The pancreas and the control of blood glucose

Sugar in the blood

After a sweet drink, the sugar of the blood rises. A few hours later, it is back to normal. A hungry person does not run out of sugar either. Two hormones of the pancreas keep the level steady. The reading in a healthy person who has not eaten for eight hours is about 70 to 100 mg/dL.

The pancreas

The pancreas is a composite gland. It is exocrine, because it makes digestive juice for the duct that leads to the duodenum. It is also endocrine, because it makes hormones for the blood.

The endocrine part is made of the islets of Langerhans. A normal human pancreas has about 1 to 2 million of them. They are only 1 to 2 per cent of the tissue of the pancreas.

The islets have two main types of cells.

• The alpha cells make glucagon.
• The beta cells make insulin.

Glucagon

Glucagon is a peptide hormone. It acts mainly on the liver cells, the hepatocytes.

• It stimulates glycogenolysis, the breakdown of glycogen to glucose. This raises the glucose of the blood, which is hyperglycaemia.
• It stimulates gluconeogenesis, the making of glucose from other substances. This too adds to the hyperglycaemia.
• It reduces the uptake and the use of glucose by the cells.

So glucagon is a hyperglycaemic hormone.

Insulin

Insulin is a peptide hormone with a major part in the regulation of glucose. It acts mainly on the hepatocytes and the adipocytes, the cells of fat tissue.

• It increases the uptake and the use of glucose by the cells. Glucose moves quickly from the blood into the liver cells and the fat cells.
• It stimulates the change of glucose to glycogen in the target cells, which is glycogenesis.
• As a result the glucose of the blood falls. This is hypoglycaemia when it goes too low.

So insulin is a hypoglycaemic hormone. A-Level and IB add that insulin also acts on muscle.

Insulin and glucagon work against each other. Together they keep the level steady, and this is called glucose homeostasis.

Diabetes mellitus

A long period of high blood glucose leads to a complex disorder called diabetes mellitus.

• Glucose is lost through the urine. When the blood glucose goes above about 180 mg/dL, the kidney cannot take back all of it.
• Harmful compounds called ketone bodies are formed.
• The person passes much urine, is thirsty and loses weight.

A person is treated by insulin therapy. A-Level and IB teach two main kinds. In type 1 diabetes the beta cells are destroyed, so there is no insulin. In type 2 the cells respond poorly to the insulin that is made. NCERT does not name the types.

How it is diagnosed. The World Health Organization counts a person as diabetic if one of two levels is reached. The first is a fasting glucose of 126 mg/dL (7.0 mmol/L) or more. The second is 200 mg/dL (11.1 mmol/L) or more two hours after a drink of 75 g of glucose. A level may be reported in mg/dL or in mmol/L. One mmol/L is 18 mg/dL. These cut-offs are not in the NEET syllabus text, but A-Level and IB use them.

You should find that the normal curve peaks at about 140 mg/dL after 45 minutes and is back near 115 mg/dL at 2 hours. When the cells respond less to insulin, the fasting level is higher, the peak is higher and later, and the glucose stays high at 2 hours. This is a teaching model. It shows the idea of the test, not real data.

Try it in Python

The program prints the glucose at five times for a chosen response of the cells to insulin.

Practise with new numbers

Common mistakes

• Mixing up insulin and glucagon. Insulin lowers the glucose and glucagon raises it. Beta cells make insulin and alpha cells make glucagon.
• Saying that insulin is made by the exocrine pancreas. It comes from the beta cells of the islets.
• Mixing up diabetes mellitus and diabetes insipidus. Mellitus is a lack of insulin or its action, with sugar in the urine. Insipidus is a lack of ADH, with no sugar.
• Saying that the islets are most of the pancreas. They are only 1 to 2 per cent of its tissue.
• Forgetting the unit of a glucose report. A number in mmol/L is about 18 times smaller than the same level in mg/dL.

Remember these

• Pancreas: exocrine and endocrine. The islets of Langerhans (1 to 2 million, 1 to 2 per cent of the tissue) have alpha cells and beta cells.
• Glucagon: alpha cells, hyperglycaemic, acts on the liver (glycogenolysis, gluconeogenesis).
• Insulin: beta cells, hypoglycaemic, acts on the liver and fat cells (glucose uptake, glycogenesis).
• Diabetes mellitus: long hyperglycaemia, glucose in the urine, ketone bodies. Insulin therapy.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which cells of the islets make insulin, and which make glucagon?

2. Why is glucagon called a hyperglycaemic hormone?

3. On which cells does insulin mainly act, and what happens to the glucose of the blood?

4. What three things happen in a person with untreated diabetes mellitus?

5. A report gives a fasting glucose of 8.0 mmol/L. Is this above the cut-off of 126 mg/dL for diabetes?

References

• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 17, The Endocrine System.
• World Health Organization (2006). Definition and diagnosis of diabetes mellitus and intermediate hyperglycaemia, report of a WHO/IDF consultation, Geneva.
• Hall J. E. and Hall M. E. (2021). Guyton and Hall Textbook of Medical Physiology, 14th edition, Elsevier, the chapter on insulin, glucagon and diabetes mellitus.
• NCERT (2023). Biology, Textbook for Class XI, chapter 19.