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Human biochemistry refers to the chemical processes and substances that occur within living human cells. It encompasses a vast array of chemical reactions that are essential for maintaining life, including the metabolism of macronutrients, the synthesis of biomolecules, and the regulation of pH and fluid balance.

Understanding human biochemistry is crucial for grasping how the body uses elements to perform its functions. Elements like carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and phosphorus (P) are so fundamental that they are considered the building blocks of life. They form biomolecules such as DNA, proteins, and fats, all of which are needed for the body's structure and processes.

Elemental abundance in the human body is typically measured in terms of mass percentage or molar concentration. The five most common elements found in the human body by mass are oxygen (O), carbon (C), hydrogen (H), nitrogen (N), and calcium (Ca). Oxygen is the most abundant, making up about 65% of the body's mass, primarily through its presence in water and organic compounds.

Oxygen is followed by carbon, which constitutes about 18%, and hydrogen, accounting for roughly 10%. Together, these three elements make up 93% of the body's mass. Nitrogen, at 3%, is a key component of amino acids and nucleic acids, while calcium, which constitutes about 1.5%, is essential for bone and teeth structure as well as many physiological processes.

The biochemical composition of the human body is complex, characterized by a wide variety of molecules made from a relatively small set of elements. Proteins, lipids (fats), carbohydrates, and nucleic acids form large structures from these elements, performing a myriad of functions. Proteins, for instance, are constructed primarily from carbon, hydrogen, oxygen, and nitrogen and are involved in virtually every physiological process, from catalyzing metabolic reactions to signal transduction.

Lipids, composed mainly of carbon and hydrogen, provide energy storage and contribute to cellular structure. Carbohydrates, formed by carbon, hydrogen, and oxygen, serve as energy sources and structural components. Lastly, nucleic acids, which contain phosphorus in addition to other common elements, carry genetic information. This collective variety and functionality exemplify the remarkable efficiency and diversity of the biological elements in the human body.