Digestive System Structures and Functions: Anatomy & Physiology Study Notes

Digestive System Overview

General Functions

The digestive system is responsible for the intake, breakdown, absorption, and elimination of food and nutrients. It also plays a role in the production of neurotransmitters, hormones, and vitamins.

• Ingestion: Taking in food.

• Mechanical Breakdown: Chewing and mixing food.

• Digestion: Chemical breakdown of food into nutrient molecules.

• Absorption: Movement of nutrients into the bloodstream.

• Elimination: Removal of indigestible substances.

• Production of neurotransmitters and hormones: Includes compounds such as acetylcholine, serotonin, histamine, nitric oxide, and hormones like gastrin, ghrelin, and secretin.

• Vitamin synthesis: Bacteria in the intestine synthesize vitamins (e.g., Vitamin K, biotin).

Anatomy of the Digestive System

Alimentary Canal vs. Accessory Organs

The digestive system consists of the alimentary canal (GI tract) and accessory digestive organs.

Alimentary Canal	Accessory Organs
Mouth	Teeth
Pharynx	Tongue
Esophagus	Salivary glands
Stomach	Liver
Small intestine	Gallbladder
Large intestine	Pancreas

Additional info: The alimentary canal is a continuous tube from mouth to anus, while accessory organs aid in digestion but are not part of the tube itself.

Gastrointestinal Tract Activities

• Ingestion

• Mechanical breakdown

• Propulsion: Swallowing and peristalsis

• Digestion: Catabolism of food

• Absorption

• Compaction & Defecation

Peristalsis vs. Segmentation

Peristalsis	Segmentation
Adjacent segments contract and relax	Nonadjacent segments contract and relax
Propels food forward	Mixes food, slow propulsion
Some mixing occurs	Food moves forward and backward

Relationship to the Peritoneum

Peritoneum and Mesentery

• Peritoneum: Serous membrane of the abdominal cavity

• Visceral peritoneum: Covers external surface of digestive organs

• Parietal peritoneum: Lines the body wall

• Peritoneal cavity: Fluid-filled space between layers

• Mesentery: Double layer of peritoneum that supports organs, vessels, and stores fat

• Intraperitoneal organs: Within the peritoneum

• Retroperitoneal organs: Outside/posterior to the peritoneum (e.g., pancreas, duodenum)

Histology of the Alimentary Canal

Four Basic Layers (Tunics)

• Mucosa: Lines the lumen; functions in secretion, absorption, and protection

• Submucosa: Areolar connective tissue, blood/lymphatic vessels, nerves

• Muscularis externa: Smooth muscle for segmentation and peristalsis

• Serosa: Outermost protective layer (visceral peritoneum)

Mucosa Sub-layers

• Epithelium: Simple columnar, mucus-secreting cells

• Lamina propria: Areolar connective tissue, capillaries, lymphoid follicles

• Muscularis mucosae: Smooth muscle for local movements

Blood Supply: Splanchnic Circulation

• Arterial supply: Branches of the abdominal aorta (celiac trunk, mesenteric arteries)

• Venous return: Via inferior vena cava and hepatic portal circulation

• Hepatic portal system: Directs blood from digestive organs to the liver for processing

Enteric (Gut) Nervous System

Components

• Submucosal nerve plexus: Regulates glands and smooth muscle in mucosa

• Myenteric nerve plexus: Controls GI tract motility

• Intrinsic controls: Short reflexes (ENS)

• Extrinsic controls: Long reflexes (ANS)

• Parasympathetic impulses: Stimulate secretion and motility

Regulation of GI Activity

1. Mechanical & chemical stimuli: Receptors respond to stretch, osmolarity, pH, substrate, and end products

2. Effectors: Smooth muscle and glands; reflexes stimulate mixing and movement

3. Nervous & hormonal control: ENS, ANS, and hormones regulate digestive activity

Functional Anatomy of the Digestive System

Oral Cavity and Associated Structures

• Boundaries: Lips, cheeks, palate, tongue

• Oral orifice: Anterior opening

• Stratified squamous epithelium: Protects against abrasion

• Associated organs: Tongue, salivary glands, teeth

Lips, Cheeks, and Palate

• Lips (labia): Orbicularis oris muscle

• Cheeks: Buccinator muscle

• Hard palate: Palatine bones/processes, slightly corrugated

• Soft palate: Skeletal muscle, uvula projects downward

• Frenulum: Lingual and labial attachments

Tongue

• Intrinsic muscles: Change shape

• Extrinsic muscles: Alter position

• Functions: Mixing food, forming bolus, swallowing, speech

• Papillae: Foliate, vallate, fungiform (taste buds)

• Ankyloglossia: "Tongue-tied" condition due to short frenulum

Salivary Glands

• Functions: Cleanse mouth, dissolve food chemicals, moisten food, begin starch breakdown

• Types: Intrinsic (buccal) and extrinsic (major) glands

• Cells: Serous (watery secretion, enzymes), mucous (mucus)

Composition of Saliva

Electrolytes	Digestive Enzymes	Metabolic Waste	Protein
Na+, K+, Cl-, PO42-, HCO3-	Salivary amylase, lingual lipase	Urea, uric acid	Mucin, lysozyme, IgA, defensins

Control of Salivation

• ~1500 ml/day produced

• Major glands activated by parasympathetic nervous system (PNS)

• Stimuli: Food ingestion, swallowing, irritating foods, nausea, sight/smell of food

Teeth

• Types: Incisors (cutting), canines (tearing), premolars (grinding), molars (grinding)

• Structure: Crown (exposed), root (embedded in jawbone), connected by neck

• Mastication: Chewing process to break food into smaller fragments

Digestive Processes of the Mouth

• Ingestion

• Mechanical breakdown (chewing)

• Initiates swallowing

• Starts digestion of polysaccharides

• Mastication: Produces bolus for swallowing

• Enzymatic breakdown: Starch by salivary amylase, fats by lingual lipase