The Digestive Process: From Swallowing to Bile Secretion
Swallowing
Once food is chewed and lubricated, the swallowing process begins, moving food from the mouth to the stomach. This process involves three stages:
Oral Stage (Voluntary)
The tongue pushes food against the hard palate, the glottis opens, and the esophagus contracts, propelling food into the pharynx.
Pharyngeal Stage (Involuntary)
Lasting about one second, this stage involves the soft palate elevating to close the nasopharynx and the larynx rising to cover the trachea’s opening. This prevents food from entering the airway.
Esophageal Stage
A peristaltic wave of contractions moves food down the esophagus to the stomach. The lower esophageal sphincter (LES) relaxes to allow food to enter the stomach and then closes to prevent regurgitation.
Gastric Secretion
Gastric secretion, primarily of hydrochloric acid (HCl), occurs in three overlapping phases:
Cephalic Phase
The sight, smell, and taste of food trigger the release of acetylcholine (ACh), stimulating gastrin and histamine production, which in turn promote HCl secretion.
Gastric Phase
Gastric distension and the presence of food stimulate further HCl secretion. Protein digestion products also directly stimulate gastrin release.
Intestinal Phase
As food enters the duodenum, it triggers the release of hormones like secretin and cholecystokinin (CCK), which inhibit gastric secretion and promote intestinal processes.
Intestinal Secretion
Intestinal secretion is primarily regulated by hormones and involves two main phases:
Cephalic and Gastric Phases
Similar to gastric secretion, these phases involve neural and hormonal stimulation, preparing the intestines for incoming food.
Intestinal Phase
CCK stimulates the production of enzyme-rich pancreatic juice, while secretin promotes the secretion of bicarbonate-rich fluid to neutralize stomach acid.
Bile Secretion
Bile, produced by the liver and stored in the gallbladder, aids in fat digestion. Its secretion is regulated by hormones and neural signals:
Digestive Period
CCK, stimulated by the presence of fat in the duodenum, triggers gallbladder contractions and the release of bile.
Interdigestive Period
Secretin stimulates bile secretion, and motilin promotes gallbladder contractions to prevent bile buildup.