The Role of the Esophagus and Stomach in Digestion

The esophagus and the stomach is part of the digestive tract. Let's look at their role in the overall digestive process.The esophagus is a muscular tube that connects the throat (pharynx) to the stomach. It carries food from the throat to the stomach.Bolus is the moist and soft ball of crushed food in the mouth. Swallowing pushes bolus to the top of the esophagus. During the swallowing - once food enters the esophagus - it is pushed towards the stomach through a rhythmic muscular action called peristalsis.The inner surface of the esophagus is like that of the mouth. Esophageal glands produce mucus to give a smooth passage of the bolus through the esophagus.The esophagus doesn't secrete any digestive enzymes. The digestive process does not take place inside the esophagus. The purpose of the esophagus is purely that of a passageway. Bolus picks up a few digestive enzymes from the mouth. Hence some amount of digestion is already happening within bolus while it is passing through the esophagus.The gastroesophageal junction or cardiac (lower esophageal) sphincter is the place where the esophagus connects to the stomach. It is technically not considered a valve, although it does act as a valve. It prevents the stomach liquid to back up into the esophagus.Stomach liquid is highly acidic because of the nature of stomach juices and enzymes. Esophageal surface is not meant to handle stomach juices. The cardiac sphincter ensures that the bolus only enters from the esophagus to stomach and stomach content doesn't back up into the esophagus. Sometimes though stomach content does leak into esophagus and this causes the irritation and burn sensation in the esophagus, which we wrongly call 'heart burn'. In reality it is an esophageal burn. That would be hard to say in practice!Next let's talk about the stomach. The stomach is the muscular and hollow part of the digestive system. The stomach lies between the esophagus and the small intestine along the digestive tract.It mainly produces protein digesting enzymes and strong acid to help with the digestion process. The strong acid is the hydrochloric acid, which we commonly refer to as HCL. HCL that stomach produces doesn't directly help with digestion but it provides a proper pH environment for the protein digesting enzymes to work. It also kills bacteria that enter the body along with the food.Through the muscular peristalsis, stomach churns its contents, which facilitates mixing and some grinding action.Major activity inside the stomach is the start of protein digestion. Protein digestion is a two-step process. Complex protein molecules are first converted into peptides and eventually peptides are converted into simple amino acids. Stomach converts protein into peptides.Many times people wonder, if the stomach can digest protein, how come it doesn't digest itself. Here is the reason. Stomach wall only releases inactive enzyme called pepsinogen. This inactive enzyme is only activated by HCL to convert into pepsin. Acid proof mucous protects the stomach lining. Inactive enzyme and HCL acid only interact outside of mucous. This way, the stomach wall is protected from digestion. Once converted to pepsin, enzyme starts interacting with protein and starts converting them into peptides. The remaining protein digestion process of converting peptides into amino acids is completed in the small intestine and doesn't happen inside the stomach.The stomach acts as a buffer along the digestive tract. Given its flexible bag like nature, it can store food when needed. Stomach only passes along food to the small intestine when the small intestine is not busy with digestion. When the small intestine is busy with digestion, stomach acts as a storage and stops sending food to the small intestine.There are few other hormones that secrete within the stomach that help regulate the flow and movement of food along the digestive tract. Gastrin prepares the stomach for more incoming food, by increasing the amount of HCL and pepsinogen. Gastrin is released within stomach when it detects stomach expansion along with the presence of large amounts of undigested protein.There are other hormones, which turn off or cut stomach emptying into the small intestine. Presence of food components, which are not absorbed in the small intestine, triggers these hormones. This signals that digestion in progress in the small intestine. When enough food is present in stomach Some of the stomach hormones also signal the brain the fullness sensation.