Oxygen, carbon, hydrogen and nitrogen are the main elements found inside the human body. I find it amazing that 63% of the human body is composed of just one element: oxygen. Perhaps this should not be surprising, since a lot of this oxygen is tied up in water and water composes around 70% of the human body. Water itself is essential to keep the body hydrated, but it also plays an absolutely vital role in respiration. Oxygen is known as 'the final electron acceptor' in the process of respiration.
Respiration is the process which makes ATP and ATP (adeninetriosephosphate) is the substance which provides the human body with energy. ATP is essential for every single body process and without a supply of ATP, humans cannot survive. It is because of this that it is essential to have a supply of oxygen in the body. After a series of chemical reactions, all of which are a part of respiration, electrons are passed down what is known as an 'electron transport chain.' Oxygen is needed to accept these electrons at the end of the chain so that the enzyme ATPsynthase can join an inorganic phosphate to ADP to create ATP. Without oxygen, the electrons become backed up and ATP is not created.
Carbon is the second most prolific element found inside the human body. Carbon is excreted from the body in the form of carbon dioxide, which is a waste product of respiration. Carbon is also the main component of glucose and other sugars. Glucose is a respiratory substrate and is found in the blood. Energy stores of glycogen are also found in the liver and muscles. Carbon is ingested into the body when carbohydrates are eaten - carbohydrates include things such as bread, pasta and potatoes. The complex starches and sugars found in such food need to be broken down into simpler sugars before they can be absorbed into the body. If excess sugars are eaten, then they will be converted to fat and stored this way. Fats are made up of fatty acids and glycerol and they also contain carbon.
Hydrogen is found in the body not only as a part of water but in sugars and fats. Essentially, they are organic compounds, which means they have long carbon chains with hydrogen atoms attached. The formula for glucose is C6H12O6. Five of the hydrogen atoms in glucose are attached to the carbon atoms as part of a hydroxy (OH) group. Hydrogen is also found in amino acids; amino acids are the basic units of proteins and they also contain hydrogen.
A chain of amino acids first forms a polypeptide and then a protein. The nitrogen needed to make proteins is found in meats and is infact eaten as protein, but to be absorbed into the body the protein found in meat needs to be broken down. Proteins are used in the body for all kinds of things; perhaps most importantly enzymes are infact proteins. Enzymes facilitate all of the body's biochemical reactions - every reaction that occurs in respiration is facilitated by an enzyme.
Nitrogen is also found in DNA. The four bases which make up DNA all contain nitrogen. DNA contains all of the body's genetic information and it is essential in the transcription and translation processes of protein synthesis.
Aside from these four elements, there are other elements in the human body, although they are found in much smaller amounts. Calcium, sodium, phosphorus and magnesium are all found in the body and are known as 'trace elements.'