Digestion is the process of metabolism whereby a biological entity processes a substance in order to chemically and mechanically convert the substance for the body to use.
The oral cavity
In humans, digestion begins in the oral cavity where food is chewed . Saliva is secreted in large amounts (1-1.5 litre/day) 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. There are two types of saliva. One is a thin, watery secretion, and its purpose is to wet the food. The other is a thick, mucous secretion, and it acts as a lubricant and causes food particles to stick together and form a bolus . The saliva serves to clean the oral cavity and moisten the food, and contains digestive enzymes such as salivary amylase , which aids in the chemical breakdown of polysaccharides such as starch into disaccharides such as maltose . It also contains mucin, a glycoprotein which help soften the food into a bolus .
Swallowing transports the chewed food into the esophagus , passing through the oropharynx and hypopharynx . The mechanism for swallowing is coordinated by the swallowing center in the medulla oblongata and pons . The reflex is initiated by touch receptors in the pharynx as the bolus of food is pushed to the back of the mouth.
The esophagus , a narrow, muscular tube about 20 centimeters (8 inches) long, starts at the pharynx, passes through the thorax 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 rectum and contract slowly, over long periods of time. The inner layer of muscles is arranged circularly in a series of descending rings, while the outer layer is arranged longitudinally. At the top of the esophagus, is a flap of tissue called the epiglottis that closes during swallowing to prevent food from entering the trachea (windpipe). The chewed food is pushed down the esophagus to the stomach through peristaltic contraction of these muscles. It takes only seconds for food to pass through the esophagus, and little digestion actually takes place.
The food enters the stomach after passing through the cardiac orifice . In the stomach, food is further broken apart thoroughly mixed with a gastric acid and digestive enzymes that denature proteins. The acid itself does not break down food molecules, rather, the acid provides an optimum pH for the reaction of the enzyme pepsin . The parietal cells of the stomach also secrete a glycoprotein called intrinsic factor which enables the absorption of vitamin B-12 . Other small molecules such as alcohol are absorbed in the stomach as well by passing through the membrane of the stomach and entering the circulatory system directly.
After being processed in the stomach, food is passed to the small intestine via the pyloric sphincter . The majority of digestion and absorption occur here as chyme enters the duodenum . Here it is further mixed with three different liquids:
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). However, it is not an enzyme!
pancreatic juice made by the pancreas
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. As the acid level changes in the small intestines, more enzymes are activated to split apart the molecular structure of the various nutrients so they may be absorbed into the circulatory or lymphatic systems. Nutrients pass through the small intestine's wall, which contains small, finger-like structures called villi , and each villi contains even smaller hair-like structures called microvilli . The blood, which has absorbed nutrients, is carried away from the small intestine via the hepatic portal vein and goes to the liver for filtering, removal of toxins, and nutrient processing.
The small intestine and remainder of the digestive tract undergoes peristalsis to transport food from the stomach to the rectum and allow food to be mixed with the digestive juices and absorbed. The circular muscles and longitudinal muscles are antagonistic muscles, with one contracting as the other relaxes. When the circular muscles contract, the lumen becomes narrower and longer and the food is squeezed and pushed forward. When the longitudinal muscles contract, the circular muscles relax and the gut dilates to become wider and shorter to allow food to enter.
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 . The large intestine absorbs water from the bolus and stores feces until it can be excreted . Food products that cannot go through the villi , such as cellulose ( dietary fiber ), are mixed with other waste products from the body and become feces .