Introduction:
Digestion is the conversion of complex nutrients like carbohydrates, fats, and proteins into simpler forms which are further converted into energy or stored in the body for proper functioning. The digestion process begins in the mouth in the presence of saliva and ends in the colon (large intestine). Each part of the digestive system helps break solid food into a semi-solid state and facilitates its absorption in the body. After food intake, it takes around 5 to 7 hours for the food to cross through the stomach and the small intestine. For further absorption of water, the churning of the food and elimination takes around 24 to 36 hours, depending on the individual's digestive health.
What Is the Digestive System?
The digestive system is our body's main functioning unit, which primarily provides nutrients and energy for proper functioning. It comprises the gastrointestinal tract and supporting organs such as the liver, pancreas, and gallbladder. The gastrointestinal tract starts from the mouth and ends in the anus. Food eaten passes through the entire gastrointestinal tract and undergoes digestion. The parts of the gastrointestinal tract are the esophagus, stomach, small intestine, and large intestine (colon). The small intestine is divided into the duodenum, jejunum, and ileum. The large intestine comprises the cecum, colon, appendix, and rectum. Good bacterias are present in the large intestine, known as gut flora. They also help in the digestion of food in the large intestine.
Why Is Digestion of Food Important?
Our body needs nutrition from the food and drinks we take, which can only be absorbed when converted into a simpler, easy-to-absorb state. Our digestive system helps to convert food into an absorbable state. The carbohydrates break down into glucose (sugar), the protein into amino acids, and the fat into fatty acids and glycerol. The gastrointestinal tract, liver, gallbladder, and pancreas convert the food into simpler forms and facilitate their absorption.
How Does the Food Move in the Gastrointestinal Tract?
The phenomenon of food movement in the gastrointestinal tract is called peristalsis. The gastrointestinal tract contains a layer of muscle that causes movement of the tract's walls, leading to the forward movement of food. The muscles behind the food contract and push the food forward, whereas the muscles in front of the food dilate and relax to allow food movement toward them.
The movement of food in different parts of the gastrointestinal tract is explained as follows:
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The Mouth: After chewing, when food is swallowed from the mouth, the epiglottis (small flap of tissue) folds over the windpipe to prevent the food from entering the windpipe or trachea. The food then passes through the esophagus.
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Esophagus: Once the food is swallowed, the movement of food becomes automatic as the brain signals the muscles to begin the movement peristalsis.
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Lower Esophageal Sphincter: When food is at the end of the esophagus, there is a ring-like muscle called the esophageal sphincter that relaxes and allows the food to pass to the stomach. The esophageal sphincter is generally closed to prevent food from returning to the esophagus.
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The Stomach: The food is mixed with digestive juices, and the stomach empties the chyme into the small intestine.
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The Small Intestine: The food undergoes churning by the muscles in the small intestine in the presence of digestive enzymes secreted from the pancreas, namely amylase, peptidase, and lipase. Bile, which is secreted by the liver, reaches the small intestine through the biliary duct. After all the digestion and absorption in the small intestine, the food is passed to the large intestine.
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The Large Intestine: After water absorption in the large intestine, what is left out is dead cells from the lining of the gastrointestinal tract, along with undigested food and fluids. This is the feces stored in the rectum and then excreted through the anus.
How Does the Digestive System Break the Food Into Small Pieces?
The food is converted into small parts while passing through the gastrointestinal tract with the help of digestive juices, digestive enzymes, bile and stomach acid, chewing, and the peristalsis movement in the tract.
The function of different parts in breaking down the food is:
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The Mouth: Chewing food into smaller parts is the first step of digestion, followed by mixing it with the saliva in the mouth. The saliva also contains enzymes such as amylase, which help to break down the starch. Saliva makes the bolus soft and moistens it so it can easily pass through the esophagus.
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Esophagus: It only acts as a passage for food from food to the stomach.
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The Stomach: Once the food enters the mouth, the stomach gets the signal to secrete hydrochloric acid. When the food enters the stomach, it is mixed with gastric juices and acid. The muscles of the stomach mix the food well with the digestive juices. The food stays in the stomach for around 4-5 hours.
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The Pancreas: When food is in the stomach, the pancreas starts the secretion of digestive juices amylase, lipase, and peptidases, which help to break down carbohydrates, fats, and proteins, respectively. The pancreas delivers these digestive juices to the small intestine through small ducts.
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The Liver: Simultaneously, the liver starts the secretion of bile which facilitates the digestion of some fats and vitamins. The gallbladder is the storage unit for bile, from where it travels to the small intestine through biliary ducts for the digestion of food.
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The Gallbladder: It stores the bile secreted by the liver and delivers it to the small intestine by biliary ducts.
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The Small Intestine: The small intestine also secretes some small amount of digestive juices, which are mixed in the food along with pancreatic enzymes and bile. The food then passes through the three parts of the small intestine and undergoes churning. The absorption of nutrients and water takes place mainly in the last part of the small intestine, called the ileum. This process takes about 2 to 3 hours of duration.
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The Large Intestine: After absorption of almost all the nutrients and water, the food (chyme) enters the large intestine, where the leftover water is absorbed, and the gut microbiome helps to absorb some more nutrients. After this, the left undigested waste passes to the rectum and stays there until the rectum is full and the brain indicates the urge to excrete it.
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The Anus: It is the last part of the gastrointestinal tract which controls the passing out of feces. It consists of an internal sphincter and an external sphincter. The opening of the internal sphincter is involuntary and controlled by the brain, whereas the external sphincter is regulated voluntarily by the person.
What Factors Can Affect the Duration of Digestion of Food?
The time taken to digest the food is affected by the following factors:
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The body type of the individual alters the duration of digestion.
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The efficiency of metabolism of the body affects the time of digestion.
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The intensity and level of physical activity performed by the individuals.
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History of any surgeries in the gastrointestinal tract.
Conclusion:
Digestion of food is a long process involving breaking down food and its absorption at different parts of the gastrointestinal tract. The food stays in the different parts of the gastrointestinal tract for different time durations, making it an almost 24 to 36 hours long process. For healthy individuals, the process takes this much time; however, the duration may alter in cases of any infection or disorder. For example, in cases of diarrhea, the food may pass out partially digested and in the form of loose stool due to incomplete absorption. Whereas in cases of constipation, it may take longer than the normal time duration. Thus it is important to evaluate the time of digestion by every individual.