Question

How can an adult lose billions of cells from all parts of the body each day and still survive?

Answer 1

Human body is made up of trillions of cells out of which around 1 billion cells are lost each day. Our body has a tendency of replacing old cells with new ones. This process is called apoptosis which is a normal growth and development process.

Complete answer:
Apoptosis is the programmed cell death which takes place in multicellular organisms. Once the cell becomes mature, it changes its maturity and dies. This cell death is very essential for growth, development and repair of our body.

The cells die by killing themselves. Under the stress condition, the cell generates signals for the apoptosis which leads to cell death. The mitochondria are the main respiratory and power house of the cell. When the apoptosis signals are generated, the mitochondria become non-functional due to the enzymatic and protein activity. This leads to the respiratory arrest of the cell and ultimately the cell dies.

Once the cell dies, the removal of the dead cells is done by the phagocytes. The lysosomes also help in digesting the dead cells and cell debris.
In the human body the skin and the hairs are most sensitive cells and are prone to apoptosis. The epithelial cells present on the upper surface are constantly replaced by the cells present in the lower layer.

Note: The new cells are generated by the process of mitosis. The mitosis is activated at the areas which require growth, repair. This helps in regulating the normal functioning of the body. The damaged cells and the epithelial cells undergo mitosis rapidly.