What is the digestive system responsible for

The digestive system breaks down the food we eat into tiny parts to give us fuel and the nutrients we need to live. As food breaks down, we get amino acids from protein, simple sugars from starches, and fatty acids and glycerol from fats.

What Are the Parts of the Digestive System?

The digestive system is made up of:

• the alimentary canal (also called the digestive tract). This long tube of organs makes a pathway for food to travel through the body. It runs from the mouth to the anus (where poop comes out) and includes the esophagus, stomach, and intestines. An adult's digestive tract is about 30 feet (about 9 meters) long.
• other organs that help the process by adding enzymes and chemicals to break down the food. Important organs that help with digestion include the salivary glands, liver, gallbladder, and pancreas.

How Does the Digestion System Work?

Here’s how digestion happens:

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

Teeth tear and chop the food. Spit moistens it for easy swallowing. A digestive enzyme in saliva called amylase (AH-meh-lace) starts to break down some carbohydrates (starches and sugars) in the food.

Muscles in the tongue and mouth work together to swallow the food and move it into the throat (pharynx). The pharynx (FAIR-inks) is a passageway for food and air. A soft flap of tissue called the epiglottis (ep-ih-GLAH-tus) closes over the windpipe when we swallow to keep food and liquid out of the lungs.

The esophagus: Food travels down a muscular tube in the chest called the esophagus (ih-SAH-fuh-gus). Waves of muscle contractions called peristalsis (per-uh-STALL-sus) force food down through the esophagus to the stomach. A person usually isn't aware of the muscle movements that push food through the digestive tract.

The stomach: At the end of the esophagus is a muscular ring or valve called a sphincter (SFINK-ter). The sphincter lets food enter the stomach, then squeezes shut to keep food and liquids from flowing back up into the esophagus. The stomach muscles churn and mix the food with digestive juices that have acids and enzymes. These juices help break up food into much smaller pieces.

By the time food is ready to leave the stomach, it has been turned into a thick liquid called chyme (kime). A small muscular valve called the pylorus (pie-LOR-is) controls when chyme is released into the small intestine.

The small intestine: The small intestine is where many nutrients (like protein, carbohydrates, and fats) get absorbed into the bloodstream. It has three parts:

• The duodenum (due-uh-DEE-num). This C-shaped first part is where enzymes from the pancreas and bile from the liver are added to the chyme.
• The jejunum (jih-JU-num). This coiled middle part further digests the chyme and absorbs nutrients.
• The ileum (IH-lee-um). The final section that leads into the large intestine absorbs nutrients, vitamin B12, and bile acids.

Millions of microscopic, finger-like projections called villi (VIH-lie) line the inside of the small intestine. The villi make lots of surface area for nutrients to get absorbed into the blood. Then blood brings the nutrients to the rest of the body.

While food works its way through the small intestine, three organs that are not part of the alimentary canal help the digestion process:

• The liver makes bile, which helps the body absorb fat. It’s located in the top right of the belly. 
• The gallbladder stores bile until it is needed. It’s hidden just below the liver.
• The pancreas makes enzymes that help digest proteins, fats, and carbs. It’s found below the stomach.

Enzymes and bile travel through small pathways (called ducts) into the small intestine, where they help to break down food.

Undigested food and some water travels to the large intestine through a muscular ring called the ileocecal (ill-ee-oh-SEE-kul) valve. This valve 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: 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 includes three parts:

The human digestive system consists of a long muscular tube and several accessory organs such as the salivary glands, pancreas and gall bladder. It is responsible for food ingestion and digestion, absorption of digestion products and the elimination of undigested materials.

Ingestion – the taking in of food

Food is taken into the mouth where it is physically broken down by the teeth into smaller pieces.

The presence of food in the mouth triggers a nervous reflex that causes the salivary glands to deliver a watery fluid called saliva to the mouth.

Saliva moistens and lubricates the food, with the aid of a slippery substance called mucin, making swallowing easier as well as dissolving some of the food and allowing it to be tasted.

The presence of a digestive enzyme, known as amylase, in saliva allows chemical digestion of starches to begin.

Digestion – breaking the large into the small

The swallowing reflex allows food from the mouth to be moved into the oesophagus. Here, waves of muscular contractions known as peristalsis move food down this thin-walled tube to a muscular bag known as the stomach.

Both physical and chemical digestion occurs within the stomach. The continual churning movements of the muscular walls of the stomach mix food with a digestive fluid, known as gastric juice.

Cells lining the stomach produce this highly acidic fluid, and the enzymes present commence the chemical breakdown of the protein component of the food. Eventually, the food is reduced to a creamy paste known as chyme.

A structure at the bottom of the stomach known as the pyloric sphincter controls the entry of chyme into the first part of the small intestine, called the duodenum. Ducts from the gall bladder and pancreas feed fluids rich in bile salts and digestive enzymes into the duodenum. In addition, some of the cells lining the small intestine produce a fluid known as ‘succus entericus’ made up mostly of water, mucus and sodium bicarbonate.

Collectively, these fluids help to lubricate the partially digested food as it moves down the gut, neutralise the acidic chyme, emulsify fats and oils and enzymatically digest the protein, carbohydrate and fatty acids present. This breaking down of large molecules into small molecules is essential as it enables the absorption of these smaller molecules into the bloodstream.

Absorption – uptake of the soluble products of digestion

The small intestine is 5–6m in length, and most of the chemical digestion occurs within the first metre. Once digested into smaller molecules, absorption can take place.

Millions of tiny finger-like structures called villi project inwards from the lining of the small intestine. These structures greatly increase the surface area of contact that the products of digestion have with the small intestine, allowing for their rapid absorption into the bloodstream. Once absorbed, they are then transported to the liver by means of the hepatic portal vein.

Egestion – the removal of undigested food materials

On reaching the end of the small intestine, all the digested food products, along with the minerals and vitamins that are useful to the body, should have been removed from the watery contents. What remains consists of the indigestible components of food such as cellulose from the consumption of plant-based foods. These materials are then passed on to the large intestine.

The 4 main functions of the large intestine are:

• recovery of water and electrolytes (sodium, chloride) from indigestible food matter
• formation and storage of faeces
• fermentation of some of the indigestible food matter by bacteria
• maintaining a bacterial population.

It has been estimated that there are over 500 species of bacteria present in the large intestine, and these friendly (commensal) bacteria perform a variety of functions. For example, undigested carbohydrates (fibre) are metabolised to short-chain fatty acids, and small amounts of vitamins, especially vitamin K and the vitamin B group, are produced for absorption into the blood.

As undigested material accumulates in the rectum, it stimulates a response that leads to the evacuation of the waste through the anus.

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