The Digestive System (1964)

to sustain the complex functions of Life the body requires a constant supply of new material for energy body growth and repair but how is food changed or broken down into molecules in man the complicated process of food breakdown or digestion is carried out by a regulated system of organs known as the digestive system parts of our digestive system include the mouth the esophagus the stomach as well as the small and large intestines also certain specialized glands secrete substances that chemically break down food to control the process of digestion our digestive organs are supplied with nerves while blood vessels help to distribute the products of digestion to other parts of the body the foods we eat are almost endless in their variety taste and appearance but to the biochemist there are only six major kinds of food carbohydrates mainly starches and sugars proteins fats salts vitamins and water what starts the complicated process of food breakdown within the body the X-ray Motion Picture enables us to observe some important steps in our digestive process here we see the chewing and grinding action of the teeth that breaks food into small particles mixing it with fluids that change the food to a semi-solid form at the same time saliva Begins the chemical breakdown of food this fluid substance secreted by three pairs of salivary glands containing a lubricant mucus and a digestive enzyme tyin tyin is the enzyme that begins the digestion of starch molecules since the chemical breakdown of food cannot be observed directly we'll use symbols to represent the digestion of starch as Tylin comes into contact with a molecule of starch some of the chemical bonds that hold the molecule together are broken in this way a molecule of starch is broken down into smaller molecules known as sugars the action of chewing combined with saliva softens food and makes it ready for swallowing as food is swallowed it is pushed to the back region of the mouth or fenx into the esophagus the long tubular passage that leads to the stomach this Movement of food through the esophagus is accomplished by wavelike contractions of muscles in the esophagus wall peristaltic waves push food through the esophagus even against the pool of gravity for example food will move toward the stomach when the body is in this position or even when the body is weightless as it is during a space flight as food passes from the esophagus into the stomach a major process of digestion begins the upper portion of the stomach serves to hold food while in the lower region gastric digestion takes place the inner lining or mucosa of the stomach is made up of numerous minute glands these glands secrete a fluid called gastric juice the cells that make up the gastric glands are specialized some cells produce an inactive enzyme like substance called pepsinogen other cells secrete hydrochloric acid as these substances mix in the stomach the hydrochloric acid acts on pepsinogen to form an active enzyme pepson this enzyme breaks down protein molecules to demonstrate the digestion of protein in the laboratory we'll remove a small quantity of gastric juice directly from the stomach in this manner what effect will this gastric juice have on protein to find out we'll use protein in the form of this thin strip of beef the meat is placed in the gastric juice and observed over a period of 1 hour how does pepsin in gastric juice bring about this change in protein as the enzyme pepsin comes into contact with protein it breaks some of the chemical bonds in the protein molecule this splits a large complicated protein molecule into smaller molecules but since pepsin breaks down protein what prevents this enzyme from digesting the protein in cells that make up the stomach wall sometimes the stomach wall actually undergo digestion but normally the lining of the stomach is protected by mucus a substance that lubricates and protects cells in the stomach wall guarding them from the digestive action of gastric juice as gastric digestion continues strong waves of contraction mix food with gastric juice gradually breaking down the contents of the stomach into smaller particles these movements as well as the secretion of gastric juice are controlled in part by nerves when food enters the stomach it's stimulates cells in the stomach wall sending nerve impulses to the central nervous system in turn nerve impulses travel to the gastric glands where they stimulate an increased flow of gastric juice but the secretion of gastric juice is also controlled by another mechanism in the lower region of the stomach some foods particularly the digestion products of protein stimulate cells in the stomach lining releasing a hormone called gastron this hormone carried by the blood to the gastric glands also increases the flow of gastric juice but this mechanism is self-regulating when food has absorbed a sufficient quantity of gastric juice the acid in food acts on the cells that release gastrin causing them to reduce their secretion of gastrin thereby slowing down the flow of gastric juice into the stomach in the stomach food undergoes only partial digestion when this partially digested food becomes a semi- liquid strong waves of contraction Force quantities of the liquefied food into the first part of the small intestine known as the duodenum here the semi- liquid food or Kim stimulates the release of a hormone causing the pancreas to release an alkaline fluid known as pancreatic juice pancreatic juice contains several enzymes that carry on digestion in the small intestine one of these enzymes Tron continues the Digest of protein that began in the stomach splitting protein into smaller molecules called amino acids another enzyme amales continues the breakdown of carbohydrates the enzyme lipase breaks down molecules of fat but the digestion of fat can be carried out much more efficiently with the help of another substance bile bile is a complex fluid formed continuously by specialized cells in the liver between meals some of this fluid is stored and concentrated in a sacklike container known as the gallbladder how does bile help the digestion of fat in this mixture of oil and water we see fat in the form of large globules in this form fat could not easily be digested many food substances dissolve in water fat does not this dark green fluid is bile that has been removed from an experimental animal in the laboratory let us observe the action of bile on the oil and water mixture with the help of bile the fat emulsifies or divides into small particles taken together these smaller particles of fat have a greater surface area enabling the enzyme lipase to digest the fat particles more readily within the body the flow of bile is controlled by a regulated mechanism the presence of food in the duodenum triggers the release of a hormone that causes the gallbladder to contract this forces B into the duodenum as this this bile mixes with Kim certain substances in the bile are absorbed through the intestinal wall and carried by the blood to the liver these substances stimulate cells in the liver to increase the production of bile but as food leaves the duodenum the need for bile decreases and the production of bile by the liver slows down in the small intestine the major process of digestion is completed here the useful products of food breakdown the nutrients are absorbed under a microscope the inner surface of the small intestine reveals numerous finger-like projections called Villi these Villi enormously increase the surface area lining the small intestine and thereby play an important role in the absorption of nutrients each villis contains a network of blood and lymph vessels and is covered by a layer of absorbing cells some nutrients certain salts vitamins and Water Pass directly through the absorbing cell into the blood and lymph molecules of digested protein or amino acids are also absorbed through the Villi cells in a similar way the products of carbohydrate break down the simple sugars pass through the absorbing cells into the blood fat is absorbed in a different way as molecules of digested fat pass pass through the Villi cell they combine into larger more complex molecules this enables fat to enter the lymph instead of the blood capillaries over a period of hours the useful nutrients are absorbed gradually the unabsorbed indigestible components of food together with a large quantity of water pass into the large intestine in the large intestine or colon the food residue under go further chemical breakdown here enzymes produced by bacteria many that live permanently in the digestive tract act on the undigested food helping to convert it to waste material at the same time most of the water that is served as a liquid medium for digestion is absorbed along with other substances through the intestinal wall Gra gradually changing the waste material to a semi-solid form slowly peristaltic waves carry the food residue into the lower region of the colon where a voluntary controlled reflex results in the elimination of waste material from the body the intricate mechanisms of digestion continue throughout our lives changing ing food into molecules that supply energy and the materials for body growth and repair with the help of modern techniques such as the X-ray motion picture and through painstaking laboratory research biologists have explained many steps in the digestive process yet challenging questions remain to be answered for