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, B1, B2, B6, B12, 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 B12 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 H2O 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-HCO3 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 D3 kidney

1,25 dihydroxy-vitamin D3

Stimulates synthesis of Ca-binding protein and Ca-ATPase in enterocytes

Diarrhea 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 H2O & ions (small & large intestine)

Catecholamines = intestinal secretion Somatostatin = H2O & ions absorption

(ileum & colon) Epinephrine = NaCl absorption (ileum)

Aldosterone = synthesis of Na channel (colon)