Phases of Gastric Secretions

Food is digested through a combination of two methods: mechanical and chemical digestion, both of which involve the stomach.

Mechanical digestion

Mechanical digestion refers to the physical process of grinding, tossing Mechanical digestion refers to the physical process of grinding, tossing and churning to break food down into smaller pieces. This provides more surface area for digestive enzymes (proteins that break down nutrients into smaller pieces to facilitate their absorption by the body) to work more effectively on the smaller bits of food. Another aspect of mechanical digestion is peristalsis, which refers to the involuntary contraction and relaxation of muscles in the esophagus, stomach and gut. This movement helps to break down food and propel it through the gastrointestinal (GI) tract.

Mechanical digestion starts in the mouth with chewing, followed by churning and mixing in the stomach and segmentation in the small intestine. The steps that occur in the stomach and small intestine expose the food to gastric juices and other fluids required for chemical digestion, thereby facilitating their absorption.

Chemical digestion

Chemical digestion refers to the breakdown of complex molecules, such as carbohydrates, proteins and fats, into tinier molecules that can be absorbed by the cells. The cells then utilize the nutrients obtained from chemical digestion to produce energy and make new cells. It typically involves a process known as hydrolysis, which uses water and digestive enzymes to break the chemical bonds that hold complex nutrients together.

Chemical digestion begins in the mouth, where enzymes found in saliva break down food. It continues in the stomach, where two types of molecules, stomach acids and digestive enzymes, are responsible for the chemical breakdown of food.

The release of gastric secretions, such as digestive juices and hormones, is a highly regulated process controlled by the nervous and hormone systems. It can be divided into three phases: cephalic, gastric and intestinal. Once gastric secretion starts, all three phases can take place simultaneously.

Cephalic phase

The cephalic phase, or reflex phase, is relatively brief and occurs before food even reaches the stomach. It is triggered by the smell, taste, sight or thought of food. When any of these four senses is stimulated, receptors send messages to the brain. This in turn delivers signals that promote gastric secretion to prepare the stomach for digestion. These signals are mediated by the vagus nerve, which is a part of the nervous system.

The increased release of gastric secretions during the cephalic phase is a conditional reflex. This means it only happens when food is desired. In contrast, when appetite is suppressed, the cephalic phase is inhibited.

Gastric phase

This phase lasts between three to four hours and is responsible for 50 to 60% of total gastric secretion. It is initiated by the presence of food in the stomach. When food enters the stomach, it creates distension that stimulates stretch receptors in the organ. These receptors send signals that activate the vagus nerve to release a substance called acetylcholine, which promotes the increased secretion of gastric juice.

Gastric juice mainly consists of hydrochloric acid and other digestive enzymes, which play a key role in breaking down proteins in digestion. Notably, your body may produce up to 1.5 liters of gastric juice daily. The presence of acid helps create an acidic environment in the stomach, helping to break down food and activate digestive enzymes.

Subsequently, proteins in food are broken down into peptides and amino acids via chemical digestion. The presence of these partially digested proteins, caffeine and increasing pH (a measurement of acidity levels) in the stomach stimulates the release of gastrin, a hormone secreted by enteroendocrine cells in pyloric glands. Gastrin in turn promotes the release of histamine, which stimulates the parietal cells (found in glands lining the top of the stomach) to produce acid and intrinsic factors (proteins that help the body absorb vitamin B12). The gastric phase goes on until food has left the stomach.

Intestinal phase

The intestinal phase of secretion is activated by the emptying of partially digested food (chyme) and low pH in the duodenum, which is the first section of the small intestine. This promotes the release of gastrin and gastric acid. However, once the pH of the lower stomach has become too acidic, your body will receive signals to limit the production of gastric acid.