Week 1 Flashcards
What is digestion?
A bodily process of breaking down foods into molecules that are small enough to enter body cells.
What is the digestion system?
A tubular system extending from mouth to anus, forming an extensive surface area in contact with the external environment, essential for processing the food we eat to be absorbed and used by body cells. It is closely associated with the cardio system.
Functions of the digestive system
Ingestion: take food into mouth
Secretion: release water, acid, buffers, enzymes into lumen of GIT to aid breakdown of food (7 L/day)
Mixing and propulsion: churn and move food through GIT via (smooth) muscular contractions from oesophagus to anus (motility)
Digestion: mechanical & chemical breakdown of food
Absorption: passage of digested products from GIT into blood & lymph via epithelial cells of lumen
Defecation: eliminate feces from GIT. Wastes, indigestible substances, bacteria, cells sloughed from GIT lining
Organs of GIT
mouth, pharynx, oesophagus, stomach, small intestine, large intestine
Accessory digestive organs
teeth, tongue, salivary glands (parotid gland, submandibular gland, sublingual gland), liver, gallbladder, pancreas. (Only teeth and tongue come into direct contact with food.)
Two groups of organs composing digestive system
Gastrointestinal tract (GIT) (alimentary canal) and accessory digestive organs
Gastrointestinal tract
A continuous tube extending from mouth to anus though the thoracic and abdominopelvic cavities.
5-7 meters long in a living person, when muscles along GIT organ walls are in a state of tonus (sustained contraction).
Mechanical and chemical digestion
Mechanical: Food cut and ground by teeth, swallowed. Smooth muscles of stomach and small intestine churn food. Food molecules dissolve and mix with digestive enzymes from salivary glands, tongue, stomach, pancreas, sm int.
Chemical: large carb, lipid, protein and nucleic acid molecules in food split catalysed by hydrolysis. Vitamins, ions, cholesterol & water absorbed without chemical digestion.
Layers of GIT
Esophagus (may vary) to anal canal, deep to superficial: Mucosa Submucosa Muscularis Serosa (adventitia)
Mucosa layer of GIT
Mucous membrane forming inner lining of GIT:
Epithelium: in contact with food
Lamina propria: connective tissue
Muscularis mucosae: thin smooth muscle
Epithelium of GIT mucosa layer
Nonkeratinized stratified squamous: mouth, pharynx, oesophagus, anal canal
- protective function
Simple columnar: stomach and intestine lining. Tight junctions restrict leakage. renew every 5-7 days. Contains exocrine cells (secrete mucus & fluid into lumen) and endocrine cells (enteroendocrine cells) (secrete hormones) - secretion/absorption function
Lamina Propria of GIT mucosa layer
- Thin, flat plate of areolar CT
- Contains many blood and lymphatic vessels - routes for absorbed nutrients
Contains cells of MALT - nodules protect against disease (present along all GIT esp tonsils, sm. int, appendix, lge int.) - Supports epithelium; binds to muscularis mucosae
Muscularis mucosa of GIT mucosa layer
- Thin layer of smooth muscle fibres
- Throws stomach and small intestine membrane into small folds, increasing surface area for digestion and absorption.
- Movements ensure all absorptive cells are fully exposed to GIT contents
Submucosa of GIT
- areolar CT that binds the mucosa to muscular
- contains
- many blood and lymphatic vessels - receive absorbed molecules
- submucosal plexus - extensive network of neutrons (secretion role)
- glands & lymphatic tissue
Muscularis of GIT
Skeletal muscle - mouth, pharynx, upper/middle esophagus, anus
- controls swallowing and defecation (voluntary)
Smooth muscle - rest of GIT
- in 2 sheets - inner circular fibers, outer longitudinal fibers
Mixes, crushes, propels food (peristalsis)
Contains myenteric plexus (neuronal network) in between layers
Serosa of GIT
Superficial serous membrane layer covers GIT portions suspended in abdominal cavity.
CT covered with simple squamous mesothelium
Covers all cavity organs/walls not open to external
Secretes slippery fluid to reduce friction
Esophagus has not series
GIT regulation
Regulated by
- enteric nervous system (intrinsic nerves)
- autonomic nervous system (extrinsic nerves)
Sensory receptors in GIT walls
- chemoreceptors - respond to chemicals in food in lumen
- mechanoreceptors - stretch receptors activated when food distends GI organ wall
ENS
- Brain of gut
- arranged in 2 plexuses:
- myenteric plexus (plexus of Auerbach)
- submucosal plexus
consist of motor neurone, interneurons, sensory neurons
- interneurons interconnect myenteric & submucosal plexus neurons
- sensory neurone supply mucosal epithelium & contain receptors that detect stimuli in GIT lumen
Myenteric plexus
plexus of Auerbach
- located between longitudinal & circular smooth muscle layers of muscularis
- motor neurone supply muscularis
mostly controls GIT motility (movement), esp frequency & strength of contraction of muscularis
Submucosal plexus
plexus of Meissner
found within submucosa
- motor neurones supply the secretory cells of mucosal epithelium
- control the secretions of GIT organs
ANS
- Can function independently
- are subject to regulation by ANS neurons
- Vagus (X) nerves supply parasympathetic fibres to most parts of GIT except last half of lge int. (parasympathetic from sacral spinal cord).
Parasympathetic nerves that supply GIT form neural connections with ENS.
Stimulation of parasympathetic nerves innervating GIT cause increase in GI secretion & motility by increasing ENS neurone activity.
Sympathetic nerves that supply GIT arise from thoracic & upper lumbar regions of spinal cord. - decrease GI secretion & motility by inhibiting ENS neurons
