Fat is absorbed lymphaticaly, everything is in the intestines, but funtion diffently.
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Functions of GI organs
The GI tract begins at the mouth, where digestion begins with chewing. Saliva containing mucus and the enzyme amylase is secreted from 3 pairs of salivary glands, located in the head. Mucus moistens the food and amylase partially digests polysaccharides (starches). Food then reaches the stomach through the pharynx and eso****us.
The stomach is the sac that stores and digests food macromolecules into a solution called chyme. Glands lining the stomach secrete hydrochloric acid that dissolves food particles and protein-digesting enzymes, called pepsin.
Final stages of digestion and most of the nutrient absorption occurs in next portion of the tract: the small intestine. The small intestine is divided into 3 segments - duodenum, jejunum, and ileum.
The pancreas is a gland located behind the stomach. From its exocrine portion it secretes (1) digestive enzymes and (2) a fluid rich in HCO3- ions to neutralize the acid from stomach. The liver secretes bile. Bile contains HCO3- ions and bile salts to solubilize fats. Bile reaches the gall bladder through hepatic ducts and is stored in the gall bladder between meals. During a meal, bile is secreted from the gland by smooth muscle contraction and reaches the duodenum portion of the small intestine by the common bile duct.
Monosaccharides, amino acids and mineral salts are absorbed by transporter-mediated processes while fatty acid water diffuse passively.
Undigested material is passed to large intestine, where it is temporarily stored and concentrated by reabsorption of salts and water. Finally, contractions of rectum, the last part of large intestine, expel the feces through the anus.
Structure of GI Tract Wall
The luminar surface is covered by a single layer of epithelium containing exocrine and endocrine cells. The exocrine cells disintegrate and discharge into the lumen, releasing their enzymes. The epithelia with an underlying layer of connective tissue (lamnia propia) and muscle (muscularis mucosa) are called mucosa. Below the mucosa is a layer of inner circular and outer longitudinal smooth muscle called muscularis externa, which provides the forces for moving and mixing the GI contents. The outermost layer of the tube is made up of connective tissue called serosa. The luminar surface of the tube is highly convoluted into projections called villi and microvilli; both of which increase total surface area for absorption. The center of each villus has a single blunt-ended lymphatic vessel called lacteal. Venous drainage from the intestine transports absorbed materials to the liver for processing via the hepatic portal vein.
Digestion and Absorption
Carbohydrate
Digestion begins in the mouth by salivary amylase and completed in the small intestine by pancreatic amylase. Monosaccharides, such as glucose, galactose and fructose, are produced by the breakdown of polysaccharides and are transported to the intestinal epithelium by facilitated diffusion or active transport. Facilitated diffusion moves the sugars to the bloodstream.
Protein
Proteins are broken down to peptide fragments by pepsin in the stomach, and by pancreatic trypsin and chemotrypsin in the small intestine. The fragments are then digested to free amino acids by carboxypeptidase from the pancreas and aminopeptidase from the intestinal epithelium. Free amino acids enter the epithelium by secondary active transport and leave it by facilitated diffusion. Small amounts of intact proteins can enter interstitial fluid by endo- and exocytosis.
Fat
Fat digestion occurs by pancreatic lipase in small intestine. A monoglyceride and two fatty acids are produced in the digestive process. Large lipid droplets are first broken down into smaller droplets, by a process called emulsification. Emulsification is driven by mechanical disruption (by contractile activity of GI tract) and emulsifying agents (amphipathic bile salts). Pancreatic colipase binds the water-soluble lipase to the lipid substrate.
Digested products and bile salts form amphipathic micelles. These micelles keep the insoluble products in soluble aggregates from which small amounts are released and absorbed by epithelial cells via diffusion. Free fatty acids and monoglycerides then recombine into triacylglycerols at the smooth ER, are processed further in the Golgi and enter the interstitial fluid as droplets called chylomicrons, which are then taken up by the lacteals in the intestine.
Vitamins
Fat-soluble vitamins are absorbed and stored along with fats. Most water-soluble vitamins are absorbed by diffusion or mediated transport. Vitamin B12, because of its large size and charged nature, first binds to a protein, called intrinsic factor, which is secreted by the stomach epithelium, and is then absorbed by endocytosis.
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