**2. In vivo lysis of lipid in human body**

Lipid digestion begins in the gastrointestinal tract from oral cavity in the presence of lingual lipase enzymes secreted by Ebner's gland present on the tongue [21]. Further physical breakdown occurs due to gastric emptying, antral contraction and retropulsion. Stomach acts as the core site for the emulsification of fat molecules into coarse emulsion droplets of size approximately 0.5 μm [22, 23] followed by the enzymatic degradation of triglyceride to form a mixture of fatty acids and monoglycerides [24]. The coarse emulsion droplets when entering the small intestine stimulate the secretion of bile salts and bile lipids from the gallbladder which stabilizes the surface of droplets leading to the formation of fine droplets upon entering the intestine and further gets homogenized with bile and pancreatic juice. In general, the chain length of triglycerides determines its absorption pathway. The short- and medium-chain

*Lipid as a Vehicle/Carrier for Oral Drug Delivery DOI: http://dx.doi.org/10.5772/intechopen.109672*

length fatty acids (having carbon chain length of less than 12) are able to diffuse through enterocytes, taken up by blood vessels, enter liver and then to blood circulation. On the other hand, long-chain fatty acids (having chain length greater than 12) undergoes re-esterification and are converted back to triglyceride form. These triglycerides are taken up by the chylomicrons which are absorbed by lymph vessels and drained to the blood circulation via the thoracic duct. When these chylomicrons reach any tissue, they activate the lipoprotein lipase present on the surface of BCECs to generate fatty acids. The fatty acids so formed enter muscles and adipose tissues for use or otherwise get stored. The lipids are transported in the body through lipoprotein vehicles like chylomicrons. These vehicles remain in the circulatory system until their triglycerides are consumed. They are also taken up by liver and digested. Among lipoproteins, the chylomicrons are the largest produced by intestinal wall. Other lipoproteins include high-density lipoproteins (HDL), low-density lipoproteins (LDL) and very low-density lipoproteins (VLDL). The HDL is synthesized by liver and intestinal wall (enterocytes), VLDL by the liver and LDL partly through the metabolism of VLDL in plasma and partly by liver [25].
