Chapter 23 Part A: The Digestive System

Hepatic portal circulation

• Drains nutrient-rich blood from digestive organs • Delivers blood to liver for processing

Control of salivation

- 1500 ml/day can be produced - Minor glands continuously keep mouth moist - Major salivary glands are activated by parasympathetic nervous system

Splanchnic circulation includes

- Arteries that branch off aorta to serve digestive organs • Hepatic, splenic, and left gastric arteries • Inferior and superior mesenteric arteries

Peritonitis

- Inflammation of peritoneum - Can be caused by piercing abdominal wound, perforating ulcer, or ruptured appendix - Treatment: debris removal and megadoses of antibiotics

Associated Organs of the Mouth

- Mouth - Tongue - Salivary glands - Teeth

Reflex Arc: Neurons (intrinsic and extrinsic) and hormones control digestive activity

- Nervous system control • Intrinsic controls: involve short reflexes (enteric nervous system) • Extrinsic controls: involve long reflexes (autonomic nervous system) - Hormonal controls • Hormones from cells in stomach and small intestine stimulate target cells in same or different organs to secrete or contract

Reflex Arc: Digestive activity is provoked by a range of mechanical and chemical stimuli

- Receptors located in walls of GI tract organs - Respond to stretch, changes in osmolarity and pH, and presence of substrate and end products of digestion (e.g., fats and smaller peptides)

Main functions of the digestive system

- Take in food - Break it down into nutrient molecules - Absorb molecules into the bloodstream - Rid body of any indigestible remains

Reflex Arc: Effectors of digestive activity are smooth muscle and glands

- When stimulated, receptors initiate reflexes that stimulate smooth muscle to mix and move lumen contents - Reflexes can also activate or inhibit digestive glands that secrete digestive juices or hormones

Three key concepts regulate GI activity

1. Digestive activity is provoked by a range of mechanical and chemical stimuli 2. Effectors of digestive activity are smooth muscle and glands 3. Neurons (intrinsic and extrinsic) and hormones control digestive activity

Processing of food involves six essential activities

1. Ingestion:eating 2. Mechanical breakdown 3. Digestion 4. Propulsion 5. Absorption 6. Defecation

All digestive organs have the same four basic layers, or tunics

1. Mucosa 2. Submucosa 3. Muscularis externa 4. Serosa

Peristalsis

Adjacent segments of the alimentary canal organs alternately contract and relax. Food is moved distally along the tract

dorsal mesentery

Digestive organs are intraperitoneal and are suspended from the body wall

Retroperitoneal

Digestive organs that lost their mesentery during development.

Segmentation

Nonadjacent segments of the alimentary canal organs contract and relax. Food is moved forward, then backward

Composition of saliva

Water, acidity, electrolytes, amylase, lingual, proteins, metabolic wastes, Ect. - Mostly water (97-99.5%), so hypo-osmotic - Slightly acidic (pH 6.75 to 7.00) - Electrolytes: Na+, K+, Cl−, PO42−, HCO3− - Salivary amylase and lingual lipase - Proteins: mucin, lysozyme, and IgA - Metabolic wastes: urea and uric acid - Lysozyme, IgA, defensins, and nitric oxide from nitrates in food protect against microorganisms

Mouth and Associated Organs

food is chewed and mixed with enzyme-containing saliva that begins process of digestion, and swallowing process is initiated

Salivatory nuclei

in brain stem that stimulate parasympathetic impulses along fibers in cranial nerves VII and IX to glands

ventral mesenteries

intraperitoneal digestive organs are also suspended from the body wall