The digestive system breaks down the food we eat into small molecules that the body can absorb and use for energy, growth, and repair. The long tube that food travels through is called the alimentary canal, or gut. It includes the mouth, esophagus, stomach, small intestine, and large intestine. Helper organs such as the salivary glands, liver, gallbladder, and pancreas add juices and enzymes to help digestion. Food moves along the tube by peristalsis, which means wave-like muscle contractions that push food forward.
Digestion starts in the mouth. The teeth chew the food, and the salivary glands release saliva containing salivary amylase. This enzyme begins the digestion of starch (carbohydrates) and turns it into maltose. The food then passes down the esophagus, which is also called the “food pipe” or gullet, and reaches the stomach.
The stomach is highly acidic, with a pH of about 1.5 to 3.5, mainly because of hydrochloric acid (HCl). HCl is secreted by the parietal (oxyntic) cells of the gastric glands. The chief (peptic) cells of the gastric glands secrete pepsinogen, which is the inactive form of pepsin. HCl changes pepsinogen into active pepsin, and pepsin is the stomach enzyme that digests proteins. The stomach also secretes intrinsic factor, which is needed for the absorption of vitamin B12 later in the intestine.
Most digestion and absorption of nutrients takes place in the small intestine. Its first part is the duodenum. Here, food mixes with bile from the liver and enzymes from the pancreas. The liver is the largest gland in the human body, and it produces bile. The gallbladder only stores bile and releases it when needed. Bile does not contain enzymes. Its main role is to emulsify fats, which means it breaks large fat droplets into tiny droplets so that enzymes can digest them more easily.
The pancreas is both a digestive gland and a hormone gland. It secretes digestive enzymes and also the hormones insulin and glucagon. Trypsin is a protein-digesting enzyme produced by the pancreas, and it works best in the alkaline medium of the small intestine. It is released as inactive trypsinogen, and in the duodenum an enzyme called enterokinase (enteropeptidase) activates it into trypsin. Pancreatic lipase digests fats into fatty acids and glycerol. Proteins are finally broken down into amino acids, which are the final products of protein digestion that the body absorbs.
The inner wall of the small intestine has many finger-like projections called villi. Their main purpose is to increase the surface area for the absorption of digested food. The digested products of fat are absorbed into the lacteals, which are small lymph vessels inside the villi.
The undigested food then moves into the large intestine. Its main function is to absorb water and salts and to form and store feces. The vermiform appendix is a small tube attached to the caecum, which is the first part of the large intestine.