16.3 Hormones and regulation of blood glucose concentration

• All hormones are produced in glands, which secrete the hormone directly into the blood (endocrine glands).
• All hormones are carried in the blood plasma to the cells on which they act, known as target cells, which have specific receptors on their cell-surface membranes that are complementary to a specific hormone.
• All hormones are effective in very low concentrations, but often have widespread and long-lasting effects.
• The pancreas is a large, pale-coloured gland that is situated in the upper abdomen, behind the stomach. It produces enzymes (protease, lipase and amylase) for digestion and hormones (insulin and glucagon) for regulating blood glucose concentration.
• On a microscopic scale, the pancreas is made up largely of the cells that produce its digestive enzymes. Scattered throughout these cells are groups of hormone-producing cells known as islets of Langerhans.
• The cells of islets of Langerhans include alpha cells, which are larger and produce the hormone glucagon, and beta cells, which are smaller and produce the hormone insulin.
• The liver is located immediately below the diaphragm, has a mass of up to 1.5kg and is made up of cells called hepocytes. It has many roles, including regulating blood glucose concentration. It produces the hormones adrenaline and glucagon, however the liver is where their effects occur.
• There are three major blood glucose regulation processes that take place in the liver. These include glycogenesis, glycogenolysis and gluconeogenesis.
• Glycogenesis is the conversion of glucose into glycogen. When blood glucose concentration is higher than normal the liver removes glucose from the blood and converts it into glycogen.
• Glycogenolysis is the breakdown of glycogen into glucose. When blood glucose concentration is lower than normal, the liver can convert stored glycogen back into glucose, which diffuses into the blood to restore the normal blood glucose concentration.
• Gluconeogenesis is the production of glucose from sources other than carbohydrates. When its supply of glycogen is exhausted, the liver can produce glucose from non-carbohydrate sources such as glycerol and amino acids.
• Glucose is the substrate required for respiration, therefore maintaining a relatively constant concentration in the blood is essential.
• If the blood glucose concentration falls too low, cells will be deprived of energy and die (brain cells are especially susceptible because they can only respire glucose).
• If the blood glucose concentration rises too high, it lowers the water potential of the blood and creates osmotic problems that can cause dehydration.
• The normal blood glucose concentration is 5 mmol dm-3.
• Blood glucose comes from three main sources : directly from the diet (through hydrolysis of carbohydrates into glucose, which is then absorbed), from glycogenolysis (where glycogen stored in liver and muscle cells is hydrolysed into glucose in the small intestine), and from gluconeogenesis (the production of glucose from sources other than carbohydrates).
• Three main hormones work to maintain a constant blood glucose concentration. These include insulin, glucagon and adrenaline.
• Insulin and glucagon act in opposite directions, therefore they can be referred to as acting antagonistically. The system is self-regulating through negative feedback in that the concentration of glucose in the blood will determine the quantity of insulin and glucagon released.
• The concentration of glucose is not constant, but fluctuates around an optimum point. This is due to the nature of negative feedback loops, since the quantity of each hormone is only increased or decreased once the glucose concentration has exceeded or fallen below a set level.