Absorption

• Haustra: The sacs in the colon that give its segmented appearance
• Valsalva maneuver: Forced expiration against the closed glottis
• Paracellular pathway: Pathway between cells
• the Jejunum does most of the absorption
• the Ileum does some absorption
• the colon absorbs water and electrolytes, and houses bacteria
• Apical SGLT co-transports glucose/galactose and Na+ into enterocytes
• Fructose uses GLUT5 transporters to enter the enterocyte
• Glucose, galactose and fructose cross the basolateral membrane of enterocytes via GLUT2
• Proteins are mostly co-transported with Na+ as amino acids into the enterocyte
• Di-peptides and tri-peptides are co-transported with protons into the enterocyte
• In the cytoplasm of enterocytes, di-peptides and tri-peptides are further metabolised into amino acids
• Amino acids are transported out of the enterocyte by Na+/K+ ATPase
• In some rare cases, small peptides can be absorbed through enterocytes via transcytosis
• in babies, antibodies are absorbed as full proteins since they do not have the digestive machinery to break down the proteins
• Monosaccharides and amino acids move into the capillaries in the villi
• Venous blood passes into the liver via the hepatic portal vein
• Liver removes potentially harmful products before they can enter the systemic circulation
• Nutrients go to the heart and are distributed systemically by the aorta
• Mono-glycerides, di-glycerides and fatty acids can diffuse into the enterocyte when the micelles get close to the brush border enzyme
• Cholesterol has a polar group so cannot diffuse into enterocytes
• Cholesterol is transported into enterocytes in an energy-dependent process
• Inside the enterocyte, triglycerides are reformed
• Triglycerides, cholesterol and protein are assembled to form a chylomicron in the Golgi Apparatus in enterocytes
• chylomicrons in enterocytes undergo exocytosis and are transported into lacteals
• chylomicrons are too big to enter capillaries
• chylomicrons enter the blood stream where the thoracic duct connects with the left subclavian vein
• Iron can be absorbed as heme iron (meat) or Fe2+ (vegetables)
• Heme iron is transported into enterocytes by itself
• Fe2+ is co-transported into enterocytes with H+
• Fe2+ is transported out of the enterocyte via the ferroportin transporter
• Inside the enterocyte, heme iron is converted into Fe2+ and porphyrin
• Uses of iron:

  • Making haemoglobin in the bone marrow
  • Stored in the liver as ferritin
• Facilitated transport of ions into enterocytes creates an osmotic gradient, causing water to flow into ECF via a paracellular pathway
• K+ and water can diffuse into the ECF via the paracellular pathway
• Tenia coli: non-continuous longitudinal muscle in the muscularis externa
• Contraction of tenia coli lead to haustra
• Ileocecal valve consists of contracted circular muscle to separate the ileum and colon
• Chyme enters the large intestinethrough the ileocecal valve
• What causes the ileocecal valve to relax?

  peristaltic wave or gastroileal reflex