MUSCULAR MOVEMENTS OF SMALL INTESTINE

Peristalsis

Uni-directional propulsion of digested food forward through the digestive tract.

• Activated following a meal (postprandial).
• Rhythmic, alternating contractions of the circular and longitudinal smooth muscle layers causes peristalsis.

Key Steps of Peristalsis

1. Circular layer contracts and pinches behind the chyme
   – Decreases diameter of the lumen
   – Propels chyme forward

• Longitudinal layer relaxes around the chyme
  – Dilates the small intestine to easily receive the chyme.

2. Longitudinal layer contracts in front of pinched region
   – Shortens region of the small intestine (much like when we bunch up a tube sock to get our foot into it)
   – This step does not actively move the chyme forward → it shortens the distance the chyme must travel and prepares for its propulsion forward through the small intestine.

3. Circular layer contracts again, pinches the small intestine farther distally
   – Propels chyme further through the small intestine

Migrating Motility Complex (MMC)

Produces specific periodic peristaltic contractions that sweep digested contents, cellular debris, and bacteria through the small intestine during fasting.

• Occur every 90 minutes to two hours (during interdigestive periods – when a meal has been digested and absorbed).
• Regulated by the candidate digestive hormone motilin.

Segmentation

Alternating contractions of the circular layer between intestinal segments to breakdown food and mix chyme.

• Activated following a meal (postprandial).

Key Steps of Segmentation

1. Mixture of chyme enters the small intestine, surrounded by digestive juices: duodenal secretions, pancreatic secretions, and bile

2. Circular layer segmentally contracts → chyme separated into these segments

3. Circular layer contracts and pinches the middle of these segments to re-segment them.
   – Chyme from each segment moves in both directions (forward and backward) from the contraction site, into the other segments.

4. Circular layer contracts again in the original site and again re-segments the small intestine.
   – Chyme from the different segments mix.
   – Also mix chyme with the duodenal and pancreatic secretions and bile.

• The circular layer promotes stationary mixing of partially digested chyme with small intestine and pancreatic secretions.

Other key features of segmentation:

• Actively brings digested chyme into close contact with intestinal epithelium → efficient absorption.
• Does not contribute to the movement of chyme down the small intestine.
• Contributes to mechanical digestion of food → further breakdown of ingested macromolecules to simpler, absorbable forms.
• Segmentation contractions cease following meal absorption.

Absorption

• Uptake of digested nutrients and water from the lumen of the digestive tract into the bloodstream and lymphatic vessels.

Small intestine (specifically the duodenum)

• Major site of nutrient absorption.
• Three folded mucosal structures maximize surface area for absorption
  1 – Plicae circulares: wavy, folds on the inner walls of small intestine → form circular folds → increase surface area 3-fold.
  2 – Villi: finger-like projections that protrude from the plicae circulares → surface area by 10-fold. Arterioles, venules, and lymphatic vessels pass through the villi and uptake absorbed nutrients.
  3 – Microvilli (brush border): hair-like projections on columnar small intestine epithelial cells (face the lumen of the small intestine) → increase surface are 20-fold. Together, all folded layers = 600-fold surface area increase

Nutrients are absorbed in the small intestine

• Monosaccharides (digested carbohydrate products)
• Amino acids, di-peptides and tri-peptides (digested protein products)
• Intact proteins
• Short-chain fatty acids, long-chain fatty acids, and glycerol (which are digested lipid products)
• Vitamins
• Water and electrolytes

Nutrients cross the apical and basolateral surfaces of the intestinal epithelium for absorption into circulation or the lymphatic system.

• Apical surface: interfaces the intestinal lumen and epithelium.
• Basolateral surface: opposite to the apical surface, lines the inside of the villi.
• Capillaries and lacteals inside villi
  – Most nutrients cross the basolateral surface and pass directly into circulation.
• Fats, however, pass directly into lacteals (lymphatic system).

Key Transport Mechanisms

• Apical Surface
  – Secondary active transport: transporter moves an ion movement down its concentration gradient, which generates energy for it to move another ion (or molecule) against its concentration gradient.
  – Facilitated diffusion: Transporter passively moves an ion or molecule across the plasma membrane, down its concentration gradient.
  – Simple diffusion: in which non-charged, lipid, and hydrophobic molecules passively cross through the plasma membrane (without a transmembrane protein) down their concentration gradient.
  – Endocytosis: form of active, energy-requiring cellular ingestion, which transports large substances into the cell.

• Basolateral Surface
  – Facilitated diffusion.
  – Simple diffusion.
  – Exocytosis: the opposite of endocytosis – a vacuole actively fuses with the plasma membrane to release its contents into the extracellular environment.

• Water Absorption

Most relevant to the function of the large intestine but also occurs in the small intestine.

– Large intestine stores and concentrates fecal material before elimination.
– Mainly absorbs water and electrolytes to do so
– Also absorbs bacterial byproducts.

• Three General Steps of Water Absorption:

Step 1:

• Sodium-potassium pump on the basolateral surface
  – Pumps K+ into cell and Na+ out of cell (pump utilizes ATP to move sodium and potassium against their concentration gradients).
• Na+ passively enters via general sodium ion transporter on the apical surface (possible because of the active transport of sodium out of the cell)
• Net positive charge in the cell

Step 2:

• Chloride enters the cell through a general chloride ion transporter on the apical surfacen via facilitated diffusion down the electrical gradient.
• Higher solute concentration inside the cell relative to the lumen.

Step 3:

• Water crosses the apical surface via osmosis.

Key Functions of the Stomach (Review)

• Temporary storage to slow food transit to the small intestine and maximize nutrient absorption.
• Physical Breakdown (like in the mouth)
• Chemical Breakdown of proteins into their amino acids (at the same time that salivary amylase from the mouth continues to breakdown carbohydrates in the stomach).

Three Gastric Phases (Review)

1. Filling, in which food enters the stomach through the gastroesophageal sphincter.
2. Mixing, in which peristaltic contractions churn the food while the gastric lining secretes juices to produce chyme.
3. Emptying, in which peristaltic contractions propel chyme into the small intestine.

Mixing Phase – In Depth

• Peristalsis – contractions of circular smooth muscle, move from fundus to antrum
  – Pushes the stomach’s contents towards the pyloric sphincter.
  – Facilitates physical breakdown of food
• Pyloric sphincter almost closed
  – Forces the chyme to spill backwards into the antrum (stomach’s body) and continues mixing.

Exocrine Cells of Stomach

• Located in tubular gastric glands that comprise gastric pits
  – Epithelial cells at entrance of gastric pits: secrete thick mucus
  – Mucous layer
  – Submucosa layer
• Secrete products into stomach lumen
• Secretions convert food to chyme

Exocrine Cell Types

• Mucous cells (mucous neck cells): secrete alkaline, bicarbonate mucus, which protects our stomach wall from erosion in an acidic luminal environment.
• Chief cells: secrete pepsinogen, an inactive enzyme that, once activated, breaks down proteins.
• Pepsinogen is a zymogen
  – An inactive enzyme that, once activated, breaks down proteins.
  – A substance must convert to its active form, pepsin
• Pepsin
  – Breaks down peptide bonds to promote chemical breakdown.
• Parietal cells
  – Secrete HCl which denature proteins.
• HCl functions:
  – Converts pepsinogen → its active form: pepsin.
  – Aids in the breakdown of food → smaller particles.
  – Denatures proteins via its acidic environment.
  – Kills most of the microorganisms that we ingest with our food, thus, providing a protective function. (Tight junctions between the digestive tract epithelium and mucus production serve as a protective barrier).
  – Facilitates chemical breakdown; it denatures proteins but, unlike pepsin, it doesn’t break peptide bonds.

Stem cells also located in gastric pit

• Rapidly divide and mature into cells that produce gastric mucosa
  – Replenishes gastric mucosal cells every 3 days due to constant exposure to the harsh, acidic environment in the stomach.

Clinical correlation

• Peptic ulcers
  – Erosions that penetrate our gastric mucosal layer.
• Pepsin and HCl access exposed regions and erode the stomach wall.

GASTRIC EMPTYING IN DEPTH

• Induced by strong antral contraction
  – Antrum has thicker layer of smooth muscle, which allows more forceful contraction
• Antral contraction pushes chyme through the pyloric sphincter
• The volume of chyme that passes depends on the force of antral contraction.
  – Despite the force of antral contraction, only a little chyme enters the duodenum, which is where nutrient absorption occurs.
  – Pyloric sphincter limits the flow of chyme to promote slow and efficient absorption in the duodenum.

MOUTH

Physical breakdown

• Food into smaller particles → mastication (chewing)

Chemical breakdown

• Carbohydrates → salivary secretions

Secretions

• Secreted by salivary glands (parotid gland, sublingual, sbmandibular)
• Secreted in anticipation of and during food consumption.
• Salivary secretions:
  – Salivary amylase: breaks down polysaccharides → maltose
  – Mucus: moistens food, forms bolus
  – Lysozyme: lyses bacteria
  0.5% of saliva is enzymes and electrolytes; the rest is water.

PHARYNX AND ESOPHAGUS

Motility (movement)

• Food from oral cavity to stomach

SWALLOWING ORAL, PHARYNGEAL AND ESOPHAGEAL PHASES.

Tongue

• Pushes to the back of the pharynx to initiate swallowing.

Pharynx

• Common passageway for both food and air, continuous with trachea

Epiglottis

• Laryngeal flap that prevents the bolus from entering the trachea.

Esophagus

• Esophageal sphincter is open (relaxed) to let passage of food
  – Sphincters: modified, one-way valves that comprise smooth muscle; they regulate food movement through the alimentary canal.
  – Peristalsis: unidirectional wave-like smooth muscle contractions to push food down the esophagus and into stomach

GASTRIC DIGESTION

• Peristalsis deposits food in the stomach
• Bolus passes through the gastroesophageal sphincter

Clinical Correlation: heartburn occurs when acidic contents of the stomach backflow into the esophagus.

STOMACH KEY FUNCTIONS

• Temporary storage
• Slows food transit to the small intestine.
• Maximizes nutrient absorption.
• Physical breakdown (like in the mouth)
• Chemical breakdown of proteins → amino acids
  – Salivary amylase (from mouth) continues carbohydrate breakdown in the stomach.

GASTRIC PHASES

1. Filling: food enters the stomach (through the gastroesophageal sphincter).
2. Mixing: peristaltic contractions churn the food.
   – Gastric juices secretion to produce chyme (solution of partially digested macromolecules)
3. Emptying, in which peristaltic contractions propel chyme into the small intestine.

GASTRIC FILLING PHASE IN DEPTH

• Gastroesophageal sphincter is a passage-way for food between esophagus and stomach
• Anatomical divisions of stomach: fundus, body, antrum
• Smooth muscle lining
  – Receptive relaxation: Stomach muscles “relax” to “receive” food
• Smooth muscle walls reduce tone to expand stomach volume (in response to food reception)
  – Stomach volume = 0.5L empty → expands to 0.8L to 4.0L during receptive relaxation
  – Increase in volume allows stomach to accommodate food with little rise in pressure
  (Note: Intertextual variation exists regarding the stomach’s full capacity during receptive relaxation.)
  – Facilitates temporary storage – stomach secretes chyme slowly, gives the small intestine more time to absorb nutrients.

FIVE MAJOR PROCESSES OF THE DIGESTIVE TRACT

• Motility of digestive products through the digestive tract.
• Secretion of enzymes and fluids into the digestive tract.
• Digestion of food breakdown.
• Absorption of nutrients and water.
• Barrier from the external environment and microorganisms.

Minor Process

• Elimination of waste and undigested material from the body.

STRUCTURES

Alimentary canal and glands/organs

• Alimentary canal
  – Runs from the mouth to the anus.
  – We can imagine the alimentary canal as a donut hole within a giant donut.
• Accessory glands/organs
  – Secrete digestive juices into the alimentary canal
  – To imagine the accessory glands, we draw a donut and show jelly ooze into the donut hole to represent the accessory gland secretions.

Accessory organs

• Salivary glands secrete saliva
• Liver secretes bile into the gallbladder
• Gallbladder secretes stored bile into the duodenum after a meal
• Pancreas secretes pancreatic juice into the duodenum

Liver, gallbladder, and pancreatic secretions pass through individual ducts

– All merge at duodenum → secretions secrete into duodenum

Additional features of alimentary canal:

• Smooth muscle lining allows peristalsis
  – Peristalsis is rhythmic contractions that push food down the alimentary canal.
  – Sphincters are modified, one-way valves that comprise smooth muscle. They contract and relax to regulate movement through the alimentary canal.

DIGESTIVE TRACT PATHWAY

Oral cavity

• Site of mastication (chewing).

Esophagus

• Transports the food bolus to the stomach (via peristalsis).
• Gastroesophageal sphincter: regulates the movement of food into the stomach
  – Prevents “heartburn”: the reflux of food back into the esophagus.

Stomach

• Churns and converts food into liquid chyme.
• Pyloric sphincter regulates the movement of chyme into our small intestine; it releases chyme slowly to allow the duodenum time to absorb food nutrients.

Small intestine

• The major site of nutrient absorption.
• Three major divisions: duodenum, jejunum, ileum.
• Most nutrient absorption in duodenum.

Large intestine

• Absorbs water
• Three major divisions: cecum, colon, rectum.

Anus

• Excretes waste.

DIGESTION OF THE FOUR MAJOR MACROMOLECULES

Aided by luminal secretions/enzymes – promote digestion of macromolecules to absorbable nutrients

Oral cavity

• Luminal secretions: Saliva = amylase, mucus, lysozyme
• Digests: Polysaccharides → smaller polysaccharides, maltose
  – Enzyme is salivary amylase

Esophagus

• Luminal secretions: Mucus, which lubricates passage of food to the stomach.  NO digestion

Stomach

• Luminal secretions: Gastric juice = HCl, pepsin, mucus
• Digests: Polypeptides → small polypeptides
  – Enzyme is pepsin

Small intestine

• Luminal secretions are:
  – Bile
  – Pancreatic enzymes: trypsin, chymotrypsin, carboxypeptidase, amylase, lipase and nuclease
  – Brush border enzymes (not secreted): dissaccharidases, aminopeptidases
• Digests:
  – Polysaccharides → disaccharides, maltose. Enzyme: pancreatic amylase
  – Disaccharides → monosaccharides. Enzyme: Disaccharidases (brush border)
  – Polypeptides → smaller polypeptides. Enzyme: trypsin & chymotripsin
  – Smaller polypeptides → amino acids. Enzymes: carboxypeptidases & aminopeptidases (brush border)
  – DNA/RNA → nucleotides. Enzyme: nuclease
  – Fat globules → glycerol, fatty acids. Enzyme: lipase and bile salts

Large Intestine

• Luminal secretions: mucus.  NO digestion. Instead absorbs water and electrolytes.

• Branches course within the pelvis to carry oxygenated blood to the pelvic viscera and musculoskeletal structures, the gluteal and medial thigh regions, and the perineum.

Divisions:

Anterior division

• Umbilical artery, which becomes patent after birth.
• Superior vesicular artery, which travels to the urinary bladder.
• Obturator artery, which travels through the obturator canal of the pelvis.
• Inferior vesicular/Vaginal artery, which serves reproductive structures,
• Uterine artery, which, in females, travels to the uterus.
• Middle rectal artery, which travels to the rectum.
• Internal pudendal artery, which supplies the external genitalia.
• Inferior gluteal artery, which exits the pelvis to travel to the gluteal muscles of the posterior hip.
  This vessel may also arise from the posterior division of the internal iliac artery.

Posterior division

• Iliolumbar artery, which, as its name suggests, travels along the ilium and lumbar vertebrae.
• Lateral sacral arteries, which travel along the sacrum.
• Superior gluteal artery, which exits the pelvis to serve the gluteal muscles.

Clinical correlation:

Blood loss from post-partum uterine hemorrhage can be so great as to be life-threatening, so surgeons are trained to seek out and ligate (tie off) the anterior division of the internal iliac artery to stop the blood loss. Ligation may be temporary, or permanent; ischemia of the pelvic viscera is uncommon, due to collateral arterial supply.

MESENTERIC ARTERIES

• Both the superior and inferior arteries arise from the abdominal aorta and travel through the mesentery to deliver blood to organs of the digestive tract.

Superior mesenteric artery

• Supplies the pancreas, segments of the small intestine, and the proximal segments of the large intestine
• Gives rise to the:
  • Inferior pancreaticoduodenal a.
  • Middle colic a.
  • Right colic a.
  • Ileocolic a.
  • The intestinal arteries (which supply the jejunum and ileum).

Inferior mesenteric artery

• Supplies the distal segments of the large intestine.
• Gives rise to the:
  • Left colic a.
  • Multiple sigmoid aa.
  • Superior rectal a.

Marginal artery

• Forms from anastomoses of the superior and inferior mesenteric arteries.

Clinical correlations:

• The marginal artery provides an alternative blood pathway for the distal portion of the large intestine if the inferior mesenteric artery is infarcted.
• However, if the superior mesenteric artery is infarcted, necrosis of the small and large intestine results.
• Mesenteric arterial networks are highly variable; for example, it is not uncommon for the right or middle colic arteries to be absent.
• “Watershed regions” where the colon receives dual blood supply:
  • Both the SMA and the IMA supply blood to the splenic flexure.
  • Both the final sigmoidal artery branch and the superior rectal artery supply the rectosigmoid junction.
  • These regions are vulnerable in colonic ischemia.
• Nutcracker Syndrome occurs when the left renal vein is compressed by the superior mesenteric artery and the aorta; patients experience flank pain, elevated renal vein pressure, and hematuria.
• Superior mesenteric artery syndrome occurs when the Superior mesenteric artery and aorta compress & obstruct the transverse portion of the duodenum; patients experience postprandial pain. It is most likely to occur in patients with weight loss, but not always.

Celiac trunk

• Supplies the liver, pancreas, stomach, and spleen with oxygenated blood.

Three main branches:

Left gastric artery travels to the stomach:

• Esophageal branches

Splenic artery travels to the spleen:

• Short gastric artery
• Left gastro-omental artery

Common hepatic artery travels to the liver, stomach, and abdomen.

• Gastroduodenal artery
  – Superior pancreaticodudenal artery
  – Right gastro-omental artery
• Hepatic artery proper
  – Right gastric artery (varies)
  – Left hepatic artery
  – Right hepatic artery -> Cystic artery

Clinical Correlation:

In celiac artery compression syndrome (aka, Dunbar’s syndrome), compression of the celiac trunk, usually by the median arcuate ligament of the diaphragm, reduces blood flow and causes abdominal pain after eating.

Hepatic portal system

• A special circulation system that transports venous blood from the digestive organs to the liver.
• Transports blood from the stomach, spleen, pancreas, and small and large intestines to the liver. This distinct circulatory pathway exists to allow the liver to metabolize nutrients and toxins from blood that leaves the digestive organs.

Primary tributaries of the hepatic portal vein:

Superior mesenteric vein

Drains tissues of the right side of the abdomen.

• Ileocolic vein drains blood from the distal small intestine and the proximal large intestine
• Right colic vein courses from the right side of the abdomen to drain blood from the large intestine
• Middle colic vein drains blood from the large intestine.
• Intestinal veins drain the jejunum and ileum of the small intestine. These drain into the left side of the superior mesenteric vein.
• Pancreatic and duodenal veins
• Right gastro-omental vein, which runs along the inferior border of the stomach (aka, greater curvature), drains into the superior mesenteric vein.
• The “omental” portion of the gastro-omental name is derived from the greater “omentum,” the apron-like fold of peritoneum that drapes over the intestines anteriorly.

Splenic vein

Drains structures on the left side of the abdomen.

• Merges with superior mesenteric vein to form hepatic portal vein
• Short gastric veins from stomach
• Left gastro-omental vein, which courses along the inferior border of the stomach and meets the right gastro-omental vein.
• Pancreatic veins
• Inferior mesenteric vein

Inferior mesenteric vein

Drains tissues of the lower left side of the abdomen into the splenic vein.

• Superior rectal vein
• Multiple sigmoid veins, which drain the sigmoid colon
• Left colic vein, which drains the descending and transverse colon segments.

Additional information:

• Hepatic vein
  • Drains blood from liver to inferior vena cava
  • Helps secure liver in place
• Smaller tributaries of the hepatic portal vein:
  • Cystic veins from gallbladder
  • Left and right gastric veins from stomach

Clinical Correlations:

• Portal hypertension occurs when the hepatic portal vein becomes blocked.
• Blood clots can cause obstruction of the hepatic portal vein, but, more commonly, portal hypertension is the result of cirrhosis, in which the liver tissue is replaced by scar tissue in response to damage from a variety of causes, including chronic alcohol intoxication or hepatitis C.
• As blood flow is obstructed, blood pressure in the hepatic portal system increases, and venous engorgement occurs. These enlarged veins form thin-walled varices, which are susceptible to trauma.
• Rupture of esophageal varices, which form at the junction of the stomach and esophagus, can cause fatal gastrointestinal hemorrhage.

Key organs of Biliary and Pancreatic Duct System:

• Liver produces bile
• Gallbladder stores and concentrates bile
• Pancreas produces pancreatic juices
• Duodenum receives bile and pancreatic juice, and mixes them with the contents of the GI tract

Key terms:

• Duct is a small tube.
• Ampulla is the dilated terminal end of the duct.
• Sphincter is a circular muscle.
• Papilla is a protuberance of target organ where the ampulla releases secretions.

Drainage Pathways:

• Right, left, and common hepatic ducts drain bile from the liver; their function is reflected in their name, as “hepatic” refers to the liver.
• Cystic duct carries bile to and from the gallbladder; it’s helpful to know that “cystic” refers to the gallbladder (recall the “cystic” artery).
• Bile duct receives bile from the cystic and common hepatic ducts, and delivers it to the duodenum. Notice that its name simply reflects its contents (bile), not a specific organ.
• Main pancreatic duct drains pancreatic juice from pancreas
• Main pancreatic and common bile duct merge at hepatopancreatic ampulla (regulated by hepatopancreatic sphincter)
• Pancreatic juice and bile is drained into duodenum via major duodenal papilla.
• When present, accessory pancreatic duct drains pancreatic juice into duodenum via minor pancreatic papilla.

Embryologic Origins

Clinical correlations:

• Post-hepatic jaundice (aka, obstructive jaundice) refers to the yellow staining of body tissues, including skin and mucous membranes; it is the result of accumulation of yellow bile pigments in the blood, and is a sign that bile is not exiting the gallbladder appropriately.
• Gallstones, which are formed from crystalized bile, can become lodged within and obstruct the ducts of the biliary system, with serious health consequences.
  For example:
• Cholecytitis (inflammation of the gallbladder) occurs when obstruction of the cycstic duct causes excessive bile accumulation in the gallbladder; subsequent swelling and irritation of the gallbladder can cause intense pain.
• Pancreatitis (aka, pancreatic inflammation) can result from obstruction of the hepatopancreatic ampulla; instead of entering the small intestine, bile and pancreatic juices back up into the pancreas, causing inflammation.

Duct/Artery associations:

• Cystic duct travels with the cystic artery
• Right and left hepatic ducts travel with the right and left hepatic arteries
• Common hepatic duct arises near the hepatic artery proper.