Digestive System Processes and Regulation

Digestion involves the mechanical and chemical breakdown of food for absorption.
The system includes the Alimentary Canal (mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus) and Accessory Digestive Organs (teeth, tongue, gallbladder, salivary glands, liver, pancreas).
Primary functions: Taking in food, breaking it down into nutrients, absorbing molecules into the bloodstream, and ridding the body of indigestible remains.

Ingestion: The act of eating.
Propulsion: Movement of food via swallowing and peristalsis (alternating waves of contraction).
Mechanical breakdown: Includes chewing, mixing with saliva, churning in the stomach, and segmentation (local intestinal constriction).
Digestion: Catabolic enzymatic breakdown of food into chemical building blocks.
Absorption: Passage of digested fragments from the GI tract lumen into blood or lymph.
Defecation: Elimination of indigestible substances as feces.
Movement types:
- Churning: Mixing food with gastric juices in the stomach.
- Segmentation: Circular muscle contractions in the intestines that mix food with enzymes.
- Peristalsis: Sequential contractions pushing materials forward from mouth to anus.

Peritoneum: Serous membranes of the abdominal cavity.
- Visceral peritoneum: Covers external surfaces of most organs.
- Parietal peritoneum: Lines the body wall.
Peritoneal cavity: Fluid-filled space that lubricates mobile organs.
Mesentery: Fused double layer of peritoneum that provides routes for blood vessels, lymphatics, and nerves; it also holds organs in place and stores fat.
Organ locations:
- Intraperitoneal: Located within the peritoneum.
- Retroperitoneal: Posterior to the peritoneum (e.g., most of the pancreas, duodenum, parts of the large intestine).
Peritonitis: Dangerous inflammation of the peritoneum caused by wounds, ulcers, or a ruptured appendix; treated with debris removal and high-dose antibiotics.

Mucosa: Innermost layer lining the lumen. Functions include secretion (mucus, enzymes, hormones), absorption, and protection. Sublayers include the epithelium, lamina propria (contains MALT for defense), and muscularis mucosae.
Submucosa: Connective tissue containing blood/lymphatic vessels, lymphoid follicles, and elastic tissue.
Muscularis externa: Responsible for segmentation and peristalsis; contains inner circular and outer longitudinal smooth muscle layers. Circular layers form sphincters.
Serosa: Outermost layer (visceral peritoneum); replaced by fibrous adventitia in the esophagus.
Structural specializations: Villi and microvilli increase surface area for absorption. Lacteals are specialized lymphatic vessels that absorb fats.

Splanchnic circulation: Includes hepatic, splenic, left gastric, and mesenteric arteries.
Hepatic portal circulation: Drains nutrient-rich blood to the liver for processing.
Enteric Nervous System (Gut Brain):
- Submucosal plexus: Regulates glands and smooth muscle in the mucosa.
- Myenteric plexus: Controls GI tract motility.
Autonomic Nervous System:
- Parasympathetic: Increases digestive activities.
- Sympathetic: Inhibits digestive activities.

Stimuli: GI activity is provoked by mechanical (stretch) and chemical (osmolarity, pH, presence of substrates) stimuli.
Effectors: Smooth muscle and glands that respond by mixing/moving contents or secreting juices.
Reflex arcs:
- Short reflexes: Mediated by the enteric nerve plexuses.
- Long reflexes: Involve the autonomic nervous system and extrinsic controls.
Hormonal controls: Hormones released from the stomach and small intestine stimulate specific target cells to secrete or contract.