Absorption, the Liver, the Large Intestine and the Control of Digestion

Digested is not the same as delivered

At the end of the last lesson, your grilled cheese sandwich had become glucose, amino acids, fatty acids and monoglycerides, floating inside the small intestine. But they are still outside your body, in a tube that runs through you. To reach your cells they must cross the lining of the small intestine and enter the blood or lymph. That step is absorption, and the small intestine is built for it.

The absorbing surface

The small intestine increases its surface area at three scales: large circular folds in its wall, millions of villi on the folds, and microvilli (the brush border) on each epithelial cell. Older textbooks often said the total surface was the size of a tennis court. More careful measurements published in 2014 estimated the inner surface of the whole digestive tract at about 32 m², almost all of it in the small intestine. That is a good reminder that scientific numbers get revised when better methods arrive.

Each villus has a one-cell-thick epithelium, a network of capillaries and a central lacteal (a small lymph vessel). The epithelial cells are packed with mitochondria, which supply ATP for active transport.

How each nutrient crosses

• Glucose and amino acids are taken into the epithelial cells mainly by active transport (using ATP-powered pumps), so they can be absorbed even when their concentration in the gut is lower than in the cells. They then pass into the capillaries.
• Fatty acids and monoglycerides are fat-soluble, so they diffuse through the cell membrane. Inside the cell they are rebuilt into triglycerides, wrapped with protein into tiny particles called chylomicrons, and released into the lacteal. Lymph carries them up the body and empties into the blood in a large vein near the heart.
• Water follows by osmosis as nutrients and salts are absorbed. Most of the water entering the tract (from food, drinks and secretions) is absorbed in the small intestine.
• Fat-soluble vitamins (A, D, E and K) travel with fats. Vitamin B12 needs a helper protein from the stomach (intrinsic factor) and is absorbed at the end of the ileum.

First stop: the liver

Blood from the capillaries of the intestines does not go straight to the heart. It collects in the hepatic portal vein and goes to the liver first. (A portal vein connects two capillary beds.) This lets the liver process everything you absorbed before it reaches the rest of the body. The liver:

• stores extra glucose as glycogen and releases glucose when blood sugar falls (with the help of the pancreatic hormones insulin and glucagon);
• removes the nitrogen from extra amino acids and turns it into urea, which the kidneys excrete;
• detoxifies alcohol, many drugs and other harmful substances;
• makes bile, cholesterol and most plasma proteins, including many clotting factors;
• stores iron and vitamins A, D and B12.

The large intestine

About 1.5 m long and wider than the small intestine, the large intestine includes the cecum (with the small, finger-like appendix attached), the colon (ascending, transverse, descending and sigmoid sections) and the rectum.

• It absorbs water and salts from the remaining material, turning liquid into semi-solid feces.
• It hosts most of the gut microbiota: trillions of bacteria. They ferment fibre our enzymes cannot digest, producing short-chain fatty acids that colon cells use for energy, and they make vitamin K and some B vitamins. A healthy, diverse microbiota also helps crowd out disease-causing microbes.
• Feces are about three-quarters water; the rest is mostly bacteria, undigested fibre, dead cells from the lining and bile pigments (which give the colour).
• When the rectum fills and stretches, a reflex creates the urge to defecate. If material moves too slowly, too much water is absorbed and constipation results; if it moves too fast, too little is absorbed (diarrhea).

Controlling digestion: nerves and hormones

Your digestive organs need to work at the right times: there's no point releasing bile into an empty intestine.

• Nerves: the sight, smell, taste or even the thought of food sends signals along nerves (mainly the vagus nerve) that start saliva and gastric juice before you take a bite. Stretch receptors in the stomach wall also trigger nerve reflexes.
• Hormones:
• Gastrin, from the stomach lining when food (especially protein) arrives, stimulates gastric juice and stomach churning.
• Secretin, from the duodenum when acidic chyme arrives, stimulates the pancreas to release bicarbonate.
• Cholecystokinin (CCK), from the duodenum when fats and proteins arrive, stimulates the gallbladder to contract (bile) and the pancreas to release enzymes. It also helps you feel full.

Secretin was the first hormone ever discovered, by the British physiologists William Bayliss and Ernest Starling in 1902. They found that the pancreas still released its juice when acid was put into the duodenum even after the nerves to the area were cut, so the signal had to travel in the blood. Starling later coined the word hormone, from a Greek word meaning "to set in motion".