Animals grow by ingesting a variety of nutrients, including carbohydrates, lipids, sugars, fats and proteins, then digesting them into their chemical components. These components are distributed throughout the body and used to build new cells. Hormonal and environmental factors can trigger animal growth, which usually ceases once the animal has reached adult size. This is a different process from plants, which grow continuously over their lifetime by enlarging existing cells.
For most vertebrate animals, such as humans, the pituitary gland is the command center for growth. The pituitary gland releases hormones to signal growth in different parts of the body. In humans, the primary excretions are testosterone, estrogen, pituitary gonadotropic hormones and pituitary growth hormones. Abnormalities in pituitary function can cause overgrowth or undergrowth in animals, such as dwarfism or gigantism.
As new cells are created, parts of the animal's body expand or change shape to make room for them, causing growth. The femur, or thigh bone, provides a good example of visible growth for many animals. As the cells receive the hormonal signal to expand, the cells divide and create new rows of bone cells, extending the leg. Once the animal has reached maturity, the pituitary gland typically reduces hormonal output.