Hormone

• A hormone is a chemical substance produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.

• Hormones are active in minute quantities and are destroyed by the liver and excreted by the kidneys.

• They affect cellular metabolism and coordinate the growth, development and activity of an organism.

• Glands are classified into two groups: exocrine glands and endocrine glands.

• Exocrine glands are glands that secrete their products via ducts. Examples include sweat glands and salivary glands.

• Endocrine glands are glands that secrete their products directly into the bloodstream. Examples include the pituitary gland, thyroid gland, adrenal gland and the gonads.

• Some glands are both exocrine and endocrine. An example would be the pancreas, which secretes pancreatic juice via the pancreatic duct, and insulin and glucagon from the islets of Langerhans into the bloodstream.

The pancreas as an endocrine gland

• The islets of Langerhans in the pancreas are areas in the pancreas that contain groups of endocrine cells.

• These cells produce the hormones insulin and glucagon.

• Insulin and glucagon are antagonistic hormones that participate in homeostatic control of blood glucose level by negative feedback mechanism.

• When blood glucose level exceeds the normal level, more insulin is released and acts to lower the glucose level.

• When blood glucose level falls below the normal level, more glucagon is released and acts to increase the glucose level.

• Insulin decreases blood glucose concentration by:

  (a) Stimulating body cells to increase glucose uptake by increasing permeability of plasma membranes to glucose

  (b) Stimulating the liver and muscle cells to store glucose in the form of glycogen

  (c) Decreasing production of glucose from glycogen breakdown in the liver

  (d) Decreasing the conversion of fatty acids and amino acids to glucose in the liver

• Glucagon increases blood glucose concentration by stimulating liver cells to:

  (a) Convert glycogen to glucose

  (b) Convert amino acids and fatty acids to glucose

  (c) Convert lactic acid into glucose

Diabetes mellitus

• Diabetes mellitus is a condition in which the body does not produce sufficient insulin or does not respond to insulin.

• The excess glucose cannot be completely reabsorbed by the kidneys and are excreted in the urine.

• Symptoms include: (a) A persistent high blood glucose concentration (b) Presence of glucose in the urine (c) Excessive urination, excessive thirst and weight loss

• Diabetes can cause: (a) Poor immune response – increased susceptibility to infections (b) Damaged blood vessels leading to vision loss and a decreased sensation in the limbs (c) Kidney failure and heart failure

• Diabetic individuals can control their disease by receiving regular injections and controlling their carbohydrate intake.

Adrenaline

• Adrenaline is a hormone produced by the adrenal glands located above the kidneys. It is responsible for the ‘fight-or-flight response’ triggered by stress (emotional or physical threats to the organism).

• In response to stress, the adrenal medulla secretes adrenaline into the blood.

• The adrenaline travels to target organs, causing:

  (a) Increased conversion of glycogen to glucose in the liver and skeletal muscles

  (b) Increased glucose release into blood by liver cells

  (c) Increased metabolic rate, causing more energy to be released in cellular respiration

  (d) Increased heart rate and volume of blood pumped per unit time, increasing oxygen and glucose supply to muscle cells

  (e) Dilated bronchioles and increased breathing rate and depth, allowing more oxygen to be taken in for cellular respiration

  (f) Decreased blood supply to the digestive system, the kidneys and the skin as vasoconstriction occurs in several body parts, diverting blood supply to the heart, brain and skeletal muscles

  (g) Vasodilation occurring in other body parts, increasing blood supply to these organs

  (h) Dilated pupils, enhancing vision

  (i) Contracted hair erector muscles, producing ‘goose pimples’