The alimentary canal is a tube that extends from the mouth to the anus, about thirty feet in length, and lined throughout by mucous membrane. The alimentary canal walls have four basic layers or tunics. These are the mucosa, the submucosa, the muscularis externa, and either a serosa or adventitia. Each of these layers has a predominant tissue type and a specific function in the digestive process. The mucosa is the wet epithelial membrane abutting the alimentary canal lumen.
The major functions of the mucosa are secretion, absorption of digested foodstuffs, and protection. The submucosa’s major function is nutrition and protection. The muscularis externa is the major regulator of GI mobility. The serosa functions to reduce friction, anchors and protects the surrounded organs. The mouth is at the beginning of the alimentary canal. The mastication of food takes place here. The lining of the mouth, a thick stratified squamous epithelium, protects it from abrasion by sharp pieces of food during chewing.
From the mouth, swallowed food passes into the oropharynx and then the laryngopharynx, which are lined by a stratified squamous epithelium, which protects them against abrasion. They contract in sequence to squeeze bolus into the esophagus. The pharyngeal constrictors are skeletal muscles, as swallowing is a voluntary action, innervated by the vagus nerve. The esophagus is a muscular tube that propels swallowed food to the stomach. The stomach then is the widest part of the alimentary canal, it’s a temporary storage tank where food is churned and turned into a paste called chime.
Also, the stomach starts the breakdown of food proteins by secreting pepsin. The small intestine is the longest part of the alimentary canal and the place of most enzymatic digestion and virtually all absorption of nutrients. The small intestine shuffles the chime back and forth contacting with the nutrient-absorbing mucosa. Peristalsis propels chime through the small intestine in about 3-6 hours. The large intestine is the last major organ of the alimentary canal. The material that reaches it is a largely digested residue that contains few nutrients.
The 12-24 hours that the residue remains in the large intestine, little breakdown of food occurs. Its main function is to absorb water and electrolytes from the digested mass, resulting in semisolid feces. Movements through the large intestine are sluggish and weak, except for mass peristaltic movements, which pass over the colon a few times a day to force the feces powerfully toward the rectum. The rectum generates strong contractions for defecation. The anal canal is the last subdivision of the large intestine.
The wall of the anal canal contains two sphincter muscles an internal and external anal sphincter. The external sphincter contracts voluntarily to inhibit defecation and the internal sphincter contracts voluntarily, both work to prevent feces from leaking from the anus between defecations and to inhibit defecation during emotional stress. Function of the pancreas- it functions as an exocrine organ by producing digestive enzymes and as an endocrine organ by producing hormones, with insulin being the most important hormone produced by the pancreas.
Function of the gallbladder-its function is to store and concentrate bile Function of the liver-functions to produce substances that break down fats, convert glucose to glycogen, produce urea (the main substance of urine), make certain amino acids, filter harmful substances from the blood, storage of vitamins and minerals, and maintain a proper level of glucose in the blood, also is responsible for producing cholesterol. Produces about 80% of the cholesterol in the body