primary function of digestive system
transfer nutrients, water, and electrolytes from food into body's internal environment; disassembly line to break macromolecules into individual components
Structure of GI tract
mucosa, submucosa, muscularis externa, serosa
mucosa
Innermost layer of GI tract, specialized to selective transport across membrane; has goblet cells that secrete mucous, exocrine cells that secrete digestive enzymes and juices, and endocrine cells that produce GI-specific hormones; inner sub-layer is epithelial barrier and mucous membrane that has constant production of mucous; middle sub-layer is lamina propia or basement membrane with acellular connective tissue; external layer is muscularis mucosa
submucosa
Middle layer, thick connective tissue
muscularis externa
inner sublayer of circular muscle and outer sublayer of longitudinal muscle; circular contracts to change diameter of tube, longitudinal contracts to shorten tube
serosa
serous membrane, outermost layer
peritoneum
Double-layered membrane surrounding the abdominal organs in abdominal cavity; parietal lines body cavity, visceral lines organs, peritoneal cavity in between
Mesentary
connects visceral and parietal peritoneum; connects visceral structures to each other and visceral structures to cavity wall; also conduit for blood vessels, nerves, and lymphatic vessels
4 basic processes of digestive system
motility, secretion, digestion, absorption
motility in digestion
2 types of movements: mixing and propulsive; mixing movements mostly caused by segmentation contractions of circular muscles; propulsive movements move things forward, usually caused by peristalsis; different frequencies at different parts of GI tract (higher in stomach, lower in large intestine)
peristalsis
the process of wave-like muscle contractions of the alimentary tract that moves food along; most common propulsive movement
secretion in digestion
mucus secreted throughout gut; acid (HCl in stomach), digestive enzymes (also from pancreas), bile salts (from liver); secretions dependent on where in the GI tract; timing usually done by feedforward + feedback mechanisms, anticipatory secretions as well as once food enters canal
digestion in digestion
Chemical breakdown of carb, proteins, fats via enzymes and movements
absorption in digestion
taking nutrients into the bloodstream; job of the small intestine for the most part; happens after digestion after food is broken up into small enough fragments
non-digestive functions of GI tract
excretion of waste, fluid + electrolyte balance (regulating amount of water + sodium lost v. reabsorbed), immunity (largest barrier against pathogens)
GALT
gut associated lymphoid tissue; immune tissue associated with mucous membrane (part of MALT); pathogens must get through GI wall to enter ICF/plasma; lymphocytes and mast cells located in between epithelial cells of lamina propia; pathogen that leaves GI tract goes to liver which uses hepatic phagocytic cells called Kupffer cells to destroy
control and regulation of digestion
done via enteric system (local control, short loop, controlled by the gut) and extrinsic system (long loop, controlled by brain) as well as enteric endocrine system
short loop of digestion
enteric nervous system, division of parasympathetic NS, gut innervated by its post ganglionic fibers; completely separate from central nervous system, can regulate digestive function autonomously; contains myenteric plexus and submucosal plexus
myenteric plexus
nerve supply between the two layers of the muscularis externa; regulates motility, sensors that feed it information, interprets sensation to stimulate effector aka muscles; provides smooth, coordinated contraction for peristalsis
submucosal plexus
within submucosa; receives information re: composition of what is within tract, controls secretions, stimulates release of appropriate digestive enzymes; indirectly influences motility by communication with myenteric plexus; indirectly influences absorption based on secretions and how they influence motility, i.e. can increase number of enzymes, slowing down motility, and increasing absorption; detects presence of toxins and increases mucous secretion and stimulates myenteric plexus to increase motility and decrease absorption
extrinsic control of gut
long loop; autonomic nervous system; from sympathetic nerves, innervate GI tract to slow digestion; from parasympathetic (vagus) nerve to increase digestion; has ability to override enteric NS, but doesn't need to
enteric endocrine system
secretes hormones from cells scattered throughout GI tract in response to specific stimuli; can be instructed by submucosal plexus or by vagus nerve; acted upon by other GI hormones
esophagus
Tube connecting the mouth to the stomach; chewed food gets pushed to back of pharynx, causes distention and triggers swallow reflex, opening of pharyngoesophageal sphincter, triggers wave of peristalsis down the GI tract; peristalsis continues until all food goes down and distention stops
stomach
Lower esophageal sphincter opens up to stomach; fundus (top), body, antrum (bottom); pyloric sphincter + duodenum at end of antrum; motility is mostly peristalsis and receptive relaxation; enteric nervous system controls via myoenteric plexus; with each peristaltic wave, small amount of chyme pushed through pyloric sphincter, when wave reaches sphincter it shuts it and chyme rebounds to cause more mixing
pacesetter cells
have set frequency, waves of depolarization when stomach empty not high enough to reach threshold, when we eat, waves become more active and trigger action potentials and wave of peristalsis
receptive relaxation
as food travels in the esophagus, stomach muscles relax in preparation to receive food; controlled by swallowing center of brain; minimum capacity of stomach is 50 mL when relaxed and empty, up to 20x that until distention begins
mucosal lining of stomach
deep gastric pits within mucosa, little holes that lead to deep invaginations, at bottom are cells responsible for secreting digestive enzymes; differs in fundus/body versus antrum in terms of which cells are present
oxyntic mucosa
lines the body and fundus of stomach; contains mucosa cells (produce highly alkaline mucous to create protective coating against stomach acids), parietal cells (secrete HCl and ICF for B12), chief cells (produce pepsinogen that becomes pepsin when hits acids), enterochromaffin (ECL) cells (endocrine cells that release histamine to act on chief and parietal cells to increase their secretions)
pepsin
released as pepsinogen, activated into pepsin when in contact with gastric juices, proteolytic enzyme
pyloric gland area
lines the antrum of stomach; contains G cells (secrete gastrin to stimulate gastric secretion + motility and go into bloodstream and hit intestines to stimulate bowels), D cells (secrete somatostatin to decrease gastric secretion + motility)
4 phases of gastric acid secretion
interdigestive, cephalic, gastric, intestinal
interdigestive phase
peristaltic contractions pass over large segments of the intestine; no active digestion taking place; basically when we're asleep, controlled by circadian diurnal rhythm: secretions lowest in morning, highest in evening
cephalic phase
all cues anticipatory, no food in the stomach yet; comes from vagal stimulation which stimulates submucosal and myenteric plexus; in response to thinking about, smelling, seeing food, brain sends signals to increase motility of stomach and increase production of pepsinogen, HCl, gastrin
gastric phase
food enters stomach and causes distention, submucosal plexus detects it and stimulates secretion of pepsinogen and HCl; also sensed by G-cells which act on parietal cells to make more HCl, ECL cells to make histamine which makes more HCl; also sensed by vagus nerve and triggers stimulation of same cells
intestinal phase
food leaving the stomach and entering the duodenum distends the intestine, triggering D-cells to release somatostatin to decrease stomach motility and slow the secretion of gastric juices; things need to be slower so absorption can happen, pancreas and liver have time to release bile and digestive enzymes; fats also slow down gastric emptying, create a bottleneck in the duodenum because they take so long to break down; duodenal mucosa cells sense changes in fat, acid, hypertonicity, and distention and trigger neuronal reflex through enteric system and hormonal response of enterogastrones
enterogastrones
Duodenal enteroendocrine cells release two important hormones that inhibit gastric secretion: secretin increases pancreatic secretion of sodium bicarbonate to act on duct cells, CCK releases pancreatic enzymes + bile to slow down gastric emptying
acinar cells
enzyme-secreting cells of the pancreatic ducts (exocrine portion), acted upon by CCK
duct cells
cells in pancreatic ducts; secrete bicarbonate, acted upon by secretin to neutralize acids
pancreatic enzymes
proteolytic enzymes (trypsin, chymotrypsin, carboxypeptidase), pancreatic amylase, pancreatic lipasen (breaks down fat, slows down digestion the most)
bile
composed of bile salts, cholesterol, lecithin, bilirubin; role is to emulsify fats by making lipid droplets into smaller droplets surrounded by bile salts to prevent droplets from re-combining; increases surface area for breakdown of fat; bile flows from liver down bile duct and backs up into gallbladder when not actively digesting; CCK opens sphincter of Oddi and contracts gallbladder, bile flows into duodenum, some bile is reabsorbed in terminal of small intestine and goes back to liver, triggers production of even more bile; positive feedback loop that shuts off when fat digestion is complete
small intestine
duodenum, jejunum, ileum; where all digestion and absorption is completed; uses segmentation contraction for both mixing and propulsion; gentle slow propulsion increases contact of food with absorptive areas; mucous membrane has villi and microvilli to further increase surface area that contain enterokinases, disaccharidases, aminopeptidases, called brush border membrane
fat digestion
dietary fat as large TG, bile salts result in lipid emulsion, pancreatic lipase breaks down into monoglycerides and free fatty acids alonf with bile salts to form micelles; micelle enters enterocyte and reassembles into TG, is coated with lipoprotein to become chylomicron which goes out central lacteal
central lacteal
The initial lymphatic vessel that supplies each of the small-intestinal villi; has capillaries, lymphatic vessels
large intestine
water, electrolytes and minerals absorbed here; feces moves from ilium to ileocecal valve to cecum to ascending colon to transverse colon to descending colon to sigmoid colon to rectum and out anal sphincter; motility handled with Haustral contractions, segmentation contractions that are slow and nonpropulsive; mass movements are mostly propulsive and happen 3-4x a day, usually after meals, to move feces down; eventually trigger defecation reflex
defecation reflex
stretch receptors in rectum trigger peristaltic waves due to distention of walls; reflexively relaxes internal anal sphincter and allows for voluntary use of external anal sphincter to control defecation