Chapter 2 Endocrine System

• Homeostasis


  The maintenance of a stable (constant) internal environment
• Even though conditions in the body fluctuate constantly, the nervous system and endocrine system are constantly working to maintain a "steady state" (or, homeostasis) of the conditions inside the body
• Homeostasis is a state of balance between continuing processes (i.e. a "dynamic equilibrium" is maintained)
• Examples of internal body conditions that are kept at homeostasis

  • Body temperature
  • Blood-sugar levels
  • Water balance
  • Blood-calcium levels
  • Ion concentration (Na+, K+, Cl-)
  • Blood pH
  • CO2 concentration in blood
• Conduction of nerve impulses depends upon the differing concentrations of Na+ and K+ inside and outside the cell


  Glucose levels must be maintained as a source of energy for cells
• Homeostasis maintains the body near a "set point" or ideal value for each factor


  For instance, body temperature is maintained near a set point of 36.7oC
• Dynamic equilibrium


  Factors fluctuate up and down around the set point
• Endocrine system


  A body system composed of special organs called "glands" that produce and secrete hormones
• Hormones


  Chemicals that act as chemical messengers
• Hormones


  1. Secreted by the glands into the blood stream

  2. Travel in the bloodstream until they reach their target cell
• Target cell


  The specific cell on which a specific hormone will have an effect
• The two types of glands of the body


  • Exocrine glands
  • Endocrine glands
• Exocrine glands


  Secrete secretions into a duct that carries the secretion to the body surface or to one of the body cavities
• Examples of exocrine glands


  • Sweat glands
  • Mucous glands
  • Salivary glands
  • The pancreas
  • Tear glands
  • The liver
  • Mammary glands
• Endocrine glands


  Ductless glands that produce and secrete hormones into the extracellular fluid surrounding the cells of the gland
• Hormones


  Change the functioning of their target cells by changing the type, quantity, or activities of proteins produced in the target cell
• Hormonal effects on enzyme activity in target cells

  1. Activating certain genes

  2. Changing the shape of the active site of an enzyme

  3. Changing the rate of production of an enzyme or structural protein
• The 2 types of hormones


  • Steroid hormones
  • Protein hormones and Amine hormones
• Steroid hormones


  Lipid-soluble hormones that do not dissolve in water
• Examples of steroid hormones


  • oestrogen, progesterone, cortisol, aldosterone
• How steroid hormones work

  1. Steroid hormones travel bound to transport proteins

  2. Separate from the transport protein and diffuse across the cell membrane

  3. Combine with a receptor protein in the cytoplasm or nucleus

  4. Hormone-receptor complex activates genes controlling protein synthesis
• Protein hormones and Amine hormones

  Water-soluble hormones that dissolve in water
• How protein and amine hormones work


  1. Cannot diffuse across the cell membrane

  2. Bind to receptor proteins on the outside of the cell membrane

  3. Combination of hormone and receptor causes a secondary messenger substance to diffuse through the cytoplasm and activate certain enzymes
• Hormone receptors


  Specific receptor proteins that only bind with one specific molecule (the "lock-and-key" model)
• Saturation of hormone protein receptors

  There is a limited number of receptor proteins, when each is bound the cell's activity cannot increase further
• Enzyme amplification


  One hormone molecule triggers a cascading effect that increases the number of reacting molecules by hundreds or thousands of times at each step
• Hormone clearance


  Hormone molecules are broken down by the liver and kidneys, then excreted in bile or urine
• To maintain homeostasis, the amount of hormone produced by an endocrine gland must be very closely regulated
• Negative feedback systems


  The response produced by the secretion of hormones is opposite to the stimulus that caused the secretion
• Hypothalamus


  Located at the base of the brain, regulates many basic body functions and connects the nervous and endocrine systems
• How the hypothalamus regulates hormone secretion


  1. Secretes releasing factors that stimulate the pituitary to secrete hormones

  2. Secretes inhibiting factors that slow down pituitary hormone secretion
• Pituitary gland


  The "master gland" that regulates many other endocrine glands, consists of an anterior and posterior lobe
• The anterior lobe of the pituitary is controlled by releasing and inhibiting factors from the hypothalamus
• Hormones secreted by the anterior lobe of the pituitary


  • Gonadotropins
  • Thyroid-stimulating hormone (TSH)
  • Adrenocorticotropic hormone (ACTH)
  • Growth hormone (GH)
  • Prolactin
• Adenohypophysis


  Releases a number of hormones that regulate a great range of bodily activities
• Secretions from the anterior lobe


  1. Controlled by releasing and inhibiting factors from the hypothalamus

  2. Factors are secreted into the extracellular fluid around the cells of the hypothalamus

  3. Carried by the blood to the anterior lobe
• Hormones released by the anterior lobe of the pituitary gland


  • Gonadotropins
  • Growth Hormone (GH)
  • Thyroid-stimulating hormone (TSH)
  • Adrenocorticotropic hormone (ACTH)
  • Prolactin (PRL)
• Gonadotropins


  Hormones that affect the gonads (ovaries and testes)
• FSH (follicle stimulating hormone)


  In females: stimulates the development of follicles; in males: stimulates productions and maturation of sperm
• LH (luteinising hormone)


  Males: causes interstitial cells of testes to produce and secrete testosterone; Females: causes ovulation and formation of corpus luteum