Valves ensure blood flows toward your heart. Vein valves work to bring the blood that flowed down via arteries back up to your heart.
C. Genetic variation
Bacteria are very numerous, and random mutation of bacterial DNA generates a wide variety of change, including resistance. It happens through mutation and selection. Not only are they capable of sharing genetic bits of DNA to related and unrealted species they can develop ways to remove the antibiotic before it reaches its intended area via biochemical pumps or enzymes to inactiavte the antibiotic.
A group of organ systems working together is an organism.
Whildlife centers are made to preserve the lion population.
