It is a cell. A cell is a mystical being. Each cell has its own unique function and shape. Cells with the same function and shape are gathered to form tissues, such as the brain, heart, lung, liver, and muscle, and again, various kinds of tissues are gathered and form organs to make up our body. The number of cells that make up our body is 60 ~ 100 trillion. Cells are very small substances that can be seen by an ultra-high-performance microscope, and their sizes, shapes, and functions are various; however, all cells have all the functions in them commonly like a big city. A nucleus that functions as a central control station and directs, adjusts, and controls everything, a ribosome that functions as a production plant and synthesizes proteins, which can be called the source of life activity, a endoplasmic reticulum that functions as a storehouse storing proteins made from the ribosome, a Golgi body that functions as a transportation sending proteins stored in the endoplasmic
reticulum out of the cell, a cytoskeleton, which maintains a shape of cell, a cell membrane that protects cells, and a mitochondria functions as a power plant producing an energy needed for the cells to survive are organized in the cell. In addition, cells function in an orderly manner through more than 500 physical and biochemical reactions and maintain our lives. Therefore, in order to stay healthy and young, 100 trillion cells should be maintained.
In our body, millions of cells are dying every second. Fortunately, substituting for dying cells, healthy cells divide into millions of new cells and maintain the number; however, cells are not newly born infinitely. The number of times being newly born is fixed, and over time, the rate a new cell is born slows down. Moreover, like a neuronal cell, some cells cannot be born again by other cells once they are dead. Cells in our body have a fixed number of divisions. As we get older, the division rate slows down, and some cells do not divide but only shrink like neurons. This is an aging and a lifespan program designed in genes. Cells making up our body can be summed up by saying, "Engaging in all of our work from the moment we were born till we die". Ultimately, the cell is our life. The reason we can maintain our life is because cells receive necessary substances (nutrients, oxygen) from extracellular nuclei, release unnecessary substances out of cells, and maintain the environment in cells balanced.
The maintenance of environmental conditions within the cell like this is called homeostasis. A state that homeostasis is maintained is a healthy state. The most surprising thing about the human body is that it adapts. Cells adapt in response to stimulation and become stronger by adaption. If cells are not stimulated, adaptive capacities are degenerated, and life activities are attenuated. Atrophy of life activity means aging, and aging accompanies a disease inevitably. In order to be healthy, the cells should be healthy.