THE DIGESTIVE SYSTEM

DISGESTION Digestion is the process of metabolism

where by a biological entity processes a substance in order to chemically and mechanically convert the substance for the body to use.

OVERVIEW

1. Ingestion: placing food into the mouth, 2. Mechanical digestion & chemical

digestion: mastication, the use of teeth 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,

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

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

PHASES OF HUMAN DIGESTION

ORAL CAVITY

Digestion begins in the oral cavity where food is chewed. Saliva is secreted by three pairs of 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.

ESOPHAGUS The esophagus, a narrow, muscular tube about 20

centimeters 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. 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.

STOMACH 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.

SMALL INTESTINE 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:

1. 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!

2. pancreatic juice made by the pancreas 3. 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.

LARGE INTESTINE

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.

CARBOHYDRATE DIGESTION The digestive system begins the process of breaking

polysaccharides in the mouth through the introduction of amylase, a digestive enzyme in the saliva. The high acid content of the stomach inhibits enzyme activity, thus suspending the digestion of carbohydrates in the stomach. After emptying into the small intestine, the potential of hydrogen (pH) changes dramatically from a strong acid to alkaline content. The pancreas secretes bicarbonate to neutralize stomach acid, and the mucus secreted in the tissue lining of the intestines, is alkaline, which promotes the activity of the digestive enzyme. Amylase is present in the small intestine and works with other enzymes to complete the breakdown of carbohydrates in a monosaccharide which absorbs around the capillaries of the villi. The nutrients in the blood is transported to the liver via the hepatic portal circuit or loop, where final carbohydrate digestion takes place in the liver. The liver complies with the digestion of carbohydrates in response to the hormones insulin and glucagon. As increasing levels of blood glucose after digestion of a meal, the pancreas secretes insulin causing the liver to convert glucose into glycogen, which is stored in the liver, adipose tissue, and muscle cells, preventing hyperglycemia. A few hours after a meal, the blood glucose is lowered due to the muscle activity, and now the pancreas that secrete glucagon causes glycogen to be converted into glucose to prevent hypoglycemia.

RECTUM & ANUS The rectum is a part of the gastrointestinal system. It is where

feces is stored before coming out of the anus. Feces in the rectum causes a desire to make that feces come out. A toilet is a place where feces can be put.

The anus is the hole in the human body that is between the buttocks. It is at the end of the gastrointestinal system (including the organs that digest food), where waste (called feces, shit or stool) comes out of the body.

AUTORES

Francisco Miguel Carrasquilla Rodríguez Córdoba

Antonio Galán Galán José Luís Bedmar Jurado