Glutamine
In human blood, glutamine is the most abundant free amino acid.
Arginine
Arginine is a conditionally essential amino acid in humans and rodents, as it may be required depending on the health status or lifecycle of the individual. For example, while healthy adults can supply their own requirement for arginine, immature and rapidly growing individuals require arginine in their diet, and it is also essential under physiological stress, for example during recovery from burns, injury, and sepsis, or when the small intestine and kidneys, which are the major sites of arginine biosynthesis, have been damaged. It is, however, an essential amino acid for birds, as they do not have a urea cycle. For some carnivores, for example cats, dogs and ferrets, arginine is essential, because after a meal, their highly efficient protein catabolism produces large quantities of ammonia which need to be processed through the urea cycle, and if not enough arginine is present, the resulting ammonia toxicity can be lethal. This is not a problem in practice, because meat contains sufficient arginine to avoid this situation.