Describe how the stomach is involved in human digestion. Include enzyme and hormone actions that ...

The stomach is involved in both mechanical and chemical digestion. During chemical digestion the hormone gastrin is released prior to the food entering the stomach to ensure the readiness of the stomach. Gastrin signals the release of gastric juices, including hydrochloric acid (HCl), pepsinogen, mucus and gastric lipase. HCl increased the acidity of the stomach contents, denatures proteins and activates pepsinogen to pepsin. Pepsin is a protease that hydrolyzes the peptide bonds between amino acids once the protein has been denatured. Since the stomach lining is made out of protein, it must be protected from the activity of the proteases. To accomplish this, mucus is secreted from the gastric pits. A small amount of gastric lipase is also secreted and breaks down lipids in the stomach.

Very little is actually absorbed by the stomach, but the few substances that are include: water, minerals alcohol, aspirin, ibuprofen and medium chain fatty acids. Most absorption along the digestive tract occurs in the small intestine.

Mechanical digestion involves the physical separation and breakdown of molecules. In the stomach, this is achieved through the activation of three layers of muscles surrounding the stomach. These layers differ in their direction of orientation. The longitudinal muscles run along the length of the GI tract. The circular muscles run across the stomach transversely, which the diagonal muscles run in a diagonal pattern across the stomach. Together, they are responsible for the churning and movement of the stomach contents.

There is no carbohydrate digestion within the stomach due to the high acidity and the inactivation of the salivary amylase that was present in the bolus of food when it entered the stomach.

The stomach is also a 'holding tank' and is responsible for storing the majority of the chyme while the pyloric sphincter controls the entry of the food into the small intestine.

Makes sense actually, thank you