GI Mod 2A Flashcards
Small Intestine Anatomy
(total length 5-6 meters)
A. Begins at pyloric sphincter and ends at ileocecal valve
B. Regions: duodenum (25-30 cm), jejunum (2.5 m) and ileum (3.5 m)
Duodenum
a. begins at duodenal bulb and ends at the ligament of Treitz
b. Hepatopancreatic ampula (Sphincter of Oddi) : allow bile and pancreatic enzyme secretions
c. Brunner’s Glands (submucosal glands): located in proximal duodenum
• Secrete “bicarbonate rich” mucus to protect against acidic chyme entering the duodenum and lubricate intestinal wall
Enzymatic digestion/breakdown of food
• stimulates release of digestive enzymes from pancreas and gall bladder
Acidic chyme (fats, partially digested protein, hyper/hypotonic fluids) entering duodenum stimulate Long Loop Reflexes
(i) Long loop reflexes
1. Mechanoreceptors/chemoreceptors stimulate CNS to increase sympathetic and decrease parasympathetic to stomach
Acidic chyme (fats, partially digested protein, hyper/hypotonic fluids) entering duodenum stimulate Short Loop Reflexes
(ii) Short loop reflexes
1. Mechanoreceptors/chemoreceptors in duodenum stimulate enteric neurons in stomach to decrease stomach motility
Acidic chyme (fats, partially digested protein, hyper/hypotonic fluids) entering duodenum stimulates the Release hormone messengers to inhibit stomach motility/acid production
(iii) Release hormone messengers to inhibit stomach motility/acid production
1. Secretin, VIP, CCK released from duodenum play role in inhibiting gastric motility and secretion
Hormones of the Duodenum- Secretin
• “regulate pH in duodenum, inhibitory to gastric activity and facilitates digestion”
• Stimulus:
(i) Acidic (pH < 4-5 pH) chyme containing fats, partially digested protein, hyper/hypotonic fluids
• Target Organ/Action:
(i) Stomach – inhibits gastric secretions (inhibits gastrin) and inhibits motility
(ii) Pancreas - stimulate pancreas to secrete watery bicarbonate solution
(iii) Liver – stimulates bile output (promote fat digestion)
(iv) Brunner’s glands of duodenum – stimulate secretion of “alkaline rich mucus”
Hormones of the Duodenum CCK (cholecystokinin)
promotes bile/pancreatic enzyme release and inhibitory to gastric activity”
• Stimulus:
(i) Fatty chyme (fats, partially digested protein) entering duodenum
• Target Tissue/Action:
(i) Stomach – inhibitory to gastric motility/secretions
(ii) Pancreas – stimulate release of pancreatic enzymes
(iii) Liver - stimulates bile output (promote fat digestion)
(iv) Gallbladder – stimulates contraction to release stored bile
Hormones of the Duodenum GIP (gastric inhibitory peptide)
• “relative” of secretin
• Stimulus:
(i) Chyme entering duodenum
• Target Tissue/Action:
(i) Stomach - inhibitory to gastric motility/secretions
1. some resources question this role and propose secretin is stimulus
(ii) Pancreas – stimulate insulin release
Hormones of the Duodenum VIP (vasoactive intestinal peptide)
• Stimulus:
(i) Chyme entering duodenum
• Target Tissue/Action:
(i) Stomach – inhibit gastric acid secretion
(ii) Intestine – vasodilate BV, promote intestinal motility
Hormones of the Duodenum Intestinal Gastrin
• “Similar role as gastrin released in the antrum of stomach”
• Stimulus:
(i) Chyme entering duodenum
• Target Tissue/Action:
(i) Stomach - Stimulate gastric motility and secretions
(ii) Intestine – stimulate motility
Hormones of the Duodenum Motilin
• “housekeeper of intestinal tract”
• Stimulus:
(i) fasting or periodic release (every few hours)
• Target Tissue/Action:
(i) Intestine - initiates MMC (migrating motor complex)
Jejunum and ileum
- Anatomy
a. Begins at ligament of Treitz and ends at ileocecal valve - Function:
a. “continued digestion, absorption and secretion of brush borders enzymes to assist in digestion/absorption”
Jejunum
• Major site of digested fat, carbohydrate, protein, water and electrolyte absorption
Ileum
• Absorb Vitamin B12, bile salts and remaining digested nutrients and water not absorbed in jejunum
2 Anatomical features that optimize absorption in jejunum and ileum
Mucosal Folds– plica) that physically slow the passage of food
Villi
Villi
• Cover the mucosal folds
• Considered functional unit of small intestine: site of both secretion & absorption
• Composition of each villi
(i) Goblet cells: mucus secreting cells
(ii) Absorptive columnar cells: site of absorption
1. Microvilli - located on end of each columnar cell
a. form the “brush border” of the mucosal surface
i. a thin layer of fluid is found along surface of brush border
b. function of “brush border fluid layer”:
i. facilitates absorption of all substances except water/electrolytes
Lamina propria layer - “extends” into each villi contains the central arteriole function:
(i) Central arteriole function:
1. capillary transport substances directly to liver via hepatic portal vein
a. carbohydrates, proteins and some fats (glycerol & FFA) to directly to liver
Lamina propria layer - “extends” into each villi contains the Lacteal Function:
- transport fat molecules/substances (fats – monoglycerides & fatty acids) to systemic circulation via thoracic duct
Crypts of Lieburkuhn at base of villi composed of Precursor cells
of SI epithelium (goblet cells and columnar cells)
- Function:
a. turnover of intestinal epithelial cells
i. precursor cells migrate to tip of villi
ii. mature SI epithelial cells slough off into intestine and serve as a source of (endogenous) protein
iii. Complete turnover of SI is approx 4-7 days
Crypts of Lieburkuhn at base of villi composed of Paneth cells
Immune function - produce/secrete antibiotic peptides
Crypts of Lieburkuhn at base of villi composed of Secretory Cells
a. Secrete brush border digestive enzymes
Small intestine Motility
A. Average transit time in small intestine:
1. 1-3 hours (large variations: 15 min to 5+ hours)
B. Stimulus:
1. Small Intestine Hormones: secretin, CCK, gastrin (intestinal gastrin), motilin
2. Intestinal reflexes: see below
Three motility patterns occur in SI
- segmentation
- peristalsis
- migrating motor complex (MMC)
Segmentation
a. Occurs during feeding
b. Frequent small rhythmic contractions of circular muscle (8-12 per minute)
c. Occurs more frequently than peristalsis
d. Function:
• Mixes chyme to allow contact with brush border
3 motility patterns of the small intestine
segmentation
peristalsis
migrating motor complex
Peristalsis
a. Occurs during feeding
b. Coordinated waves of contraction/relaxation of longitudinal muscles
• Waves consist of short segments 10 cm
c. Wave of contraction moves slowly (1 – 2 cm/sec) to allow for digestion/absorption
d. Function:
• Moves chyme toward large intestine
Migrating Motor Complex (MMC)
a. Occurs during fasting
b. Slow periodic waves of peristalsis that occurs every 1-2 hours
• Originate in stomach and pass through small intestine
c. Motilin plays role in stimulus
d. Function: “House cleaning” - sweeps out stomach/SI
• “Pushes along” residual chyme, non-digested substances (fiber, bone, etc..)
• Bacteria homeostasis:
(i) transport bacteria to LI
(ii) prevent reflux of bacteria from LI
Ileogastric reflex
a. Stimulus: ileum distended
b. Action: inhibits gastric motility
• Result: slows down adding more chyme to SI to allow terminal ileum to empty contents to LI
3 Intestinal motility reflexes
Ileogastric reflex
Gastroileal reflex
Intestinointestinal reflex (inhibitory-protective reflex)
Gastroileal reflex
a. Stimulus: increased stomach (gastric) motility/secretion
b. Action: promotes terminal ileum motility/ileocecal valve relaxation
• Result: empty SI so it can receive more chyme from stomach
Intestinointestinal reflex (inhibitory-protective reflex)
a. Stimulus: section of small intestine becomes distended
b. Action: relaxes distal small intestine
• Result: allows chyme to be moved toward LI
Ileocecal valve and SI motility
- Intrinsically regulated and normally “closed”
- Regulation to open ileocecal valve
a. Valve opens similar to LES esophagus and pyloric valve in stomach
b. Stimulus: approaching peristaltic wave in the ileum
c. Action: relaxation of the ileocecal sphincter - Regulation to close ileocecal valve
a. Stimulus: distension of cecum/ascending large intestine
b. Action: constriction of ileocecal valve
Characteristics: arrangement of circular and longitudinal muscles is unique in LI
- Teniae coli: 3 longitudinal bands if muscle along length of colon
- Haustra: pouches formed from circular muscle layer of intestine
- O’Bierne sphincter controls passage of chyme from sigmoid colon into rectum
Large Intestine
A. Begins at ileocecal valve and ends at anus
B. Regions: Cecum, appendix, colon, rectum, anus
1. Colon consists of four regions: ascending, transverse, descending and sigmoid colon
4 Large Intestine Motility
Haustral segment contractions
Multihaustral segmentation
Mass movement peristalsis
Intestinal reflexes of the colon
Haustral segment contractions
a. When does it occurs? during fasting
b. Individual haustra segments push contents (fecal mass) back and forth
c. Function:
• maximize absorption of water and electrolytes in large intestine
• Occurs a majority (90%) of the time
Multihaustral segmentation
a. When does it occurs? during fasting
b. Several haustral segments contract and relax as a single unit
c. Function:
• Push contents (fecal mass) a short distance forward
• Occurs less often (10%) of the time
Mass movement peristalsis
a. Function:
• promote emptying of the intestine into the sigmoid colon & rectum
Intestinal reflexes of the colon
a. Orthocolic reflex (occurs upon awakening)
• Stimulus: standing first thing in morning stimulates peristalsis of LI
• Action: peristalsis propels fecal mass into sigmoid colon and rectum
b. Gastrocolic reflex
• Stimulus: ingestion of food (occurs during or immediately after eating)
• Action: peristalsis propels fecal mass into sigmoid colon and rectum
(i) “continuation” of gastroileal reflex – increase in motility of stomach promotes motility of SI
Bowel Movement (deification)
- Stimulus: stretch of rectum wall
- Action: relaxation of internal anal sphincter and the urge to have BM
- Rectal reflex is usually inhibited by voluntary control
a. cerebral cortex input/motor pathways in spinal cord: contract the external anal sphincter
b. Clinical:
• Cauda equina syndrome (damage to nerve roots of lower spine)
(i) Result? loss of bowel control
Mouth–Carbohydrate Digestion
a. Enzyme: salivary amylase
b. Action: Starches are broken into smaller molecules (dextrin and oligosaccharides)
Small Intestine–Carbohydrate Digestion
a. Pancreatic role
• Enzyme: pancreatic amylase secreted from pancreas
• Action: further breakdown starches into major oligosaccharides (lactose, maltose and sucrose)
Brush Border–Carbohydrate Digestion
- Enzyme: Brush-border enzymes (lactase, maltase and sucrase) secreted from secretory cells of base of villi (Crypts of Lieburkuhn)
- Action: breakdown oligosaccharides (lactose, maltose and sucrose) into monosaccharides (galactose, glucose and fructose)
Absorption of Carbohydrates
- Location: brush border of Villi
- Action: monosaccharides (galactose, glucose and fructose) are absorbed by villi capillaries and transport directly to liver (via hepatic portal vein)
- NOTE: insulin is not required for intestinal uptake of glucose
Stomach– Protein Digestion
a. Enzyme: Pepsin
• NOTE: pepsinogen released from chief cells is “converted” into pepsin when combined with gastric acids (hydrochloric acid)
b. Action: break down protein into smaller protein molecules (proteoses, peptones)
• (requires)
• NOTE: this site of protein digestion is not essential…small intestine is primary site
Small Intestine– Protein Digestion
a. Pancreatic role
• Enzyme: pancreatic enzymes (trypsin, chymotrypsin, carboxpeptidase) secreted by pancreas into duodenum
• Action: breakdown protein molecules into smaller peptides (polypeptides and dipeptides)
Brush Border Role– Protein Digestion
b. Brush border role
• Enzyme: Brush border enzymes (aminopeptidases/dipeptidase) are secreted from secretory cells of base of villi (Crypts of Lieburkuhn)
• Action: breakdown smaller peptides (polypeptides and dipeptides) into amino acids
Absorption of Protein
- Location: Villi in small intestine
2. Action: Amino acids are absorbed by villi capillaries and transported directly to liver (via hepatic portal vein)
Digestions of Fat requires emulsification
A. Digestion of fats requires emulsification
- Emulsifying agents in GI tract
a. Fatty acids, monglycerides, lecithin, cholesterol, protein, bile salts - What do emulsifying agents do?
a. Surround small fat articles and prevent them from re-forming into larger fat droplets
b. Emulsification reduces the surface tension and separates the molecules
Stomach- Digestion of Fat
a. Enzyme: gastric lipase
b. Action: initiate breakdown of fats
Small intestine– digestion of Fat
a. Emulsification of fat droplets
• Emulsyfing agents: Fatty acids, monglycerides, lecithin, cholesterol, protein, bile salts
• Action: smaller fat droplets are formed in the presence of emulsifying agents
Lypolysis– Digestion of Fat
- Enzymes: lipase, phosholipase and hydrolipase secreted from pancreas
- Action: Emulsified fat molecules are broken down into diglycerides, monoglycerides, free fatty acids, glycerol
Absorption of Fat
- Fat molecule must be “made” water-soluble to “reach” intestinal epithelium
a. This is accomplished by the formation micelles (water soluble molecule)
3 Steps in Fat Absorption
a. Micelles formation
• Bile salts, fat molecules, cholesterol, fat-soluble vitamins (A,D,E and K) combine to form a micelle
b. Transport of fats into columnar epithelial cell of SI
• Micelle can pass through aqueous brush border fluid and contact epithelial cell
• Fat contents of micelle then diffuse into epithelial cell
• Left over bile salts from micelle remain in intestine and are reabsorbed in the distal ileum
c. Transport of fats into lacteals
• Inside the epithelial cell the fat molecules are resynthesized into triglycerides and phospholipids
• These “new” fat particles are called chylomicrons
• The chylomicrons exit the intestinal wall (epithelial cell) into lacteals of villi and are transported to heart and systemic circulation
(i) Chylomicrons exit the cell via exocytosis
Vitamin B 12
- absorption occurs in terminal ileum
- absorption requires B-12 to bind with intrinsic factor
a. intrinsic factor (along with hydrochloric acid) is secreted by gastric parietal cells - Clinical application:lack of B-12 causes per
a. poor RBC formation thus causes pernicious anemia (macrocytic anemia)
b. liver and kidneys store enough B-12 to last for years
Fluid is ingested or secreted into GI tract throughout all regions
- Drink: 2 L per day
- Saliva: 1.5 L per day
- Gastric secretions: 2 L per day
- Bile: 0.5 - 1 L per day
- Pancreas: 1.5 L per day
- Total ingested/secreted into GI tract: 7.5 – 8 L/day
Fun Facts
B. Small intestine will absorb most of fluid back into blood stream
1. How much is absorbed? Approximately 85 – 90% is reabsorbed in SI
C. Large intestine will secrete and absorb final fluid balance….driven by electrolyte balance
1. The LI ”fine tunes” fluid and electrolyte homeostasis
D. Approximately 100 ml (1/10 of L) is excreted in stool in normal circumstance
1. NOTE: Renal excretion approximately 1 – 2 L/day
