Regulation of blood glucose level Transport of the monosaccharide glucose to all cells is a key function of the blood circulation. In humans, the normal level of blood glucose is about 90 mg of glucose/100 cm3 of blood, but this can vary.

For example, during an extended period without food, or after prolonged and heavy physical activity, the blood glucose level may drop to as low as 70 mg. After a meal rich in carbohydrate has been digested, the blood glucose level may rise to 150 mg.

Respiration is a continuous process in all living cells. To maintain their metabolism, cells need a regular supply of glucose, which can be quickly absorbed across the cell membrane. Glucose is the principal fuel used for respiration Most cells hold additional glucose reserves in the form of glycogen, which is quickly converted to glucose during prolonged physical activity; however, glycogen reserves may be used up quickly. In the brain, glucose is the only fuel the cells can use and there are no glycogen reserves held there at all.

If our blood glucose falls below 60 mg/100cm3 a condition called hypoglycaemia develops. If this is not quickly reversed, the person may faint. If the body and brain continue to be deprived of adequate glucose levels, then convulsions and hypoglycaemic coma follow, which can be fatal.

An abnormally high concentration of blood glucose, known as hyperglycaemia, is also a problem. Since high concentrations of any soluble metabolite lower the water potential of the blood plasma, water is drawn out of the cells and tissue fluid by osmosis, back into the blood. As the volume of blood increases, water is excreted by the kidney in an attempt to maintain the correct concentration of blood. As a result the body tends to become dehydrated, and the circulatory system is deprived of fluid. Ultimately, the correct blood pressure cannot be maintained.

Regulation of blood glucose After the digestion of carbohydrates in the gut, glucose is absorbed across the epithelial cells of the villi into the hepatic portal vein. The blood carrying the glucose therefore reaches the liver first. If the blood glucose level is too high then glucose is removed from the blood and stored as glycogen. But, even so, blood circulating in the body after a meal has a raised level of glucose.

At the pancreas the presence of an excess of blood glucose is detected by groups of cells within the organ, known as the islets of Langerhans. These islets are hormone- secreting glands (endocrine glands); they have a rich capillary network, but no ducts that would carry secretions away. Instead, their hormones are transported all over the body by the blood. The islets of Langerhans contain two types of cell, known as alpha cells and beta cells.

How blood glucose is regulated by insulin and glucagon in relation to diabetes In the presence of a raised blood glucose level, the beta cells are stimulated. They secrete the hormone insulin into the capillary network. Insulin stimulated the uptake of glucose into cells all over the body, but especially in cells in the liver and the skeletal muscle fibres.

It also increases the rate at which glucose is used in respiration, in preference to alternative substances (such as fats). Another effect of insulin is to trigger conversion of glucose to glycogen for storage in cells (glycogenesis), and of glucose to fatty acids and fats, and finally the deposition of fat around the body.

As the blood glucose level reverts to normal this is again detected in the islets of Langerhans. The beta cells respond by stopping insulin secretion. Meanwhile, the hormone is excreted by the kidney tubules, and the blood insulin level falls.

When the blood glucose level falls below the normal, the alpha cells of the pancreas are stimulated. These secrete a hormone called glucagon. This hormone activates the enzymes in the liver that convert glycogen and amino acids to glucose (gluconeogeneis) Glucagon also reduces the rate of respiraton.

Low blood glucose also triggers the secretion of other hormones in the body, particularly adrenaline by the adrenal glands. One effect of adrenaline is to stimulate the conversion of glycogen and amino acids to glucose, which raises the level of blood glucose. As the blood glucose level reverts to normal, glucagon production and adrenaline production cease, and these hormones in turn are removed from the blood in the kidney tubules.

Diabetes Diabetes is the name for a group of diseases in which the body fails to regulate blood glucose levels. Type I diabetes results from a failure of insulin production by the beta cells. Type II is a failure of the insulin receptor proteins on the cell membranes of target cells.

As a consequence, blood glucose levels are more erratic and, generally, permanently raised. Glucose is also regularly excreted in the urine. If this condition is not diagnosed and treated, it carries an increased risk of circulatory disorders, kidney failure, blindness, strokes and heart attacks.

Questions 1. Describe a simple test the nurse could use at the clinic to detect glucose in urine. 2. What result would you expect to see if the patient were excreting glucose? 3. The nurse decided to send the patient to hospital for some further health checks. In the hospital, the patient was asked, by a specialist nurse, to take a glucose tolerance test. Write down the stages involved in conducting a glucose tolerance test.

4. Some people can control diabetes by insulin injection. Explain two other ways that diabetes could be managed by somebody who suffers from the condition.

Answers 1. Indicator sticks/clinistix/multistix/unistix dipped in urine. 2. Multistix/unistix indicator turns from green to brown OR clinistix turns from pink to purple OR indicator/stick changes colour.

3. Patient fasts for several hours Measured amount of glucose given/ingested Blood glucose (not urine) levels monitored/measured at regular intervals over next few hours Blood glucose level taken before and after glucose meal to detect change.