The Stomach

Swallowing.

    Even if early digestion begins in the mouth with some enzymes, it is the stomach that does the major work. With acid secretions and muscle contractions, the stomach breaks the tissue and cell membranes of food. But before the food (the food bolus) arrives in the stomach, one must swallow it. This process is called swallowing or deglutition. It is important to coordinate the deglutition so it does not interfere with respiration and vocalization. Otherwise food or water could reach the trachea and provoke coughing or even choking.

     Deglutition occurs in three steps: 1) a voluntary step that initiates the process, 2) an involuntary pharyngeal reflexe that runs for 1-2 seconds, pushing the bolus into the esophagus, and 3) the esophageal involuntary contraction, pushing the bolus toward the stomach.

    1) The voluntary step is to trap a small amount of food (bolus) between the tongue and the palate. The bolus is then pushed towards the pharynx where the swallowing reflex is triggered.

    2) The involuntary swallowing reflex is initiated at the entrance of the pharynx, where the bolus stimulates ciliary cells, sending information to sensory centers in the brain stem (medulla). In response to this stimulus, the brain triggers a series of muscle contractions that propel the bolus into the esophagus. First, the soft palate rises and blocks the entrance to the nasal cavity, preventing reflux of food into the nose. Then, the palato-pharyngeal folds toward the center in order to limit the passage only to the smaller pieces of food. The presence of larger pieces can inhibit swallowing, and push the bolus back to the mouth. When the bolus reach the pharynx, the epiglottis closes the entrance of the trachea to prevent food from entering the respiratory tract. The next step is, using hyoid muscles, to move the pharynx forward and upward to fully open the entrance of the esophagus. Finally, the upper part of the esophagus relaxes and forms a funnel to receive food, and a serie of contractions pushes the bolus into the esophagus.

    3) The oesophageal phase consist of pushing the bolus throughout the esophagus into the stomach. This is done by a wave of contractions called peristalsis. It takes about 5 to 10 seconds for the wave to travel along the esophagus and the bolus to reach the stomach. This peristaltic reflex is initiated by the presence of food itself, which stimulates some afferences of the vagus nerve. The signal travels to the medulla, and returns by vagal efferents, hence the esophageal contraction.

The stomach.

    The stomach function's is to start the process of food digestion. It is in the stomach that are secreted the different acids and enzymes that break down the food particles to molecules as small as possible. The stomach can hold about 1 liter of food and its contraction ensure a good mixing.

    First, upon receiving food, the stomach relaxes. Then, before beginning the mixing, a ring of muscle (sphincter), at the lower level of the esophagus near the stomach, closes to prevent reflux of food and stomach acid back into the esophagus.

    The stomach acts as a reservoir for the time it empties itself into the duodenum. In the mean time, the mixing of food with the stomach acids liquefy them. This semi-liquid mixture is called the chyme and will slowly go into the duodenum.

    The mixing and emptying of the stomach is under the control of a host of variables. There is the presence of food itself, where the stretching of the wall enables a reflex vagal which increases the peristaltic contractions towards the pylorus. Stretching also promote the secretion of a hormone at the pyloric antrum, gastrin. This will stimulate the mucosa of the stomach to secrete more gastric juices. Finally, there is another reflex, even more important, that inhibits and may even stop emptying of the stomach. This vagal reflex is initiated when the duodenum is overloaded and it controls the amount of food to be transferred to the rest of the intestines.

    It is the mucus lining of the stomach that forms the acids juices. The muscular wall is responsible for the contractions and mixing of the food. And the external envelope of the stomach is the serous coat.

Cross section of the stomach wall.
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