Oxygen enters our body as a gas with two oxygen atoms attached to each other to make one molecule of oxygen. It exits the body as either two oxygen attached to a carbon atom as carbon dioxide or as one oxygen atom bonded to two hydrogen atoms as water. The processes in between are complex and result in the execution of the metabolic functions of the body.
The process starts in the lungs.
As air moves into the lungs, small sacs called aveoli are inflated. Capillaries surround each sac and force the red blood cells to flow nearly single file while each blood cell drops off its load of carbon dioxide and picks up oxygen. From the lungs, the oxygen is transported by arterial blood to every cell in the body. In the tissues of the body, the blood cells again pass through another series of capillaries where they drop off oxygen and pick up carbon dioxide for their return trip to the lungs.
Cellular processes require oxygen to burn the calories from food.
The general process that cells use to turn food into energy is called respiration. Respiration is the opposite of photosynthesis. What plants do in photosynthesis to convert energy into sugar, respiration does in reverse to change sugar into energy. Within the cell, this operation is driven by a circular sequence called the citrus acid cycle or the Krebs cycle. This process stores energy in a molecule called adenosine triphosphate (ATP) when it is synthesized from adenosine diphosphate (ADP). The operation ultimately uses oxygen and creates carbon dioxide and water.
Cells use the ATP anytime that energy is required.
When ATP is converted back to ADP, energy is released and the ADP is recycled along with the extra phosphate back into the system to be remade into ATP. It is the presence of oxygen that allows the body to use this food to energy conversion process. This happens essentially the same way in nerves, muscles, the heart, and all body tissues. When deprived of oxygen, cellular respiration eventually comes to a halt, and the cell will die.
Oxygen is a component in many of the molecules in the human body.
Amino acids which are the building blocks of proteins have oxygen in them. Consequently, proteins also have oxygen as part of their molecular structure. The same is true of carbohydrates and fats. Since these molecules comprise most of the tissue in the body, oxygen is a vital component to materials that comprise the body itself.