Gastrointestinal system anatomy and physiology

The gastrointestinal tract consists of a long tube, where food travels through, which runs from the mouth to the anus, as well as a number of accessory organs that sprout off the sides of that tube.

The gastrointestinal tract is made up of the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and finally the anal canal.

The accessory organs include the teeth, tongue, salivary glands, the liver, gallbladder, and the pancreas.

The main job of the gastrointestinal system is ingestion - taking in food, digestion - breaking it down into nutrients, absorption - pulling these nutrients into the bloodstream, and finally, excretion - getting rid of waste.

All right, so let’s say we eat a slice of pizza. The pizza goes in our oral cavity where we use our teeth to masticate, or chew the food up into small fragments.

These fragments get tasted and rolled around by the tongue, which is basically a huge muscle that lines the floor of the mouth.

The roof of the mouth, which separates it from the nasal cavity, is made up by the anterior hard palate, which provides a hard surface for the tongue to mash food against and the posterior soft palate, which moves together, along with the pendulum- like uvula to form a flap or valve that helps makes sure food flows down instead of going up into the nose.

At the same time, the three sets of salivary glands - the sublingual, below the tongue, the submandibular, below the mandible, and the parotid gland, which is near the ear all secrete saliva to lubricate the food.

The saliva helps to make the food compact down into a soft, warm ball, called a “bolus”.

Saliva also contains salivary amylase, an enzyme that breaks long carbohydrates down into smaller sugars.

Once that bolus of food gets swallowed through the pharynx it goes into the esophagus. Right at that moment, there’s a spoon-shaped flap of cartilage called the epiglottis which acts like a lid and seals the airway off so that the food doesn't end up in the lungs by accident.

Now if we zoom into a cross-section of the rest of the gastrointestinal tract, anywhere from the esophagus till the anus, the walls are typically lined by the same four layers of tissue.

The outermost layer is either the adventitia, a thick fibrous connective tissue, or the serosa, a slippery serous membrane.

Next is the muscularis externa, a smooth muscle layer, which contracts automatically, without you even having to think about it. If we look closer at this muscle layer, it’s actually composed of an inner circular muscle layer, arranged in circular rings which contract and constrict the tract behind the food, which keeps it from moving backward, while the outer longitudinal muscle layer, arranged along the length of the tract, relaxes and lengthens and therefore pulls things forward. Together, they perform what’s called peristalsis, which is a series of coordinated wave-like muscle contractions that helps squeeze the food bolus in one direction.

In specific places along the tract, like the esophageal sphincter, the circular layer thickens, forming sphincters that keep food from passing from one part of the gastrointestinal tract to another.

Also, between the circular and the longitudinal muscle layer, there’s a plexus, or networks of nerves, which help coordinate muscle contraction and relaxation. This is the myenteric plexus, also called as Auerbach’s plexus, which when activated, causes smooth muscle relaxation.

Now, surrounded by the muscularis externa is the submucosa, which consists of a dense layer of tissue that contains blood vessels, lymphatics, and nerves.

Specifically, buried in the submucosa, there’s a second plexus, the submucous plexus, also called as Meissner’s plexus, which is responsible for helping to control the size of the blood vessels as well as the secretion of digestive juices.

And finally, there’s the inner lining of the intestine called the mucosa, which itself consists of three cell layers.

The outermost layer of the mucosa is the muscularis mucosa or muscularis interna, and it’s a layer of smooth muscle that contracts and helps break down food.

The middle layer is the lamina propria and it contains blood and lymph vessels.

Finally, there’s the innermost epithelial layer and it absorbs and secretes mucus and digestive enzymes because this is the layer that comes into direct contact with food.

Now, the esophagus has a particularly thick muscularis externa that propels the bolus of food down to the esophageal sphincter, which opens, allowing the bolus to pass into the stomach.

In the stomach, there are four regions - the cardia, the fundus, the body, and the pyloric antrum.

There’s also a pyloric sphincter, or valve, at the end of the stomach which closes while eating, keeping food inside for the stomach to churn over and over again.

To help churn the food, the stomach has an extra layer of oblique smooth muscle within its muscularis externa that allows it to contract and expand like a big accordion.

Also, the inner lining of the stomach has millions of tiny gastric pits that dive down to gastric glands. These glands contain a variety of secretory cells which produce gastric secretions.

Gastric secretions are made up of hydrochloric acid, which help destroy any pathogens that slipped through the food, an enzyme called pepsin, which chops up proteins, mucus which protect the stomach, as well as water, which turns the bolus into a liquidy pulp, called chyme.

Now, once the stomach is done, doing what stomachs do, the pyloric sphincter opens, allowing the chyme to pass into the small intestine.

The small intestine has three parts: the duodenum, the jejunum, and the ileum.