What happens in the stomach?

This hollow organ is the mixing-machine of the digestive system

(Image source: Future PLC)

The period after we’ve just finished a meal is a crucial time for digestion. The recently arrived food bolus – a ball of food and saliva-  has already been partially digested in the mouth, but next comes exposure to an onslaught of digestive enzymes that will prepare the food’s molecules for absorption in the intestines. The bolus first arrives through the pyloric sphincter into the stomach – a hostile alien environment where molecules are continuously secreted from the walls and activated by the pool of gastric acid that fills the chamber.

Alongside the stomach, a collection of other organs are busily producing and secreting their own cocktail of digestive enzymes. These will unite with the partially digested food in the duodenum, the organ that follows the stomach. The stomach is a fascinating organ because one of its main jobs is storing and slowly releasing food into the small intestine. Without this limiting point, food would rapidly travel through the digestive tract, and we’d miss out on a huge amount of precious nutrients simply because our intestines wouldn’t have time to absorb them. But the stomach is able to stretch and hold onto food for hours, methodically churning the material and setting upon it with its powerful stomach acid.

By the time the converted food leaves the lower chamber of the stomach and greets the awaiting digestive enzymes secreted by the neighbouring organs, it has been thoroughly prepared for its next step. It’s nearly time for the food to become a part of you.

When food meets the stomach

1. Propulsion

From the oesophagus, peristaltic waves move the food bolus into the stomach, where it collects and meets the stomach acids.

2. Grinding

Waves of peristalsis move from the fundus (top of the stomach) to pylorus (bottom of the stomach), churning the acids and food into a mixture called chyme.

3. Retropulsion

Eventually the chyme is pushed through the pylorus into the duodenum, small amounts at a time, while the rest of the contents is pushed back into the stomach.

(Image source: Future PLC)

The many powers of stomach acid:

1. Bacteria killer
The highly acidic environment is harmful to most microorganisms, including pathogenic bacteria.

2. Pepsin switch
Hydrochloric acid in the gastric juices converts pepsinogen, which is secreted from the stomach’s walls, into protein-digesting pepsin.

3. Protein unraveller
Available pepsin in the acid breaks down protein structure, cutting the molecule into smaller chains of amino acids.

4. Vitamin absorption
Gastric acid helps stimulate the secretion of a glycoprotein known as intrinsic factor into the stomach, which will later bind to vitamin B12 in the small intestine.

5. Bile delivery
The presence of stomach acid in the small intestine helps to stimulate the release of fat digesting bile.

6. Blocks acid reflux
The acidity of gastric juices helps to trigger the contraction of the lower oesophageal sphincter, helping to keep harmful acid away from the unprotected tubing.

7. Aiding migration
The pressure from the volume of stomach acid helps to open the pyloric sphincter briefly, transferring chyme and acid into the small intestine.


This article was originally published in How It Works issue 128, written by James Horton 


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