Digestion, Part 2 (Chemical) 1939?

one involving muscular and the other chemical activities take place within the elementary Canal or digestive tract at the same time the elementary Canal is a long tube extending from the mouth to the anus and coiled in parts of its course part of the canal is now being displaced so that the coiled regions can be clearly SE during the passage of food along the canal complex substances in it are changed by the action of digestive juices into simple soluble forms which can be absorbed into the blood and carried to the tissues after the useful substances have been absorbed the remaining material is passed out of the body digestive juices secreted into the mouth stomach and small intestine contain ferments or enzymes which bring about chemical changes in food stuffs gland three on each side secrete into the mouth through ducts a fluid called saliva by chewing food is broken up and well mixed with saliva which dissolves soluble substances and Begins the first chemical change which takes place during digestion let us examine the food in the mouth the main types of food stops to be digested are carbohydrates consisting of starches shown as squares and sugars shown as triangles proteins shown as rods and fats shown as circles chemical changes taking place during digestion will be indicated by the splitting of these symbols during digestion starches shown as squares are changed into sugars an enzyme in saliva called tyin begins this chemical process by acting on cooked starch saliva also acts as a lubricant and makes the process of swallowing easier after swallowing the mixture of food and saliva is pushed down the esophagus the food then enters the stomach the upper part of the stomach is known as the cardiac end this is the upper part of the stomach in section the inner lining of the stomach contains glands which secrete a digestive fluid called gastric juice gastric juice contains an enzyme called pepsin which acts on protein it also contains hydrochloric acid which comes from special cells found only in the cardiac end of the stomach and mainly at the necks of the glands this is the whole of the stomach in section A bolus of food is passing down the esophagus and entering the stomach at the cardiac end rhythmic muscular contractions in the stomach wall mix the food with the gastric juice and press it on towards the lower end of the stomach at the lower the pyloric end vigorous waves of muscular movement take place these movements churn the contents of the stomach and bring about a thorough mixing of the food and digestive fluids let us examine the food in the stomach the proteins are shown as rods changes in the proteins are begun by the pepsin In the gastric juice which begins to break them down into to simpler substances proteos and peptones the starches are shown as squares the saliva swallowed with the food continues to act for a short while on cooked starch changing it into a form of sugar the squares representing cooked starch lying near the stomach wall are not being split because as food is pushed against the wall the hydrocloric acid in the gastric juice stops the action of saliva sugar is shown as triangles in the stomach some of the sugar may be changed into simpler forms by the action of the hydrolic acid after thorough mixing of the food and digestive juices has taken place small quantities of the partly digested food are released at intervals from the stomach into the small intestine some of the food stuffs are sufficiently broken down to be absorbed into the blood others are not yet digested this absorption takes place mainly in the small intestine here shown in section with the food taken away we can see that the lining is thrown up into many folds this greatly increases the area available for the passage of digested materials into the blood glands in the inner lining secrete a fluid called intestinal juice or sucus ticus which with two other juices brings about further chemical changes in the food stuffs through this duct bile which is made by the liver flows into the duodenum the first part of the small intestine from this second duct pancreatic juice secreted by the pancreas also enters the duodenum the mixture of food and digestive juices is passed along the length of the small intestine this forward movement is brought about by muscular contractions which take place in the wall of the intestine these peristaltic movements also mix the food and the juices together further digestion takes place whilst the contents of the intestine are being moved onwards let us examine the food in the small intestine the circles represent the fats bile breaks up the fats into a fine Emulsion this increases the surface on which lipase the fat splitting enzyme in the pancreatic juice can act pancreatic juice acts on all three types of food Stuffs the proteins are represented by the rods pancreatic juice completes the digestion of the proteins breaking them down into amino acids carbohydrates consist of starches and sugars undigested starches large squares now being split into smaller squares representing the first stages in the digestion of starch sugar taken into the mouth as sugar is represented as large triangles this sugar and the partly digested starches the small squares are converted into simpler sugars now shown as small triangles the fats are represented as circles the fats are broken down by the pancreatic juice into fatty acids and glycerol all the food is now ready for absorption into the blood most of the absorption of food into the blood takes place in the small intestine this process is helped by segmentation which brings the wall of the in into close contact with its contents the intestines are supported by a fine membrane the messent through which run arteries veins and lymphatic vessels the mestic arteries and the lymphatic pass into the intestinal wall where they Branch into smaller vessels on the surface of the inner lining of the small intestine are numerous fine projections called villi which greatly increase the absorbing surface this is a group of vli greatly enlarged absorption is aided by movements of the vli between the Villi lie the glands which secrete intestinal juice each Villas has a small artery which breaks up into capillaries the capillaries unite again to form a small vein the end products of digested proteins and carbohydrates amino acids and sugars are shown here as solid symbols in the body the amino acids and sugars are in solution and so are able to pass through the walls of the Villi into the capillary blood and on into the veins in the center of each villis there is also a small lymphatic vessel carrying lymph and called a lacal the end products of digested fats fatty acids and glycerol pass into the cells of the Villi where they are reformed into fats and passed on into the lacle the contents of the lacle are carried away in the lymphatic vessels e part of the colon and the mesent have been removed to show how the mesenteric veins unite to form a short wide blood vessel the portal vein which leads directly to the liver blood carrying digested food away from the small intestine passes through the mesic veins and portal vein to the liver where some of the substances in it are stored for future use some some undergo further changes and some are passed on into the general circulation blood containing some of the sugar and certain other substances leaves the liver through the hepatic veins and enters the inferior vena cava in which it is carried to the right oricle of the heart in the same way the lymphatic vessels from the intestine unite in the mesent carrying lymph containing the products of digested fat into the receptaculum kyly and then into the thoracic duct which lies behind the heart the contents of the duct pass into the blood in a vein just below the neck and then through the superior vinaa into the right oricle of the heart blood from the superior and the inferior vena CA enters the heart and is pumped through the pulmonary arteries to the lungs where it picks up oxygen and is then returned through the pulmonary veins into the left oricle pumped by the heart through the main artery the aorta this Blood passes into Branch arteries which carry it all over the body Distributing food and oxygen to the tissues e