The Digestive System

Food is our body’s fuel. The nutrients in food gives our cells energy and substances that they need to function. Before our body can get nourished by food however, it must be processed into tiny pieces that the body can absorb and use.

How the digestive system works

Digestion starts in the mouth. When we see, smell, taste, or just think about a delicious meal, the salivary glands, which are located under the tongue and near the lower jaw start producing saliva. This is where the expression "mouth watering" comes from.

Saliva production is triggered by a brain reflex that is set off when we think about eating or when we sense food. In response, the brain sends impulses through the nerves that control the salivary glands that tell them to get ready to eat.

As we chew, saliva moistens the food to make it easier to swallow. The saliva also contains a digestive enzyme called amylase that starts to break down some of the carbohydrates in the food while still in the mouth.

Swallowing moves food from the mouth down to the throat or pharynx. The pharynx is about 5 inches long, serves as a passageway for food and air. To prevent us from choking when we swallow, a flap of tissue called the epiglottis reflexively closes over the windpipe.

From the pharynx, food travels down the esophagus, where muscle contractions called peristalsis push food down into the stomach.

At the end of the esophagus, a valve called the sphincter lets food enter the stomach end them closes shut to keep food or fluid from flowing back up the esophagus. The stomach muscles mix the food with acids and enzymes that break it into even smaller pieces. An acidic environment is necessary for digestion to take place. Glands in the stomach lining produce about 3 quarts of digestive juices per day.

Most food substances need to be digested further before they can be absorbed. Thus they need to travel into the intestine. An empty adult stomach has a volume of 1.6 fluid ounces. But it can expand to hold more the 64 fluid ounces of food after a large meal.

By the time the food leaves the stomach, it has already been processed into a thick liquid called chime. The pylorus, a valve the size of a walnut located at the outlet of the stomach, keeps the chime in until it reaches just the right consistency to pass into the small intestine. Chime is then squeezed into the small intestine where digestion continues so that the body can absorb nutrients into the bloodstream.

The small intestine

The small intestine is divided into three parts:

  • the duodenum – is the C-shaped primary part.
  • the jejunum – is the coiled midsection.
  • the ileum – is the end section leading to the large intestine.

The inner wall of the small intestine is covered with microscopic, finger-like protrusions called villi – through them, nutrients can be absorbed by the body.

The liver, gallbladder, and pancreas

These three organs are not really part of the alimentary canal, but they are essential to digestion.

The liver produces bile which helps the body absorb fat. The gallbladder then stores the bile until it is needed. The pancreas in turn, produces enzymes that help digest proteins, fats and carbs. It also produces a substance that neutralizes stomach acid. These enzymes are transported directly into the small intestine where they assist in breaking down food. The liver also plays a role in the processing of nutrients. These nutrients are carried to the live in the blood from the small intestines.

The large intestine

From the small intestine, undigested food and some water are transported to the large intestine via the pylorus. By the time food gets here, the absorption of nutrients is almost done. The large intestine’s primary function is to remove water from the undigested food and create water that can be excreted.

The large intestine is divided into three parts:

Cecum – a pouch located that connects the large intestine to the small intestine. The cecum expands in diameter to allow food to pass from the small intestine to the large intestine. At the end of the cecum is the appendix, a tiny, hollow, finger-like pouch.

Colon – the colon stretches from the cecum up the right of the abdomen, across the upper abdomen, and then down the left side of the abdomen, and then finally connecting to the rectum. The colon has three parts:

  • the ascending colon
  • the transverse colon – absorb fluids and salts.
  • the descending colon – holds the resulting waste.

The rectum – this is where the feces are stored until they are expelled from the digestive system through the anus.

Source: MSN Health