The digestive system Flashcards
Processes of the GI tract
motility, secretion, digestion and absorption
Mucosa tissue layer
Innermost layer - lines luminal surface and tightly folded.
Mucous membrane: serves as a protective surface, secretes and absorbs, contains exocrine gland cells, endocrine gland cells and epithelial cells.
Lamina propria: houses gut-associated lymphoid tissue (GALT) = defence against disease
Muscularis mucose: smooth layer of muscle
Submucosa tissue layer
Thick layer of connective tissue giving distensibility and elasticity. Contains larger blood and lymph vessels and a nerve network
Muscular externa tissue layer
Major smooth muscle coat of the digestive tube. Has a circular inner layer in which contraction decreases the diameter of the lumen, and a longitudinal outer layer in which contraction shortens the tube. Produces propulsive and mixing movements.
Serosa tissue layer
Secretes serous fluid to lubricate and prevent friction between digestive organs and surrounding viscera. Is continuous with mesentery throughout the tract - attachment provides relative fixation, supports digestive organs in place
Enteric nervous innervation
Intrinsic - myenteric plexus and sub mucous plexus. Interconnected motor, secretory and sensory neurons
Extrinsic nervous innervation
Autonomic - parasympathetic increases activity and sympathetic decreases
Salivary glands
3 pairs; secrete salt, water, mucus, amylase. pH rises as secretion increases as bicarbonate ions not absorbed as fast
Tongue
Secretes lingual lipids, IgA, lysozyme
Stomach
Secretes HCl, pepsin, music, HCO3- to stabilise food, protein digestion and protection
Pancreas
Secretes many enzymes, HCO3- to digest fats, CHO3, protease and neutralise chyme
Liver
Secretes bile salts, HCO3- and waste products to solubilise fats, adjust pH and remove toxic material
Gall bladder
Stores and concentrated bile used in fat digestion
Small intestine
Secretes enzymes, salt, water and mucous for digestion and to maintain fluidity of contents
Carbohydrate
Salivary/pancreatic amylase hydrolyses 1:4 alpha link –> oligosaccharides –> monosaccharides. Absorbed by active co-transport with Na+ and facilitated diffusion into capillary
Proteins
Pepsin in stomach hydrolyses various size aromatic AA-PPs. Small intestine endopeptidases, exopeptidase and dipeptidase. Absorbed by transcytosis or co-transport with Na+.
Fats
Lingual lipase digests up to 30% triglyceride, duodenum = pancreatic lipase - triglyceride –> monoglyceride + 2FAs. Fats aid digestion by forming large liquid droplets, lipases are water double and emulsification. To speed up, micelles form due to bile salts much smaller than emulsion droplets.
Sodium, Cl- and HCO3- absorption
Follow Na+ => absorbed with glucose
Water absorption
Passive, follows solutes to maintain osmotic equilibrium
Vitamin absorption
Fat soluble with fats, water soluble by diffusion/mediated transport. B12 bind to intrinsic factor and then specific sites on epithelial cells of ileum by endocytosis
GI control systems respond to…
distention of walls by luminal contents, chyme osmolality, chyme acidity, conc of specific digestive products
Types of GI control systems
Neural - autonomic Enteric - stomach Hormonal - endocrine Paracrine - local Local muscle response
Long and short reflects and stimuli outside GI tract
Sub mucous plexus endocrine and exocrine secretions, myenteric plexus control of peristaltic activity, interconnections between the two, neural info to and from tract
Hormonal control of the GI tracts
Cells in GI epithelium, receptors on surface in contact with luminal content, hormones released into interstitial fluid - endocrine action via blood and paracrine action via luminal content, each hormone subject to -ve feedback. Hormones together produce a bigger effect
Phases of GI control
Cephalic - receptors in head, efferent pathway, primarily neural
Gastric - receptors in stomach (vol), efferent pathway, neural, long and short reflexes, hormonal
Intestinal - receptors in intestine (vol), efferent pathway, neural, long and short reflexes, hormonal
Mouth
Mastication - somatic nerves to skeletal muscle, voluntary control, reflex in gyms and hard palate
Saliva secretion
Swallowing - complex reflex
Oesophagus
Peristalsis. upper 1/3 skeletal, lower 1/3 smooth. Skeletal and autonomic nerves, secondary peristalsis = afferent/efferent pathway. Food bolus moves in one direction
Stomach gastric motility
Filling involves receptive relaxation - extra volume of food with little rise in pressure, mediated by vagus nerve.
Storage in body, mixing in antrum
Emptying controlled by factors n duodenum and stomach (vol chyme). Either neural or autonomic nerves
Stomach gastric secretory activity
2 regions of gastric mucosa secrete gastric juice - oxyntic mucosa and pyloric gland area. Gastric pits at base of gastric glands. 3 types of gastric exocrine cells: mucous, chief and parietal
Hal stomach secretion bu parietal
Pepsinogen stimulated by nerves
Control of Hal during a meal: cephalic (sight etc in HCl), gastric (distention etc in HCl), intestinal (dec HCl)
Bile secretion
HCO3- by epithelial cells of bile duct stimulate by secretion
Bile salts by rate of secretion, storage of bile in gall bladder, CCK
SI motility
Mixing, decreased frequency of segmentation from duodenum to ileum, slow forward movement of chyme, force affected by hormonal and neural influences, gastro-ileac reflex increases segmentation, migrating motility complex, short peristaltic waves, contents moved to LI in 2hrs
LI motility
Ilocecal sphincter relaxed by gastroileal reflex - chyme moves in to LI, reflux prevented by closure of ilocecal spinchter, enteric NS, LI segmentation, mass movement 3-4 times a day
Defection
Anus - internal anal sphincter (smooth) abd external sphincter (skeletal), neural control, voluntary modification
Rectal distention - defecation reflex: internal s relaxes, external contracts, inc peristalsis in sigmoid colon, inc rectal pressure, external s relocation
