How Digestion Works: 5 Stages of Human Digestion
Introduction to Digestive System
The human digestive system consists of the food tube, organs, and glands, which secrete juices into it to help in the digestion of food. They are listed in the table below. The process of digestion includes a mechanical and a chemical phase. Digested food is absorbed by the body with the help of the circulatory and lymphatic systems. Undigested materials are passed through the anus into the external environment.
Accessory Organs and Glands
Ingestion is the first stage of digestion. The food tube in man is about nine meters long (9m), extending from the mouth down to the anus. Food travels through the entire length of the food tube in 24 hours. This is why defecation is usually done once a day. It is not advisable to keep the feces in the intestine longer than three days. The decomposition products can reach the bloodstream and poison the body. Listed below is the step-by-step procedure on how we excrete food in our digestive system.
• The food we swallow goes down the esophagus with the help of peristalsis. Peristalsis is the wave-like contraction of muscles that push food down the digestive tube.
• The food stays a while at the lower end of the esophagus, the cardiac sphincter, which is a circular muscular valve that relaxes to allow the food into the stomach.
• After two hours, the pyloric sphincter which guards the opening on the lower end of the stomach relaxes.
• Food enters the duodenum. This is the upper part of the small intestine.
• Final digestion occurs in the small intestine. Undigested food passes on to the large intestine, where it undergoes decomposition by the action of bacteria.
• The resulting feces are thrown out of the body through the anus by the process of defecation or bowel movement.
2. Mechanical Phase of Digestion
Mechanical digestion, the second stage, involves a change in the physical properties of food.
• Food is cut and chewed into small pieces with the use of our teeth.
• Saliva produced from three pairs of salivary glands moistens the food. The tongue mixes the food with saliva. The back of the tongue mixes the food with saliva. The back of the tongue secretes mucus, which makes the food easier to swallow.
• The food tube churns and mixes the food with digestive juices in the stomach and small intestine.
• When the body happens to take in harmful substances, peristalsis in reverse direction helps protect our body by causing us to vomit.
3. Chemical Phase of Digestion
The chemical phase of digestion involves the change in the chemical composition of food, converting the complex molecules of proteins, carbohydrates, and fats into the simpler molecules of amino acids, simple sugars, fatty acids, and glycerol. This takes place in the presence of special protein molecules called enzymes.
3a. Chemical Digestion of Carbohydrates
Enzymes that are involved in the digestion of proteins are known as proteinases. Those involved in the digestion of carbohydrates (such as starches and double sugars) are known as carbohydrates. The enzyme involved in the digestion of fats, which are also called lipids, is known as lipase. These names give you an idea of how digestive enzymes are named. The names have two parts:
a. The substance on which they act, or the substrates; and
b. The suffix -ase.
The figure above shows that the products of chemical digestion of food are amino acids, fatty acids, glycerol, and simple sugar. Chemical digestion of carbohydrates brought about? Chemical digestion of starch starts from the mouth. Man has three parts of salivary glands. They are the parotid glands, submaxillary glands, and sublingual glands.
Organs, Glands, and Enzymes
Salivary Glands (Amylase or Ptyalin)
Pancreas (Amylase or Ptyalin)
Intestinal Glands (Maltase, Sucrase, Lactase)
Maltose, Sucrose, Lactose
Glucose, Fructose, Galactose
Saliva contains a starch digesting enzyme called salivary amylase, or ptyalin. Amylase is an example of a carbohydrate. It changes starch, also called amylum, into a double sugar called maltose. Maltase in the small intestine completes the digestion of starch by changing maltose to simple sugar.
When we eat and swallow starchy food without chewing it well, there is hardly any digestion of starch in the mouth. Fortunately, the pancreas produces a digestive juice which contains another starch-digesting enzyme called pancreatic amylase, or amylopsin. It is emptied into the small intestine by way of a fine tube or duct. It converts starch into maltose.
The small intestine has numerous glands along its inner wall. These glands secrete a digestive fluid called intestinal juice, which contains several enzymes. Among them are carbohydrases which help digest double sugars. For example, the enzyme sucrase changes cane sugar, or sucrose, into simple sugars. The enzyme lactase helps digest milk sugar, or lactose, into simple sugars.
3b. Chemical Digestion of Proteins
The stomach has a great number of glands along its inner wall. These glands secrete a digestive fluid called gastric juice, which contains two important substances: pepsinogen and hydrochloric acid (HCl, about 0.2% to 0.5%). In the presence of hydrochloric acid, pepsinogen is converted into the enzyme pepsin, which is a proteinase. The chemical change can be presented as follows.
Pepsinogen -> Pepsin
Pepsin changes the long protein molecules into shorter protein molecules called polypeptides. Another proteinase called trypsin, in pancreatic juice also changes proteins into polypeptides. The other proteinases called peptidases, secreted by the pancreas and intestinal glands complete the digestion of proteins by changing the polypeptide into amino acids.
Site of Digestion
Digestive Juices and Their Properties
pepsinogen, protein, milk protein
Pancreatic and Intestinal Juice
polypeptides, amino acids
The other protein-digesting enzyme, trypsin, is also produced by the intestinal glands as inactive trypsinogen. It is changed to trypsin when it combines with enterokinase, which is another secretion of the intestinal glands.
It has been found that another proteinase, rennin, is present in the stomach of infants. Rennin curdles the milk in preparation for the action of other proteinases. In adults, pepsin performs the function of rennin.
3c. Chemical Digestion of Fats
The large digestive gland in the body is the liver. It secretes a yellow-green liquid known as bile which is stored in the gall bladder. The gall bladder releases the bile the moment food is present in the duodenum. It empties the bile into the duodenum. It empties the bile into the duodenum by way of a fine tube or duct. Bile has no enzyme. It changes fat into tiny droplets, something like the action of soap suds on oil. In other words, fat is changed into an emulsion. The enzyme lipase can act on fats better when they are in the form of very tiny droplets.
Pancreatic juice contains several enzymes. One of these is lipase. One of the enzymes in intestinal juice is also lipase. Thus, the body has three adaptations that ensure the digestion of fats.
a. Bile, which emulsifies fats
b. Lipase in pancreatic juice
c. Lipase in the intestinal juice
In spite of these adaptations, it is not advisable, especially for elderly people to take in too much fat. This is because of a substance called cholesterol that the body manufactures from fatty foods and which, when present in great quantities, gets deposited along the inner surface of blood vessels and thereby makes the blood vessels narrower.
Absorption, the fourth stage of digestion, is the process by which substances are taken in by the cells of the food tube. The final digestion of food takes place in the small intestine. It is also here, especially at the lower portion of the small intestine, that most of the digested food is absorbed.
Digested foods in the form of molecules of amino acids, simple sugars, fatty acids, and glycerol diffuse into the capillaries and reach the blood. Molecules of fatty acids and glycerol diffuse into the lacteals and reach another circulating fluid, the lymph. The process of absorption of food includes the diffusion of digested food from the food tube to the cells lining the food tube until it reaches the circulating fluids, that is, blood and lymph. Beyond this point is another process, circulation. The circulating fluids distribute the digested food to all the cells of the body.
Below is a video that shows a portion of the inner surface of the intestinal wall. It is covered by very small projections called villi. These are structures that absorb digested food from the small intestine. Each villus is provided with two kinds of vessels: capillaries and lacteals.
5. Excretion (Elimination)
The last stage of digestion is the elimination or excretion. In the elimination phase, undigested food or food molecules that cannot be absorbed by the body need to be excreted. Elimination is sometimes called defecation. This is where indigestible wastes in the form of feces, are removed from the body. The feces, before leaving the anus, are stored in the rectum, which is the last part of the large intestine.
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