What is Digestion

As kids, we've always been listening to our parents reiterating the importance of proper digestion. Digestion is defined as the mechanical and chemical breakdown of macro food particles into smaller and simpler forms, so as to facilitate the absorption of nutrients by the body. Does the definition clear all doubts? No! In fact, it raises a few more. Let's take a look at the digestive system and try to find out what happens after the food goes into your mouth.

Digestion: Why is It Necessary

Digestion is necessary to facilitate metabolism of food. The food that we eat contains various nutrients like proteins, vitamins, carbohydrates, minerals, etc. that provide energy, immunity, serve as building blocks, and nourishes the body. In the food that we eat, nutrients are in the form of large molecules that are physically and chemically bound together. Digestion breaks down this bond of complex molecules into simple nutrient molecules that can be absorbed by the body to support its day-to-day functions. For example the complex protein molecules are broken down into simple molecules of amino acids, which can be easily absorbed by the intestinal walls, and then can be diffused into the blood.


The Digestion Process: A Detailed Explanation

Several organs of the digestive system are involved in the digestion process. The process starts in the mouth and passes through the esophagus, stomach, small intestines, large intestines, rectum and ends with the anus. But, before we go further, it is important to know what controls the digestive system. There are hormones that control the functioning of the digestive system. These are produced by cells in the stomach and small intestine. Gastrin, secretin, and cholecystokinin (CCK) are the three hormones that mainly control digestion. Let us go step by step to see what happens after food enters the mouth.

• The process of digestion begins in the mouth or the oral cavity. The mechanical process of cutting and chewing the food is the very basic step of digestion. This process also involves the mixing of food with saliva where the tongue is actively involved. There are certain enzymes in the saliva which softens the food and prepares it for digestion, before the core process starts in the stomach.
• The next organ worth mentioning is the esophagus. It is a tunnel like organ that connects the mouth to the stomach and delivers the chewed food mixed with saliva to the stomach. It has a set of ring shaped muscles aligned parallel to each other that push the food towards the stomach. This is the reason why one can successfully swallow the food, even while in an upside down position (please, don't try this at home!).
• The next organ in the functioning of the digestive system is the stomach. The stomach is the most important organ in the digestive system of mammals. Any disorder in the functioning of the stomach can cause digestion problems. The stomach is a small J-shaped pouch like organ, walls of which are made of highly elastic muscles. The inclusion of the stomach in the entire process of digestion is called the gastric phase. There are four parts into which the stomach is divided. The part that receives food from the esophagus is known as cardia. The food then passes on to the curvature of the stomach known as the fundus. The central part of the stomach breaks down the food particles. After the food molecules has been broken down, it is transferred to the antrum which passes it to the small intestine. When the food enters the stomach, the lining of the fundus (called the gastric fundal mucosa) produces hydrochloric acid (HCl). Basically, this acid does not directly contribute to the digestion process. It only optimizes the PH level required for the digestive enzymes produced by the liver and the pancreas for breaking down food.
• The small intestine is the next organ that the food passes to in the process of digestion. It is the longest organ in the entire digestive system and can be as long as 7-8 meters. The small intestine is also divided into three parts. The duodenum is the part that connects the stomach to the rest of the intestine. In spite of being the shortest track in the small intestine, a lot of chemical digestion takes place here. In the duodenum, the digestive juices generated by the pancreas and liver mix to speed up the process. Here, the pH levels gradually change and turn into a base. The central part of the small intestine is known as the jejunum. This is the longest part of the small intestine and it is here that the absorption of nutrients take place. Here, the inner surface has small finger-like structures called villi. Each villi is covered with even smaller hair-like structures called microvilli. These improve the process of nutrient absorption by increasing the surface area of the intestine. Hepatic portal veins carry the blood containing the absorbed nutrients from the small intestine to the liver for filtering, removal of toxins, and nutrient processing. The remainder of the food is passed on to the ileum. The job of the ileum is to absorb the nutrients missed by the jejunum. Most vitamins are absorbed by the ileum.
• After this, the remaining food particles sans nutrients are transported to the large intestine, the next organ in the system. One of the functions of the large intestine in digestion is fermentation, done by the gut bacteria, which breaks down the remaining nutrients after processing in the small intestine. There are three parts of the large intestine: the cecum, the colon and the rectum. The cecum is attached to the appendix and is also the connection between the small and the large intestines. The colon is responsible for the abrogation of water and salts from the digested foods. This is the final stage of digestion. The third and the final part of the large intestine is the rectum, and along with the anus, which allows for controlled elimination of stool. It is interesting to note that most digestion and absorption occurs before the food enters the large intestine but the food spends more time in the large intestine than anywhere else in the entire process.

It's interesting to know how long does digestion take. On an average, food takes ½ an hour to 2 hours to travels through the stomach, two to six hours to continue through the small intestine, and can spend six to 72 hours in the large intestine before being removed by defecation. So, in all the entire process of digestion can take anywhere between 24 to 72 hrs.

Now, after understanding digestion, it is important to follow healthy practices like chewing food properly, avoiding alcohol and decreasing caffeine intake; can work wonders in keeping your digestive system up and running.