endocrine system

• Endocrine and Exocrine glands:

  • exocrine glands secretes into a duct exiting the body
  • endocrine glands secretes into the extracellular fluid
• Hormones:

  main chemical structural hormone - proteins, steroids and amines

  • these may target all cells or only target cells/organs
• Receptors:

  • hormones can only influence a cell if there is a correct receptor
  • all receptors are bound by hormones → saturation occured and hormone effect will not be amplified further
  • protein and amine bind to receptors on the cell membrane, the receptor causes a secondary messager to be diffused through the cell
  • steroid hormones attach to receptors inside the cell and will act to switch genes ‘on’ to synthesise specific proteins
• Cellular effects of hormone:

  often linked to enzymes, hormones may:

  • activate a gene to produce an enzyme or structural protein
  • change the shape of an enzyme
  • changes produce rate of an enzyme or structural protein by changing transcription or translation rate
• Protein and amine are functionally the same but its chemical structure is different.
• • Cascade effect amplification of the effect of one hormone up to a billion times
• Hormone clearance:

  hormone molecules are broken down by target cells and transported to the liver and kidney for excretion

  • hormone must be turned off after the desired effect is produced
• Negative feedback controlled by the hypothalamus:

  • the hypothalamus can regulate hormone activity by releasing regulating factors
  • regulating factors regulate the function of the pituitary gland
  • releasing factors - stimulate hormone release
  • inhibiting factors - slows hormone secretion
• Hypothalamus controls: (via control of the pituitary gland)

  • body temperature
  • water balance
  • heart
• Hypothalamus location:

  • at base of the brain
  • joined to the pituitary gland via the infundibulum (stalk)
• Pituitary Gland: (neurohypophysis)

  • contains anterior and posterior lobes
  • posterior - joined via nerves; does not create hormones
  • anterior - joined via blood vessels
  • pituitary gland = size of pea and shaped as two lobes
• Role of the hypothalamus:

  • produces hormones → carried to pituitary gland
  • anterior lobe = bloodstream
  • posterior lobe = nerve fibres
• Anterior refers to the ‘front’ whilst the posterior refers to the ‘rear’
• Anterior lobe release:

  They also make and release their own hormones

  Gonadotropins - targets the ovaries/testes

  • eg. FSH stimulates follicular development and production and maturation of sperm
  • eg. LH forms the corpus luteum and stimulates male testes
• Oxytocin

  • stimulates contraction of the uterus and cells in the mammary glands → releases milk
• Antidiuretic hormone (ADH)

  • causes increased reabsorption of water from the kidneys → decreases amount of urine
• Growth hormone (GH)

  • stimulates skeletal growth
  • increases amino acid uptake to build proteins
  • secreted through life to maintain organ size
• • Thyroid stimulating hormone (TSH) stimulates production and releases hormones
• Adrenocorticotropic hormone (ACTH) - controls production and release of hormones from the adrenal gland’s cortex
• Prolactin (PRL) - works with other hormones to initiate and maintain milk production
• Parathyroid glands:

  • parathyroid hormone (PTH) - controls calcium and phosphate levels in the blood
• Low calcium levels → release of PTH

  • calcium released from bones into blood
  • increased absorption of calcium in the intestine
  • increased REabsorption in the kidney
• The thyroid glands:

  • effects nearly every tissue
  • regulates the base metabolic rate
  • important in body changes needed during seasonal changes
• Triiodothyronine (T3)

  • increases metabolic rate
  • converted from T4 by enzymes
• Adrenal medulla - secretes adrenaline and noradrenaline

  • adrenaline prepares body for flight-or-flight response
  • noradrenaline increases the rate and force of the heartbeat
• The pancreas:

  • islets of Langerhans release insulin and glucagon which regulate blood sugar level
  • insulin causes the conversion of glucose to glycogen
  • glucagon promotes breakdown of glycogen to glucose
• Pineal Gland:

  • decreases in size
  • secretes: melatonin (involved in sleep pattern regulation)
• Thyroxine (T4)

  • major hormone - stimulates carbs, protein and fat metabolism
  • less active than T3 but lasts longer
  • regulates reactions where complex molecules are broken down
  • targets the liver and the kidneys
• Calcitonin

  (regulates calcium and phosphate levels in the blood)

  • secreted by C- cells
  • reduces reabsorption of calcium
  • moves phosphate into bone and reduces its reabsorption
• Thymus:

  • thymosins - influences the maturation of T-lymphocytes (immunity cells)
• Adrenal cortex - secretes more than 20 hormones (corticosteroids)

  • adosterone reduces sodium amount and incease potassium amount in urine
  • cortisol promotes NORMAL metabolism - helps the body withstand stress
• The gonads:

  • testes produce androgens which maintain male sexual characteristic
  • ovaries produce oestrogen and progesterone which stimulate and maintain female sexual characteristics; gondadotropins regulate the menstrual cycle
• Other endocrine tissues:

  • stomach and small intestine secrete hormones
  • kidneys secrete hormones
  • eg. erythropoietin (EPO) - stimulates RBC production
  • heart secretes a hormone to reduce blood pressure
  • placenta secretes hormones to maintain pregnancy