Anatomy and Physiology of GIT
The gastrointestinal (GI) tract, also known as the alimentary canal or gut, is a hollow tube extending from the mouth to the anus. It measures about 7–11 meters in length and appears shorter due to the folds in the gut wall. The GIT performs the following functions:
- Ingestion (receiving food)
- Peristalsis
- Mechanical breakdown of food
- Chemical breakdown of food
- Absorption of nutrients
- Excretion of unabsorbed food residue
Components of the Alimentary Canal
- Mouth cavity
- Pharynx
- Esophagus
- Stomach
- Small intestine
- Large intestine
- Rectum and anus
Mouth Cavity
The mouth is the first part of the GIT, consisting of lips, cheeks, palate, teeth, and tongue. The palate separates the oral and nasal cavities. Teeth are responsible for mastication, and the tongue aids in taste, speech, mastication, and swallowing. Saliva from parotid, submandibular, and sublingual glands lubricates food, begins starch digestion with ptyalin (amylase), and provides antibacterial action.
Pharynx
The pharynx is a muscular cone-shaped tube behind the nasal and oral cavities and larynx. It serves as a passage for both food and air and has three parts:
- Nasopharynx: Connected to the nasal cavity, lined with pseudostratified columnar epithelium; auditory tubes open here.
- Oropharynx: Behind the mouth, lined with stratified squamous epithelium; contains palatine tonsils.
- Laryngopharynx: Extends to the esophagus and larynx, lined with stratified squamous epithelium.
Esophagus
A muscular tube about 25 cm long connecting pharynx to stomach. Upper third is striated muscle for voluntary swallowing, lower two-thirds smooth muscle for peristalsis. Lined with epithelial tissue and mucus-producing glands. Sphincters at the upper and lower ends prevent backflow and aid swallowing.
Stomach
The stomach is a hollow, J-shaped organ performing mechanical churning, storage, and chemical digestion. It consists of fundus, body, and antrum. Gastric secretions include hydrochloric acid (parietal cells), pepsin (chief cells), and mucus (goblet cells). The pyloric sphincter controls chyme passage into the duodenum.
Phases of Gastric Juice Secretion
- Cephalic phase: Stimulated by sight, smell, or taste of food via vagus nerve.
- Gastric phase: Food presence stimulates gastrin release, enhancing gastric juice secretion.
- Intestinal phase: Partially digested chyme in duodenum triggers secretin and cholecystokinin to slow gastric emptying and mix chyme with bile and pancreatic juice.
Small Intestine
Extends from the pyloric sphincter to the ileocecal valve, about 5 meters long, divided into:
- Duodenum: 25 cm, curved around pancreas, receives bile and pancreatic ducts through the sphincter of Oddi.
- Jejunum: 2 meters, suspended by mesentery.
- Ileum: 3 meters, opens into caecum of large intestine.
The small intestine has villi and microvilli to increase surface area for absorption. Intestinal glands (crypts of Lieberkühn) secrete succus entericus, while Brunner’s glands and goblet cells secrete mucus. Movements include peristaltic, rhythmic, and pendulum motions to mix food and enhance absorption. Digestion is aided by pancreatic juice, bile, intestinal enzymes, and intestinal microflora.
Large Intestine
About 1.5 meters long, surrounding the small intestine. It consists of:
- Caecum (with appendix)
- Ascending colon
- Transverse colon
- Descending colon
- Sigmoid colon
Functions include water absorption, microbial synthesis of vitamins K and folic acid, and defecation via peristalsis, rectal muscle contraction, and sphincter relaxation.
Rectum and Anus
The rectum connects sigmoid colon to the anal canal. The anal canal (about 3.8 cm) leads to the exterior and is controlled by two sphincters: internal (involuntary) and external (voluntary), facilitating controlled defecation.
Detailed Notes
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