Final answer:
Only water-soluble monomers like amino acids and simple sugars can enter the circulatory system through the intestinal mucosa, while certain synthetic monomers can pose health risks.
Step-by-step explanation:
Only monomers that are water-soluble actually enter the circulatory system, since the GI tract is open to and continuous with the environment outside the body. Nutrients such as amino acids and simple sugars are absorbed in their monomeric form and can pass through the apical surfaces of epithelial cells into the interstitial fluid. After absorption, water-soluble nutrients enter the capillary blood in the villi and are transported to the liver via the hepatic portal vein. Other molecules like fats are reassembled into triglycerides and, along with other lipids and proteins, are formed into chylomicrons that can enter the lymphatic system through lacteals.
It is important to note that the intestinal mucosa has tight junctions which prevent substances from passing between cells. Therefore, absorbed molecules must be small enough, like the water-soluble monomers, to diffuse directly through the cell membranes of the epithelial cells to enter the bloodstream.
The presence of residual monomer can occur in synthetic materials and not all monomers are benign. Certain monomers such as formaldehyde and bisphenol-A are known carcinogens and their presence in materials that come into contact with food may pose health risks, even though current evidence may suggest that the risks associated with low-level exposure are minimal. Public policy is adapting to regulate these exposures more strictly.