Digestive system basics

Digestive system cartoonA big part of a healthy lifestyle is eating the right things, and avoiding the wrong things.

When talking about diet and nutrition the digestive system is mentioned a lot. So I thought I would provide a brief summary of what happens in the various parts of the digestive system.

At its simplest, you can think of the digestive system as a tube. You put food in one end of the tube.

All the nutrients your body needs are extracted from the food as it passes through, and the waste comes out the other end.

Various parts of the digestive system

So that your food can be digested and the nutrients absorbed, there are various specialised parts within your digestive system at various points in “the tube”.

Waste that needs to be removed from your blood stream also flows into the digestive system to be excreted.

This is intended to be a simple description of what happens at the various points in your digestive system. You can find plenty of other descriptions of the process but they mostly go into more detail than you need to understand how the digestive system works.


As you already know, food enters your body through your mouth. Your teeth chop the food into smaller parts and as you chew, the food is ground up to make swallowing and digestion easier.

As you chew, saliva moistens the food, and enzymes in your saliva start the process of digesting starch. Enzymes are chemicals your body produces to speed up chemical reactions. Starch is a complex carbohydrate, made up of lots of individual sugars all joined together.

The enzymes in your saliva start the breakdown of this starch. Starch is found in foods like wheat, rice and potatoes, and anything made from them such as bread and pasta.

After the food has been chewed for a little while your tongue rolls it up and the muscles in your throat (also know as your pharynx) pull the food down into your esophagus.


Digestive system diagramYour esophagus is a tube that connects your throat (pharynx) with your stomach. The muscles in the walls of your esophagus pull the food downwards.

The waves of muscle contraction is called peristalsis, which happens throughout the digestive system.

It’s peristalsis that pushes your food through your digestive system from one end to the other.

When the food reaches the bottom a circular muscle called the cardiac sphincter opens the the food enters your stomach.


Your stomach is a “stretchy bag” that lets you eat food quickly and store it inside your body until the rest of the digestive system is ready for it.

The food stays in your stomach for between three and five hours. While in the stomach food is mixed with gastric juice, which is produced in your stomach. The muscles in the stomach walls contract and mix the food and gastric juice together.

Gastric juice contains strong acid and enzymes. The acid helps to kill any bacteria that might be present in your food, while the enzymes start breaking down protein in the food.

When the food has been mixed with gastric juice in the stomach is it called “chyme”.

The stomach is closed off at the other end by another circular muscle called the pyloric sphincter. Every now and again the pyloric sphincter opens and a little bit of chyme passes through into the small intestine.

Small intestine – duodenum, jejunum and ileum

The small intestine is about 23 feet (seven metres) long. It is during its journey through the small intestine that most of the digestion of food takes place. The nutrients released by digestion are absorbed into your bloodstream through the walls of the small intestine.

Although it is actually one long tube, the small intestine has three parts: the duodenum, jejunum, and ileum.


The first 18 inches (45 cm) of the small intestine is called the duodenum. As the chyme (food mixed with gastric juice) passes through into the duodenum, other chemicals are added to it from the bile duct and pancreatic duct.

Bile duct

The bile duct brings bile from the liver into the duodenum. Bile contains bilirubin, a waste product from the breakdown of old red blood cells, which needs to be removed from your body via your intestines. Bile also contains bile salts, which helps to break up fats to make fat digestion easier.

Pancreatic duct

The pancreatic duct brings pancreatic juice from the pancreas to the duodenum. Pancreatic juice is alkaline, which helps to neutralise the stomach acid in the chyme. Pancreatic juice also contains a mixture of enzymes: protease enzymes to digest proteins, lipase enzymes to digest fats, plus amylase and other enzymes that complete the digestion of carbohydrates.

As your food (now called chyme, remember?) passes through the rest of the small intestine these enzymes break down and release the nutrients so that they can be absorbed into the blood stream.

Jejunum and ileum

The second section of the small intestine is called the jejunum, and the part that follows that is called the ilium.

The absorption of digested nutrients really starts in the jejunum and by the time the food has reached the end of the ileum, just about everything that can be digested will have been, and the nutrients absorbed through the walls of the small intestine into the bloodstream. It’s actually a little more complicated for fats.

The food material that reaches the end of the small intestine is mostly indigestible (things like dietary fiber) and waste that your body wants to get rid of (like bilirubin, which gives the waste its typical colour).

This waste material enters the large intestine

Large intestine (colon)

The large intestine, also know as the colon, starts with the caecum. The end of the ileum “plugs into” the caecum allowing the food waste to enter the large intestine.

Attached to the caecum is the appendix. The appendix is a little pocket-shaped bag with no known function. Food residues can get trapped in the appendix, causing it to become inflamed, which often needs surgery.

The colon (large intestine) is about five feet (1.5m) long, and has a number of sections. From the caecum the colon progresses upwards, and this section is called the ascending colon. Then the colon goes horizontally – the transverse colon, before going downwards – the descending colon. The part that comes next is “s-shaped”, called the sigmoid colon.

Since almost all of the digested nutrients, and most of the water, have been absorbed into the blood through the walls of the small intestine, it is mostly remaining water that is absorbed in the large intestine. This makes the waste material more compact before it moves through the sigmoid colon on to the rectum.

Rectum and anus

The waste material is stored in the rectum before being removed from the body by defecation. As the amount of waste builds up in the rectum the walls are stretched, which stimulates the desire to defecates.

As the rectum becomes full the waste material (feces) moves through the anus and out of the body.

Just the digestive system basics

As I mentioned at the beginning of this article, this is just a basic overview of the digestive system. It’s important to have a picture in your mind of the structure and function of the digestive system to understand things that affect its function. Changes in the digestive system can have a positive or negative effect on your health.

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