Homeostasis and Controlling Blood Glucose

The cells of your body are fussy. The enzymes that run their chemistry only work well within a narrow range of temperature, pH and water content, and they need a steady supply of glucose. Yet the world outside the body is constantly changing, and inside the body, activities such as exercise and eating cause continual upsets. The body's solution is homeostasis — keeping the internal conditions stable despite all these changes. This lesson explains what homeostasis is, sets out the negative-feedback model that controls it, and then works through one vital example in detail: the control of blood glucose, including diabetes and its link to body mass. This is part of Topic B3 of OCR Gateway Science A, and it brings together the nervous and endocrine systems from the previous lessons.

By the end of this lesson you should be able to define homeostasis, describe the general negative-feedback model, explain how insulin (and, at Higher tier, glucagon) controls blood glucose, distinguish Type 1 and Type 2 diabetes, and carry out a BMI calculation.

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What Is Homeostasis?

Homeostasis is the maintenance of a constant internal environment in the body, despite changes in the external and internal conditions. It keeps conditions stable so that cells and enzymes can work properly.

The main conditions kept constant by homeostasis are:

• body temperature (so enzymes are not denatured and reactions run at the right rate);
• blood glucose concentration (so cells have a steady supply of fuel for respiration);
• the water and ion content of the body (so cells are not damaged by water moving in or out).

Why does this matter so much? Because the body's enzymes are sensitive: if the temperature or pH moves too far from the optimum, enzymes denature and reactions stop; if the water balance is wrong, cells can shrink or burst. Homeostasis protects the delicate conditions that life depends on.

Exam Tip: A precise definition earns the mark: homeostasis is the maintenance of a constant (stable) internal environment. Add "so that enzymes and cells can work properly" to show you understand why it matters.

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The Negative-Feedback Model

All homeostatic control systems work in the same way, using a mechanism called negative feedback. The general model has three parts:

flowchart LR
  A["Receptor<br/>detects the change"] --> B["Coordination centre<br/>(brain, spinal cord, pancreas)<br/>processes the information"]
  B --> C["Effector<br/>(muscle or gland)<br/>brings about a response"]
  C -.->|"corrects the change,<br/>returning to normal"| A

• Receptors detect a change in a condition (for example, a rise in blood glucose).
• A coordination centre (such as the brain, spinal cord or pancreas) receives and processes the information.
• Effectors (muscles or glands) bring about a response that counteracts the change.

The key word is counteracts. If a level rises too high, the system acts to lower it; if it falls too low, the system acts to raise it. Either way, the condition is brought back to normal. Because the response always opposes the change, this is called negative feedback. You met exactly this pattern with thyroxine in the last lesson; you will now see it controlling glucose, and then temperature and water in the next lesson.

Exam Tip: Learn the trio receptor → coordination centre → effector and the phrase "the response counteracts the change, returning the level to normal". This single framework answers most homeostasis questions, whatever the specific condition.

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Controlling Blood Glucose

Glucose is the fuel for respiration, so cells need a steady supply. The concentration of glucose in the blood is controlled by the pancreas, which monitors it and releases hormones to keep it stable.

When blood glucose is too high

After a meal — especially a sugary or starchy one — blood glucose rises. The body must bring it back down.

1. The pancreas detects that blood glucose is too high.
2. The pancreas releases the hormone insulin into the blood.
3. Insulin causes body cells (especially liver and muscle cells) to take up glucose from the blood.
4. In the liver, excess glucose is converted into glycogen, an insoluble store.
5. As glucose moves out of the blood and into store, the blood glucose level falls back to normal.

So the rule to remember is: insulin lowers blood glucose, partly by converting glucose into glycogen in the liver.

When blood glucose is too low (Higher tier only)

Higher tier only: If blood glucose falls too low — for example during exercise or fasting — a second hormone corrects it.

1. The pancreas detects that blood glucose is too low.
2. The pancreas releases the hormone glucagon into the blood.
3. Glucagon causes the liver to convert stored glycogen back into glucose.
4. This glucose is released into the blood, so blood glucose rises back to normal.

The two hormones therefore work as an antagonistic pair: insulin lowers blood glucose (glucose → glycogen) and glucagon raises it (glycogen → glucose). Between them they hold the level steady.

flowchart TD
  A["Blood glucose TOO HIGH"] --> B["Pancreas releases INSULIN"]
  B --> C["Liver and cells take up glucose;<br/>liver stores it as glycogen"]
  C --> D["Blood glucose falls to normal"]
  E["Blood glucose TOO LOW<br/>(Higher)"] --> F["Pancreas releases GLUCAGON"]
  F --> G["Liver breaks glycogen back into glucose"]
  G --> H["Blood glucose rises to normal"]

Exam Tip: Do not confuse the two similar words. Glycogen is the storage molecule in the liver; glucagon is the hormone that releases glucose from it. A reliable memory aid: glucAGON makes glucose go into the blood.

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Diabetes

Diabetes is a condition in which the body cannot control blood glucose properly, so it can rise dangerously high (and sometimes fall too low). There are two types.

Feature	Type 1 diabetes	Type 2 diabetes
Cause	The pancreas stops producing enough insulin (often from childhood)	Body cells stop responding properly to insulin (insulin resistance)
Typical onset	Often begins in childhood	Usually develops in later life
Risk factors	Not linked to lifestyle	Strongly linked to obesity, poor diet and inactivity
Symptoms	High blood glucose; glucose in the urine; thirst; tiredness	Similar, often milder at first
Treatment	Insulin injections (and controlling diet)	Diet, exercise and weight loss first; sometimes medication

The key contrast is in the cause and the treatment. Type 1 is a problem of not enough insulin, so it is treated by injecting insulin (it cannot be taken as a tablet because it would be digested). Type 2 is a problem of cells not responding to insulin, and it can often be controlled by losing weight, eating a healthier diet and exercising more.

Exam Tip: Insulin for Type 1 diabetes must be injected, not swallowed, because it is a protein and would be digested in the stomach. This "why injected?" point is a common exam question.

Why uncontrolled blood glucose is dangerous

It is worth understanding why the body works so hard to control blood glucose, because this explains the symptoms of diabetes. Glucose is the fuel for respiration: every cell needs a steady supply to release the energy it requires. If blood glucose is too low (hypoglycaemia), cells — especially brain cells — cannot respire fast enough, which causes confusion, weakness and, in severe cases, unconsciousness. If blood glucose is too high (hyperglycaemia, as in untreated diabetes), the excess glucose begins to appear in the urine (the kidney cannot reabsorb it all), the person becomes very thirsty and tired, and over time high glucose damages blood vessels, harming the eyes, kidneys and circulation. Keeping blood glucose within its normal range therefore protects both the day-to-day energy supply of cells and the long-term health of the body.