How can the human liver regain its original mass after part of it is removed?

When part of the human liver is removed, it does not regenerate the missing lobes themselves. Instead, the organ restores its original mass and function through a process known as compensatory hypertrophy.1

In this process, the residual liver tissue enlarges to compensate for the loss. The remaining portions of the organ grow as large as necessary to restore the liver's total mass to a level equivalent to the original organ. This ability to adjust its size in response to a reduction in mass is a characteristic shared by several other mammalian organs, including the kidneys, pancreas, thyroid, adrenal glands, gonads, and lungs.1