What is the main substance of digesting?

Food is our fuel, and its nutrients give our bodies' cells the energy and substances they need to work. But before food can do that, it must be digested into small pieces the body can absorb and use.

The first step in the digestive process happens before we even taste food. Just by smelling that homemade apple pie or thinking about how delicious that ripe tomato is going to be, you start salivating — and the digestive process begins in preparation for that first bite.

Almost all animals have a tube-type digestive system in which food:

• enters the mouth
• passes through a long tube
• exits the body as feces (poop) through the anus

Along the way, food is broken down into tiny molecules so that the body can absorb nutrients it needs:

• Protein must be broken down into amino acids.
• Starches break down into simple sugars.
• Fats break down into into fatty acids and glycerol.

The waste parts of food that the body can't use are what leave the body as feces.

How Does Digestion Work?

The digestive system is made up of the alimentary canal (also called the digestive tract) and other organs, such as the liver and pancreas. The alimentary canal is the long tube of organs — including the esophagus, stomach, and intestines — that runs from the mouth to the anus. An adult's digestive tract is about 30 feet (about 9 meters) long.

Digestion begins in the mouth, well before food reaches the stomach. When we see, smell, taste, or even imagine a tasty meal, our salivary glands in front of the ear, under the tongue, and near the lower jaw begin making saliva (spit).

As the teeth tear and chop the food, spit moistens it for easy swallowing. A digestive enzyme in saliva called amylase (pronounced: AH-meh-lace) starts to break down some of the carbohydrates (starches and sugars) in the food even before it leaves the mouth.

Swallowing, done by muscle movements in the tongue and mouth, moves the food into the throat, or pharynx (pronounced: FAIR-inks). The pharynx is a passageway for food and air. A soft flap of tissue called the epiglottis (pronounced: ep-ih-GLAH-tus) closes over the windpipe when we swallow to prevent choking.

From the throat, food travels down a muscular tube in the chest called the esophagus (pronounced: ih-SAH-fuh-gus). Waves of muscle contractions called peristalsis (pronounced: per-uh-STALL-sus) force food down through the esophagus to the stomach. A person normally isn't aware of the movements of the esophagus, stomach, and intestine that take place as food passes through the digestive tract.

At the end of the esophagus, a muscular ring or valve called a sphincter (pronounced: SFINK-ter) allows food to enter the stomach and then squeezes shut to keep food or fluid from flowing back up into the esophagus. The stomach muscles churn and mix the food with digestive juices that have acids and enzymes, breaking it into much smaller, digestible pieces. An acidic environment is needed for the digestion that takes place in the stomach.

By the time food is ready to leave the stomach, it has been processed into a thick liquid called chyme (pronounced: kime). A walnut-sized muscular valve at the outlet of the stomach called the pylorus (pronounced: pie-LOR-us) keeps chyme in the stomach until it reaches the right consistency to pass into the small intestine. Chyme is then squirted down into the small intestine, where digestion of food continues so the body can absorb the nutrients into the bloodstream.

The small intestine is made up of three parts:

1. the duodenum (pronounced: due-uh-DEE-num), the C-shaped first part
2. the jejunum (pronounced: jih-JU-num), the coiled midsection
3. the ileum (pronounced: IH-lee-um), the final section that leads into the large intestine

The inner wall of the small intestine is covered with millions of microscopic, finger-like projections called villi (pronounced: VIH-lie). The villi are the vehicles through which nutrients can be absorbed into the blood. The blood then brings these nutrients to the rest of the body.

The liver (under the ribcage in the right upper part of the abdomen), the gallbladder (hidden just below the liver), and the pancreas (beneath the stomach) are not part of the alimentary canal, but these organs are essential to digestion.

The liver makes bile, which helps the body absorb fat. Bile is stored in the gallbladder until it is needed. The pancreas makes enzymes that help digest proteins, fats, and carbs. It also makes a substance that neutralizes stomach acid. These enzymes and bile travel through special pathways (called ducts) into the small intestine, where they help to break down food. The liver also helps process nutrients in the bloodstream.

From the small intestine, undigested food (and some water) travels to the large intestine through a muscular ring or valve that prevents food from returning to the small intestine. By the time food reaches the large intestine, the work of absorbing nutrients is nearly finished.

The large intestine's main job is to remove water from the undigested matter and form solid waste (poop) to be excreted.

The large intestine has three parts:

1. The cecum (pronounced: SEE-kum) is the beginning of the large intestine. The appendix, a small, hollow, finger-like pouch, hangs at the end of the cecum. Scientists believe the appendix is left over from a previous time in human evolution. It no longer appears to be useful to the digestive process.
2. The colon extends from the cecum up the right side of the abdomen, across the upper abdomen, and then down the left side of the abdomen, finally connecting to the rectum.

   The colon has three parts: the ascending colon and the transverse colon, which absorb fluids and salts; and the descending colon, which holds the resulting waste. Bacteria in the colon help to digest the remaining food products.

3. The rectum is where feces are stored until they leave the digestive system through the anus as a bowel movement.

It takes hours for our bodies to fully digest food.

The digestive system is made up of the digestive tract and other organs that help the body break down and absorb food. It is a long, twisting tube that starts at the mouth and goes through the oesophagus, stomach, small intestine, large intestine and anus.

The digestive system breaks down food into nutrients such as carbohydrates, fats and proteins. They can then be absorbed into the bloodstream so the body can use them for energy, growth and repair. Unused materials are discarded as faeces (poo).

Other organs that form part of the digestive system are the pancreas, liver and gallbladder.

How does the digestive system work?

Each organ of the digestive system has an important role in digestion.

Mouth

When someone eats, the teeth chew food into very small pieces. Glands in the cheeks and under the tongue produce saliva that coats the food, making it easier to be chewed and swallowed.

Saliva also contains enzymes that start to digest the carbohydrates in food.

Oesophagus

The oesophagus is the muscular tube that carries food from the mouth to the stomach after it is swallowed. A ring of muscle at the end of the oesophagus relaxes to let food into the stomach and contracts to prevent stomach contents from escaping back up the oesophagus.

Stomach

The stomach wall produces gastric juice (hydrochloric acid and enzymes) that digests proteins. The stomach acts like a concrete mixer, churning and mixing food with gastric juice to form chyme — a thick, soupy liquid.

Small intestine

Bile from the gallbladder and enzymes in digestive juices from the pancreas empty into the upper section of the small intestine and help to break down protein into amino acids and fat into fatty acids. These smaller particles, along with sugars, vitamins and minerals, are absorbed into the bloodstream through the wall of the small intestine.

It is called small because it is about 3.5cm in diameter, but it is about 5m long to provide lots of area for absorption. Most of the chemical digestion of proteins, fats and carbohydrates is completed in the small intestine.

Large intestine and anus

The lining of the large intestine absorbs water, mineral salts and vitamins. Undigested fibre is mixed with mucus and bacteria — which partly break down the fibre — to nourish the cells of the large intestine wall and so help keep the large intestine healthy. Faeces are formed and stored in the last part of the large intestine (the rectum) before being passed out of the body through the anus.

What are common conditions related to the digestive system?

Gastro-oesophageal reflux

Gastro-oesophageal reflux (GORD) occurs when acidic stomach contents move from the stomach back up the oesophagus. It causes a burning sensation in the chest or throat.

Diverticulitis

Diverticulitis is caused by inflammation or infection of abnormal pouches in the lower part of the large intestine. It can cause mild or severe pain on the lower left-hand side of the abdomen.

Stomach ulcers

Stomach ulcers are commonly caused by the bacterium Helicobacter pylori that can live in the stomach of about 4 in 10 Australians. They can cause long-term, low-level inflammation of the stomach lining in some people. It is not well understood why they cause stomach ulcers in some people and not in others.

Haemorrhoids

Haemorrhoids are itchy or painful lumps that occur in and around the anus. The lumps contain swollen blood vessels. Haemorrhoids can cause bleeding when you go to the toilet (do a poo) — you might notice bright red blood on the toilet paper or in the toilet. If you find blood on the toilet paper or in the toilet, always seek medical advice.

Last reviewed: April 2021