Absorption of Digested Food

2. Absorption of Digested Food Absorption is the uptake of digested food molecules from the alimentary canal (gastrointestinal tract) into the blood or lymph The principal site for absorption of nutrients is the ileum of the smallintestine where the epithelialcells lining this gut region are ideally suited to this role Glucose and aminoacids are transported across the epithelial cells to the bloodcapillaries within the villi, and reconstituted fats are absorbed into the lymphcapillaries (lacteals)

3. The gross and histological structure of the ileum adapt this region for its absorptive and secretory roles Absorption of Digested Food Absorption involves the transportprocesses of simple diffusion, facilitated diffusion, active transport and osmosis (for water absorption)

4. gland within the submucosa (Brunner’s glands) gland outside the gut, e.g. pancreas mucosa villi muscularis mucosa submucosa (connective tissue) circular muscle longitudinal muscle outer serosa lumen

5. The duodenum is the site where pancreaticjuice (containing many enzymes) is secreted into the gut and where Brunner’sglands secrete an alkalinemucus to help neutralise and protect its lining from the acidchyme arriving from the stomach The ileum is the principal site for the absorption of nutrients; it is verylong (about 4 metres in length) and the numerous villi with their epitheliallinings increase the surfacearea for absorption

6. Adaptations of the Ileum The ileum displays adaptations for both absorption and secretion: • The ileum is very long and absorption can occuralong its length • The mucosa is highlyfolded and the numerous ‘finger-like’ projections, the villi, vastly increase the surfacearea of the epithelium for both digestion and secretion • The epithelial cells of the villi bear microvilli at their luminal surface that project into the lumen of the gut; these microvilli form the brushborder and further increase the surface area available for both absorption and secretion • The villi are well supplied with a networkofbloodcapillaries into which glucose and amino acids are transferred and then transported to the liver along the hepatic portal vein • A single, permeable lacteal within each villus transports reconstituted fats away from the intestine

7. Epithelial cells, bearing microvilli, project into the lumen of the gut Intestinal gland between the villi; contains enzyme secreting cells and hormone releasing cells

8. T.S. Ileum intestinal gland Lumen villi mucosa

9. Microvilli forming the brush border Numerous mitochondria Nucleus Microvilli further increase the surface area of the intestine for efficient absorption of food molecules Numerous mitochondria provide energy, in the form of ATP, for the active transport of various molecules and ions

10. Membrane-bound protein carriers are involved in the transport of glucose molecules by facilitated diffusion and active transport Glucose is transported across the epithelialcells of the villi from the lumen of the gut to the blood capillaries by facilitateddiffusionand sodium-linked activetransport Facilitated diffusion is dependent upon the existence of a glucoseconcentrationgradient across the epithelial cell membranes Active transport allows for the transfer of glucose against its concentration gradient

11. Specific protein carriers in the mucosal membranes of intestinal epithelial cells possess binding sites for both glucose molecules and sodium ions The transport of these molecules into the epithelial cells from the gut lumen is dependent upon the sodium ion concentration gradient; so long as this gradient exists, glucose can be transported against its own gradient

12. The sodium gradient is maintained by sodium/potassium pumps located within the serosal membrane of the epithelial cells These energy-requiring pumps transfer sodium ions out of the cell into the blood in exchange for potassium ions The operation of these Na+/K+ pumps results in a permanently low Na+ concentration within the epithelial cells and the maintenance of a Na+ concentration gradient across the mucosal membrane