The first stage of the digestive system is the mouth and digestion starts here before you even eat your food. The smell of food alone triggers salivary glands in your mouth causing your mouth to water. When you begin to eat your food the teeth begin to break down the food into smaller pieces with a larger surface area and the food forms into a bolus (ball.) The saliva produces contains water to dissolve soluble substances. The saliva also contains lysozymes to kill bacteria and amylase to digest starch.
The second stage in the digestive system is the oesophagus. It connects the mouth and the stomach. It is a soft tube that can be closed, unlike the trachea, which is a hard tube, held open by rings of cartilage. Here the food bolus is swallowed by an involuntary reflex action through the pharynx (the back of the mouth). During swallowing the trachea is blocked off by the epiglottis to stop food entering the lungs. The bolus is pushed through the oesophagus and into the stomach by means of a series of contractions called peristalsis.
The third stage in the digestive system is the stomach. The stomach is a sac-like organ with strong muscular walls. There are three layers of muscle to churn the food into a liquid called chyme.. The stomach secretes acid. The gastric juice, which contains: hydrochloric acid (pH 1) to kill bacteria (the acid does not help digestion, in fact it hinders it by denaturing most enzymes); mucus to lubricate the food and to line the epithelium to protect it from the acid; and the enzymes pepsin and rennin to digest proteins. It continues the process of breaking the food down and changing it to a consistency of liquid or paste.
The fourth stage in the digestive system is the small intestine, this is about…