Digestive System Overview and Oral Cavity

Digestive System Overview

• The digestive system breaks down complex nutrients into simpler organic and inorganic molecules.
• These molecules are absorbed into the blood or lymph, transported to cells, and used for energy and building new tissues.
• The digestive system is divided into the alimentary tube and accessory organs.
• The alimentary tube runs from the mouth to the anus, including the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.
• Digestion occurs in the oral cavity, stomach, and small intestine, with most nutrient absorption happening in the small intestine.
• Undigestible material, mainly cellulose, is eliminated by the large intestine.
• Accessory organs, such as teeth, tongue, salivary glands, liver, gallbladder, and pancreas, also play crucial roles in digestion.

Oral Cavity

• Food enters the oral cavity through the mouth.
• The oral cavity's boundaries include the hard and soft palates superiorly and the cheeks laterally.
• Teeth and tongue are located within the oral cavity, along with openings for salivary glands.

Teeth

• Teeth function in chewing, mechanically breaking down food and mixing it with saliva.
• Deciduous teeth erupt around 6 months of age, totaling 20, becoming fully developed by 2 years old.
• Permanent teeth gradually replace deciduous teeth throughout childhood, with molars appearing around 6 years old.
• A complete set of permanent teeth comprises 32 teeth: incisors, canines, premolars, and molars.
• The visible part is the crown, the root is embedded within the mandible or maxilla.
• The periodontal membrane lines the socket and creates bone-like cement that anchors the tooth.

Enamel and Dentin

• Enamel, produced by ameloblasts, forms the outermost layer of the crown; it's hard, resists decay, and provides a chewing surface.
• Dentin, very similar to bone, is produced by odontoblasts and forms the roots as well as most of the tooth.
• The pulp cavity sits within the dentin; it contains blood vessels and nerve endings from the trigeminal nerve.

Tooth Decay

• Dental caries (cavities) may result when bacterial acids erode enamel and dentin.
• Tooth decay can lead to bacterial invasion of the pulp cavity, causing significant toothache.

Tongue

• The tongue is made of skeletal muscle innervated by the hypoglossal nerves (cranial nerve 12).
• Papillae are small projections on the tongue's upper surface.
• Nerve supply includes the facial nerve and glossopharyngeal nerve.
• The tongue plays a vital role in swallowing by elevating and forcing food toward the pharynk.

Salivary Glands

• Salivary glands produce saliva, a digestive secretion in the oral cavity.
• There are three pairs of salivary glands: the parotid glands, submandibular glands (or submaxillary glands), and sublingual glands.
• Parotid glands are located behind and in front of the ear.
• Submandibular glands are located at the posterior corners of the mandible.
• Sublingual glands are positioned underneath the tongue.
• Saliva secretion is continuous, though the amount varies based on circumstances.
• The presence of food increases saliva secretion via a parasympathetic response regulated by facial and glossopharyngeal nerves. Sight and smell of food also increase saliva production.
• Stress can decrease saliva production, leading to dry mouth.

Esophagus

• The esophagus is a muscular tube that transports food from the pharynx to the stomach.
• No digestion occurs in the esophagus.
• Peristalsis is the muscular contractions that propel food in one direction, ensuring it reaches the stomach regardless of body position.
• The lower esophageal sphincter (LES) controls the passage of food into the stomach, preventing stomach contents from returning to the esophagus (heartburn).

Stomach

• The stomach is located in the upper left quadrant of the abdominal cavity, adjacent to the liver and spleen.
• It's a sac-like structure extending from the esophagus to the small intestine.
• The stomach serves as a reservoir for food, permitting gradual digestion.
• Both mechanical and chemical digestion take place within the stomach.
• Regions include the cardiac orifice (opening from the esophagus), fundus (upper dome-shaped region), body (largest portion), and pylorus (lower region connecting to the small intestine).
• The pyloric sphincter is a circular muscle surrounding the stomach's opening into the small intestine.

Small Intestine

• The small intestine, approximately 6 meters long, runs from the stomach to the large intestine's cecum.
• The duodenum, the first 10 inches (25 cm), is where the common bile duct enters.
• The jejunum, roughly 8 feet long, and the ileum, about 11 feet, are the longer parts of the small intestine.
• Digestion in the small intestine is completed with end products being absorbed into the blood and lymph.
• Its mucosa consists of columnar epithelium with microvilli and goblet cells that secrete mucus, enabling nutrient absorption.
• Enteroendocrine cells within the intestine produce hormones.
• Peyer's patches, lymph nodules, are abundant in the ileum, aiding in pathogen destruction.

Small Intestine Layers

• The small intestine's external layer comprises two smooth muscle layers: circular and longitudinal.
• These layers mix chyme (partially digested food) with digestive secretion and propel the mixture toward the colon.
• Three sources of digestive secretions act within the small intestine: liver, pancreas, and the small intestine itself.

Layers of Alimentary Tube

• The alimentary tube's inner lining, or mucosa, is made up of epithelial tissue, connective tissue, and thin smooth muscle layers.
• The esophagus has stratified squamous epithelium; the stomach and intestines have simple columnar epithelium.
• The submucosa is made up of areolar connective tissue, containing numerous blood vessels, lymphatic vessels, and nerve fibers (Meissner's plexus).
• Parasympathetic impulses stimulate secretions; whereas sympathetic impulses decrease secretions.
• The external layer typically consists of two smooth muscle layers: an inner circular layer and an outer longitudinal layer.

Accessory organs (assignment)

• Liver: Produces bile, crucial for fat digestion; involved in detoxification, protein synthesis, and nutrient metabolism.
• Pancreas: Secretes pancreatic juice, containing enzymes for digesting carbohydrates, proteins, and fats.
• Gallbladder: Stores and concentrates bile produced by the liver.
• Large Intestine: Absorbs water and electrolytes remaining from digested food. Forms and expels feces.

Explore the intricate workings of the digestive system, including the breakdown of nutrients and the roles of various organs. This quiz covers the alimentary tube and accessory organs, as well as the critical functions of the oral cavity in digestion. Test your understanding of how food is processed and absorbed in the body.