The Earth's atmosphere is composed of 79% nitrogen, 20.59% oxygen and the rest consists of rare gases. Oxygen is a key part of human respiration. Oxygen binds with the four available sites on the hemoglobin molecule. Hemoglobin, which is part of human blood carries the oxygen to the cells. In the cells, the oxygen is metabolized and carbon dioxide is the product, along with water of that metabolism.
Oxygen is critical to the brain and oxygen deprivation can cause brain damage.