What Part of Digestion Involves Chemical Actions?

Chemical digestion breaks food down to molecules that tissues can assimilate on the cellular level.

Digestion involves the process of converting food into chemical forms that can be absorbed by the blood and delivered to body tissues for use on the cellular level. The first stage of digestion involves the breakdown of solid food by the mechanical actions of chewing and swallowing. The second stage is the chemical digestion of food into forms that can be easily assimilated.

Chemical Digestion Process

Think of the digestive system as one long tube winding through the body where all these chemical digestion processes occur in different areas: the mouth, esophagus, stomach, duodenum, small intestine and large intestine. The accessory glands and organs that aid in chemical digestion are the salivary glands, the gastric glands, pancreas and liver, among others.

This process involves the digestive enzymes combining with water molecules to hydrolyze fats, proteins and carbohydrates into smaller molecules that the cells can easily absorb and metabolize. Some of these digestive enzymes include amylase, chymotrypsin, collagenase, elastase, nuclease, protease and trypsin. Depending on the amount of food consumed and the level of digestive enzymes present in the stomach, the entire chemical digestion process can take from a few hours up to three days.

Mouth and Pharynx

Chemical digestion of food begins in the mouth as the salivary glands secrete an amylase enzyme which breaks the polysaccharides down to a disaccharide form called maltose.

Peristalsis is a series of wavelike contractions that move the swallowed food downward through the esophagus into the stomach. Nausea is an uncomfortable sensation which occurs during illness. It induces an involuntary anti-peristaltic motion of contractions, moving unacceptable material upward from the stomach out to the mouth where it can be expelled. After vomiting, the mouth and pharynx usually experience a burning sensation from the gastric acid present in the partially chemically digested material thrown up from the stomach.

Stomach

The stomach pouch can contain up to two liters of food at a time, and its gastric glands produce juices containing hydrochloric acid and pepsin, which digest proteins. This muscular organ contracts vigorously, mixing the food with gastric juices to produce a well-blended acidic mixture called "chyme." When the chyme spurts from the stomach into the first part of the small intestine called the duodenum, all the proteins and carbohydrates are partially chemically digested, but fat or lipid digestion has not yet begun.

Duodenum

Chemical digestion continues as the acidic chyme stimulates the duodenum cells to produce secretin, which further activates the pancreas to produce sodium bicarbonate to neutralize the hydrochloric acid, thus protecting the lining of the duodenum. Secretin also stimulates the liver to produce bile, which is stored in the gall bladder.

The presence of food in the duodenum activates the production of cholecystokinin, further stimulating the gall bladder to release its stored bile and the pancreas to produce its enzymes. The presence of food also induces the production of gastric inhibitory peptide. GIP slows the churning, mixing and grinding action of the stomach, reducing its rate of spurting chyme into the duodenum until the duodenum can pass along the food it already contains further into the small intestine.

Small Intestine

This section of the long digestive tube is about 10 feet long and contains projections called villi in addition to its numerous ridges and furrows. It functions to absorb nutrients broken down by chemical digestion of the food. Chemical digestion proceeds further with the peptidase and maltase enzymes breaking down the protein peptides to amino acids, and maltase completing the breakdown of dissacharides. Fat digestion completes its conversion of fats into glycerol and fatty acids, which enter the villi by diffusion, as do other nutrients in the small intestine.

Large Intestine

The colon or large intestine collects about 10 liters of water per day and about 1.5 liters of that comes from ingested food. The remaining 8.5 liters accumulates from secretions of the digestive system during chemical digestion. Dehydration results if this water is not absorbed and this causes diarrhea and electrolyte imbalances. The chemical digestion of the food has been completed at this point and any waste or fecal material accumulates in the last eight inches of the large intestine or the rectum, where it is finally discharged from the digestive system.

References

• Right Health: What is chemical digestion?
• Healthline: Peristalsis
• Colorado State University: The Physiology of Peristalsis

Resources

• National Geographic: Health and Human Body: Digestive System