalamic neurons secrete releasing/inhibiting hormones into the hypophyseal portal system

  2. Hormones travel through the infundibulum via the hypophyseal portal system to the anterior pituitary

  3. Releasing/inhibiting hormones stimulate/inhibit the secretion of anterior pituitary hormones into the bloodstream

  
• Hypothalamic Releasing/Inhibiting Hormones


  • Thyrotropin-releasing hormone (TRH)
  • Corticotropin-releasing hormone (CRH)
  • Growth hormone-releasing hormone (GHRH)
  • Growth hormone-inhibiting hormone (GHIH or somatostatin)
  • Gonadotropin-releasing hormone (GnRH)
  • Prolactin-inhibiting hormone (PIH)

  
• Gonadotropins


  Follicle-stimulating Hormone (FSH) and Luteinising Hormone (LH)

  
• Stimulus for gonadotropin secretion

  Gonadotropin-releasing hormone (GnRH) stimulates the secretion of FSH and LH from the anterior pituitary

  
• Actions of FSH and LH

  • FSH stimulates the development of oocyte (egg) containing follicles and production/secretion of oestrogen in ovaries, and regulates sperm production in testes
  • LH stimulates ovulation and production/secretion of progesterone (mainly) & oestrogen in ovaries, and stimulates the production & secretion of testosterone in testes

  
• Prolactin (PRL)


  Prolactin-inhibiting hormone (PIH) primarily controls the secretion of PRL from the anterior pituitary, and decreased PIH secretion leads to an increase in PRL secretion. PRL targets the mammary glands and stimulates milk production.

  
• Adrenocorticotropic Hormone (ACTH)


  Corticotropin-releasing hormone (CRH) stimulates the secretion of ACTH from the anterior pituitary, and ACTH targets the adrenal cortex and stimulates the adrenal cortex to secrete the hormone cortisol

  
• Thyroid-stimulating Hormone (TSH)


  Thyrotropin-releasing hormone (TRH) stimulates the secretion of TSH from the anterior pituitary, and TSH targets the thyroid gland and stimulates the thyroid gland to secrete the thyroid hormones T3 (triiodothyronine) and T4 (thyroxine)

  
• Growth Hormone (GH)


  Growth hormone-releasing hormone (GHRH) stimulates the secretion of GH, and Growth hormone-inhibiting hormone (GHIH or Somatostatin) inhibits the secretion of GH. GH promotes the growth of body tissues, especially skeletal muscle and bone, by stimulating cells to secrete insulin-like growth factors (IGFs).

  
• The hyposecretion of GH leads to dwarfism and hypersecretion leads to gigantism (in children) and acromegaly (in adults)
• Thyroid hormones (TH)


  Thyroid-stimulating hormone (TSH) stimulates the secretion of TH from the thyroid gland. TH affect virtually every cell in the body and primarily increase basal metabolic rate, body heat production, heart rate and force of contraction, and promote the growth of muscles and bones (in conjunction with growth hormone) and nervous system development.

  
• Deficiencies in TH during fetal development, infancy or childhood lead to mental retardation and stunted bone growth. Hyposecretion of TH during adulthood can result in a low BMR, heart rate and blood pressure, cold intolerance, muscular weakness and weight gain. Hypersecretion of TH can result in an above-normal BMR, increased body temperature, rapid heart rate, high blood pressure and weight loss.
• Calcitonin


  Secreted by the parafollicular cells of the thyroid gland when blood Ca2+ levels increase above the normal range. Calcitonin targets bone, and decreases blood Ca2+ to normal levels by inhibiting the activity of osteoclasts and stimulating calcium uptake from the blood into bone.

  
• Parathyroid Hormone (PTH)


  Secreted by the parathyroid glands when blood Ca2+ levels decrease below the normal range. PTH targets bone, kidneys and small intestines, and increases blood Ca2+ to normal levels by stimulating bone-resorbing osteoclasts, the kidneys to return more Ca2+ to the blood, and the kidneys to secrete calcitriol which increases the absorption of Ca2+ from the small intestines.

  
• Hypoparathyroidism can be caused by parathyroid gland trauma or their removal during thyroid surgery, resulting in hypocalcaemia (low blood Ca2+ levels) that can affect body functions such as blood clotting, neural communication and muscle contraction. Hyperparathyroidism is often caused by a parathyroid gland tumour, leading to excessive resorption of the bone matrix and loss of bone mass (osteoporosis).
• Erythropoietin (EPO)


  Secreted by the kidneys when blood oxygen levels drop below their normal range, and it targets the bone marrow and stimulates the production of red blood cells.
• The adrenal glands consist of an outer adrenal cortex that produces and secretes glucocorticoids (primarily cortisol) and mineralocorticoids (primarily aldosterone), and an inner adrenal medulla that produces and secretes the hormones adrenaline and noradrenaline.

  
• Cortisol


  The secretion of cortisol from the adrenal cortex is stimulated by the anterior pituitary hormone, ACTH.

  
• Erythropoietin (EPO)


  Hormone secreted by the kidneys when blood oxygen levels drop below their normal range (hypoxemia)
• Erythropoietin (EPO)


  • Targets the bone marrow and stimulates the production of red blood cells
• Adrenal glands


  Paired, pyramid-shaped organs that sit on top of the kidneys, consisting of an outer adrenal cortex and an inner adrenal medulla