Many Amino Acids Are Removed by the Liver on "First Pass," Never Reaching the Systemic Circulation

The hepatic-portal circulation is arranged in such a way that all nutrients leaving the gut via the blood pass through the liver before entering the systemic circulation (see Figure 30-23).

This arrangement places the liver in a “sentinel” position, from which it can modify the nutrient composition of portal blood before the blood is distributed to other tissues. The function of the liver in modifying portal blood composition is well illustrated in the case of amino acid absorption. Many of the amino acids absorbed into portal blood are removed as the blood passes the liver, so they never reach the general circulation. Figure 32-7 illustrates that in the dog, only about 23% of the amino acids reaching the liver during the absorp­tive period pass into the general circulation; the liver thus helps keep blood amino acid concentrations stable during periods of amino acid absorption. The blood amino acid con­centration, as with the blood glucose concentration, is usually kept relatively constant.

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