Human Digestive System

Digestion is the breaking down of food in the body, into a form that can be absorbed and used or excreted. It is also the process by which the body breaks down food into smaller components that can be absorbed by the blood stream.

Mechanical and chemical digestion begin in the mouth where food is chewed, and mixed with saliva to break down starches. The stomach continues to break food down mechanically and chemically through the churning of the stomach and mixing with enzymes. Absorption occurs in the stomach and gastrointestinal tract, and the process finishes with excretion.

Digestion is usually divided into mechanical processing to reduce the size of food particles and chemical action to further reduce the size of particles and prepare them for absorption

Ingestion: placing food into the mouth.

Mechanical digestion & chemical digestion: mastication to tear and crush food, and churning of the stomach. Addition of chemicals (acid, bile, enzymes, and water) to break down complex molecules into simple structures.

Absorption: movement of nutrients from the digestive system to the circulatory and lymphatic capillaries through osmosis, active transport, and diffusion.

Egestion: Removal of undigested materials from the digestive tract through defecation.

The Stages of Digestion:

Picture of the human digestive system

2/ Digestion begins in the oral cavity where food is chewed. Saliva is secreted in large amounts by three pairs of exocrine salivary glands (parotid, submandibular, and sublingual) in the oral cavity, and is mixed with the chewed food by the tongue.

The Esophagus, a narrow, muscular tube about 30 centimeters (12 inches) long, starts at the pharynx, passes through the larynx and diaphragm, and ends at the cardiac orifice of the stomach. The wall of the Esophagus is made up of two layers of smooth muscles, which form a continuous layer from the Esophagus to the oten and contract slowly, over long periods of time.

3/ The food enters the stomach after passing through the cardiac orifice. In the stomach, food is further broken apart, and thoroughly mixed with a gastric acid and digestive enzymes that break down proteins. The acid itself does not break down food molecules, rather, the acid provides an optimum pH for the reaction of the enzyme pepsin.

4/ After being processed in the stomach, food is passed to the small intestine via the Pyloric sphincter. The majority of digestion and absorption that occurs here as chyme enters the duodenum. Here it is further mixed with three different liquids:

 a) bile, which emulsifies fats to allow absorption, neutralizes the chyme, and is used to excrete waste products such as bilin and bile acids (which has other uses as well). It is not an enzyme, however. The bile juice is stored in a small organ called the gall bladder.

 b) pancreatic juice made by the pancreas.

 c) intestinal enzymes of the alkaline mucosal membranes. The enzymes include: maltase, lactase and sucrase, to process sugars; trypsin and chymotrypsin are also added in the small intestine.

Most nutrient absorption takes place in the small intestine.

5/ After the food has been passed through the small intestine, the food enters the large intestine. The large intestine is roughly 1.5 meters long, with three parts: the cecum at the junction with the small intestine, the colon, and the rectum. The colon itself has four parts: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon.

Email article    Print