Human Digestive System/Printable version

Human Digestive System

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Diagram of a human mouth.

The mouth is the entry point for our nutrition. The most obvious function is to masticate, or chew, food to a size and texture able to pass through the esophagus to the stomach. Many more things happen in the mouth. Saliva is produced by glands called the salivary glands. Saliva contains enzymes that begin to break down food. The nerves of the brain send signals to the rest of the digestive tract to prepare for digestion. Both fluids and chewed foods have to be passed to the back of the throat and into the esophagus without blocking the breathing tube (the trachea). The teeth grind food. The tongue tastes food and crushes food against the hard palate. The tongue moves food particles into a bolus that is pushed into your pharynx. The soft palate and uvula close off the nasopharynx when swallowing.


Diagram of the neck with Pharynx labeled.

The pharynx moves food from the mouth to the esophagus.


Esophagus anatomy
1. Pharynx
2. Epiglottis
3. Larynx
4. Esophagus

The esophagus is a soft tube that is usually about 25 centimeters in length. When food or liquids are passed from the mouth, it is pushed down the esophagus by circular muscles. These muscles squeeze in top to bottom order, pushing the food and liquid down, like toothpaste being pushed through a tube. At the bottom of the esophagus is a barrier called a sphincter. This muscle is like a door that opens to let the esophagus contents enter the stomach. The sphincter then closes to keep stomach contents from coming up. Sometimes this muscle doesn't work well and stomach juices splash into the esophagus causing acid reflux and heartburn.


Anatomy of a human stomach

The stomach is the place where food and liquids that pass from the esophagus are stored and prepared for digestion. The upper part of the stomach is called the fundus (See part A). This part of the stomach stretches to make room for large meals. The food and liquid is then ground up by the lower part of stomach, called the pylorus (See part D). As the food is broken down to tiny particles, stomach juices, including acid and pepsin, begin to break down the food further. Liquids tend to leave the stomach first. When the food is small enough, it passes through the pyloric valve, into the small bowel.

Small intestine

Intestine's anatomy

The small intestine is where food breakdown continues and absorption of nutrition occurs. The small intestine is about 7 meters long and is divided into three sections: duodenum, jejunum and ileum. Different types of nutrition are absorbed in the different sections of the small bowel.

As food passes from the stomach, it is bathed in secretions of the liver (bile) and pancreas, breaking down fats and proteins and carbohydrates. There are small, finger-like projections from the surface of the small bowel, called villi. If you look under a microscope you can see where much of the nutrition breakdown and absorption occurs.

Eventually, most of the nutrients are extracted from the food and the waste is passed into the colon.



The liver is the largest visceral organ and performs over 200 functions. Some of the main functions include:

  • the metabolism of absorbed substances

- carbohydrate metabolism: synthesize and store glycogen when glucose levels become elevated and breakdown of glycogen when glucose levels drop below normal

- lipid metabolism: triaglycerides and cholesterol are released from the liver when blood levels drop

- amino acid metabolism: amino acids are synthesized into proteins or can be changed into glucose or lipids when there is demand for energy.

- vitamin and mineral storage: liver serves as a reservoir for most vitamins and minerals.

  • blood regulation

- drug, waste product and toxin removal

- elimination of antibodies

- processing of hormones circulating in the blood like adrenal, thyroid, hormones, vitamin D, etc.

- makes plasma proteins such as albumin and clotting factors

- produces bile

The production of bile is important for digestion. Bile salts help emulsify fats by creating micelles (lipid like droplets). The formation of micelles creates a hydrophobic region inside the core and a hydrophilic region outside. Inside a micelle there are lipids and cholesterol and lipase enzymes secreted from the pancreas can break down triaglycerides into monoglycerides, fatty acids and glycerol. After this process, bile salts are recycled.

When the liver does not produce adequate bile salts to create micelles properly, cholesterol breaks off and causes gallstones

Biliary tract

Diagram of the bile ducts.

The biliary tract (biliary tree or biliary system) functions to make, store, and secrete bile. First the liver makes bile, which travels through the right and left hepatic ducts into the common hepatic duct. Where will it go next? The common bile duct or the cystic duct that can be stored in the gallbladder. Then the bile from the gallbladder will get squeezed out by cholecystokinin. That bile then goes into the common bile duct. The bile from the common bile duct will go into the duodenum. The other bile that went into the common bile duct will go into the duodenum too.



Pancreas is a digestive gland which helps to digest the food by secreting pancreatic juice. It is located inside the abdomen, just behind the stomach. It plays an essential role in converting the food we eat into fuel for the body's cells.

The pancreas has two main functions: an exocrine function that helps in digestion and an endocrine function that regulates blood sugar.

Exocrine FunctionEdit

The pancreas contains exocrine glands that produce pancreatic juices important to digestion. The pancreatic juices and bile that are released into the duodenum, help the body to digest fats, carbohydrates, and proteins.

Endocrine FunctionEdit

The endocrine component of the pancreas create and release important hormones directly into the bloodstream. Two of the main pancreatic hormones are insulin, which acts to lower blood sugar, and glucagon, which acts to raise blood sugar.


Diagram of the intestines.

The main function of the colon is to absorb water. The water and other things get absorbed that feces are formed. The feces is stored in the rectum. The internal sphincter then opens. The external sphincter then decide when to open. When it is open, the feces go out the anus. The appendix is a little pouch that stores good bacteria which can squirt it out. It squirts it out to help digest food.