More, and bigger
Organs can grow by getting more cells, or larger ones
An organ can grow in two ways. Adding more cells through mitosis makes an organ bigger, but so does increasing the size of individual cells (called ‘hypertrophy’). Humans use both methods. The embryonic heart increases in size by adding extra cells, but after birth, our hearts grow through hypertrophy.
By exercising, we stimulate this process, and the hearts of athletes can grow much larger still. This so-called physiological hypertrophy is normal; when you stop training, your heart will adapt and slowly reduce again. In heart failure, which can result from many things (including infection, poor diet and high blood pressure), there is an increased load on the heart that stimulates hypertrophy. In this case, despite the increase in size, heart performance actually worsens as the thickened muscle can become stiff or obstruct blood flow.