The human body has several systems that perform specific essential jobs to keep the body alive and healthy. The respiratory system is one of these, performing the essential function of taking in fresh air and expelling waste gases. The oxygen in the fresh air is the component that the body needs to take in, and it is carbon dioxide that the body has to expel. Once inside the body, oxygen forms an essential part of cell activity, the waste product of which is carbon dioxide.
A human body has two entrances for fresh air into the body. These are the nose and mouth, and they also act as the exit points for waste gas. The nose and mouth join at the windpipe, which is a long tube that runs down to two lungs. The lungs are the areas that the body uses to absorb oxygen from fresh air, and expel carbon dioxide waste from the body.
Humans need to make incoming air warm and wet, to avoid damage to the lungs. The nose and mouth of the respiratory system add warmth to the air, as does the journey down the windpipe. Inside the lungs, the oxygen in the air moves through tiny sacs into the bloodstream.
Carbon dioxide moves the other way, from the bloodstream to the lungs. To and from the lungs, the body uses the circulatory system to transport these two molecules. Fresh, oxygenated air moves through arteries, pumped by the heart, and around the body to the cells that need a supply of oxygen. The veins carry the carbon dioxide the opposite way, back to the lungs.
Oxygen is an important substance, without which people cannot live. Humans, and other animals, need to have oxygen for an essential process inside the individual cells of the body. This process is the conversion of food to stored energy. Each cell needs to be able to release energy from storage in order to perform other jobs inside the cell, and without a supply of stored energy, the cells die off.
Adenosine triphosphate (ATP) is a molecule that cells use to store energy in. The digestive system breaks down food into simpler molecules, such as glucose. Each cell then grabs glucose molecules, breaks the bonds between the atoms that form the glucose molecule, and makes them into a different molecule, which is ATP. To do this, the cell needs to use the energy and atoms inside oxygen molecules.
In science, the chemical reaction for this is; 6O2 + C6H12O6 = 6CO2 + 6H2O + 36ATP. This represents the cell collecting six individual oxygen molecules (O2) and one glucose molecule (C6H12O6.) The cell then jumbles the individual atoms in these molecules up to make six carbon dioxide (CO2) molecules, six water molecules (H2O,) and 36 ATP molecules.
This chemical reaction uses up oxygen and produces carbon dioxide. As the body does not require carbon dioxide for any purpose, and in fact needs to move it out of the cells for health reasons, the body expels it through the respiratory system. Where the oxygen molecules came into the body, the carbon dioxide molecules move back out. The respiratory system therefore provides the cells with a source of essential oxygen, and carries the waste carbon dioxide away from the cells once they have used up oxygen.