The Human Body: Systems Working Together

What Is a System?

A system is a collection of parts that interact together for a common purpose. But a system is not just any old collection of parts. The parts are related in such a way that each depends on the others to do whatever job there is to be done. No single part can do the job alone, and any malfunction or delay is likely to affect the whole system.

A body system is a set of body parts that do a particular task. The human body itself is an example of a complex system—many sets of interacting parts that work to keep the human machine running. On any single day, we can estimate that your heart beats 103,689 times, your blood travels 168,000,000 miles, your digestive system processes 7.8 pounds of waste, and your lungs take in 438 cubic feet of air. These are only a few of the multitude of functions the human body performs. And while the least little mishap could cause a glitch in the system, amazingly, day in and day out over most of our lifetime, our bodies operate almost flawlessly.

Health and Wellness

The basic living unit of the body is the cell. There are about 200 different kinds of cells in our bodies, and about 5 trillion cells overall.

How One Cell Makes Five Trillion

Each human organism begins as a single cell—the fertilized egg, no bigger than the period at the end of this sentence. It becomes two cells, then four, then eight, and so on. As cells grow and multiply, they take on special roles that fit into four broad categories. For example, some cells develop force and movement. These are muscle cells, able to adapt flexibly to stress. Cells called neurons

Genes, which are inherited from parents, are made of DNA. Short for deoxyribonucleic (de-OK-se-ri-bo-noo-KLE-ik) acid, DNA is the primary material of the cell nucleus. It usually takes the shape of a double helix (a pair of spirals). Imagine it as a flexible ladder that has been twisted into a spiral. The two long edges of the ladder are made of chemical molecules called nucleotides. Each nucleotide includes a chemical called a base. There are four bases that occur in pairs, called base pairs. These are adenine and thymine (A-T) and cytosine and guanine (C-G). The base pairs form the rungs in the ladder, held together by chemical links called hydrogen bonds. If any of the base pairs are missing, out of place or repeated, the defective gene may cause a health problem. Or the defect may have such a minor effect that the person never notices it.