The Digestive System

Slide 1:

The Digestion and Excretory Systems The Digestion Process

Objectives:

Objectives Relate the four major functions of the digestive system to the processing of food. Summarize the path of food through the digestive system and the major digestive processes that occur in the mouth, stomach, small intestine, and large intestine. Describe how nutrients are absorbed from the digestive system into the blood stream or lymphatic system. Identify the role of the pancreas and liver in digestion

Breaking Food Down:

Breaking Food Down Before your body can use the food you eat, it must be broken down into smaller molecules. The process of breaking down food is called digestion. The digestive system breaks food molecules down into small enough molecules that the body can use them and then gets rid of undigested food and waste.

The Tube:

The Tube The digestive system is made up of a long winding tube that travels through the core of the body. It begins at the mouth and ends at the anus. The digestive system includes the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and rectum. Although the liver and pancreas are not part of the digestive system, they deliver secretions into the digestive tract through ducts.

Starting Digestion:

Starting Digestion The digestion of food begins when teeth in the mouth chew and rip food into smaller pieces. This process also mixes the food with a watery substance called saliva. This lubricates the food to ease swallowing. Saliva also contains amylases. These are enzymes that break down carbohydrates such as starches into monosaccharides. After chewing the food passes through the pharynx. This triggers a swallowing response in the throat which also closes off the trachea to prevent food from entering the respiratory system. Food passes into the esophagus.

The Esophagus:

The Esophagus The esophagus is a long tube that carries food into the stomach. No digestion takes place in the esophagus. The esophagus is approximately 25 cm (10 in) long. The last two thirds are wrapped in smooth muscle. Food does not simply fall into the stomach. It is carried by successive rhythmic waves of muscle contraction called peristaltic contractions.

The Stomach:

The Stomach The majority of digestion occurs in the stomach. The food enters through a valve in the upper portion called a sphincter valve. This prevents food or stomach acid from backing up into the esophagus. The stomach is a sack like organ located just beneath the diaphragm. Its job is to temporarily store food and provide a place for chemical and mechanical breakdown. When food enters the stomach, gastric juice is secreted by cells that line the stomach. Gastric juice is a combination of hydrochloric acid and pepsin. The acid breaks the bonds of proteins and unfolds large protein molecules into single chain protein strands. The pepsin is an enzyme that cuts the single protein strands into smaller chains of amino acids. The stomach also continues physical digestion by crushing and churning the food through peristaltic contractions. Swallowed food can spend anywhere from 2 to 6 hours in the stomach.

The Small Intestine:

The Small Intestine Food passes through another sphincter valve at the bottom of the stomach and enters the small intestine. This is a tube shaped organ roughly 6 m (19.8 ft) long which functions mainly to digest and absorb nutrients. The first section, the duodenum, receives secretions from the liver and pancreas, and gall bladder that break proteins into amino acids, carbohydrates into monosaccharides, and lipids into fatty acids. The gall bladder secretes bile that emulsifies lipids into tiny globules. These can then be digested into fatty acids by enzymes called lipases. Most absorption into the blood or lymphatic system occurs in the small intestine. The organ is lined with finger like projects called villi which increase the surface area of the inside of the organ. They are too small to be seen without a microscope. Sugars and amino acids enter capillaries through the villi and are carried by blood to the liver. Fatty acids and glycerol enter lymphaic vessels in the villi and eventually make their way to the blood stream.

The Large Intestine:

The Large Intestine All components of food that are not for energy production are considered waste. These wastes move into the large intestine. No digestion takes place here. Instead, water is reabsorbed from the waste product. The volume of material that flows through the digestive system each day is large, approximately 10 liters, mostly water. The volume that leaves the body is small by comparison because 90 to 95% of the water is reabsorbed in the large intestine along with some useful minerals. The final segment of the large intestine is called the rectum. Solid waste is temporarily stored here until it can be eliminated by muscular contractions similar to peristalsis. The water balance is important. Feces rushed through the large intestine retains too much water. This results in diarrhea. Too long a time in the large intestine can result in hard dry feces. The condition is called constipation.