5.1.4 - Hormonal Communication

• Endocrine Gland

  Secretes hormones directly into the blood.
• Exocrine Gland

  Secrete hormones into a duct, which then carries the molecule to a particular target.
• Detection of Hormones

  Hormones only affect target cells with complementary receptors, making them very specific.

  
  

  Endocrine glands can be stimulated by electrical impulses, change in conc. of a specific substance, etc.
• Structure of Adrenal Glands - Adrenal Cortex

  Outer region, produces essential hormones.
• Adrenal Cortex - Cortisol

  Regulates metabolism.

  
  

  Regulates blood pressure and cardiovascular function in response to stress.
• Adrenal Cortex - Corticosterone

  Works with cortisol to regulate immune response and suppress inflammatory reactions.
• Adrenal Cortex - Aldosterone

  Controls blood pressure through maintaining balance of salt and water conc. in the blood.
• Adrenal Cortex - Androgens

  Release a small amount of male and female sex hormones.

  
  

  Important in women after menopause.
• Adrenal Medulla - Adrenaline

  Increases heart rate (sends blood quickly to muscles and brain).

  
  

  Raises blood glucose levels by converting glycogen to glucose in the liver (glycogenolysis).
• Structure of Adrenal Glands - Adrenal Medulla

  Darker inner region, produces non-essential hormones.
• Adrenal Medulla - Noradrenaline

  Works with adrenaline in response to stress.

  
  

  Increases heart rate.

  
  

  Dilates pupils and air passages.

  
  

  Constricts blood vessels in non-essential organs (raises blood pressure).
• Histology of Pancreas - Islets of Langerhans

  Alpha cells produce and secrete glucagon.

  
  

  Beta cells produce and secrete insulin.

  
  

  Alpha cells are larger and more numerous than beta cells, however differential staining is required to distinguish them.

  
• Histology of Pancreas - Pancreatic Acini

  Small, berry-like clusters.

  
  

  Produce and secrete digestive enzymes (exocrine).

  
• Function of the Pancreas - Exocrine Gland

  Most of the pancreas is made of exocrine tissue (acini).

  
  

  Produces digestive enzymes - amylases, proteases and lipases - and pancreatic juice.

  
  

  Enzymes and juice are secreted into ducts, which lead to the pancreatic duct, which is then released into the duodenum (top of small intestine).
• Function of the Pancreas - Endocrine Gland

  Islets of Langerhans (endocrine) produces insulin and glucagon.

  
  

  Secreted directly into the blood.
• Regulation of Blood Glucose - Insulin

  Islets of Langerhans detects when blood glucose is too high, and beta cells secrete insulin (negative feedback).

  
  

  - Insulin increases rate of glucose absorption and respiratory rate of cells (increasing need for glucose).

  - Increases glycogenesis (glucose → glycogen).

  - Increases rate of glucose to fat conversion.

  - Inhibits release of glucagon.

  
  

  Insulin must be constantly secreted as it is broken down by enzymes in the liver.
• Regulation of Blood Glucose - Glucagon

  Islets of Langerhans detects when blood glucose is too low, and alpha cells secrete glucagon.

  
  

  - Increases glycogenolysis - liver breaks store of glycogen down into glucose.

  - Reduces amount of glucose absorbed by liver cells.

  - Increases gluconeogenesis - conversion of glycerol and amino acids into glucose in the liver.
• Control of Insulin Secretion

  At normal blood glucose levels, ATP-sensitive potassium channels in plasma membranes of beta cells are open, so K⁺ ions diffuse out.

  
  

  Increase in blood glucose means glucose enters the cell via a glucose transporter.

  
  

  Glucose is metabolised, producing ATP.

  
  

  ATP binds to ATP-sensitive potassium channels, causing them to close.

  
  

  K⁺ ions no longer diffuse out and potential difference reduces, causing depolarisation.

  
  

  Depolarisation means voltage-gated calcium channels open.

  
  

  Ca²⁺ ions enter the cell, causing secretory vesicles to release insulin by exocytosis.
• Type 1 Diabetes

  Unable to produce insulin.

  
  

  The cause is unknown - evidence suggests an autoimmune disease that targets beta cells.

  
  

  Symptoms develop quickly and appear in childhood.
• Treatment of Type 1 Diabetes

  Controlled by regular injections (insulin-dependent).

  
  

  Hyperglycaemia - insulin dose too low, can result in unconsciousness and is fatal if untreated.

  
  

  Hypoglycaemia - insulin dose too high.
• Type 2 Diabetes

  Beta cells either do not produce enough insulin, or body cells do not response properly to insulin.

  
  

  Cells lose responsiveness to insulin, and so do not take up enough glucose.

  
  

  Caused by obesity, physical inactivity, etc.
• Treatment of Type 2 Diabetes

  Regulate carbohydrate intake through diet.

  
  

  Increase physical activity.

  
  

  Lose weight.
• Medically Produced Insulin

  Originally obtained from cows and pigs' pancreas, but this was difficult and expensive (low yield).

  
  

  Now made using genetically modified bacteria.
• Benefits of Medically Produced Insulin

  Less likely to cause allergic reactions - produced in its pure form.

  
  

  Can be produced in higher quantities.

  
  

  Cheaper.

  
  

  Religious and ethical concerns are overcome.
• Stem Cells in Treatment of Diabetes

  Could potentially produce new beta cells.

  
  

  Donor availability no longer an issue.

  
  

  Reduced likelihood of rejection. Can also be made using SCNT.

  
  

  No longer have to inject with insulin.