after digestion and absorption, an amino acid not used to build protein will first be subjected to

After digestion, an amino acid not absorbed by the body and not used to build proteins will first be subjected to removal from its amino group. If they are not used for protein synthesis, amino acids becomes part of the metabolism process. The body has the capability to create a subset of amino acids, also known as non-essential amino acids because we don’t have requirement for them in our diet. 

the tricuspid valve is present between right atrium and right ventricle