Once mixed micelles reach the brush border of the enterocyte, two different lipid uptake mechanisms are believed to occur, but lipid uptake is not completely understood. One mechanism is that individual components of micelles may diffuse across the enterocyte. Otherwise, it is believed that some components may be taken up through unresolved transporters. For example, cholesterol transporters have been identified, but their overall mechanism of absorption is not well understood. The individual compounds are taken up as shown below.
Figure 4.71 Uptake of mixed micelle components into the enterocyte
Once inside the enterocyte, there are different fates for fatty acids, depending on their length. Short- and medium-chain fatty acids move through the enterocyte and enter circulation through the capillaries; they are transported by the protein albumin. They will be carried to the liver by the portal vein, like monosaccharides and amino acids. Long-chain fatty acids, 2-monoglyceride, lysolecithin, and cholesterol will be re-esterified forming triglycerides, phosphatidylcholine, and cholesterol esters, respectively. These re-esterified lipids are then packaged into chylomicrons, which are lipoproteins, that are described in further detail in the next section. These chylomicrons are too large to fit through the pores in the capillaries, but they can fit through the larger fenestrations (openings) in the lacteal.
Figure 4.72 Fates of lipids in the enterocyte
Lacteals (shown below) are small vessels that feed into the lymphatic system. Thus, the chylomicrons enter the lacteals and enter into lymphatic circulation.
Figure 4.73 Anatomy of a villus, with the lacteal shown in blue1
The lymphatic system is a system similar to the circulatory system in that it contains vessels that transport fluid. However, instead of blood, the lymphatic system contains a clear fluid known as lymph. There are a number of lymph nodes (small glands) within the lymphatic system that play a key role in the body's immune system. The figure below shows the lymphatic system.
Figure 4.74 The lymphatic system2
The following videos describe and illustrate how the lymphatic system and lymph functions.
The lymphatic system enters general circulation through the thoracic duct that enters the left subclavian vein as shown below. General, in this case, means that it is not directed to the liver like other components that have been absorbed.
Figure 4.75 The thoracic duct is where the lymphatic system enters circulation.
The animation below is an overview of lipid digestion, uptake, and initial transport.
References & Links