S4.5.5 Chapter 5
Blood sugar and diabetes
You eat a sweet meal one hour and nothing the next. Why does the sugar in your blood stay about the same?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
- Explain how insulin controls blood sugar
- Compare type 1 and type 2 diabetes and how each is managed
Every cell uses glucose as fuel. Glucose comes from the carbohydrate in food, and the blood carries it to the cells. The amount in the blood is called the blood sugar level, or blood glucose. It must stay steady. Too little starves the cells, and too much harms the body.
Between meals the level of glucose in blood is about 0.7 to 1.0 g/L. In an adult the blood holds only about 4 or 5 g of glucose in all. Keeping it steady is called homeostasis. The body keeps many things steady in this way.
The pancreas does the main work. It lies behind the stomach and has cells that sense the glucose in the blood. They release two hormones: insulin and glucagon. They have opposite jobs.
Draw it first
After a meal
1 You eat. The carbohydrate is digested to glucose, which passes into the blood. The blood glucose rises.
2 The pancreas senses the rise and releases insulin into the blood. The blood carries it to the liver and the muscles. Cells there take in glucose.
3 The liver turns glucose into glycogen, a store. The glucose in the blood falls back to normal.
Insulin lowers blood glucose. It makes liver and muscle cells take glucose in. The liver joins the glucose into long chains of glycogen and keeps them as a store. Muscle cells use glucose, or store glycogen too.
Between meals, or during hard exercise, the level can fall. Then the pancreas releases glucagon. Glucagon makes the liver turn glycogen back into glucose and release it into the blood. The level rises again.
In both loops the last step undoes the first. A rise leads to a fall, and a fall leads to a rise. This is negative feedback. It keeps the level close to the same value, rising and falling a little around it.
How much glucose is in all your blood? The amount is the volume times the concentration. Look at the table, then work it out for 5 L of blood.
Worked example
An adult has 5 L of blood, with 0.9 g of glucose in each litre. How much glucose is in all the blood?
- Multiply the volume by the amount in each litre.
- 5 × 0.9 = 4.5 g.
Answer: 4.5 g
That is about one teaspoon of sugar in all the blood. A meal can add far more glucose than that. So the control must work fast. Look at the graph of a healthy person after a meal. The level rises, but only a little, and it soon returns.
Worked example
At 120 minutes, how much higher is the glucose of the person with untreated diabetes than that of the healthy person?
- Read the line for untreated diabetes at 120. It is at 2.4 g/L.
- Read the healthy line at 120. It is at 1.0 g/L.
- Subtract: 2.4 − 1.0 = 1.4 g/L.
Answer: 1.4 g/L
Diabetes is a condition in which the body cannot keep blood glucose steady. The level goes too high and stays high. A health worker can test for it with a drop of blood. Over the years a high level damages blood vessels, nerves, eyes and kidneys. Early care prevents much of this harm.
In type 1 diabetes the pancreas makes too little insulin. The body's own defences destroy the cells that make it. It often starts in children and young people, but it can start later in life. Eating sugar does not cause it. A person with type 1 diabetes needs insulin from a health worker, with care over food and exercise. Insulin is a protein, so it would be digested if it were swallowed. A health worker shows how it is given.
In type 2 diabetes the cells stop responding well to insulin. The pancreas may also make too little. It is the more common kind. It is linked to body mass, to little exercise, to family history and to growing older. A healthy diet, more exercise and a healthy body mass help. A health worker may also give medicines. Some people need insulin later.
Someone with very high glucose may be very thirsty, pass a lot of urine and feel tired. Someone whose glucose falls too low may feel shaky, confused or faint. Both need a health worker. If a person with diabetes is confused or faint, get help at once. Diabetes is not catching. You cannot get it from another person.
Try it
A model of the control
You need: two bowls, a cup, water, a piece of tape or a pen mark on the inside of one bowl, two or three people, a tray or a place where spills do not matter.
Be safe: Work on a tray or outside, and wipe up any spills so that no one slips. Use only water.
- Fill bowl A, the blood, with water up to the mark. Bowl B, the liver store, has a little water in it.
- One person is the food. At times of their choice they pour a cup of water into bowl A.
- One person is the pancreas. If the water in bowl A is above the mark, they say "insulin". The liver, the third person, moves a cup of water from bowl A to bowl B.
- If the water is below the mark, the pancreas says "glucagon". The liver moves a cup from bowl B back to bowl A. Play ten rounds and notice the level.
- Now play again with no pancreas. Nobody says "insulin" or "glucagon", and the liver does nothing. Watch what the level does.
What you will see: With the pancreas, the level stays close to the mark. Without it, every cup of food pushes the level up and it stays high. This is a model of type 1 diabetes, where too little insulin is made. The model is basic, and the real body is far more detailed.
A wrong idea many people have
Diabetes is caused by eating too much sugar.
Why it feels right: A person with diabetes has to take care over sugar, and sweet food does raise blood glucose.
The test that breaks it: In type 1 diabetes the pancreas makes too little insulin, and food does not cause this. Type 2 is linked to body mass, to little exercise, to family history and to age. Many people eat a lot of sugar and do not have diabetes.
What is true: Sugar on its own does not cause diabetes. Diabetes means the body cannot control its blood glucose. There is too little insulin, or the cells do not respond to it well.
Key words
- homeostasis
- keeping conditions inside the body steady.
- insulin
- a hormone from the pancreas that lowers blood glucose.
- glycogen
- a store of glucose, kept in the liver and the muscles.
- glucagon
- a hormone from the pancreas that raises blood glucose.
- negative feedback
- a change that sets off a response which reverses the change.
- diabetes
- a condition in which the body cannot keep blood glucose steady.
- type 1 diabetes
- diabetes in which the pancreas makes too little insulin.
- type 2 diabetes
- diabetes in which the cells respond badly to insulin.
Back to your guess
The answer is B: The pancreas releases insulin, and insulin makes cells and the liver take glucose out of the blood.
After a meal the pancreas releases insulin. Insulin makes the liver and muscle cells take glucose out of the blood, and the liver stores it as glycogen. Glucagon does the opposite job, when the level is low.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
What does insulin do?
See the answer
It makes liver and muscle cells take glucose out of the blood.
Insulin lowers blood glucose. The liver stores the glucose it takes in as glycogen.
- If you chose A: That is the job of glucagon. Insulin does the opposite.
- If you chose B: Food is broken down by enzymes in the digestive system. Insulin is a hormone, and it acts after the glucose is in the blood. Go to S3.3.2.
Between meals, a person's blood glucose starts to fall. What happens next?
See the answer
The pancreas releases glucagon, and the liver turns glycogen into glucose.
Glucagon makes the liver break down glycogen to glucose. The glucose enters the blood, and the level rises.
- If you chose B: More insulin would lower the level further. When the level is low, the pancreas releases glucagon.
- If you chose C: Insulin comes from the pancreas, not the liver. The liver releases glucose when glucagon tells it to.
Why is the control of blood glucose called negative feedback?
See the answer
The response reverses the change that started it.
A rise in glucose leads to insulin, which lowers it. A fall leads to glucagon, which raises it. The response reverses the change, so the level stays near the same value.
- If you chose A: That would be positive feedback. Here the response undoes the change.
- If you chose C: There is a response. Insulin or glucagon is released.
What is the main problem in type 1 diabetes?
See the answer
The pancreas makes too little insulin.
In type 1 diabetes the body's own defences destroy the cells that make insulin. Without insulin, glucose stays high in the blood.
- If you chose A: That mixes up two things. In type 1 the problem is too little insulin, and eating sugar does not cause it.
- If you chose B: Glucagon comes from the pancreas, not the liver. In type 1 the problem is that the pancreas makes too little insulin.
- If you chose C: Sensing is not the main problem. The cells that make insulin have been destroyed, so little insulin is released, however high the level goes.
Which is a main way to manage type 2 diabetes?
See the answer
A healthy diet and more exercise, with medicines from a health worker if needed
In type 2 diabetes the cells respond badly to insulin. A healthy diet, more exercise and a healthy body mass help them respond better. A health worker may also give medicines.
- If you chose A: Diabetes is not catching. You cannot get it from another person.
- If you chose B: Diet and exercise do matter in type 2. A health worker may also give medicines.
Which statement about diabetes is true?
See the answer
You cannot catch diabetes from someone who has it.
Diabetes is not an infection. It cannot pass from one person to another.
- If you chose A: Sugar on its own does not cause it. Type 1 is not caused by diet, and type 2 has many linked causes.
- If you chose C: Little exercise is linked to type 2. Type 1 happens when the cells that make insulin are destroyed.
From an earlier chapter
Which hormone does the thyroid gland make?
Look at the graph. How much higher is the highest glucose of the untreated diabetes line than the highest glucose of the healthy line, in g/L?
See the answer
1.2 g/L
The highest healthy value is 1.4 g/L. The highest untreated value is 2.6 g/L. 2.6 − 1.4 = 1.2 g/L.
A child has 3 L of blood with 0.9 g of glucose in each litre. How much glucose is in all the blood?
See the answer
2.7 g
3 × 0.9 = 2.7 g.
At 60 minutes after a meal, one person's blood glucose is 1.8 g/L and another person's is 1.2 g/L. How much higher is the first?
See the answer
0.6 g/L
1.8 − 1.2 = 0.6 g/L.
Keep it
Copy this into your exercise book.
Glucose high: pancreas releases insulin, cells and liver take in glucose, liver stores glycogen. Glucose low: glucagon, liver releases glucose. Type 1: too little insulin. Type 2: cells respond badly. See a health worker.
Draw: Two loops side by side: glucose rises, insulin, glucose falls; and glucose falls, glucagon, glucose rises.