MCQ Detailed View

The human body is mainly composed of a few essential elements, and their relative abundance shows how important they are for sustaining life. The most abundant element in the human body is oxygen, which accounts for about 65% of body mass. This is largely because water, which makes up around two-thirds of the body, contains oxygen. Oxygen is also critical in respiration, where it helps release energy from food molecules.

The second most abundant element is carbon, making up about 18% of the body. Carbon forms the backbone of all organic molecules including proteins, carbohydrates, lipids, and nucleic acids like DNA and RNA. Its unique ability to form stable bonds with many other atoms allows the construction of complex structures required for life.

The third most abundant element is hydrogen, contributing around 10% of the human body’s mass. Hydrogen is a key part of water and is present in almost every organic compound. It plays a central role in metabolism, energy transfer, and maintaining the body’s acid–base balance. Despite its small atomic size, hydrogen is vital for the chemistry of life.

Calcium, although important, does not come in the top three by percentage. Calcium makes up only about 1.5–2% of body mass, mainly in bones and teeth. It is, however, essential for functions like muscle contraction, nerve signaling, and blood clotting.

To summarize the order of abundance in the human body:

1. 
   

   Oxygen (~65%)

   
   


2. 
   

   Carbon (~18%)

   
   


3. 
   

   Hydrogen (~10%)

   
   

Thus, the correct answer to this question is Hydrogen (10%), as it ranks third in overall body composition. This highlights how basic elements combine to form the complex structures and processes that keep the human body alive.