The Digestive System

One Long Tube, Plus Helpers

The digestive system is a single continuous tube running from the mouth to the anus, roughly nine meters long in an adult, with several accessory organs emptying into it along the way. Its job is breaking food into pieces small enough to be absorbed, absorbing them, and disposing of what is left.

Technically, the inside of that tube is outside the body. Nothing has entered you until it crosses the intestinal wall into the blood.

The Path Food Takes

Mouth. Chewing breaks food apart mechanically. Saliva adds amylase, an enzyme that begins breaking down starch, so chemical digestion starts here too.

Pharynx and esophagus. Swallowing pushes the food bolus into the pharynx, the epiglottis folds over the larynx, and the bolus enters the esophagus. From there, peristalsis, a wave of smooth muscle contraction, moves it downward. Peristalsis is why you can swallow while lying down or upside down.

At the bottom, the lower esophageal sphincter keeps stomach contents from traveling back up. A weak one produces reflux.

Stomach. Sits in the left upper quadrant. It stores food, mixes it, and secretes hydrochloric acid and pepsin, which begins protein breakdown. Stomach acid runs around pH 1 to 2, strong enough that the stomach lines itself with mucus to avoid digesting itself. A failure of that protection produces an ulcer.

Food leaves the stomach as a semi liquid called chyme, released through the pyloric sphincter in controlled amounts.

Small intestine. About six meters, in three parts: duodenum, jejunum and ileum. Nearly all chemical digestion and nearly all absorption happen here. The duodenum receives bile from the gallbladder and enzymes from the pancreas.

The inner surface is covered in villi, tiny finger like projections, each covered in still smaller microvilli. That folding turns a modest tube into an enormous absorptive surface. Nutrients cross into the capillaries inside each villus, and fats are absorbed into lymphatic vessels called lacteals.

Large intestine. About 1.5 meters: cecum, ascending colon up the right side, transverse colon across, descending colon down the left, sigmoid colon, then rectum and anus. Very little digestion happens here. Its job is reabsorbing water and electrolytes and forming stool. Gut bacteria living here produce some vitamin K and B vitamins.

The appendix hangs off the cecum in the right lower quadrant. Inflammation of it is appendicitis, which classically starts as vague pain around the umbilicus and then localizes to the right lower quadrant.