5 min read

Glucose Physiology

In this series of articles, medical students from across the country will share their knowledge and insight of medical physiology, anatomy and biochemistry to give you a taster of medical school. This will be a fantastic opportunity to build upon your A-Level Knowledge.

These articles give you a basic overview of the principles; we have attached videos and useful websites to develop a more detailed insight.

Glucose is a really important molecule in our bodies which makes controlling how much of it we have really key in allowing our body to function properly. In fact, if we can't control it properly we develop conditions like diabetes which come with a whole host of other problems. So how exactly does our body do this?

As a general point, our bodies really like having things within certain ranges. If something strays outside this range it will do its absolute best to reel it back in (homeostasis), if this doesn't happen it can get a bit problematic. If you understand this basic principle, you'll not only understand how diabetes works but many other other medical problems.

Fig. 1 - Graph showing variation of blood glucose concentration over time

We can see this principle on the graph above which shows the upper and lower limits of the amount of sugar your body likes to have in your blood. When you have a meal, the glucose from it will be absorbed into your bloodstream and so the blood glucose concentration rises - at this point the glucose is just floating about in the blood as it flows past all the cells. Notice how the concentration always levels off before it hits the upper limit, and then as time goes on and your body uses up glucose in processes like muscle contraction and respiration, the concentration starts to dip until the next meal.

This leaves us with 2 important questions to answer:

Different meals have different glucose concentrations so why does the amount your blood glucose increases stay roughly the same and level off?
What happens if you don't eat something once your glucose concentration gets low?

The role of insulin

To answer question 1 we need to look at the role of insulin. Insulin is a hormone produced by the pancreas - in particular it's produced by a special type of cell called a beta-cell that's found in Islets of Langerhans (these are just clusters of cells that Mr Langerhans figured he could get famous off when he discovered them). When your pancreas detects blood glucose concentration starting to get close to that upper limit, it releases insulin into the bloodstream. The purpose of insulin is to get glucose out of the blood to avoid the concentration getting too high (this is important because high blood glucose concentration can lead to other health issue like the micro and macrovascular complications of diabetes).

The question is where do you put it? The quickest and easiest place to move it would be the cells that the blood is flowing past, which is exactly what happens! Insulin acts as a chemical signal that tells the cells around your body to absorb a bit of the glucose as it flows past, which is especially true of muscle and liver cells which absorb lots. This means that the amount of glucose in your blood starts to drop as bits of glucose are absorbed by cells around the body.

As the concentration of glucose in the blood drops, insulin secretion decreases to stop blood glucose from dropping too low. This is an example of a negative feedback system - where an increase in one substance (glucose) stimulates the release of a second substance (insulin) which has the effect of stopping the level of the first substance from rising too much.

Where does glycogen fit in

Once glucose has moved into the cell it has to be stored in a different form. Glucose is osmotically active meaning that wherever you have a lot of glucose, you have a concentrated solution (and therefore a solution with a low concentration of water relative to the concentration of the solute - in this case glucose). As a result water will try to move into that compartment via osmosis. This means that if we put a lot of glucose in a cell for storage, water would follow it in and inflate the cell. The cell has a fairly flexible membrane but just like a water balloon, if you put in too much water it would burst.

To avoid this glucose is converted to an osmotically inactive substance called glycogen for storage. Glycogen has many features that make it a good storage molecule, but the most relevant is that it will not draw water into the cell as glucose would. This process is outlined in the diagram below

Fig. 2 - Diagram showing movement of glucose into a cell and its subsequent conversion into glycogen for storage

What if I can't eat in time?

Looking at Fig 1 we can see that the blood glucose concentration starts to spike upwards again once we've eaten a meal, but not everybody can eat at the exact same time every day. Just as our body has a hormone to stop the blood glucose concentration passing the upper limit, it has a second hormone to stop blood glucose dipping below the lower limit. This hormone is called glucagon (in hindsight, maybe whoever named glucose, glycogen and glucagon could've done a better job so they didn't all sound so similar). It signals cells to convert that glycogen back into glucose and release these glucose molecules back into the blood stream. As a result the blood glucose concentration starts to rise again.

Do these systems ever go wrong?

As long as you have some glycogen stored in cells and your cells continue to respond to the signals of insulin and glucagon your body can regulate its concentration of blood glucose. One of the most common diseases where there's a problem with these processes diabetes which is subdivided into Type 1 and Type 2. Type 1 diabetes involves insulin not being produced, so whilst cells around the body can still respond to the signals of insulin, that signal never arrives. In contrast, type 2 diabetes occurs when your pancreas can still produce insulin but the cells don't respond to the signal as well as they used to. Both lead to increases in blood glucose above the upper limit of normal (hyperglycaemia), and as a result of their different mechanisms they are treated slightly differently.

This article gives a basic overview of the principles behind how glucose concentration is controlled in our bodies. Whilst this was all very much done in a conceptual way, and as with anything there is much more detail to everything discussed, if you have a good understanding of these homeostatic principles you will have no problem understanding what happens when this system goes wrong in conditions like diabetes.

Key words

Homeostasis - maintenance of a constant internal environment
Insulin - Hormone released by beta cells in the pancreas that works to decrease blood glucose concentration
Glycogen - glucose is converted into this molecule to allow it to be stored within cells
Glucagon - a hormone released by alpha cells in the pancreas that works to increase blood glucose concentration