Small Intestine, Large Intestine, and Appendix Lecture Notes

Small Intestine Function

  • Digestion and absorption of nutrients, water, electrolytes, and minerals.

Nutrient Absorption in the Small Intestine

  • Duodenum:
    • Absorbs amino acids, disaccharides, iron, electrolytes, and water.
  • Jejunum:
    • Absorbs disaccharides, monosaccharides, and free fatty acids.
    • Monoglycerides are absorbed here.
    • Vitamins A, D, E, and K, calcium, magnesium, zinc, and copper are absorbed, along with water and electrolytes.
    • Thiamine, biotin, and B6 are primarly absorbed in the jejunum
  • Ileum:
    • Absorbs bile acids and Vitamin B12 with intrinsic factor.

Blood Supply to the Small Intestine

  • Superior mesenteric artery (SMA) originates from the aorta.
  • Superior mesenteric vein (SMV) merges with the splenic vein to form the portal vein leading to the liver.
  • The SMA crosses the duodenum and can cause obstruction of the fourth portion of the duodenum, known as SMA syndrome.

Distinguishing Jejunum from Ileum

  • No clear demarcation between the jejunum and ileum exists on the surface of the bowel.
  • Distinguished by the vascular network within the mesentery.
    • Jejunum: Long vasa recta and fewer vascular arcades.
    • Ileum: Shorter vasa recta and multiple arcades.

Microscopic Anatomy of the Small Intestine

  • Villi increase the surface area for absorption.
  • Layers:
    • Mucosa
    • Submucosa (Meissner's plexus located here)
    • Muscularis (longitudinal and circular muscle layers, Auerbach's plexus located here)
    • Serosa
    • Subserosal layer
  • Crypts of Lieberkühn.
  • Blood supply within the villi and lymphatic capillaries (lacteals).

Carbohydrate Digestion and Absorption

  • Salivary amylase starts digestion but is deactivated in the stomach.
  • The bulk of carbohydrate digestion occurs in the small intestine.
  • Enzymes in the brush border (maltase, dextrinase, glucoamylases) break down starches into simple molecules like glucose.
  • Glucose is absorbed via carrier-mediated active transport.
  • Fructose is absorbed through facilitated diffusion.

Protein Digestion and Absorption

  • Pancreatic proteases and peptidases break down proteins into oligopeptides and free amino acids.
  • Brush border peptidases further break down oligopeptides.
  • Dipeptidases and tripeptidases are brought into the cell and broken down into individual amino acids, which enter the bloodstream.

Fat Digestion and Absorption

  • Fats are predominantly absorbed in the jejunum.
  • Bile acids help form micelles that diffuse into the epithelial cells of the intestine.
  • Fats are broken down into fatty acids, triglycerides, and monoglycerides.
  • These are coated with proteins from the smooth endoplasmic reticulum to form chylomicrons, which enter the lymphatic lacteals and eventually join the bloodstream in the chest.
  • Medium-chain fatty acids do not require chylomicrons and can diffuse directly into the bloodstream.
  • Bile acids are absorbed in the jejunum by passive diffusion and reabsorbed in the enterohepatic circulation of the distal ileum for recycling.
  • Loss of the ileum can lead to significant bile salt loss, impacting gallstone formation.

Gastrointestinal Hormones

  • The small intestine is the largest endocrine organ in the body.
  • Plays a major role in pancreatic, biliary, and intestinal secretion and motility.
  • Gastrin:
    • Role not specified in this section, but its importance is emphasized.
  • Cholecystokinin (CCK):
    • Stimulated by fats and some peptides and amino acids in the duodenum and jejunum.
    • Stimulates pancreatic enzyme secretion and gallbladder contraction.
    • Relaxes the sphincter of Oddi and inhibits gastric emptying.
  • Secretin:
    • Stimulates the release of water and bicarbonate into the pancreatic ducts to neutralize stomach acid.
  • Somatostatin:
    • A universal off switch.
    • Inhibits gastric acid secretion, motility, and gastrin release.
  • Motilin:
    • Stimulates upper GI tract motility and may initiate the migrating motor complex, important for downstream peristalsis.
  • Vasoactive Intestinal Polypeptide (VIP):
    • Found in neurons throughout the GI tract.
    • Stimulates pancreatic and intestinal secretion; VIP-secreting tumors can cause secretory diarrhea.

Immune Function of the Small Intestine

  • Contains abundant Peyer's patches and gut-associated lymphoid tissue.
  • Produces IgA, which inhibits the adherence of bacteria to epithelial cells, preventing colonization and multiplication.
  • Antigens contact specialized microfold cells (M cells) overlying the Peyer's patches.
  • Antigens are processed by the immune system, leading to antibody formation.
  • Peyer's patches are prominent in the ileum, possibly contributing to ileocolic intussusception in children due to hypertrophy from viral infections.

Small Bowel Obstruction (SBO)

  • Common Causes:
    • Adhesions (most common in industrialized countries).
    • Tumors.
    • Hernias.
    • Other causes: Crohn's disease, gallstones, enteroliths, bezoars, foreign bodies.
  • Pathophysiology:
    • Obstruction leads to increased peristalsis initially, then fatigue and dilation of the intestine.
    • Accumulation of water and electrolytes intraluminally and in the bowel wall (third spacing).
    • Fluid loss from the plasma space leads to dehydration, hypovolemia, hypotension, oliguria, and hemoconcentration.
    • Increased intra-abdominal pressure decreases venous return to the heart, elevating the diaphragm and compromising ventilation.
    • Bacterial fermentation leads to gas production and air-fluid levels.
    • Distension compresses venous return, impairing oxygenation.
    • Bacterial translocation can lead to systemic sepsis.
  • Symptoms:
    • Abdominal pain.
    • Nausea and vomiting.
    • Distension.
    • Inability to pass feces or flatus.
    • Borborygmi (vigorous rushes of peristalsis).
    • Late obstruction: bowel fatigue, dilation, and loss of bowel sounds.
    • Tenderness, guarding, and rebound may indicate strangulation and peritonitis.
  • Diagnosis:
    • Meticulous history and physical examination.
    • Check for hernias and rectal examination for intraluminal masses or blood.
    • X-rays: distension of the small bowel, plicae circularis (stack of coins appearance), air-fluid levels in a step-ladder pattern.
    • CT scan: Transition point from distended to collapsed bowel.
  • Treatment:
    • NG tube for decompression.
    • IV fluids.
    • Electrolyte correction.
    • Antibiotics.
    • Monitor urine output.
    • 80% of patients with simple SBO from adhesions respond to conservative management.
    • Surgery is indicated for non-responders, incarcerated hernias, or suspected strangulation.

Ileus

  • Slowing of the passage through the bowel without mechanical obstruction.
  • Causes:
    • Neurogenic.
    • Infectious.
    • Narcotics.
    • Post-op abscess.
    • Retroperitoneal inflammation or hemorrhage.
    • Intestinal ischemia.
    • Systemic sepsis.
  • Treatment involves addressing the underlying cause.
  • Pain in SBO is more colicky, whereas pain in ileus is minimal with possible distention

Small Intestine Neoplasms

  • Rare, accounting for only 5% of all GI neoplasms and 1-2% of all malignant GI tumors.
  • Types:
    • Carcinoids (most common).
    • Adenocarcinoma.
    • Lymphoma.
    • Mesenchymal tumors.
  • Older patients more commonly affected; higher incidence in males.
  • Carcinoid Tumors:
    • After the appendix, the small intestine is the second most frequently affected site.
    • Almost always occur within the last two feet of the ileum.
    • Arise from enterochromaffin cells.
    • Most patients are in their fifth decade of life.
    • Foregut: Produce low levels of serotonin, may secrete 5-hydroxytryptophan or ACTH.
    • Midgut: Produce serotonin and substance P; most commonly within two feet of the ileocecal valve.
    • Hindgut: Rarely produce serotonin but may produce other hormones such as somatostatin.
    • Primary importance: malignant potential.
    • Carcinoid syndrome occurs in a small percentage of patients.
    • Majority are asymptomatic and found incidentally.
    • Multicentric in 20-30% of patients.
    • Frequent coexistence of another primary cancer, most commonly synchronous adenocarcinoma in the colon (up to 20% of patients).
  • Clinical Manifestations:
    • Abdominal pain, partial or complete intestinal obstruction, diarrhea, weight loss. Malignant carcinoid syndrome: flushing, diarrhea, hepatomegaly, right heart valvular disease, asthma, malabsorption, and pellagra.
  • Carcinoid Syndrome:
    • Less than 10% of patients with carcinoid tumors.
    • Symptoms: Flushing, valvular heart disease on the right side, enlarged liver, diarrhea, cramps, nausea, vomiting.
    • Products produced by tumors can cause retroperitoneal and pelvic fibrosis and respiratory manifestations.
  • Diagnosis:
    • Serotonin metabolized in the liver to 5-hydroxyindoleacetic acid (5-HIAA) can be looked for in the urine (specific but not very sensitive).
    • Chromogranin A is a well-established marker for carcinoid disease.
    • Indium-labeled pentetreotide (somatostatin analog) binds to somatostatin receptors in the tumor, allowing for nuclear scanning.
  • Treatment:
    • Based on the size of the tumor and whether there is metastatic disease.
    • Resection of small lesions
    • Wide excision of the bowel and mesentery, or right hemicolectomy.
    • Anesthesia induction can precipitate a carcinoid crisis (hypotension, bronchospasm, flushing, tachycardia).
      • Treatment: octreotide before induction.
    • Surgery for debulking in patients with widespread metastatic disease.
      *Patients also need a multidisciplinary apporach

Diverticular Disease of the Small Intestine

  • Relatively common; can be true or false.
    • True diverticula: all layers of the intestinal wall, usually congenital.
    • False diverticula: protrusion through a defect in the muscle coat, acquired.
  • Duodenal diverticula: most common acquired diverticula of the small bowel.
  • Meckel's diverticulum: most common true congenital diverticulum of the small bowel; on the antimesenteric border.
  • Jejunal diverticula are false diverticula.
  • Can see diverticula in the menesenteric border where there is a natural break in the muscular layer

Small Bowel Ulcerations

  • Relatively common; can be attributed to Crohn's, typhoid fever, tuberculosis, lymphoma.
  • Drug-induced ulcerations from enteric-coated medications (potassium chloride tablets), NSAIDs, and corticosteroids.
  • Treatment: segmental resection and intestinal reanastomosis.

Miscellaneous Small Bowel Problems

  • Ingested Foreign Bodies:
    • Common in children; intentional in adults (psychiatric or incarcerated).
    • Most patients can be observed; some require removal depending on the object (battery, razor blades).
  • Enterocutaneous Fistula:
    • Communication between the bowel and the skin.
    • Usually iatrogenic (surgical misadventure); less than 2% occur spontaneously (Crohn's disease).
    • Classified as low or high output depending on enteric fluid leakage.
    • More proximal fistulas lose more fluid.
    • Can be imaged by injecting contrast (CT fistulography or CT fistulogram).
    • Complications: Sepsis, fluid and electrolyte depletion, necrosis of the skin, malnutrition.
    • Mortality can be high (15-20%).
    • Some fistulas close spontaneously with time and proper care. Factors predicting fistula will not close (FRIEND Mnemonic):
      • Foreign bodies in the tract.
      • Radiation enteritis.
      • Infection or undrained abscess.
      • Epithelialization of the fistula tract.
      • Neoplasm in the fistula.
      • Distal obstruction.
    • Management: Treat sepsis, control drainage, fluid and electrolytes, protect the skin, adequate nutrition.
    • Long-acting somatostatin analog (octreotide) can help reduce volume output.
    • Surgery:
      • Excise the fistula tract.
      • Segmental resection of the bowel.
      • Anastomosis.

Blind Loop Syndrome

  • Rare condition; diarrhea, steatorrhea, megaloblastic anemia, weight loss, deficiencies of fat-soluble vitamins, neurologic disorders.
  • Etiology: Bacterial overgrowth in stagnant areas of the small bowel (strictures, stenosis, fistula, diverticula).
  • Treatment:
    Broad spectrum antibiotics to help kill off the bacterial overgrowth.
    Surgery to get rid of stagnant areas
    Vitamin b therapy

Radiation Enteritis

  • Radiation causes obliterative endarteritis, leading to vascular insufficiency to the bowel.
  • Effects: Bowel becomes thin, strictured, and fragile.
  • Symptoms:
    * Acute phase: Pain, diarrhea, malabsorption. These effects are usually self limites
    * Late effects: Strictures, fistulas, and obstructions due to submucosal fibrosis.
  • Surgery for obstruction, fistula, perforation, bleeding.

Short Bowel Syndrome

  • Inadequate small bowel length to support adequate nutritional absorption.
  • Causes: Large resections due to mesenteric ischemia, Crohn's disease, necrotizing enterocolitis in neonates.
  • Absorption requires time and surface area (villi).
  • You can resect up to 70% of the small bowel if you preserve the terminal ileum and still not get short bowel syndrome.
  • Proximal bowel resection is tolerated much better because the more distal bowel can adapt.
  • Treatment:
    • Early phase: Treat diarrhea, fluid and electrolytes, TPN.
    • Try to see how much nutrition they can digest.
    • Small bowel transplantation (definitive surgical treatment).

Appendix

  • Lecture focuses on appendicitis and appendiceal tumors.
  • Appendicitis: Obstruction of the lumen of the appendix, where the appendix begins to swell and swell and swell. And these small little blood vessels are not enough to supply it with blood supply. It turns necrotic perforates and spills its contents into the peritoneal cavity.
  • Diagnosis: History and physical exam.
  • Patient Symptoms: Epigastric patient presents to the emergency room stating with abdominal pain that woke them up in the morning, that started around their umbilicus the periumbillical region. And there's also nausea. They don't wanna eat. They have no appetite. What the patient will tell you is the pain started in the periumbilical region. But now their pain has migration, has is no longer really there. So that's kind of on their right side.
  • Appendix Variety of Positions:
    • Pelvic.
    • Subcecal.
    • Retrocecal.
    • Postileal.
    • On top of the ileum.
      *Follow the tinea of the colon to find the appendix
  • Ultrasound vs Scans:
    • Ultrasounds are used more in children and help to pregnant women.
    • Best way to image this is the CT scan.
  • Treatment Options:
    • Appendectomy.
    • Give them fluids, antibiotics, and PO. And then you take them to the OR at early in the morning, instead of midnight or at 1 AM or 2 AM, 3. But what we've also discovered by doing that is that some people really feel much better after it and their symptoms disappear. And some people can get antibiotics. But some may never need an operation.
  • But by and large, that patient should get appendectomy. Is how we do that. And you know, can be done laproscopically now, I haven't actually done open appendic in years.
  • But there's important to use seatscan because they present late. So high why count of 20. So what's the case? You have to say, let's do cast scan because I think you do, there's probably purificate in the house this is like a big bottle.

Cancers of the Appendix

  • Most are asymptomatic.
  • Usually less than a centimeter.
  • If they are, you can basically just resect the appendix, and that's it.
    *if they are larger than 2cm, they can have a higher incidence of distance.

Colon

  • Extends from ileoeceal valve to teh rectum and anus.
  • From the colon extending to the ascending colon. Hepatic flexure, transverse colon, splenic flexure, descending colon, sigma colon and then right. From here down, is rec room.
  • Blood supply Off the SMA, remember here is you SMA that supplied all these other a little brands to the small bell. The wall is these brands supply de colon as such. So you your ileoocolik. Which has ileal branch. The colic branch. And then you have you right colic of.
  • And you have middle Colic. And then you have Mar artery Drummond. And then you have inferior mesenteric artery. It's superior mesenteric artery.
  • Inferior mesenteric artery, which with its left colic branch, and sigma branches is in superior reatal branches.
  • There Is Is the arc rheolan is the Anastomo is network between the IMA I the MMA. It likes the two vessel.
  • The colon obsorbs a large surface area basically to help recycle water. The most efficient site obsorpiton of water to obsorbption via the Gi tract per surface area, is the colon not the Ilium not the stomach Not the duodenum is the cold.
  • Motility themes: mucosa, submucosa muscularis and between the circular and long. Have our box my and then the Sub mucosa have the meiss and also plexus.
    Colonic Flora: you know, not when you get, like, very significant colne counts you get 10 to the eleventh and 10 to the 12 bacterial colony right and depending on where you are in GI tract. There's gonna have different Micro organism what You'll see is the stomac duum strep.

Diverticular Disease

  • This disease happens mostly in countries with low dietary input Intake
  • And the more muscular, is you move from right to left Okay.

Colonc Volvulus:

  • Sima volvulous presnts the majoritiy. Patients typically have slow transit. They're constipated. They may have I very redundant piece of sigma colon. They may be laxative users. They may develop a little bit of mega and that colon then twist on itself.

Neoplasms

  • It takes timer for cancer to devlop, it's a adenocarcnoma sequence
    Once you diagnose the patient on endopscy you stage the cancer stage depends on the loaction or tumor location
    TNM Staging: whether its confined to mucosa. Is submucosa. IS invading muscularisa propria.

Adjucant Therapy

*Patients need adjuvant chemo espcially those with with advanced disease metastasis with met and mph