Lower AC

  • various organs help break food down into nutrients and pass waste products out of the body

    • mesentery is your “newest” organ

      • provides support, stores fat, and regulates intestinal function

    • small intestines pull nutrients out of cyme and into the bloodstream for delivery

    • large intestines reclaim water and compact waste products into feces for elimination

    • liver, gallbladder, and pancreas (accessory organs)

      • help with chemical digestion

Mesentery

  • mesentery suspeds organs, stores fat, allows for the passage of vessels, and supports organ viability

    • continuous collection of connective tissues

      • suspends organ, allows for strenous contractions, and prevents interstinal twisting/tangling

    • fat storage also impacts inflammatory responses

      • e.g. creeping fat/fat wrapping in Crohn’s disease

    • supports various organs developmentally and as adults

      • many organs embryologically derived from mesentery

      • contributes to organ viability

        • supports splenic regeneration following spenectomy or rupture

        • supports development of accessory spleens (splenunculus)

        • supports intestinal viability (may become necrotic if separated)

      • can also support unwanted growth, such as with heterotopic pancreas, etopic pregnancies, and teratomas

Small Intestine

  • most nutrient absorption happen here

    • longest pare of the GI tract (~5m long)

  • three regions

    • duodenum (first 25cm; part retroperitoneal)

      • pyloric valve to duodenojejunal flexure

      • recieves chyme and secretions from liver/pancreas

    • jejunum (next 40% of SI; 1-1.7 m)

      • most digestion and nutrient absorption happens here

    • ileum (last 60% of SI; 1.6-2.7)

      • ends at ileocecal valves (sphincter to cecum)

      • aggregated lympoid nodules (peyer’s patches)

  • lumen with simple columnar epithelium

  • thick inner circular layer and thin outer longitudinal layer in muscularis externa

  • large internal surface area (SA) for absorption

    • circular folds (plicae circlares)

      • cause spiral path chyme (promote mixing)

      • morphologically varied in the three regions

  • duodenal glands secrete bicarbonate-rich mucin

    • in duodenal submucosa

  • peyer’s patches help with immune surveillance

  • villi function in absorption and sending nutrients to the bloodstream for delivery

    • goblet cells produce mucus

    • lacteals specialized for lipid transport

    • microvilli form a brush border

      • absorb nutrients and protect against bacteria

        • also secrete gut enzymes to help with both

  • intestinal crytps

    • progenitor cells actively divide to replace intestinal epithelial cells continuously

    • paneth cells produce granules with antimicrobial peptides and immunomodeling proteins

      • regulate the composition of the gut flora

Large Intestine

  • no circular fold or villi

    • features taenia coli (smooth m.) with more goblet cells

  • lymphatic tissues to protect from bacteria

  • measures 1.5. m long and 6.5 cm in diameter

    • cecum with appendix → ascending colon → right colic (hepatic) flexure → transverse colon → left colic (splenic) flexure → descending colon → sigmoid colon → rectum → anal canal

  • internal anal sphincter

    • inside of rectum; ANS control

  • external anal sphincter

    • outside the end of the anal canal; voluntary control

Liver

  • largest gland in the body (~3lbs)

  • inferior to the diaphragm

    • four lobes (right, left, quadrate, and caudate)

    • inferior depression for the gallbladder

      • between right and quadrate lobes

    • hilum (porta hepatis) for vessels and bile duct

  • comprised of hepatocytes

    • absorb nutrients after meals

    • break down glycogen between meals

    • remove hormones, toxins, etc.

    • secrete albumin, angiotensinogen, etc. into blood

  • histology

    • hepatic lobules

      • central v. passes down core

      • center surrounded by hepatocytes

    • hepatic triads between lobules

      • branches of hepatic portal vein and hepatic portal artery

      • bile ductule

    • hepatic sinusoids separatw hepatocytes from blood

      • channels lined by fenestrated endothelium

      • hepatocyte microvilli project into sinusoids

      • filter blood from stomach and intestines

      • hepatic macrophages phagocytize bacteria/debris

  • processes materials absorbed by the intestines

    • detoxifies potentially harmful chemicals (and meds)

    • secretes useful chemicals (e.g. hormones, albumin, and bile)

      • albumin carries hormones, drugs, and fats in blood

      • bile helps absorb fats and carry out wastes

        • creates ~500-1,000mL of bile/day

        • excess drugs and toxins are often integrated into bile for eventual excretion through the intestines

Gallbladder

  • features a fundus (head) and cervix (neck)

    • cervix attached to common hepatic duct, which becomes the bile duct that passes through the pancreas

      • hepatopancreatic sphincter regulates bile release into the SI

  • internally lined by simple columnar epithelium

  • stores and concentrates bile from the liver

    • bile contains minerals, cholesterol, phospholipids, fats, pigments (such as bilirubin), and bile acids (salts)

      • 95% water

      • bile acids are steroids that aids in fat digestion/absorption

        • 20% of bile salts excreted in feces

        • 80% reabsorbed and returned to the liver

      • bilirubin is a major pigment derived from hemoglobin

        • most comes from the breaking down of RBCs

Pancreas

  • is both endocrine gland and exocrine gland

    • exocrine (99% of pancreas is exocrine tissue)

      • secretory acinar cells → secrete into ducts

        • ducts converge into main pancreatic duct

      • accessory pancreatic duct bypasses sphincter

        • secretes pancreatic juice into duodenum without bile

      • pancreatic juice (up to 1,500mL/day)

        • water, enzymes, zymogens, sodium bicarbonate, and other electrolytes

          • enzymes = pancreatic amylase, pancreatic lipase, ribonuclease, and deoxyribonuclease

          • zymogens = trypsinoen, chymostrysinogen, and procarboxypeptidase

    • endocrine

      • pancreatic islets (islets of langerhans)

        • alpha cells → glucagon

        • beta cells → insulin and amylin (promotes satiety)

        • delta cells → somatostatin (inhibitor)

          • also exists in the pylorus and duodenum

        • PP or F cells → pancreatic polypeptide

        • epsilon cells → ghrelin (inhibits beta cells)

  • sodium bicarbonate is secreted by duct cells

    • CO2 diffuses into duct cells from the blood

    • carbonic anhydrase (CAH) converts CO2 and H20 into carbonic acid (H2CO3)

    • H2CO3 dissociates into HCO3- + H+

    • H+ exchanged for Na+ via antiporter

    • HCO3- exchanged for Cl- and moves into the lumen

      • co-transported with H2O and Na+

    • sodium bicarbonate forms in the lumen

  • regulation of pancreatic secretions

    • acetylcholine (ACh) stimulates acinar cells during the cephalic phase of gastric motility

    • cholecystokinin (CCK) secreted in response to fats in the small intestine

      • stimulates acinar cells to secrete enzymes, gallbladder contractons,and relations of hepatopancreatic spincter

    • sectretin secreted in response to the arrival of chyme in the small intestine

      • stimulates pancreatic juice secretion

        • sodium bicarbonate in pancreatic juice raises pH

          • promotes activity of digesive enzymes

          • inactives pepsin

          • buffers gastric juice in chyme