The alimentary canal is a tube that extends from the mouth to the anus, measuring about thirty feet in length. It is lined with a mucous membrane throughout its entire length. The walls of the alimentary canal are made up of four layers known as tunics: mucosa, submucosa, muscularis externa, and either serosa or adventitia. Each layer has its own primary tissue type and plays a specific role in digestion. The mucosa serves as a moist epithelial membrane that forms the boundary of the lumen of the alimentary canal.
The mucosa has several major functions, including secretion, absorption of digested foodstuffs, and protection.
Similarly, the submucosa’s main function is nutrition and protection.
On the other hand, the muscularis externa serves as the primary regulator of GI mobility.
Meanwhile, the serosa functions to reduce friction while also anchoring and protecting the surrounding organs.
Located at the beginning of the alimentary canal, the mouth is where food mastication occurs.
The lining of the mouth is a thick stratified squamous epithelium that provides protection against abrasion from sharp food pieces during chewing.
Swallowed food passes from the mouth into the oropharynx and laryngopharynx, both of which have a stratified squamous epithelium lining to protect against abrasion. These sections contract one after another, forcing the bolus into the esophagus. The pharyngeal constrictors, skeletal muscles responsible for this voluntary action, are innervated by the vagus nerve. The esophagus, a muscular tube, propels the swallowed food towards the stomach. The widest part of the alimentary canal, the stomach acts as a temporary storage tank where food is churned and transformed into a paste called chime.
The stomach secretes pepsin to break down food proteins. The small intestine, which is the longest part of the alimentary canal, is where most enzymatic digestion and absorption of nutrients occur. The chime is moved back and forth in the small intestine, contacting the nutrient-absorbing mucosa. Peristalsis helps propel the chime through the small intestine in approximately 3-6 hours. The large intestine is the final main organ of the alimentary canal and receives mostly digested residue with few nutrients.
The residue remains in the large intestine for 12-24 hours with minimal food breakdown. The primary function of the large intestine is to absorb water and electrolytes from the digested mass, resulting in semisolid feces. Movements within the large intestine are slow and weak, except for a few daily mass peristaltic movements that forcefully push the feces towards the rectum. Defecation is aided by strong contractions in the rectum. The anal canal represents the final part of the large intestine.
The anal canal is equipped with two sphincter muscles, namely the internal and external anal sphincters. The external sphincter contracts voluntarily in order to prevent defecation, while the internal sphincter contracts intentionally. Both sphincters work towards preventing feces leakage from the anus between bowel movements and inhibiting defecation during periods of emotional stress.
On the other hand, the pancreas functions as both an exocrine organ responsible for producing digestive enzymes and an endocrine organ that produces hormones. One of these hormones, insulin, plays a crucial role and is secreted by the pancreas.
The gallbladder stores and concentrates bile, while the liver has various roles. These include breaking down fats, converting glucose to glycogen, producing urea and certain amino acids, filtering harmful substances from the blood, storing vitamins and minerals, regulating blood glucose levels, and producing 80% of the body’s cholesterol.
