Digestive System Overview

Introduction

• The digestive system breaks down complex organic nutrients into simple organic and inorganic molecules.
• These molecules are absorbed into the blood or lymph for transport to cells.
• The digestive system is divided into two parts: 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; most nutrient absorption occurs in the small intestine.
• Undigestible material (like cellulose) is eliminated by the large intestine (colon).
• Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Oral Cavity/ Buccal Cavity

• Food enters via the mouth.
• Boundaries include the hard and soft palate, and cheeks.
• The oral cavity contains teeth, tongue, and salivary glands.

Teeth

• Teeth function in chewing (mechanical breakdown).
• Deciduous teeth (baby teeth) erupt around 6 months and are replaced by permanent teeth around age 6.
• A complete set of permanent teeth has 32 teeth of different types (incisors, canines, premolars, molars).
• The tooth crown is visible above the gums; the root is embedded in the jawbone.
• The tooth structure includes enamel (outermost layer), dentin, and pulp cavity.
• Erosion of enamel and dentin can lead to cavities from bacterial acids.

Tongue

• The tongue is made of skeletal muscle innervated by the hypoglossal nerves (cranial nerve 12).
• Papillae are small projections on the tongue surface.
• The tongue is involved in swallowing, pushing food to the pharynx, and sensing taste.
• The tongue has nerve supply from facial and glossopharyngeal nerves.

Salivary Glands

• Saliva is produced by three pairs of salivary glands (parotid, submandibular, sublingual).
• Saliva secretion is continuous but varies based on factors like food and stress.
• Saliva is produced by parasympathetic response of facial and glossopharyngeal nerve, increased by sight/smell of food, and decreased by stress.
• Saliva plays a role in moistening food, initiating starch digestion, and cleansing the oral cavity.

Esophagus

• The esophagus is a muscular tube that carries food from the pharynx to the stomach.
• No digestion takes place in the esophagus.
• Peristalsis (wave-like contractions) moves food through the esophagus.
• The lower esophageal sphincter (LES) prevents backflow of stomach contents.

Layers of the Alimentary Tube

• The alimentary canal has four layers including the innermost mucosa layer, submucosa, muscular and outermost serous layer.
• The Mucosa is composed of epithelial tissue, connective tissue and muscle layers.
• The mucosa layer of esophagus is stratified squamous epithelium; that of stomach and intestines has simple columnar epithelium.
• The muscular layer typically contains inner circular and outer longitudinal smooth muscle, which in the upper third of the esophagus contains striated muscle.
• The stomach has three layers to help break up food and mix it with digestive enzymes.

Stomach

• The stomach is a sac-like organ in the upper left quadrant of the abdomen.
• The stomach is responsible for holding food and initiating chemical digestion by secreting gastric juices.
• Both chemical and mechanical digestion begin in the stomach.
• The cardiac orifice connects to the esophagus, and the pylorus connects to the small intestine.

Small Intestine

• The small intestine is a long, coiled tube connecting the stomach to the large intestine.
• The small intestine is responsible for most of the absorption of nutrients.
• It includes three sections: duodenum, jejunum and ileum.
• The duodenum is the first part of the small intestine, where bile and pancreatic secretions enter.
• Sections of the small intestine are specialized for absorption.
• The lining of small intestine has villi for absorption and enteroendocrine cells for producing hormones.