Cellular respiration harvests electrons from organic compounds like glucose to produce ATP, which cells use as energy. It requires a continuous oxygen supply. The air we breathe is 21% oxygen, which enters our bodies through inhalation and transfers into blood to reach cells. Low oxygen levels cause hypoxia and anaerobic respiration, generating lactic acid waste. Breathing is vital, as stopping it lowers pH and oxygen while raising carbon dioxide levels, potentially leading to cell death.