What you'll learn
- How insulin and Higher Tier glucagon control blood glucose concentration.
- The causes and control of type 1 and type 2 diabetes.
- How BMI and waist:hip ratio can be used to discuss diabetes risk.
- For Separate Biology, how the urinary system and nephrons filter blood, control water, and remove urea.
The big idea: homeostasis
Your body works best when internal conditions stay within a suitable range. For example, cells need a steady supply of glucose for respiration, but too much glucose in the blood can be harmful.
Homeostasis
Homeostasis is the maintenance of a constant internal environment in the body, even when external conditions change.
Blood glucose concentration means the amount of glucose dissolved in the blood. Glucose is a sugar used in respiration to release energy.
Hormones and negative feedback
A hormone is a chemical messenger carried in the blood. Hormones are released by glands and affect target organs or cells.
Blood glucose is mainly controlled by the pancreas, an organ near the stomach. The pancreas monitors blood glucose and releases hormones when the level is too high or too low.
Negative feedback
Negative feedback is when a change triggers responses that reverse the change, bringing the condition back towards normal.
This diagram shows the two main control loops: insulin lowers blood glucose after a meal, while glucagon raises it when blood glucose is too low.
Insulin lowers blood glucose
After you eat carbohydrate-containing food, digestion breaks starch and sugars down into glucose. Glucose is absorbed from the small intestine into the blood, so blood glucose concentration rises.
The pancreas detects this rise and releases insulin. Insulin causes body cells, especially liver and muscle cells, to take up more glucose from the blood. In the liver and muscles, glucose is converted into glycogen, an insoluble storage carbohydrate.
So the overall effect of insulin is:
- blood glucose concentration falls
- glucose is stored as glycogen
- the level returns towards normal
Blood glucose after a meal
A student eats a large bowl of pasta. Explain how their blood glucose concentration returns to normal.
- The pasta contains starch, which is digested into glucose, so glucose is absorbed into the blood and blood glucose concentration rises.
- The pancreas detects the rise and releases insulin into the bloodstream.
- Insulin causes liver and muscle cells to take up glucose and convert it into glycogen, reducing the glucose concentration in the blood.
- This brings blood glucose back towards the normal range, so less insulin is released by negative feedback.
Insulin does not break down glucose
Insulin is a hormone, not an enzyme. It does not digest glucose. It causes cells to take in glucose and store some of it as glycogen.
Glucagon raises blood glucose
For Higher Tier, you also need to explain how glucagon controls blood glucose concentration.
If you have not eaten for a while, or you are exercising, your cells use glucose in respiration. Blood glucose concentration may fall.
The pancreas detects the fall and releases glucagon. Glucagon acts mainly on the liver, causing stored glycogen to be broken down into glucose. The liver then releases glucose into the blood.
So the overall effect of glucagon is:
- glycogen is converted back into glucose
- glucose is released into the blood
- blood glucose concentration rises towards normal
Blood glucose during exercise
A runner’s blood glucose concentration falls during a long run. Explain how glucagon helps restore it.
- During exercise, muscle cells use more glucose for respiration, so the glucose concentration in the blood falls.
- The pancreas detects the low blood glucose and releases glucagon.
- Glucagon causes the liver to break down glycogen into glucose and release glucose into the blood.
- Blood glucose concentration rises back towards the normal range.
Type 1 diabetes
Diabetes is a condition where blood glucose concentration is not properly controlled.
In type 1 diabetes, the pancreas produces little or no insulin. This is usually because the insulin-producing cells in the pancreas have been destroyed by the immune system.
Without enough insulin, cells do not take up enough glucose after meals. Blood glucose concentration can become dangerously high, and glucose may appear in the urine.
Type 1 diabetes is controlled by:
- injecting insulin or using an insulin pump
- monitoring blood glucose concentration
- matching insulin dose to food intake and exercise
- eating a balanced diet
Type 2 diabetes
In type 2 diabetes, the body cells become less responsive to insulin. This is called insulin resistance. The pancreas may still produce insulin, especially at first, but the insulin does not lower blood glucose effectively.
Type 2 diabetes is often linked with obesity, lack of exercise, age, and genetic factors. It is controlled by:
- losing body mass if needed
- doing regular exercise
- eating a balanced diet with controlled carbohydrate intake
- taking medication to improve insulin response
- sometimes using insulin if other treatments are not enough
Do not swap the diabetes types
Type 1 diabetes is usually caused by the pancreas producing little or no insulin. Type 2 diabetes is usually caused by body cells not responding properly to insulin.
Body mass, BMI and type 2 diabetes risk
There is a correlation between higher body mass and increased risk of type 2 diabetes. A correlation means two variables are linked in a pattern. It does not automatically prove that one variable directly causes the other.
One common measure is BMI, which stands for body mass index. It compares a person’s mass with their height.
BMI=mass in kg(height in m)2\text{BMI} = \frac{\text{mass in kg}}{(\text{height in m})^2}BMI=(height in m)2mass in kgA second measure is the waist:hip ratio, which compares waist circumference with hip circumference.
waist:hip ratio=waist circumferencehip circumference\text{waist:hip ratio} = \frac{\text{waist circumference}}{\text{hip circumference}}waist:hip ratio=hip circumferencewaist circumferenceA higher waist:hip ratio suggests more fat stored around the abdomen, which is linked with a higher risk of type 2 diabetes.
Calculating BMI and waist:hip ratio
A person has a mass of 72 kg, a height of 1.60 m, a waist circumference of 82 cm, and a hip circumference of 100 cm. Calculate their BMI and waist:hip ratio.
- Substitute the mass and height into the BMI equation: BMI=72(1.60)2\text{BMI} = \frac{72}{(1.60)^2}BMI=(1.60)272
- Square the height first: (1.60)2=2.56(1.60)^2 = 2.56(1.60)2=2.56
- Divide the mass by this value: BMI=722.56=28.1\text{BMI} = \frac{72}{2.56} = 28.1BMI=2.5672=28.1
- Calculate the waist:hip ratio: waist:hip ratio=82100=0.82\text{waist:hip ratio} = \frac{82}{100} = 0.82waist:hip ratio=10082=0.82
- Use these values carefully: they can suggest risk, but they do not diagnose diabetes on their own.
Evaluating BMI
BMI is quick and useful for populations, but it does not distinguish between fat and muscle. A very muscular person may have a high BMI without having high body fat.
Correlation is not proof
If a graph shows that type 2 diabetes is more common at higher BMI, you can say there is a correlation. Do not say BMI alone “proves” someone will get diabetes.
Separate Biology: the urinary system
The urinary system content in this section is for Separate Biology in Edexcel 1BI0. It is still useful background if you study Combined Science, but the spec points ending in B are Separate Biology only.
The urinary system removes waste substances from the blood and controls water and ion levels. Its main parts are:
- kidneys: filter the blood and produce urine
- ureters: carry urine from kidneys to bladder
- bladder: stores urine
- urethra: carries urine out of the body
- renal artery: carries blood to the kidney
- renal vein: carries filtered blood away from the kidney
Each kidney contains many tiny filtering units called nephrons.
Nephron
A nephron is a microscopic structure in the kidney that filters blood and forms urine.
How the nephron forms urine
Blood enters a knot of capillaries called the glomerulus. The glomerulus sits inside a cup-shaped structure called the Bowman’s capsule.
High pressure forces small molecules out of the blood and into the Bowman’s capsule. This is filtration. Substances filtered out include water, glucose, urea and ions. Large proteins and blood cells stay in the blood because they are too large to pass through the filter.
The filtered fluid then flows along the tubule. Useful substances are taken back into the blood by selective reabsorption. Glucose is normally completely reabsorbed into the blood. Some water is also reabsorbed, depending on how much the body needs.
Urine contains mainly:
- urea
- excess water
- excess ions
Explaining glucose in urine
A urine test shows glucose in a person’s urine. Explain why this could happen in diabetes.
- Normally, glucose is filtered out of the blood at the glomerulus and then selectively reabsorbed from the tubule back into the blood.
- In diabetes, blood glucose concentration can become very high, especially after meals.
- If there is too much glucose in the filtrate, the kidney may not reabsorb all of it, so some glucose remains in the urine.
ADH and water control
For Higher Tier Separate Biology, you need to explain the role of ADH, which stands for anti-diuretic hormone.
ADH controls how much water is reabsorbed from the collecting duct back into the blood. It does this by changing the permeability of the collecting duct. Permeability means how easily substances can pass through a surface.
When the body has too little water, more ADH is released. The collecting duct becomes more permeable to water, so more water is reabsorbed into the blood. This produces a smaller volume of more concentrated urine.
When the body has too much water, less ADH is released. The collecting duct becomes less permeable to water, so less water is reabsorbed. This produces a larger volume of more dilute urine.
Urea and kidney failure
Urea is produced in the liver from the breakdown of excess amino acids. Amino acids come from digested proteins. The body cannot store excess amino acids, so the liver converts the nitrogen-containing part into urea, which is carried in the blood to the kidneys and excreted in urine.
If the kidneys fail, urea and excess ions can build up in the blood, and water balance becomes difficult to control.
Treatments for kidney failure include:
- Kidney dialysis: blood is passed through a machine with a partially permeable membrane. Urea diffuses out of the blood into dialysis fluid.
- Kidney transplant: a healthy donated kidney is placed into the patient. This can give a better quality of life than dialysis, but there is a risk of rejection.
Dialysis versus transplant
Dialysis can keep a person alive but takes time and must be repeated regularly. A transplant can be more long-term, but it needs a donor kidney and usually requires drugs to reduce rejection.
Testing simulated urine for sugar
A suggested practical is to test simulated urine or body fluids for sugar. A common test uses Benedict’s solution, which tests for reducing sugars such as glucose.
If glucose is present, Benedict’s solution changes from blue to green, yellow, orange or brick-red when heated in a water bath. A stronger colour change suggests more glucose is present.
Practical safety
Benedict’s solution is heated in a water bath, not directly over a flame. Wear eye protection and follow your teacher’s safety instructions.
In the exam
- For blood glucose questions, always state the starting change first: “blood glucose rises” or “blood glucose falls”.
- Link the correct hormone to the correct effect: insulin lowers blood glucose; glucagon raises it.
- For BMI or waist:hip questions, calculate carefully, then evaluate limitations instead of claiming the number proves someone has diabetes.
Check yourself
- How does insulin reduce blood glucose concentration after a meal?
- Why can type 2 diabetes be linked with body mass but not caused by BMI alone?
- In a nephron, which useful substance is normally selectively reabsorbed completely?
