How does the pancreas work?

Your pancreas is a soft, pale, bumpy organ about 15 cm long and about 80 g, roughly the weight of a small banana. It lies deep in the upper belly, behind the stomach, across the back wall. We are looking from the front, so the patient's right is on your left.

It has four parts. The fat head sits snugly in the C-shaped curve of the duodenum, the first part of the small intestine (see IntestineClear). A little hook, the uncinate process, curls behind a big vein. Then come the neck, the body, which crosses in front of the aorta and spine, and the tail, which reaches up to the spleen.

Down its middle runs the main pancreatic duct, like the spine of a leaf. In the head it meets the common bile duct from the liver and gallbladder, and they empty together through a tiny opening in the duodenum, the ampulla of Vater.

The pancreas is really two organs in one. About 98% of it is an exocrine factory that makes digestive juice and sends it down the ducts. Scattered through it, like islands, are about a million tiny islets of Langerhans, only 1 to 2% of its weight. They are endocrine: they release hormones such as insulin straight into the blood.

Almost all of the pancreas is made of tiny clusters of cells called acini (from the Latin for "grapes"). Each acinar cell is a busy factory. It builds digestive enzymes, packs them into little parcels called zymogen granules at its tip, and releases them into a tiny channel in the middle. Channels join into bigger ducts, like streams into a river, and end at the duodenum. You make about 1 to 1.5 litres of pancreatic juice a day.

The juice holds three main kinds of enzyme: amylase cuts starch, lipase cuts fats, and proteases such as trypsin cut proteins. Here is the clever part: the pancreas is made of protein too. So the proteases are made switched off, as trypsinogen and friends. Only in the duodenum does an enzyme on the gut lining, enteropeptidase, snip trypsinogen into active trypsin, which then switches on the others. The pancreas never digests itself, as long as the lock holds. You can watch these enzymes cut up a real meal in IntestineClear.

The cells lining the ducts add water and bicarbonate, the same stuff as baking soda. Food arrives from the stomach (see StomachClear) as an acidic soup at about pH 2, and bicarbonate neutralises it.

How does the pancreas know food has arrived? Hormones. Acid in the duodenum makes it release secretin into the blood, which says "send bicarbonate". Fat and protein make it release cholecystokinin (CCK), which says "send enzymes", and also squeezes the gallbladder. Secretin, found in 1902, was the first hormone ever discovered.

An islet of Langerhans is a ball of about a thousand to a few thousand cells, smaller than a grain of salt, wrapped in tiny blood vessels. There are about a million of them, scattered through the pancreas like islands in a sea of juice-making cells. The German student Paul Langerhans first saw them in 1869, without knowing what they did.

An islet holds three main kinds of cell. Beta cells (about 55 to 60% in people) make insulin, which lowers blood sugar. Alpha cells (about 30 to 40%) make glucagon, which raises it. Delta cells (under 10%) make somatostatin, which calms both down. In humans the cell types are mixed together, and every cell is close to a blood vessel so its hormone can leave quickly.

A beta cell does not need a brain to decide. It measures sugar by burning it. Glucose flows in, the cell's power stations turn it into the energy molecule ATP, and ATP closes potassium (K⁺) channels. That changes the voltage across the cell's skin, which opens calcium (Ca²⁺) channels. Calcium rushing in is the signal for thousands of tiny insulin granules to fuse with the surface and pour insulin into the blood. More sugar, more ATP, more insulin.

Every cell in your body burns glucose, a simple sugar, for energy, and your brain uses about 100 g of it a day. Yet all your blood holds at any moment is about 4 to 5 grams, roughly a teaspoon. Keep too little and the brain struggles; keep too much for years and blood vessels get damaged. So the islets run a thermostat.

After a meal, glucose pours in from the gut (see IntestineClear). Beta cells sense the rise and release insulin. Insulin is like a key: it lets muscle and fat cells open doors for glucose, and tells the liver to store it as glycogen (see LiverClear). Sugar falls.

Hours later, or overnight, sugar starts to dip. Insulin drops and alpha cells release glucagon, which tells the liver to release its stored glucose. Sugar rises again. The brain does not need insulin to take in glucose, which is why keeping it supplied matters so much.

In most people this keeps blood glucose around 70 to 100 mg/dL before breakfast and usually below about 140 mg/dL two hours after eating. A walk helps too: working muscles pull in glucose even without extra insulin.

This chapter shares general facts, not medical advice. Only a doctor can diagnose diabetes. If you are worried, talk to a doctor.

Diabetes means blood glucose stays too high because the body makes too little insulin, or cannot use it well, or both. It is very common and nobody's fault. Around 589 million adults live with it worldwide, and a large national study, ICMR-INDIAB, estimated about 101 million people in India in 2021, with another 136 million who have prediabetes.

In type 1 diabetes, the body's own immune system destroys the beta cells, so almost no insulin is made. It is most often found in children and young adults but can start at any age, and it is not caused by eating sugar or by anything a person did. People with type 1 need insulin every day, prescribed by their doctors.

Type 2 diabetes is the most common kind, about 9 in 10 cases. The body's cells respond less to insulin (insulin resistance), and over time the beta cells cannot keep up. Genes, family history, age, body weight and activity all play a part, and in South Asia it often appears at younger ages.

High glucose can make people very thirsty, pass a lot of urine and feel tired. Above about 180 mg/dL the kidneys let sugar spill into urine: the "honey urine", madhumeha, that ancient Indian doctors described. Over years, high glucose can damage blood vessels in the eyes, kidneys, nerves and heart (see EyeClear and HeartClear), which is why diagnosis and care matter.

Doctors often use the HbA1c blood test. Glucose sticks to haemoglobin in red blood cells, which live about three months, so HbA1c shows the average of the last 2 to 3 months. By ADA thresholds, 6.5% or more is in the diabetes range, and 5.7 to 6.4% is prediabetes.

This chapter shares general facts, not medical advice. If you are worried about your own health, talk to a doctor.

Pancreatitis means an inflamed pancreas. It happens when digestive enzymes switch on inside the pancreas instead of in the gut, so the gland starts to digest itself. The two most common causes are gallstones, which can block the shared opening at the ampulla, and heavy alcohol use. The typical sign is sudden, severe pain in the upper belly that may spread to the back. That needs urgent medical care. Most mild cases get better in about a week in hospital.

Pancreatic cancer usually starts in the cells lining the ducts. Because the pancreas lies so deep, behind the stomach, it often causes no signs early and is hard to find. Possible signs include yellow skin or eyes (jaundice, when a growth in the head squeezes the bile duct), belly or back pain, and losing weight without trying. Risk rises with age and smoking. Anyone with such signs should see a doctor.

Type 2 diabetes risk can often be lowered. In two famous studies, one in the USA and one in India, adults with raised glucose who were supported to be more active and eat more healthily were much less likely to develop type 2 over three years. The WHO suggests adults get at least 150 minutes of moderate activity, like brisk walking, a week. Type 1 diabetes cannot be prevented this way.