Upper Digestive Pathway

Buccal Cavity (Oral Cavity)

The buccal cavity, also known as the oral cavity, is the entry point of the digestive system. It is lined by the oral mucosa, which protects underlying tissues and aids in the initial processing of food.

• Gingivae: The gums, which surround and support the teeth.

• Hard palate: The bony anterior portion of the roof of the mouth, separating the oral and nasal cavities.

• Soft palate: The posterior, muscular part of the palate, which closes off the nasopharynx during swallowing.

• Uvula: A small, fleshy extension at the back of the soft palate, involved in speech and preventing food from entering the nasal cavity.

Salivary Glands

Salivary glands produce saliva, which is essential for moistening food, beginning digestion, and protecting oral tissues.

• Saliva: Contains water, mucus, buffers, enzymes, and immune components.

• Salivary amylase: Enzyme that begins starch digestion.

• Lingual lipase: Enzyme that starts lipid digestion.

• Lysozyme: Antibacterial enzyme.

• Immunoglobulins: Proteins that provide immune defense.

Tooth Structure

Teeth are specialized for mechanical breakdown of food. Their structure supports mastication and protection.

• Periodontal ligament: Connects the tooth to the jawbone.

• Crown, neck, & root: The crown is the visible part, the neck is at the gum line, and the root anchors the tooth.

• Enamel: Hard, outer layer covering the crown; the hardest substance in the body.

• Dentin: Calcified tissue beneath enamel, forms most of the tooth.

• Pulp cavity: Central chamber containing nerves and blood vessels.

• Root canal: Passage for nerves and vessels from the pulp cavity to the jaw.

• Apical foramen: Opening at the tip of the root for entry of nerves and blood vessels.

Dental Succession

Humans have two sets of teeth during their lifetime: primary (deciduous) and adult (permanent).

• Eruption: Process by which teeth emerge through the gums.

• Primary teeth: Also called baby teeth; typically 20 in number.

• Adult teeth: Permanent teeth; usually 32, including wisdom teeth.

• Wisdom teeth: Third molars, often erupt in late adolescence or early adulthood.

Tooth Varieties

Teeth are classified by shape and function.

• Incisors: Chisel-shaped for cutting.

• Cuspids (canines): Pointed for tearing.

• Bicuspids (premolars): Flattened for crushing.

• Molars: Broad for grinding.

• Mastication: The process of chewing.

• Bolus: The mass of chewed food ready for swallowing.

Pharynx

The pharynx is a muscular tube connecting the oral cavity to the esophagus and larynx. It plays a key role in swallowing and directing food to the esophagus.

Esophagus

The esophagus is a muscular tube that transports food from the pharynx to the stomach.

• Esophageal hiatus: Opening in the diaphragm through which the esophagus passes.

• Upper esophageal sphincter: Controls entry of food into the esophagus.

• Lower esophageal sphincter: Prevents reflux of stomach contents.

Swallowing (Deglutition)

Swallowing is a complex process involving multiple phases to move food from the mouth to the stomach.

• Buccal phase: Voluntary; food is pushed into the oropharynx.

• Pharyngeal phase: Involuntary; food passes through the pharynx.

• Esophageal phase: Involuntary; peristalsis moves food down the esophagus.

• Peristalsis: Coordinated, wave-like muscle contractions.

• Secondary peristaltic waves: Occur if initial swallowing is incomplete.

Stomach

The stomach is a muscular organ responsible for mechanical and chemical processing of food.

• Mechanical processing: Churning converts the bolus into chyme.

• Storage: Holds food temporarily.

• Gastric juice: Contains acid and enzymes for digestion.

• Chemical action: Digestion of proteins and fats begins.

• Gastrin secretion: Hormone that stimulates gastric activity.

Stomach Anatomy

• Pylorus: Distal region leading to the small intestine.

• Pyloric sphincter: Controls passage of chyme into the duodenum.

• Rugae: Folds in the stomach lining that allow expansion.

Stomach Function

• Preliminary digestion: Proteins and fats are partially digested.

• No nutrient absorption: Most nutrients are not absorbed in the stomach.

• Some chemicals can be absorbed: Examples include alcohol and certain drugs.

Gastric Glands and Pits

Gastric glands are located in the stomach lining and secrete various substances for digestion.

• Gastric pits: Openings in the mucosa leading to glands.

• Mucous cells: Secrete mucus to protect the stomach lining.

• Surface & neck mucous cells: Located at different regions of the gland.

• G cells: Secrete gastrin.

• Parietal cells: Secrete hydrochloric acid and intrinsic factor.

• Chief cells: Secrete pepsinogen and gastric lipase.

Parietal Cells

Parietal cells are essential for producing stomach acid and facilitating vitamin absorption.

• Intrinsic factor: Required for vitamin B12 absorption.

• Hydrochloric acid (HCl): Maintains acidic environment for digestion.

• Proton pumps: Transport H+ ions into the stomach lumen.

• Carbonic acid: Formed during acid production.

• Alkaline tide: Temporary increase in blood pH after eating due to bicarbonate release.

Chief Cells

Chief cells secrete enzymes necessary for protein and fat digestion.

• Pepsinogen: Inactive precursor of pepsin.

• Pepsin: Active enzyme that digests proteins.

• Gastric lipase: Enzyme for fat digestion.

• Rennin in infants: Enzyme for milk protein digestion (primarily in infants).

Example: Acid Production Equation

Parietal cells produce hydrochloric acid via the following reaction:

Hydrogen ions (H+) are secreted into the stomach, while bicarbonate (HCO3-) enters the bloodstream, causing the alkaline tide.

Additional info: Gastric lipase is more active in infants, and rennin is primarily present in the infant stomach for milk digestion. The textbook may contain an error regarding rennin in adults.