We have three sources of blood glucose; we can have it direct in our diets from carbohydrates like starch, lactose, sucrose and maltose. Also the breakdown of glycogen (glycogenolysis), which is stored in our liver and muscle cells.
The average humans liver contains 75-100g of glycogen, this is made by converting excess glucose taken in the diet, and this process is called glycogenesis.
Glycogenesis means production of new glucose other than in the diet. Our liver can make glucose from glycerol and amino acids. Glucose is used up at different rates depending on the individuals’ mental and physical activities.
With this we rely on three main hormones to maintain a constant level of blood glucose, these hormones are insulin, glucagon and adrenaline.
Our pancreas has specialised cells that are called the islets of langerhans, there is two types of these cells, the large alpha cells and the smaller beta cells. Beta cells will respond to a rise in blood glucose levels, the cells will secrete the hormone insulin directly into the blood. Insulin is made up of 51 amino acids. Once insulin combines with receptors, these receptors are found in the membranes of all body cells except for red blood cells.
Insulin will then increase the membranes permeability and the enzyme action so blood glucose levels are lowered by cellular respiratory rate being increased which will use up more glucose and increasing it’s uptake by cells. Or by the rate of conversion of glucose to glycogen, this increases in the cells of the liver and muscles. Another example is to convert glucose to fat in adipose tissue or the rate of absorption of glucose into the cells increases mostly in muscle cells.
All of these processes remove glucose from the blood so it returns to normal level; the lowering of the blood glucose levels causes the beta cells to reduce their secretion of insulin. This is an example of negative feedback.
If there is a fall in blood glucose the hormone glucagons is secreted directly into the blood. Cells in the liver respond by activating the enzyme phosphorylase, this converts glycerol into glucose, this increases the conversion of amino acids and glycerol into glucose is an example of gluconeogenesis.
The effect is to increase the amount of glucose in the blood and to return it to its normal level. The raise in blood glucose levels makes the alpha cells to reduce the secretion of glucagons; this is an example of negative feedback.
