How Digestion Works

Understanding digestion

Digestion involves putting some food (or a drink) in your mouth and swallowing it. The body then extracts energy, vitamins and minerals from the food or drink you just had and excretes the rest. Fairly simple, right? You know I’m kidding. Digestion is really complex— entire books have been dedicated to this essential body process. Here’s a very brief overview.

The mouth

I’m sure you’ve noticed that your mouth waters as soon as you smell food and when you take your first bite. Well, that’s your digestive system gearing up and stimulating the salivary glands to release saliva. Chewing partly breaks down the food while saliva moistens it — these two ‘processes’ makes it easier to swallow food. Saliva also allows the dissolved food molecules to reach the taste buds which emit signals — these signals and the food’s aroma are what enable you to enjoy the taste of your meals. Moreover, saliva contains salivary amylase, an enzyme which initiates the chemical digestion of food by splitting starch molecules into smaller carbohydrate molecules.

The pharynx and the esophagus

When you swallow, the bolus of food slides down the pharynx, or the throat, and into the esophagus which is also known as the food pipe; a muscular tube connecting the mouth to the stomach. Rhythmic contractions of the smooth muscles in the esophagus propel the food towards the stomach — an action called ‘peristalsis’. Once the bolus reaches the lower esophagus, it exerts pressure on the lower esophageal sphincter, a muscular valve. The pressure signals the sphincter to relax, allowing food to enter the stomach.

The stomach

Once in the stomach, the smooth muscles in this large sac-like organ churns the food with highly acidic gastric secretions to form a semi-liquid food mass called ‘chyme’. This gastric ‘juice’ is secreted by gastric glands in the stomach’s lining and consists of water, hydrochloric acid, mucus and an inactive form of pepsin, a protein-digesting enzyme. The hydrochloric acid kills the bacteria in the food, inactivates salivary amylase and helps begin protein digestion by activating pepsin.

With the exception of some water, alcohol, and drugs like aspirin and acetaminophen, very little absorption occurs here. Once the chyme is well mixed, the stomach walls propel about an eighth of an ounce of this liquefied blob through the pyloric valve and into the duodenum, the beginning of the small intestine.

The rate at which chyme empties from the stomach depends on the size and the composition of the meal. Obviously, a large meal stays in the stomach much longer than a small one. And since solid foods need to be liquefied, they linger in the stomach whereas liquid meals empty more quickly. A meal that consists mostly of carbohydrate from starch, refined products or sugar leaves the stomach quickly — which is why you’ll feel hungry soon after a high-carb meal. On the other hand, meals that are high in protein and fiber take more time to be converted to chyme and therefore take longer to leave the stomach. . A high-fat meal stays in the stomach the longest. What this means is that a meal with the right balance of protein, fats and fiber (from whole grains, fruits or vegetables) will keep you satisfied longer.

The small intestine

This narrow, coiled tube which is over 20-feet long, is the primary site for the chemical digestion of food as well as the absorption of water, vitamins, minerals, and the products of carbohydrate, fat, and protein digestion. It is made up of three segments: the duodenum, jejunum, and ileum. In the small intestine, secretions from the pancreas, the gallbladder, and the small intestine itself aid digestion as follows:
The pancreas secrete pancreatic juice which consists of bicarbonate and the digestive enzymes pancreatic amylase, proteases and lipase. The bicarbonate neutralizes the acid in the chyme, thereby allowing enzymes from the small intestine and the pancreas to function. The pancreatic amylase completes the digestion of carbohydrates into sugars; the pancreatic proteases (trypsin and chymotrypsin) break protein into amino acids and short-chain amino acids while pancreatic lipase digests fat into fatty acids. The pancreas also produces the hormones insulin and glucagon, which help regulate the level of sugar (glucose) in your blood.

The liver has over 500 functions but its main digestive function is to produce bile; a solution that allows lipases to access and digest the fat molecules more efficiently. The bile and digested fats then form small droplets, facilitating fat absorption into mucosal cells where the products of fat digestion are incorporated into transport particles which are then absorbed into the lymph before passing into the blood.

The gallbladder stores and secretes the bile through the cystic duct and into the duodenum when fat molecules enter the small intestine.

Digestion continues as bile and pancreatic secretions mix with digestive juices secreted by the wall of the small intestine and your now unrecognizable meal moves to the second portion of your small intestine, the jejenum where it is further broken down and nutrients absorbed. The ‘food’ then continues its journey to the last part of the small intestine — the ileum — where practically all of the nutrients left are absorbed via the lining of the ileum’s wall. What remains at this point is a combination of water, electrolytes (like sodium) and waste products including undigested plant fiber and dead cells shed by the lining of the digestive tract. This residue is then propelled towards the large intestine.

The large intestine

The large intestine, which ends with the anus, is about 5 feet long and is made up of two segments: the colon followed by the rectum (about 8 inches long). A small amount of water and a few vitamins and minerals are absorbed in the colon. Material which is not absorbed in the colon then moves into the rectum for temporary storage before being evacuated as feces via the anus. The volume and consistency of your feces depends on your overall nutrition, particularly on the amount of water you drink and fiber you consume. For soft stools that are easily passed, make sure to keep yourself well hydrated and choose high fiber foods over refined ones.