Endocrine system

• Endocrine system


  A collection of specialised organs (endocrine glands) scattered throughout the body that act to produce hormones
• Endocrinology


  The study of this system and the diagnosis and treatment of its disorders
• Functions of the Endocrine System

  • Co-ordination
  • Homeostasis
  • Glandular secretion
  • Metabolism
  • Behaviour
  • Growth and development
  • Physical appearance
  • Reproduction
  • Digestion
  • Circadian rhythm
  • Body fluids
  • Immune response
  • Mood and emotions
  • Hunger
• Circadian rhythm


  The endocrine system is involved in the establishment of circadian rhythms
• Body fluids


  The endocrine system regulates the volume and composition of body fluids
• Immune response


  The endocrine system regulates the body's immune response
• Mood and emotions


  The endocrine system controls many aspects of mood and influences emotional states
• Hunger


  The endocrine system initiates hunger cravings and satiety
• Hormones


  Chemical messengers that are transported by the bloodstream and stimulate another tissue or organ
• Functions of hormones


  • Control
  • Regulation
  • Balance systems
• Lipid-soluble hormones


  Circulate bound to transport proteins
• Water-soluble hormones


  Circulate freely in the plasma
• Lipid-soluble hormones


  Bind to receptors within target cells
• Water-soluble hormones


  Bind to receptors on the exterior surface of the target cell
• Factors that determine how a target cell responds to a hormone

  • The hormone's concentration in the blood
  • The number of hormone receptors on the target cell
  • Influences exerted by other hormones
• Synergistic effect


  Some hormones work more effectively when a second hormone is present to assist them
• Antagonistic effect


  Some hormones oppose the action of others
• Hormone secretion


  Secretion is regulated by signals from the nervous system, chemical changes in the blood, and other hormones
• Negative feedback


  Increased target organ hormone levels inhibit (stop) release of hormones
• Positive feedback


  Stretching of uterus increases OT release, causes contractions, causing more stretching of uterus, etc. until delivery
• The endocrine system affects almost every aspect of life, controlling and regulating many body functions
• Hormones are secreted in short bursts when needed
• Hypothalamus


  Regulates primitive functions of the body from water balance and thermoregulation to sex drive and childbirth
• Functions regulated by the hypothalamus

  • Heart rate and blood pressure
  • Body temperature
  • Fluid and electrolyte balance, including thirst
  • Appetite and body weight
  • Glandular secretions of the stomach and intestines
  • Production of substances that influence the pituitary gland to release hormones
  • Sleep cycles
• Pituitary gland (hypophysis)


  Suspended from hypothalamus by a stalk - infundibulum, located in sella turcica of sphenoid bone, size and shape of kidney bean, composed of two structures with independent origins and separate functions - adenohypophysis (anterior pituitary) and neurohypophysis (posterior pituitary)
• Hormones secreted by the anterior pituitary lobe

  • Thyroid-stimulating hormone (TSH)
  • Follicle-stimulating hormone (FSH)
  • Luteinizing hormone (LH)
  • Adrenocorticotropic hormone (ACTH)
  • Human growth hormone (hGH)
  • Prolactin (PRL)
• Somatostatin


  Inhibits the pituitary gland's secretion of growth hormone and thyroid stimulating hormone, also produced in the pancreas and inhibits the secretion of other pancreatic hormones such as insulin and glucagon, and in the gastrointestinal tract where it acts locally to reduce gastric secretion, gastrointestinal motility and to inhibit the secretion of gastrointestinal hormones, including gastrin and secretin
• The hypothalamus and pituitary gland interact as a major brain centre that controls activities of other organs
• Hypothalamo-pituitary-target organ relationships


  • GH
  • ACTH
  • TSH
  • TRH
  • GnRH
  • CRH
  • IGF
  • GHRH
  • PRL
  • LH
  • FSH
• Anterior pituitary hormones and their actions

  • Growth hormone (GH) - Stimulates liver, muscle, cartilage, bone, and other tissues to synthesize and secrete insulin-like growth factors (IGFs); IGFs promote growth of body cells, protein synthesis, tissue repair, breakdown of triglycerides, and elevation of blood glucose level
  • Thyroid-stimulating hormone (TSH) - Stimulates synthesis and secretion of thyroid hormones by the thyroid gland
  • Follicle-stimulating hormone (FSH) - In females, initiates development of oocytes and induces secretion of estrogens by the ovaries; in males, stimulates testes to produce sperm
  • Luteinizing hormone (LH) - In females, stimulates secretion of estrogens and progesterone, ovulation, and formation of corpus luteum; in males, stimulates testes to produce testosterone
  • Prolactin (PRL) - In females, stimulates milk production by the mammary glands
  • Adrenocorticotropic hormone (ACTH) - Stimulates secretion of glucocorticoids (mainly cortisol) by the adrenal cortex
  • Melanocyte-stimulating hormone (MSH) - Exact role in humans is unknown but may influence brain activity; when present in excess, can cause darkening of skin
• Posterior pituitary hormones and their actions

  • Oxytocin - Stimulates contraction of smooth muscle cells of uterus during childbirth; stimulates milk ejection from mammary glands
  • Antidiuretic hormone (ADH) - Conserves body water by decreasing urine output; decreases water loss through sweating; raises blood pressure by constricting (narrowing) arterioles
• Growth hormone (GH)


  Has widespread effects on the body tissues, especially cartilage, bone, muscle, and fat, induces liver to produce growth stimulants insulin-like growth factors (IGF-I) or somatomedins (IGF-II) that stimulate target cells in diverse tissues, bone growth, thickening, and remodeling influenced, especially during childhood and adolescence, secretion high during first two hours of sleep, can peak in response to vigorous exercise, GH levels decline gradually with age
• Pineal gland


  Attached to roof of third ventricle beneath the posterior end of corpus callosum, after age 7 it undergoes involution (shrinkage) down 75% by end of puberty, tiny mass of shrunken tissue in adults, may synchronize physiological function with 24-hour circadian rhythms of daylight and darkness, synthesizes melatonin from serotonin during the night, seasonal affective disorder (SAD) occurs in winter or northern climates - symptoms are depression, sleepiness, irritability and carbohydrate craving, 2 to 3 hours of exposure to bright light each day reduces the melatonin levels and the symptoms (phototherapy)
• Thymus


  Plays a role in three systems: endocrine, lymphatic, and immune, site of maturation of T cells important in immune defense, secretes hormones (thymopoietin, thymosin, and thymulin) that stimulate development of other lymphatic organs and activity of T-lymphocytes
• Thyroid gland


  Ductless alveolar gland found in the anterior neck, just below the laryngeal prominence (Adam's apple), roughly butterfly-shaped, with two lobes wrapping around the trachea and connected in the middle by an isthmus, one of the main regulators of metabolism, T3 and T4 act via nuclear receptors in target tissues and initiate a variety of metabolic pathways, high levels of them cause these processes to occur faster or more frequently
• Metabolic processes increased by thyroid hormones

  • Basal Metabolic Rate
  • Gluconeogenesis
  • Glycogenolysis
  • Protein synthesis
  • Lipogenesis
  • Thermogenesis
• Thyroid follicles


  Thyroid epithelia form follicles filled with colloid - a protein-rich reservoir of the materials needed for thyroid hormone production, these follicles range in size from 0.02-0.3mm and the epithelium may be simple cuboidal or simple columnar
• Parafollicular cells


  Found in the spaces between the thyroid follicles, secrete calcitonin, which is involved in the regulation of calcium metabolism in the body by stimulating osteoblast activity and bone formation
• Thyroid hormone synthesis


  Begins with the recognition of thyroid-stimulating hormones (TSH) by TSH-receptors at the follicular cells, iodine is taken into the cell, iodine is used in the conversion of tyrosine into thyroxine (T4) and triiodothyronine (T3)
• Parathyroid glands


  Secrete parathyroid hormone (PTH) which increases blood Ca2+ levels, promotes synthesis of calcitriol, increases absorption of Ca2+, decreases urinary excretion, increases bone resorption