Functions of Human Life


man running on side of road
Photo by Pixabay on


The human body is made up of trillions of cells that are arranged in a way that maintains distinct internal compartments. These compartments keep body cells away from dangers in the outside world and keep them moist and nourished. They also keep body fluids inside the body separate from the millions of microorganisms that grow on body surfaces, such as the lining of some passageways that lead to the outside of the body. For example, there are more bacterial cells in the intestinal tract than there are human cells in the whole body. However, these bacteria are outside the body and cannot move freely inside the body.

Cells have a cell membrane (also called the plasma membrane) that separates the fluids and organelles inside the cell from the fluids and organelles outside the cell. Blood vessels keep the circulatory system closed, and nerves and muscles are separated from structures around them by sheaths of connective tissue. In the chest and abdomen, major organs like the lungs, heart, and kidneys are kept separate by different membranes.

The skin, hair, and nails are all part of the integumentary system, which is the largest organ system in the body. The top layer of skin is a barrier that keeps harmful microorganisms and other toxins out of the body’s organs and fluids.


The first law of thermodynamics says that energy can neither be created nor destroyed; it can only change form. As an organism, your main job is to take in (ingest) the energy and molecules in the food you eat, turn some of that energy into fuel for movement, keep your body functions going, and build and maintain your body structures. This is done through two types of reactions: anabolism and catabolism.

Anabolism is the process of combining smaller, simpler molecules to make bigger, more complicated substances. Using energy, your body can put together the complex chemicals it needs by putting together small molecules from the food you eat.

Catabolism is the process of breaking down bigger, more complicated molecules into smaller, simpler ones. This process gives off energy. The body breaks down the complex molecules in food so it can use the parts to build the structures and substances it needs to live.

When these two things happen together, they are called metabolism. Metabolism is the sum of all of the body’s anabolic and catabolic reactions. Anabolism and catabolism happen at the same time and all the time to keep you alive.

A chemical called adenosine triphosphate (ATP) is used by every cell in your body to store and use energy. The cell stores energy by making ATP, which is called anabolism. The cell then moves the ATP molecules to the place in the cell where energy is needed. The ATP is then broken down (catabolism), which releases a controlled amount of energy that the cell uses to do a certain job.


The ability of an organism to adapt to changes in its internal and external environments is called responsiveness. One way to respond to outside stimuli is to move toward sources of food and water and away from things that seem dangerous. Changes in an organism’s internal environment, like a rise in body temperature, can induce sweating and open up blood vessels in the skin to cool down.


Movement in a person is not just what happens at the joints, but also what happens in each organ and even in each cell. As you read this, your red and white blood cells are moving around your body, your muscle cells are contracting and relaxing to keep your posture straight and your eyes focused, and glands are releasing chemicals to control how your body works. Your body coordinates the actions of groups of muscles so that you can move air in and out of your lungs, move blood around your body, and move food through your digestive system. Consciously, you contract your skeletal muscles to move the bones of your skeleton and do all the other things you need to do in your daily life.

Development, growth and reproduction

Changes that happen to the body over time are called development. In the process of development, unspecialized cells change their shape and function to do specific jobs in the body. This is called differentiation. Cells also change during the processes of growth and repair, which are both part of development.

Growth is the process of getting bigger. Humans, like all multicellular organisms, grow by adding more cells, putting more non-cellular material around the cells (like mineral deposits in bone), and, within very narrow limits, making the cells bigger.

Reproduction is the process by which one organism gives rise to another. Both the male and female reproductive systems help people have children. Because all complex organisms eventually die, the line of organisms would end if they didn’t reproduce.


Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Cruz, D. H., … DeSaix, P. (2013). Anatomy and Physiology. Houston, Texas: OpenStax. Retrieved from