One of the interesting discoveries about intestinal physiology was that the molar ratios of the different amino acids in the lumen of the intestine stay relatively constant. The absolute levels of amino acids goes up and down, but the relative composition stays the same. The gut constantly renews its lining, the cells at the bottom of the crypts are constantly dividing, the cells move up the sides of the villi and are sloughed off from the tip. So the gut renews its lining approximately every 4 days. There is also an exocrino-enteric circulation of amino acids in the gut.
Amino acids pass into the lumen of the intestine from the mucosa and are reabsorbed again.
So the amino acids in the mucosa and in the gut lumen are in equilibrium. The relatively constant molar ratios of amino acids in the lumen is important from the point of view of amino acid uptake mechanisms. Amino acids are transported into cells by a series of specific carriers. Each carrier transports a group of related amino acids, so there is a carrier for the acidic amino acids, another for the basic amino acids and another for the aromatic amino acids and so on. All the amino acids transported by any one carrier all act as competitive inhibitors for each other because they are all competing for the same transport site.