The location and integration of metabolic pathways are released by studies at lower levels of organisation, namely the following:

1. At the subcellular level, each cell organelle (mitochondria) or compartment (cytosol) carries out specific biochemical roles that from part of a subcellular pattern of metabolic pathway.
2. At the tissue and organ level, the nature of the substrate entering and metabolites are leaving tissues and organs.

At the tissue and organ level, the blood circulation integrates metabolism. Amino acids resulting from digestion of dietary protein and glucose resulting from the digestion of carbohydrate share a common route of absorption via the hepatic portal vein. This ensures that both of these metabolites and other water-soluble products of digestion are initially directed to liver.

Liver has the primary metabolic function of regulating the blood concentration of most metabolites, particularly glucose and amino acids. In the case of glucose, this is achieved by taking up excess glucose and converting it to glycogen (glycogenesis) or fat (lipogenesis). Between meals, it can draw upon its glycogen.