Digestion Part 1

the body obtains energy to carry out its living processes from substances known as food within the elementary Canal complex substances in food stuffs are changed into simpler forms which can be absorbed into the blood this process is called digestion part of the elementary Canal or digestive tract is now being displaced so that some of its coiled regions can be more clearly seen during the passage of food along the canal digestive fluids act upon it changing complex substances into simple soluble forms which can be absorbed into the blood and carried to the tissues by the action of muscles in the canal wall food stuffs are thoroughly mixed with digestive fluids and are moved along the tract during this process useful substances are absorbed into the blood the remainder is finally expelled from the body let us first consider the muscular and mechanical processes of digestion these begin in the mou the funnel shaped space at the back of the mouth is called the FX two tubes open out from the lower part of the FX this one starts as the larynx and continues as the tracha it carries air to the lungs the second tube is the esophagus it carries food to the stomach in the mouth food is cut and ground by the teeth this is called mastication the tongue has been made transparent so that the teeth on the far side can be seen the food is moved about and gradually pushed towards the back of the mouth by the action of the tongue and the cheek muscles the food is gradually collected onto the back of the tongue and swallowed during mastication food is well mixed with a digestive fluid called saliva the mixture of food and saliva is collected onto the back of the tongue in the form of a bolus in swallowing the Linx goes up and slightly forward to prevent food from entering the traa let us see the chewing and swallowing actions once again the movement of the bolus is no longer under voluntary control once the action of swallowing is started the linkx goes up and forwards the bolus is propelled through the FX and immediately it enters the esophagus it is pushed along it by contractions of the muscles in the wall of the tube muscles behind the food contract while those in front of it relax the pincer likee contractions behind the bolus push it into the relaxed portion of the tube ahead of it the muscles in the wall of the esophagus are arranged in two layers the inner ones are placed in a circular Direction and the outer ones in a longitudinal Direction the wall of the elementary Canal along its whole length has this outer coat of muscles and an inner lining called the mucus membrane which varies in structure in different regions the esophagus passes down inside the chest through the diaphragm the Dome shaped sheet of muscle separating the chest from the abdomen and then leades into the stomach the outer part of the stomach wall is largely made up of muscle fibers running in three directions the outermost coat is longitudinal the middle coat circular and the third incomplete coat is oblique this is the stomach in section the swallowed food collects in the stomach it is gradually pressed from the upper part the fundus towards the lower the pyloric end this is brought about by rhythmic contractions of the muscles in the stomach wall at the upper end of the stomach small contraction waves occur often in groups and at intervals of about 20 seconds the stomach shown here is about half full the rate of expansion of the stomach and the spreading of the food from the center towards the wall are greatly speeded up in the lower the pyloric end of the stomach sometimes called the pyloric Mill vigorous waves of muscular movement take place in an active stomach about three large waves of contraction take place a minute these contractions churn the food and thoroughly mix it with the digestive fluid secreted by the inner lining of the stomach as a result of this churning and mixing the food becomes a thin liquid mass this is the pyic end of the stomach its opening into the small intestine is normally kept closed by contraction of the circular muscles it opens at intervals and small quantities of partly digested food are shot out of the stomach into the first region of the small intestine The Duo denum for the small intestine is a coiled tube about 20 ft in length the first region the duodenum is soall because it is about 12 in Long the middle region is the junam the lower region is the ilum in the muscular outer wall of the small intestine the muscles run in two directions some in a l itudinal Direction and some in a circular Direction these muscle fibers contract behind and relax in front of the food squeezing it along in the same wav likee movement seen in the esophagus this type of movement by the involuntary intestinal muscles is called peristalsis and takes place along the whole length of the elementary Canal only part of the small intestine is shown moving here so that the movements can be clearly seen two other kinds of muscular movement take place in the small intestine in segmentation pins movements by the muscles squeeze the intestine and its contents into a series of segments new pins movements then occur in the middle of the existing segments and the previous contractions relax segmentation AIDS the mixing and absorption of the food a swaying pendulum like movement also takes place for the intestines are supported by a delicate membrane containing blood vessels the mesentary to show the mesentary some of the intestines have been moved out of position digested food is gradually absorbed into the blood in the blood vessels in the intestinal wall and this Blood passes through the mestic veins into the portal vein the blood flows through the portal vein to the liver some forms of food pass into lymphatic vessels and not into blood vessels the undigested and indigestible part of food and a great deal of water and salts pass out of the small intestine into the large intestine the colon the appendix is at the beginning of the colon the colon passes up the right side of the abdomen and turns below the liver it then crosses to the left side of the body the curen is about 5 ft long and is a much wider tube than the small intestine the contents of the colon are moved Along by peristalsis as in the esophagus and the small intestine but the movement is slower as a result of this forward movement the contents of the colur accumulate in the lower end of the descending colur out of which they are finally expelled from the body during their Passage through the first parts of the colon much of the water and some of the salts are absorbed into the blood in the curan wall blood vessels and carried away through the mesic vein and the portal vein to the liver this absorption of water and salts makes the remaining waste material a semisolid which is eventually expelled from the body via the rectum as a result of the mechanical and muscular processes we have seen and the chemical changes which accompany them the body is able both to select from an immense variety of food stuffs substances necessary to its life and to get rid of surplus and waste materials e