GIT Blood Supply & Intrinsic Factor

• There are review questions about the intrinsic factor and its role in digestion
• There are three main trunks/arteries off the descending aorta that supply blood to the guts
• Venous drainage of the abdominal gastrointestinal tract, spleen, pancreas, and gallbladder (except the inferior rectum) use the portal system of veins
• Blood enters the liver from GIT via the portal vein and exits via the hepatic veins to enter the inferior vena cava

Blood Supply to GIT

• Celiac trunk supplies the foregut, from the stomach to where the bile duct enters the duodenum

• Important arterial branches of the celiac trunk are:

  • Common Hepatic
    • Hepatic proper
      • Left Hepatic
      • Right Hepatic
    • Right Gastric
    • Gastroduodenal
  • Left Gastric
  • Splenic

• The superior mesenteric artery supplies the midgut from where the bile duct enters the duodenum to 2/3 across the transverse colon

• Relevant arterial branches of the superior mesenteric artery:

  • Right colic
  • Middle colic
  • Ileocolic
  • Ileal and jejunal branches

• Supply occurs via the inferior mesenteric artery to the hindgut (from 2/3 across the transverse colon to the rectum)

• Arterial branches of the inferior mesenteric artery:

  • Left colic
  • Sigmoid
  • Superior rectal

• Accessory organs include the liver, pancreas and gall bladder

Liver Functions and Bile Physiology

• Bile consists of bile acids, cholesterol, phospholipids, bile pigments, electrolytes, and water
• Bile serves as a means for eliminating waste products, including bilirubin and cholesterol excesses
• Bile production occurs at 100 mL/day
• Bile salts (bilirubin, biliverdin), cholesterol, fats, fat-soluble hormones, lecithin, neutralize and dilute stomach acid
• Bile salts emulsify fats.
• Most bile salts are reabsorbed in the ileum
• Secretin stimulates bile secretions (increasing water and bicarbonate ion content)
• The liver stores glycogen, fat, vitamins, copper, and iron
• Hepatic portal blood comes to the liver from the small intestine
• The nutrient interconversion process converts amino acids to energy producing compounds and hydroxylates vitamin D
• Vitamin D travels to kidney where it is hydroxylated again into its active form to promotes bone growth and absorption of calcium
• Hepatocytes detoxify removing ammonia and converting it to urea
• Kupffer cells phagocytize worn-out and dying red and white blood cells, and some bacteria
• The liver synthesizes albumins, fibrinogen, globulins, heparin, and clotting factors
• Bile acid-dependent components are produced by hepatocytes, secreted into canaliculi, and transported through the bile ducts
• Bile acid- independent components are made by the ductal cells, these cells secrete alkaline fluid made by pancreatic duct cells which is stimulated by the hormone secretin
• Dependent and independent bile components enter the intrahepatic bile ducts which drain into the biliary tree
• The biliary tree transports bile from the liver to the gallbladder and to the duodenum
• The gallbladder concentrates and stores bile by removing water and ions, regulated by hormone cholecystokinin released from the duodenum

Gall Bladder

• The maximum volume that the gallbladder can hold is 30 to 60ml, but it stores about 450ml
• The gallbladder mucosa absorbs water, sodium, chloride, and other small electrolytes, concentrating the remaining bile components
• Gallstones forms via precipitated cholesterol when excess cholesterol due to high-cholesterol diet and not enough bile salts accumulate
• Gallstones can block the cystic duct and lead to pancreatitis if moving far in the duct
• Gallstones occur because of drastic dieting/fasting as the liver secretes extra cholesterol into bile causing gallstones

Regulation

• Vagus nerve stimulation causes the contraction of the gallbladder
• Secretin stimulates bile secretion by the liver
• Cholecystokinin stimulates contraction of gall bladder
• Bile salts reabsorbed in the ileum stimulate additional bile secretion

Enterohepatic Circulation

• Enterohepatic Circulation (EHC) is the movement of bild acid from the liver to the small intestine and back

Carbohydrate Absorption

• Glucose and galactose absorption occurs via secondary active transport through the Sodium-Glucose co-transporter (SGLT1)
• Glucose and galactose exit the cell via GLUT2 receptors across the basolateral membrane into the blood
• Fructose enters the cell by facilitated diffusion via GLUT5
• Fructose is transported into the blood via GLUT2 receptors

Protein Absorption

• Amino acids are absorbed via a Sodium cotransporter, in a similar mechanism to the monosaccharides
• Transport is across the basolateral membrane via facilitated diffusion
• Di and tripeptides are absorbed via separate H+ dependent cotransporters and are hydrolysed to amino acids within the cell
• Whole protein molecules are absorbed by transcytosis by newborns

Fat Absorption

• The products of digestion are released at the apical membrane and diffuse into the enterocyte
• Inside the cell, the products are re-esterified to form the reassembled lipids, triglycerides, cholesterol and phospholipids.
• The lipids are then packaged inside apoproteins to form a chylomicron
• Chylomicrons are too large to enter circulation, so they enter the lymphatic system via lacteals

Water Absorption

• Most water and electrolyte absorption occurs in the small intestine, with a little absorbed in the colon
• Water absorption depends on solute absorption, such as Na+ and Cl-
• Na+ is absorbed from the intestinal lumen, most use the cotransport with glucose and amino acids and the Na^/H+ exchange
• Water can follow transcellularly or paracellularly (between enterocyte tight junctions)
• Water and Na+ ions diffuse into the capillaries

Absorption of Other Ions

• Calcium is absorbed in the blood especially from the duodenum
• Calcium ion absorption is controlled to body need by parathyroid hormone and vitamin D
• Iron ions are actively absorbed from the small intestine, regulated to body's need for hemoglobin formation
• Actively absorbed through the intestinal mucosa are potassium, magnesium, phosphate,

Vitamins

• Vitamin K, vitamin B12, thiamine, and riboflavin may be found in the colon
• Gases that contribute to flatus in the colon are carbon dioxide, hydrogen gas, and methane
• Vitamin K is important because what you eat doesn't have enough to maintain blood coagulation
• Intrinsic factor is necessary for B12 absorbtion

Review questions about the intrinsic factor and its role in digestion. Arteries off the descending aorta supply blood to the guts. Venous drainage of the abdominal gastrointestinal tract, spleen, pancreas, and gallbladder use the portal system of veins.