Gastrointestinal system anatomy and physiology

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Introduction0:00–0:48

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.

Oral cavity0:48–2:08

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.

Histology2:08–4:26

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.

Esophagus and Stomach4:26–5:32

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.

Small Intestine5:32–6:13

The small intestine has three parts: the duodenum, the jejunum, and the ileum. Now, despite its name, the small intestine can be as long as 10.5 meters or about 35 feet, with lots of tiny ridges and grooves, each of which projects little finger-like fibers called villi.
And in turn, each villus is covered in teeny tiny little microvilli. All of this gives the small intestines plenty of surface area to absorb nutrients.
But almost no nutrient absorption can take place without the help of the trio of accessory organs - the liver, gallbladder, and the pancreas.

Liver, Gallbladder, and Pancreas6:13–7:45

First up, the liver is a massive organ that sits under the right dome of the diaphragm, and it makes bile. So, going back to the pizza slice - at this point, the fats in the cheese are part of the chyme, and that stimulates the enteroendocrine cells or hormone secreting cells of the small intestine to secrete a hormone called cholecystokinin into the blood.
The cholecystokinin makes its way to the gallbladder, the thin walled, green sac snuggling up against the liver, and signals it to squeeze out some bile through the cystic and bile ducts into the small intestine.
This is pretty much your gallbladder’s job—store and concentrate the bile that’s made by the liver until the time comes to squirt it out into the small intestine.
That bile emulsifies the fat, essentially making fat organize into small “micelles”, which are tiny bubbles of mixed lipids and bile acids, rather than clumping together, and making them easier to absorb.
And this is where the pancreas comes in. The pancreas is a long, skinny gland the length of a dollar bill, snuggled around by the duodenum.
Cholecystokinin stimulates acinar cells in the pancreas to secrete digestive enzymes that travel through the pancreatic ducts and into the duodenum.
One enzyme is pancreatic lipase which grabs the triglycerides hanging out in those “micelles” and breaks them down into fatty acids and glycerol.
There’s also pancreatic amylase which breaks down carbohydrates into shorter oligosaccharides, and proteases, like trypsin, which cleaves proteins down into smaller peptides.
Now, to prevent the hydrochloric acid from the stomach from damaging the intestinal mucosa, enteroendocrine cells also secrete another hormone, called secretin, which stimulates the pancreatic duct cells to secrete water and bicarbonate.

Intestinal secretions7:45–9:00

Bicarbonate helps neutralize the acidic chyme, and raising the pH in the intestinal lumen also helps digestive enzymes work more effectively.
A generous amount of bicarbonate is also secreted by a number of glands lying in the submucosa of the duodenal wall. At this point, we’re almost ready for absorption.
Fatty acids and glycerol can easily pass through the small intestinal epithelial and into the lymphatics. But to help absorb sugars, there are special enzymes, on the top surface or brush border of the intestinal cells, called brush border enzymes.
These are maltase, sucrase, and lactase, which break down the short chains of sugars called oligosaccharides into simple sugars, called monosaccharides.
These include glucose, fructose, and galactose. Similarly, there are peptidases which break down peptide chains into single amino acids.
The epithelial cells can absorb these nutrients into the bloodstream, and from there they can go to various tissues around the body.
All right, so whatever isn’t absorbed, like fiber, continues its journey onward through the ileocecal sphincter and into the very last part of the gastrointestinal tract: the large intestine, also known as colon.

Large intestine9:00–10:48

The large intestine is basically a one and a half meter or five feet long loop that frames the small intestine and consists of six parts: the cecum, which has a tiny worm-like little outpouching called the appendix, the ascending colon that climbs along the right side of the abdomen, the transverse colon right beneath the diaphragm, the descending colon, running down the left side of the abdomen, the S-shaped sigmoid colon and finally, the rectum.
When chyme hits the cecum, it’s met by trillions of bacteria that colonize the large intestine - they’re collectively called the gut microbiome.
This microbiome is still being studied and understood, but we know that the bacteria help produce essential B and K vitamins, as well as gases like carbon dioxide, methane and sulfurous compounds which sometimes get released in awkward moments.
The chyme slowly moves through the large intestine through small waves of peristalsis that take place over hours or even days.
The large intestine absorbs excess water from the chyme, and that helps condense it into dry fecal matter. And that feces eventually ends up in the rectum.
Once the rectum is filled and stretched, signals travel to parasympathetic neurons in the spinal cord, initiating the defecation reflex.
These parasympathetic neurons make the rectum contract and the internal anal sphincter relax. Meanwhile, signals are sent to the brainstem and thalamus, and when these decide the right moment has come, they allow the external anal sphincter to relax - and feces goes, hopefully, into the toilet bowl.

Review10:48–11:58

Alright, as a quick recap, food is ingested through the mouth, chewed by the teeth, mixed with saliva and turned into a bolus.
The bolus is then moved by peristalsis through the esophagus and into the stomach, where hydrochloric acid is secreted and pepsin begins the digestion process.
Next is the small intestine, where most of the digestion and absorption occurs, with the help of the liver’s bile coming from the gallbladder and the pancreatic enzymes.
Finally, the large intestine absorbs water, the feces forms and gets excreted through the anus.