Human biology is an academic field which focuses upon the study of human biology; it is closely related to medicine, primate biology, and a number of other fields.

A human being is a multicellular eukaryote made up of an estimated 100,000 billion cells. As a species, humans are primates and can be distinguished from other primates by their more highly evolved brains. Even though humans are multicellular animals, many of the basic life processes of human cells are basically the same as in simple unicelluar eukaryotes such as yeast and even prokaryotes.

One of the major biological differences betweens humans and yeast is that yeast are unicellular organisms — when a yeast cell divides, it produces a new and independent organism. A human being is initially just one cell, a zygote, but when that cell divides, it forms an embryo. The early embryo is made up of totipotent stem cells, which go on to differentiate. Differentiation is the process by which an unspecialized cell becomes specialized into one of the many cells that make up the body, such as a heart, liver, or muscle cells. During differentiation, certain genes are turned on, or become activated, while other genes are switched off, or inactivated. This process is intricately regulated. As a result, a differentiated cell will develop specific structures and perform certain functions. Differentiation can involve changes in numerous aspects of cell physiology; size, shape, polarity, metabolic activity, responsiveness to signals, and gene expression profiles can all change during differentiation.

When developed, the human body is made up of many different types of cells — see list of distinct cell types in the adult human body. These cells make up tissues, groups of cells that perform a similar function. For example, muscle cells make up muscle tissue. Similarly, groups of tissues which perform some function make up an organ, and groups of related organs makes up an organ system. The human body depends on all its organ systems in order to survive and reproduce.

Major organ systems of the human body:

• Cardiovascular system: blood circulations with heart and blood vessels
• Digestive system: processing food with mouth, stomach and intestines
• Endocrine system: communicating within the body using hormones
• Excretory system: eliminating wastes from the body
• Immune system: defending against disease-causing agents
• Integumentary system: skin, hair and nails
• Muscular system: moving the body
• Nervous system: collecting, transferring and processing information with brain and nerves
• Reproductive system: the sex organs
• Respiratory system: the organs used for breathing, the lungs
• Skeletal system: structural support and protection through bones

The study of development from fertilized cell to fully developed body is studied in developmental biology, the structure of the developed human body is studied in human anatomy and its function in human physiology. The human body can develop problems, and these are studied in medicine.