Human digestive system explained: organs, enzymes and how food is absorbed
The journey of food from mouth to large intestine: the organs involved, the enzymes that break down carbohydrates, proteins and fats, and where nutrients enter the blood.
Digestion breaks food into small molecules the body can absorb, through four steps: ingestion, mechanical and chemical breakdown, absorption, and elimination. Food travels from the mouth through the oesophagus and stomach to the small intestine, where most digestion and absorption happen, and the large intestine then reabsorbs water before waste leaves the body.
Mouth and oesophagus: where digestion starts
Digestion begins in the mouth. Teeth chew food into smaller pieces, and saliva from three major salivary glands moistens it into a soft mass called a bolus. Saliva contains salivary amylase, an enzyme that begins converting starches into maltose, and lipase from the tongue begins breaking down fats.
The tongue pushes the bolus into the oesophagus, a muscular tube that carries it to the stomach by peristalsis, wave-like contractions that move food in one direction. A flap called the epiglottis keeps food out of the windpipe during swallowing, and a ring-like sphincter at the stomach end of the oesophagus stops stomach contents travelling back up.
Stomach: acid, pepsin and chyme
The stomach is a sac-like organ that churns food and mixes it with gastric juices. Its environment is strongly acidic, with a pH between about 1.5 and 2.5, and this acid converts inactive pepsinogen into pepsin, the enzyme that begins protein digestion by breaking proteins into smaller polypeptides. The acid also kills many microorganisms in food.
A thick mucus lining protects the stomach wall from its own juices; when that lining is damaged, ulcers can form. Churning mixes the food into a semi-liquid mixture called chyme, which the pyloric sphincter releases into the small intestine a little at a time over a few hours after a meal.
Small intestine, accessory organs and the large intestine
The small intestine, over 6 metres long and divided into the duodenum, jejunum and ileum, is where digestion is completed and most nutrients are absorbed. Its lining is folded into finger-like villi covered in microscopic microvilli, hugely increasing the surface area for absorption into the bloodstream. Pancreatic juices neutralise the acidity of chyme and supply enzymes for starches, proteins and fats, while bile, made in the liver and stored in the gallbladder, emulsifies fats into small droplets so lipase can digest them.
Key enzymes to remember are amylase for carbohydrates, proteases such as pepsin and trypsin for proteins, lipase for fats, and lactase for milk sugar; people who make too little lactase cannot fully digest lactose. Absorbed nutrients travel via the hepatic portal vein to the liver, which regulates their distribution. What is left passes to the large intestine, which reabsorbs water and mineral salts and stores waste until it is eliminated through the rectum and anus.
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Frequently asked questions
What are the four steps of digestion?
Ingestion, mechanical and chemical breakdown of food, absorption of nutrients, and elimination of indigestible waste.
Which enzyme begins starch digestion and where?
Salivary amylase, in the mouth, begins converting starches into maltose. Pancreatic amylase continues carbohydrate digestion in the small intestine.
What is the role of bile in digestion?
Bile, made in the liver and stored in the gallbladder, emulsifies fats into small droplets in the duodenum so that lipase can break them down more easily. Bile is not itself an enzyme.
Where does most nutrient absorption happen?
In the small intestine, especially the jejunum, whose villi and microvilli greatly increase the surface area for absorption into the bloodstream.
What does the large intestine do?
It reabsorbs water and mineral salts from undigested food, hosts bacteria that aid digestion, and stores waste before elimination through the rectum and anus.