Digestive System

Anatomy and Physiology

The GI tract, also called the gastrointestinal (GI) system. Consists of digestive tube called GI tract or alimentary canal and several accessory organs whose primary function is to break down food, absorb it, and eliminate waste. Extends from mouth to anus. Waste is not absorbed by the bloodstream. Accessory organs of digestion: the liver, gallbladder, and pancreas.

Mouth

The mouth, or oral cavity, is receptable for food. Its formed by the cheeks (bucca), lips, teeth, tongue, and hard and soft palates. Located around the oral cavity are three pairs of salivary glands that secrete saliva. Food is broken mechanically in mouth (by the teeth through chewing) and chemically (by saliva) and then formed into a bolus.

Teeth

Teeth play an important role in the initial stages of digestion by mechanically breaking down food (mastication) into smaller pieces. Teeth are covered by enamel, giving them a smooth white appearance. Beneath the enamel is dentin, the main structure of the tooth. The Intermost is the pulp, containing nerves and blood vessels. Teeth are embedded in gums (gingiva).

Tongue

The tongue assists in the chewing process by manipulating the bolus, moving it to the back of the mouth for swallowing (deglutition). The tongue also aids in speech production and taste. Rough projections on the surface are called papillae, containing taste buds. Sweet, sour, salty, and bitter.

Hard and Soft Palates

The two structures forming the roof of the mouth are the hard (anterior) and soft (posterior) palate. The soft palate forms a partition between the mouth and nasopharynx, continuous with the hard palate. The entire oral cavity is line with a mucous membrane.

Pharynx, Esophagus, and Stomach

Pharynx (throat), it is guided by the V-shaped structure called the uvula. The funnel-shaped pharynx serves as a passageway to the respiratory and GI tracts and provides resonating chamber for speech. The lowest portion divides into two tubes: one leading to lungs (trachea) and one leading to the stomach (esophagus). The small flap of cartilage called the epiglottis folds back to cover the trachea during swallowing, forcing food down the esophagus. At all other times, the epiglottis remains upright, allowing breathing.

The stomach, located in LUQ of the abdominal cavity, serves as a food cavity and continues both mechanical and chemical digestion. The stomach extends from the esophagus to the small intestine (duodenum). The terminal portion of the esophagus, the lower esophageal (cardiac) sphincter, is composed of muscle fibers that constrict once food passed through the stomach. Prevents regurgitation to esophagus. The body of the stomach, the large central portion, together with the upper portion (fundus) are mainly storage areas. Most digestion occurs in the pylorus (terminal portion). The lining of the stomach contains fibers, called rugae, which unfold as the stomach fills. HCl and enzymes help to chemically digest food, turning it into chyme that slowly leaves through the pyloric sphincter (controls movement and prohibits backflow) and enters the duodenum. Food flows through the GI tract by muscle contractions call peristalsis.

Small Intestine

20 feet log begins at pyloric sphincter:

1) Duodenum: uppermost segment 10 inches long

2) Jejunum: 8 feet long

3) Ileum: 12 feet long

Digestion is completed in the small intestine with the help of enzymes and secretions from the pancreas and liver. Villi absorb nutrients from chyme. Nutrients enter the bloodstream and lymphatic system for distribution to body. Muscle called ileocecal valve allows undigested food/unabsorbed material to pass into the large intestine and eventually is excreted.

Large Intestine

5 feet long, begins at end of ileum and extends to anus. No digestion. Secretes mucus in the colon, lubricating fecal matter so it can pass. Three main compartments: cecum, colon, rectum. The cecum (2-3 inches) hangs inferior to the ileocecal valve. The appendix (no apparent function and when inflected and inflamed becomes appendicitis) is connected to the cecum. Cecum merges into colon, which main functions are absorbing water and minerals and eliminate undigested material. The colon is divided into the ascending, transverse, descending, and sigmoid portion. The ascending colon forms the hepatic flexure.

Accessory Organs of Digestions

Some organs lay outside of the GI tract but are still playing an important role in digestion.

Liver

Largest glandular organ in the body, weighs 3-4 pounds. Located beneath diaphragm and in RUQ and LUQ of abdominal cavity.

Important functions:

-Reducing bile (aids in digestion of fat)

-Removing glucose (sugar) from blood and synthesizing glycogen (starch), retaining it for later usage.

-Storing vitamin B12, A, D, E, and K

-Destroying or transforming toxic products into less harmful compounds

-Maintaining normal glucose levels in the blood

-Destroying old and releasing bilirubin

-Synthesizing proteins in blood, such as albumin for fluid balance and prothrombin and fibrinogen for coagulation

Pancreas

Performs endocrine and exocrine functions. In the endocrine gland, the pancreas secretes insulin directly into the bloodstream to maintain normal blood glucose levels. As an exocrine gland, the pancreas produces enzymes for digestion that pass through the pancreatic duct. That duct extends along the pancreas with the hepatic duct from the liver, entering the duodenum.

Gallbladder

Storage area for bile. Common bile duct, bile to duodenum. Bile is drained from the liver through the left and right hepatic duct. These two eventually form the hepatic duct. The cystic duct of the gallbladder merges with the hepatic duct to form the common bile duct, leading to the duodenum. Bile production is stimulated by hormone secretion, produced in the duodenum once food enters the small intestine. Without bile, no fat digestion.

Disease Focus

Although some digestive disorders are asymptomatic, many are associated with nausea, vomiting, bleeding, pain, and weight loss. Clinical signs, like jaundice and edema can indicate a hepatic disorder. Gastroenterology is the branch of medicine concerned with digestive diseases.

Peptic Ulcer Disease (PUD)

An ulcer is circumscribed open sore on skin or mucous membranes of body. Peptic ulcers are one of most common that primarily develop in the stomach and duodenum, but can also occur in the lower esophagus. Helicobacter pylori bacteria can cause PUD. Most common in duodenum. Risk factors that contribute to PUD include smoking, tobacco usage, stress, caffeine.

Hernia

Protrusion of any organ, tissue, or structure of the wall of cavity which it is naturally contained. In general, term is applied to protrusions of abdominal organs (viscera) through abdominal wall. Inguinal hernia develops in the groin. In the early stages, usually reducible; can be pushed back into place. Strangulated hernia may develop, leading to necrosis with gangrene. An umbilical hernia part of intestine at the navel. Diaphragmatic hernia a congenital disorder, and hiatal hernia occur in the diaphragm. GERD- Gastroesophageal reflux disease.

Hepatitis

Inflammatory condition of liver. Hepatitis A (infectious) through contaminated food, hepatitis B (serum hepatitis) blood transfusions or sexual contact (for C too), and hepatitis C has no vaccine available. No cure for hepatitis.

Diverticulosis

Small, blister like pockets (diverticula) develop in inner lining of large intestine and may balloon through intestinal wall. Pockets most common in sigmoid colon.

Oncology

Neoplasm nearly always develops from the epithelial or mucosal lining of the stomach in the form of cancerous glandular tumor (gastric adenocarcinoma). Persistent indigestion = stomach cancer. Colorectal cancer is one of the most common types in the US. It originates in epithelial lining of the colon or rectum and can occur anywhere in the large intestine.