Insulin is a hormone produced by the pancreas. It has many actions, but is particularly important in keeping the blood glucose level normal.
Question: How does insulin help to keep the blood glucose level within the normal range?
Insulin allows glucose to enter cells and the tissues of the body. If there is no insulin then glucose cannot leave the blood and enter the tissues. This means that the blood glucose level just becomes higher and higher. If there is too much insulin, more glucose enters the tissues than it should and the blood glucose level can fall too low. This is what happens when someone takes more insulin than they need.
As insulin helps glucose to enter cells it makes sense that we produce insulin when we eat and the blood glucose level is going up. The insulin prevents the glucose level in the blood from going too high. Between meals and overnight the insulin level drops down. This allows the glucose level to stay within the tight range that the body needs to function normally. If the insulin level did not drop then the blood glucose level would become too low. You have already learnt that the liver, muscle and fat all store glucose when there is plenty about. Insulin allows these tissues to take up glucose and store it. Insulin also stops glucose from being released from the liver.
The brain is the only organ in which glucose uptake is not controlled by insulin. This is important, because otherwise the amount of glucose available for the brain to use would vary as insulin levels went up and down. Instead the brain relies on mechanisms in other parts of the body to keep the blood glucose levels within a narrow range. The brain does not function properly if glucose levels in the blood drop below normal.
It is clear that insulin is needed to stop blood glucose levels from going too high. Diabetes occurs when there is no insulin or not enough insulin (insulin deficiency). Figure 5 summarises the actions of insulin and the consequences of a lack of, or insufficient, insulin. Diabetes can also occur when the insulin present does not function properly because the body cannot respond to (‘is resisting’) its actions (insulin resistance). Obesity is a common cause of insulin resistance and can lead to diabetes. Obesity is defined as greatly elevated body weight in relation to height (see body mass index in Activity 6), to an extent which is associated with a serious increased risk to health.
Click to view larger PDF version of Figure 5