The liver transports bile salts and antimicrobial molecules (IgA, angiogenin 1) to the intestinal lumen through the biliary tract. This maintains gut eubiosis by controlling unrestricted bacterial overgrowth. Bile salts also act as important signaling molecules via nuclear receptors (such as FXR, TGR5) to modulate hepatic bile acid synthesis, glucose metabolism, lipid metabolism and energy utilization from diet. On the other hand, gut-products such as host and/or microbial metabolites and MAMPs translocate to the liver via the portal vein and influence liver functions. Additionally, systemic circulation extends the gut-liver axis by transporting liver metabolites from dietary, endogenous or xenobiotic substances (eg. FFAs, choline metabolites, ethanol metabolites) to the intestine through the capillary system. Owing to this medium of transport and ease of diffusion of systemic mediators across blood capillaries, these could affect the intestinal barrier both, positively (eg. butyrate) or negatively (eg. acetaldehyde)
(TMA: Trimethylamine; TMAO: Trimethylamine N-oxide; MAMPs: Pathogen-associated molecular patterns; VLDL: Very low-density lipoprotein; FXR: Farnesoid X receptor; TGR5: Takeda G-protein coupled receptor 5; FFA: Free fatty acid)