Digestive System - Lecture 6

Digestive System - Lecture 6

• Digestive system function: Breaks down complex organic nutrients into simpler organic/inorganic molecules. These absorb into blood/lymph and are transported to cells.

Stages of Digestion

• Ingestion: Taking food and liquid into the oral cavity.
• Mastication (Chewing): Breaks down solid food into digestible pieces.
• Motility: Muscular movements of materials through the digestive tract.
• Secretion: Release of lubricating/protective mucus, digestive enzymes, acids, alkaline fluids, and bile.
• Hormone release: Controls motility and secretion locally.

Chemical Digestion & Absorption

• Chemical digestion: Enzymatic breakdown of large molecules to smaller ones.
• Absorption: Uptake of small molecules and water into the blood and lymph.

Two Main Parts of the Digestive System

• Alimentary canal (GIT): Mouth to anus; includes oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.
• Accessory organs: Salivary glands, liver, gallbladder, and pancreas.

Oral Cavity Parts

• Vestibule: Space outside the teeth and gums.
• Mouth cavity proper: Surrounded by teeth, gums, hard and soft palates, and tonsils.

Tooth Anatomy

• Crown: Visible part of the tooth.
• Neck: Between crown and root.
• Root: Extends into bone, holding the tooth in place.

Deciduous Teeth

• Individuals develop a set of 20 deciduous teeth.
• Erupt around 6 months of age.
• Typically complete by 2 years of age.

Permanent Teeth

• Individuals develop 32 permanent teeth.
• Includes incisors, canines, premolars, and molars.

Tongue Functions

• Chewing, speech, taste, and swallowing.

Tongue Surfaces

• Ventral (lower) surface: Smooth.
• Dorsal surface: Irregular, with papillae and lingual tonsils.

Saliva Production

• Salivary glands produce 1-1.5 liters of saliva daily.

Saliva Composition

• 99.4% water, plus ions and enzymes.

Salivary Glands

• Types: Minor (in oral mucosa, 10%), and major (parotid, submandibular, sublingual).
• Locations: Parotid (below and in front of ears), submandibular (posterior mandible corners), sublingual (below the mouth floor).

Percentage of Saliva Origin

• Approximately 70% from submandibular glands.
• 25% from parotid glands.
• 5% from sublingual glands.

Gastrointestinal Tract Wall Layers

• Mucosa: Inner layer.
• Submucosa: Layer beneath mucosa.
• Muscularis externa: Muscle layer (circular and longitudinal).
• Serosa: Outermost layer.

Mucosa Layer Components

• Epithelial layer
• Lamina propria: Connective tissue.
• Muscularis mucosae: Thin layer of smooth muscle.

Muscularis Externa Function

• Responsible for mixing and propelling luminal contents forward.
• Composed of circular and longitudinal layers of smooth muscle.

Enteric Nervous System Function

• Located in the muscularis externa and submucosa.
• Generates and coordinates muscular contractions.

Serosa Layer Purpose

• Thin sheet of loose connective tissue.
• Rich in blood vessels, lymphatics, and adipose tissue.
• Covers the digestive tract in the abdominal cavity.

Pharynx Length

• Approximately 5 inches long.

Pharynx Division

• Nasopharynx, oropharynx, and laryngopharynx.

Pharynx Function

• Allows passage of air and food. Does not have a digestive function.

Enamel Layer Purpose

• Hardest, white outer part of the tooth.
• Protects underlying layers.

Dentin Layer Function

• Layer of hard tissue beneath enamel.
• Provides additional support and protection.

Pulp Role

• Soft, living inner structure of the tooth.
• Contains blood vessels and nerves.

Cementum Layer Function

• Connective tissue layer.
• Attaches tooth roots to gums and jawbone.

Primary Teeth Function

• Chewing (mastication).

Sulcus Terminalis Function

• V-shaped groove separating papillary and tonsillar areas on the tongue's dorsal surface.

Major Salivary Gland Ducts

• Each major salivary gland has at least one duct that takes saliva to the oral cavity.

Saliva Production Purpose

• Digestive, lubricating, and protective functions in the mouth.

Stomach - Pt2

• Function: Temporary storage, mechanical digestion (muscular contractions), and chemical digestion (acid and enzymes).
• Empty size: Approximately 50 ml.
• Stomach lining folds: Rugae.
• Semi-digested food: Chyme.
• Major regions: Cardia, fundus, body, and pylorus.

Cardiac Sphincter

• Located at the esophagus-stomach junction.
• Prevents stomach acid from reflux.

Pyloric Sphincter

• Opens to allow partially digested food (chyme) into the duodenum.

Stomach Mucosa Cells

• Mucous cells, chief cells, parietal cells, and enteroendocrine G cells.

Parietal Cells Function

• Hydrochloric acid (HCl) and intrinsic factor production (for vitamin B12 absorption).

Small Intestine Length

• Approximately 6 meters long.

Small Intestine Segments

• Duodenum, jejunum, and ileum.

Duodenum Function

• Breaks down food and absorbs nutrients.
• Allows food to pass from stomach into the small intestine.

Jejunum Function

• Absorbs sugars, fatty acids, and amino acids.

Ileum Function

• Absorbs vitamin B12 and reabsorbs conjugated bile salts.

Large Intestine Diameter/Length

• Approximately 6.3 cm in diameter and 1.5 m in length

Large Intestine Parts

• Colon, cecum, rectum, and anus.

Cecum Function

• Receptacle for liquids entering the large intestine.
• Absorbs salts, electrolytes; involved in lubrication/cellulose digestion.

Appendix Function

• Function unknown, one hypothesis is storage area for beneficial bacteria.

Rectum Length/Diameter

• Approximately 8 inches (20 cm) in length and 2.5 inches in diameter.

Liver Weight/Location

• 1.5 kg or 2% of total body weight.
• Upper right/center of the abdominal cavity, below the diaphragm.

Liver Lobes

• Two large lobes: right and left.

Liver Structural Units

• Liver lobules.

Sinusoids in Liver Lobules

• Receive blood from hepatic artery and portal vein.

Central Veins in Liver Lobules

• Unite to form hepatic veins, which take blood out the liver to the inferior vena cava.

Major Histological Components of Liver

• Stroma (capsule of Glisson) and parenchyma (hepatocytes).

Cardiac Sphincter Function

• Allows food to pass from the esophagus to the stomach.
• Prevents stomach acid from refluxing into the esophagus.

Stomach Size Change After Meal

• Can enlarge to up to 1.5 liters.

Rugae Function

• Flatten as the stomach fills, allowing the lining to expand without tearing.

Gastric Juice pH

• pH of 1 to 2.

G Cells Function

• Secrete gastrin, stimulating greater gastric juice secretion.

Small Intestine Location

• Extends from the stomach to the cecum of the large intestine.

Ileocecal Valve Function

• Prevents large intestine contents from refluxing into the ileum during peristalsis (muscle contractions).

Haustra in Colon

• Sacs in the colon wall. Produce the segmented appearance of the colon.
• Formed by the teniae coli (3 longitudinal ribbons of smooth muscle).

Colon Divisions

• Ascending colon, transverse colon, descending colon, and sigmoid colon.

Liver's Relation to Hepatic Artery/Portal Vein

• Blood is received from hepatic artery and portal vein.

Capsule of Glisson Function

• Part of the liver's stroma (supporting tissue). Consists of connective tissue and contains blood vessels

Hepatocytes Function

• Functional units of the liver. Responsible for metabolic processes.

Hepatic Vein Destination

• Takes blood out of the liver to the inferior vena cava.

Capsule of Glisson Covering

• Covered by a layer of mesothelium.