Digestion and Absorption Dr. Mohammed Alzoghaibi

Digestion and Absorption Carbohydrates, proteins and lipids are digested and absorbed in the small intestine Absorption  The surface area for absorption in the small intestine is greatly increased by the presence of the brush border  Pathways of Absorption - cellular - paracellular

Structure of intestinal wall Longitudinal folds (finger-like villi) - longest in duodenum & shortest in ileum - increases surface area 600 fold The significance of villi & microvilli - increase the surface area - maximizing the exposure of nutrients to digestive enzymes

Digestion and absorption of lipids

Abnormalities of lipids digestion 1. Pancreatic insufficiency (chronic pancreatitis and cystic fibrosis) 2. Acidity of duodenal content (zollinger-Ellison syndrome) 3. Deficiency of bile salts (ileal resection) 4. Bacterial over growth (deconjugation of bile salts) 5. Decrease intestinal cells for absorption 6. Failure of synthesis of apoproteins (abetalipoproteinemia)

Digestion and absorption of carbohydrates

Carbohydrate malabsorption Lactose malabsorption syndrome  Symptoms - gurgling noises in the intestine - flatulence - diarrhea Sucrase-isomaltase deficiency - decrease level of sucrase - suppression of transporter protein Glucose-galactose malabsorption syndrom - deficiency in transporter protein

Digestion and absorption of proteins

Vitamins and Electrolytes transport and Diarrhea Dr. Alzoghaibi

Absorption of vitamins In terms of absorption, vitamins are classified to whether they are lipid-soluble or water-soluble The fat-soluble vitamins include A, D, E, & K The water-soluble vitamins are C, B 1, B 2, B 6, B 12, and folic acid

Absorption of vitamins (cont) A. Fat-soluble vitamins are incorporated into micelles and absorbed along with other lipids B. Most water-soluble vitamins are absorbed by Na-dependent cotransport mechanisms C. Vitamin B 12 is absorbed in the ileum and requires intrinsic factor  Gastrectomy results in the loss of parietal cells and loss of intrinsic factor pernicious anemia

Absorption and secretion of electrolytes and water Electrolytes and H 2 O may cross intestinal epithelial cells by either cellular or paracellular The permeability of the tight junctions varies with the type of epithelium o A tight epithelium is the colon o Leaky epithelia are the small intestine and gallbladder

Absorption and secretion of electrolytes and water Absorption of NaCl: Na moves into the intestinal cells by the following mechanisms 1) Passive diffusion 2) Na-glucose or Na-amino acid cotransport 3) Na-Cl exchange 4) Na-H exchange

Absorption and secretion of electrolytes and water Cl absorption accompanies Na absorption by the following mechanisms: 1) Passive diffusion 2) Na-Cl cotransport 3) Cl-HCO 3 exchange Absorption and secretion of K  K is absorbed in the small intestine by passive diffusion  K secretion in the colon is stimulated by aldosterone  Excessive loss of k in diarrheal fluids causes hypokalemia

Heme transport

Ca Absorption by Enterocytes plasma Ca parathyroid hormone 25-hydroxy-vitamin D 3 kidney 1,25 dihydroxy-vitamin D 3 Stimulates synthesis of Ca-binding protein and Ca-ATPase in enterocytes

Diarrhea  To run through   ECF   arterial pressure  HCO3 (relative to Cl)  Hyperchloremic metabolic acidosis  K  Hypokalemia Causes of Diarrhea:  Decreased surface area for absorption  Osmotic diarrhea (lactase deficiency)  Secretory diarrhea

Hormonal control of absorption & secretion Glucocorticoid = absorption of H 2 O & ions (small & large intestine) Catecholamines = intestinal secretion Somatostatin = H 2 O & ions absorption (ileum & colon) Epinephrine = NaCl absorption (ileum) Aldosterone = synthesis of Na channel (colon)