Blood glucose regulation (SACE Stage 2 Biology)

What this dot point is asking

You need to name the control gland and hormones, explain the two opposing feedback responses, and explain the two main types of diabetes as a failure of this control.

The variable and its control centre

Blood glucose concentration must be kept within a narrow range. Too high (hyperglycaemia) damages tissues and changes water balance; too low (hypoglycaemia) starves cells, especially brain cells, of energy.

The control centre is the pancreas, which monitors blood glucose and releases two opposing hormones from its islet cells: insulin and glucagon. Their main target is the liver.

Response to high blood glucose

After a meal, blood glucose rises above the set point:

1. The pancreas detects the rise and releases insulin into the blood.
2. Insulin makes liver and muscle cells take up glucose and convert it to glycogen for storage (and promotes glucose use by cells).
3. Blood glucose falls back toward the set point, and insulin release decreases.

Response to low blood glucose

Between meals or during exercise, blood glucose falls below the set point:

1. The pancreas detects the fall and releases glucagon.
2. Glucagon makes the liver break stored glycogen back down into glucose, which is released into the blood.
3. Blood glucose rises back toward the set point, and glucagon release decreases.

Diabetes

Diabetes mellitus is a failure of blood glucose regulation, so glucose stays dangerously high.

• Type 1 diabetes: the pancreas cannot produce enough (or any) insulin, often because the insulin-producing cells are destroyed by the immune system. It usually appears early in life and is managed with insulin injections.
• Type 2 diabetes: the pancreas produces insulin, but body cells become resistant and no longer respond to it properly. It is linked to lifestyle and diet and is often managed with diet, exercise and medication.

In both, glucose cannot be taken into cells effectively, so blood glucose remains high.

Why this is endocrine control

Blood glucose regulation is a hormonal (endocrine) example of homeostasis, which is why it contrasts with fast nervous control. Insulin and glucagon are secreted into the blood and travel to target cells everywhere, so the response is slower to start but longer-lasting and widespread. The pancreatic islets contain two cell types: beta cells secrete insulin in response to high glucose, and alpha cells secrete glucagon in response to low glucose. Because the two hormones have opposite effects on the same variable, they are described as antagonistic, and having opposing effectors is what lets the system correct deviations in either direction rather than only one.

How insulin actually lowers glucose

Strong answers explain the mechanism, not just the outcome. Insulin binds receptors on liver, muscle and fat cells and triggers two main effects: it increases the number of glucose transporter proteins in the cell membrane so cells take up glucose faster by facilitated diffusion, and it activates the enzymes that convert glucose into glycogen (glycogenesis) for storage. The combined effect of more uptake and more storage removes glucose from the blood. Glucagon does the reverse, activating the enzymes of glycogen breakdown (glycogenolysis) in the liver. The clinical signs of untreated diabetes - glucose appearing in the urine and excess urine production - follow directly from glucose staying so high that the kidney cannot reabsorb it all, linking this dot point to osmoregulation.