L5 - The Digestive System - Topic 1 Flashcards
What is the gross anatomy of the colon ?
Walls contain typical 4 layers found in the rest of the GI tract
3 unique features :
- Teniae coli ( formed by bands of longi layer of SM thickening )
- Haustra ( sacs / pockets caused by tonic contractions of teniae coli )
- Epiploic appendages
What are the subdivisions of the colon ?
- Cecum – First part of large intestine
- Appendix – Masses of lymphoid tissue (MALT), projections off of cecum → Immunity, bacteria that may be used to recolonize colon when needed
Appendicitis : Acute inflammation of appendix; usually results from a blockage by feces that traps infectious bacteria
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Colon : Several regions, most which are retroperitoneal (except for transverse & sigmoid regions)
- Ascending colon : Travels up right side of abdominal cavity to level of right kidney, ends in right-angle turn - right colic (hepatic) flexure
- Transverse colon : Travels across abdominal cavity, ends in another right-angle turn, left colic (splenic) flexure
- Descending colon : Travels down left side of abdominal cavity
- Sigmoid colon : S-shaped portion that travels through pelvis
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Rectum : 3 rectal valves stop feces from being passed with gas (flatus)
( willful control via skeletal muscles, babies / elderly ppl ) -
Anal Canal : Last segment of large intestine that opens to body exterior at anus, 2 sphincters
- Internal anal sphincter : Smooth muscle (no willful control)
- External anal sphincter : Skeletal muscle (willful control, we decide when to relax it)
What is the relationship of the large intestine to the peritoneum ?
- Cecum, appendix, & rectum are all retroperitoneal
- Colon is also retroperitoneal, except for its transverse & sigmoid parts
- Intraperitoneal regions are anchored to posterior abdominal wall by mesentery sheets called mesocolons
if lose mesentery → Intestine can twist / turn upon itself (vulvulus – cuts off blood supply to area, tissue becomes necrotic)
What is the microscopic anatomy of the large intestine ?
- Large intestine contains thicker mucosa made up of simple columnar epithelium except in anal canal, where it becomes strat squamous epi to withstand abrasion (involved in passing of feces)
- Does NOT contain circular folds, villi, or digestive secretions
- Contains abundant deep crypts with many mucus-producing goblet cells & water absorbing cells
What is the microscopic anatomy of the anal canal ?
- Mucosa of anal canal hangs in long ridges or folds referred to as anal columns
Anal sinuses : Spaces located between anal columns; secrete mucus to lubricate + aid in moving things along/emptying
Pectinate line : Horizontal line that parallels the inferior margins of the anal sinuses
- Visceral sensory nerves innervate area superior to this line [ Region insensitive to pain (above line) ]
- Somatic nerves innervate inferior to this line
Region sensitive to pain (below line)
Superficial venous plexuses of anal canal form hemorrhoids if inflamed
Bacerial Microbiota ( Bacterial Flora )
- Consist of 1000+ different types of bacteria (outnumber our cells 10:1)
- Enter from small intestine or anus to colonize colon
Metabolic functions
- Fermentation – Ferment indigestible carbohydrates + mucin, release irritating acids & gases (~500 ml/day) as a byproduct of digestion of indigestible carbs (beans)
- Vitamin synthesis – Synthesize B complex & some vitamin K needed by liver to produce clotting factors (deficiencies in vit K may lead to bleeding disorders)
- Keep pathogenic bacteria in check –
Beneficial bacteria outnumber and suppress pathogenic bacteria, immune system destroys any bacteria that try to breach mucosal barrier :
Epithelial cells recruit dendritic cells (macrophages) to mucosa to sample microbial antigens and present to T cells of MALT, triggering production of IgA antibodies that restricts microbes
Gut Bacteria & Health
Mounting evidence supports findings that the types & proportions of gut bacteria can influence :
- Body weight
- Susceptibility to various diseases (including diabetes, atherosclerosis, fatty liver disease)
- Our moods / mental state
- Manipulating gut bacteria may become a routine health-care strategy in future
Diet influences composition of microbiota
Digestive Processes of the Colon
- Residue remains in large intestine 12–24 hours
- No food breakdown occurs except what is digested by enteric bacteria
- Vitamins (made by bacterial flora), H2O & electrolytes (especially Na+ and Cl−) are reclaimed from what is passing through
Major functions of large intestine : Propulsion of feces to anus & defecation
Motility of the Large intestine
Haustral contractions : Most contractions of colon, haustra sequentially contract in response to distension
- Slow segmenting movements, mostly in ascending and transverse colon
pocket will fill up –> distend –> once reaches certain point of distension, this is sensed by stretch receptors –> will trigger pocket to contract & pass food to next pocket
Gastrocolic reflex : Initiated by presence of food in stomach/duodenum
- Results in mass movements – Slow, powerful peristaltic waves at middle of transverse colon that are activated 3 - 4 x per day
- Descending colon & sigmoid colon act as storage reservoir
Defecation
Mass movements force feces toward rectum
- Distension initiates spinal defecation reflex
- Parasympathetic signals to stimulate contraction of sigmoid colon + rectum & relax internal anal sphincter
- Conscious control allows relaxation of external anal sphincter
Muscles of rectum contract to expel feces – Assisted by Valsalva’s maneuver
- Closing of glottis, contraction of diaphragm and abdominal wall muscles cause increased intra-abdominal pressure
- Levator ani muscle contracts, causing anal canal to be lifted superiorly and allowing feces to leave body
Steps of Defecation
- Feces move into + distend the rectum, stimulating stretch receptors which will transmit signals along afferent fibers to spinal cord neurons
- A spinal reflex is initiated, Para NS motor (efferent) fibers will stimulate contraction of the rectum + sigmoid colon, as well as relaxation of internal anal sphincter
- If convenient to defecate, voluntary motor neurons are inhibited, allowing external anal sphincter to relax so feces may pass
Mechanisms of Digestion
Digestion : Catabolic process that breaks macromolecules into monomers (chemical building blocks) small enough for absorption
- Intrinsic & accessory gland enzymes are involved
- Enzymes carry out hydrolysis -water is added to split apart chemical bonds
Mechanisms of Absorption
Absorption : Moving substances from lumen of gut into body
- Tight junctions – Molecules pass through rather than between cells
- Materials enter cell through apical membrane (lumen side) and exit through basolateral membrane (blood side)
Lipid molecules can be absorbed passively through membrane, but polar molecules are absorbed by active transport →
- Most nutrients are absorbed before chyme reaches ileum
Digestion & Absorption of Carbs
Lactose Intolerance
