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.

The Accessory Organs

Liver. Right upper quadrant, tucked under the lower right ribs, the largest solid organ in the body. It produces bile, processes everything absorbed from the intestine, stores glucose as glycogen and releases it on demand, makes clotting factors and plasma proteins, and detoxifies drugs and alcohol.

The liver holds a large volume of blood at any moment and is highly vascular, which is why liver injury bleeds heavily. Lower right rib fractures are a reason to expect it.

Liver failure has consequences worth connecting. Fewer clotting factors means the patient bleeds more easily. Less detoxification means medications last longer and ammonia accumulates, which produces confusion.

Gallbladder. A small sac under the liver that stores and concentrates bile. A fatty meal signals it to contract and push bile into the duodenum. Gallstones blocking that outflow produce right upper quadrant pain, often after eating, and often referred to the right shoulder blade.

Pancreas. Lies across the upper abdomen behind the stomach, with its head in the right upper quadrant and its body and tail extending into the left. It has two separate roles. The exocrine portion makes digestive enzymes that break down fat, protein and carbohydrate. The endocrine portion makes insulin and glucagon.

Pancreatitis is inflammation in which those enzymes activate early and begin digesting the pancreas itself. Severe upper abdominal pain radiating straight through to the back is the pattern.

Spleen. Not a digestive organ, though it sits in the left upper quadrant alongside them and belongs to the lymphatic system. It filters blood and holds a reserve of red cells. It is fragile, well supplied with blood, and a common source of serious bleeding after blunt trauma to the left side. Left lower rib fractures raise the same concern the right side does for the liver.

Solid Versus Hollow

The distinction determines what an injury does.

Solid organs are the liver, spleen, pancreas and kidneys. They are dense and vascular, and injury produces bleeding, which can be rapid and is not visible from outside.

Hollow organs are the stomach, intestines, gallbladder and bladder. Injury spills their contents into the abdominal cavity, which produces irritation and infection rather than immediate blood loss. Signs can develop over hours, and the patient's abdomen becomes rigid and exquisitely tender as the peritoneum reacts.

Both matter. The solid organ patient can bleed to death quickly, and the hollow organ patient can look deceptively well for a period before becoming very sick.

Referred Pain

Abdominal organs have relatively poor sensory mapping, and their nerve fibers enter the spinal cord alongside fibers from areas of skin. The brain sometimes attributes the signal to the skin region rather than the organ.

Patterns worth recognizing:

A patient with left shoulder pain after being tackled has nothing wrong with the shoulder. The blood irritating the diaphragm is producing the sensation.

Visceral pain comes from the organ itself and is dull, cramping and poorly localized. Parietal pain comes from the peritoneal lining and is sharp, constant and well localized, so a patient who can point to one spot has a different and generally more advanced problem than one who waves a hand over the whole abdomen.

Gastrointestinal Bleeding

Upper GI bleeding comes from the esophagus, stomach or duodenum. Vomited blood may look bright red if recent, or like coffee grounds if it has been in contact with stomach acid. Blood that travels the length of the intestine appears in stool as melena, black and tarry with a distinctive odor.

Lower GI bleeding from the colon or rectum shows as bright red blood in the stool, because it has not been exposed to acid or bacteria for long.

A patient with esophageal varices, which are dilated veins that develop in liver disease, can bleed enormously and quickly.

What to Take Away

One tube from mouth to anus, with liver, gallbladder and pancreas feeding into it. Nearly all absorption happens in the small intestine. Solid organs bleed, hollow organs spill. Learn the quadrant locations, because that is how abdominal findings are reported, and learn the referred pain patterns, because the patient will point somewhere other than the problem.