Gastrointestinal System-Anatomy and Physiology

The gastrointestinal system has two major components, which are the alimentary canal or also called as the gastrointestinal tract and the accessory organs. It has two major functions. First, it is about breaking down of food and fluid into simple chemicals that can be absorbed into the bloodstream and transported through the body and second, is about elimination of wastes through excretion of stool. It is an important system because if it malfunctions, the overall health of the person will be greatly affected.

The alimentary canal is a hollow muscular tube that begins in the mouth and extends to the anus. It also includes the pharynx, esophagus, stomach, small intestine, and the large intestine. On the other hand the accessory organs are the liver, biliary duct system, and the pancreas.

Alimentary canal includes:

1. Mouth – It primarily deals with chewing, salivating, and swallowing of food. The tongue provides the sense of taste and saliva moistens the food during chewing. Saliva is produced by the parotid, submandibular, and sublingual salivary glands.
2. Pharynx or throat – It is a cavity that extends from the oral cavity to the esophagus. It assists in swallowing and propelling the food toward the esophagus.
3. Esophagus – It is a muscular tube that extends from the pharynx through the mediastinum to the stomach. When food is swallowed, the cricopharyngeal sphincter relaxes for the food to enter the esophagus. Then peristalsis propels liquids and solids through the esophagus into the stomach.
4. Stomach – It is the reservoir for food and it is a collapsible, pouch-like structure in the left upper part of the abdomen, just below the diaphragm. Its upper border is attached to the lower end of the esophagus. The stomach contains two sphincters: cardiac sphincter and pyloric sphincter.

The stomach breaks down food into chime, a semifluid substance, produces intrinsic factor necessary for the absorption of vitamin B12, and moves the gastric contents to the small intestine.

1. Small Intestine – It is approximately 6 meters long and more or less all digestion and nutrient absorption takes place in it. It also deals with the secretion of hormones that control the secretion of bile, pancreatic juices and intestinal juice. It is divided into three: duodenum, jejunum, and ileum.

The small intestine has structures that helps in the absorption such as the villi, which are fingerlike mucosa, microvilli, which are tiny projections on the surface of epithelial cells, and the plicae circulars, which are circular folds  on the mucosa.

1. Large Intestine or colon – It absorbs excess water and electrolytes, stores food residue, and eliminates waste materials in the form of feces. It is divided into six segments: cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum.

Accessory Organs:

1. Liver – It is located in the right upper quadrant of the abdomen under the diaphragm and it has four lobes: the left, right, caudate, and quadrate lobes.  It is consists of hepatic cells that encircle a central vein and radiate outward. The hepatic cells secrete bile and do many metabolic, endocrine, and secretory functions. The liver metabolizes carbohydrates, fats, and proteins and detoxifies various toxins in the plasma. It is also responsible for converting ammonia to urea, synthesizing plasma proteins, nonessential amino acids, and vitamin A, regulating blood glucose levels and in secreting bile.
2. Gallbladder – It is a small pear shaped structure that is positioned halfway under the right lobe of liver. It stores and concentrates bile produced by the liver and releases it into the common bile duct for delivery to the duodenum.

Bile is a greenish liquid that emulsifies fat and promotes intestinal absorption of fatty acids, cholesterol, and lipids.

1. Pancreas – It can be seen horizontally in the abdomen behind the stomach. Its head and neck extend into the curve of the duodenum and its tail is against the spleen.

It is responsible for the exocrine and endocrine functions. Its exocrine function includes scattered cells that secrete many digestive enzymes every day. Clustered lobules and lobes release their secretions into the pancreatic duct then the pancreatic duct runs the pancreas and joins the bile duct from the gallbladder before entering the duodenum. While its endocrine function involves the islets of Langerhans which is composed of two types of cells: beta cells that secrete insulin and alpha cells that secrete glucagon, blood glucose levels stimulate their release.

Digestion and Elimination

Digestion begins in the mouth where mastication, salivation and swallowing takes place. When  a food is swallowed, the hypopharyngeal sphincter relaxes, making the food enter the esophagus. In the esophagus, the glossopharyngeal nerve stimulates peristalsis which moves the food toward the stomach. As the food passes the esophagus, glands secrete mucus, which lubricates the bolus. As the food bolus enters the stomach, digestive juices are secreted and the stomach stretches. When the stomach stretches, gastrin is produced. Gastrin stimulates the motor functions and secretion of gastric juices by the gastric gland. These include pepsin, intrinsic factor, proteolytic enzyme and hydrochloric acid.

Peristalsis churns the food into tiny particles and mixes it with the juices, forming chyme. Chyme is then moved into the antrum of the stomach, where it backs up against the pyloric sphincter before being released into the duodenum. Carbohydrates, fats, and proteins are being broken down by the intestinal contractions and digestive secretions, allowing it to be absorbed in the intestinal mucosa and to the bloodstream. The chyme thereafter passes through the small intestine and later on the ascending colon where it is reduced to indigestible substances.

As the bolus goes to the large intestine absorption continues without producing hormones or digestive enzymes. It travels from the ascending colon, past the right abdominal cavity, to the liver’s lower border and crosses below the liver and stomach by way of transverse colon. It descends through the left abdominal cavity to the iliac fossa through the descending colon. It then travels through the sigmoid colon then to the rectum and finally moves out to the anal canal as waste product.