Like all living organisms, our bodies are made up of cells – humans are built from trillions of them – and over time, many of them become old and worn and need to be replaced. To keep up with this constant wear and tear, our bodies produce millions of new cells every single second. The speed at which this process happens varies massively depending on the cell type, however, and can be studied using techniques which ‘mark’ each cell’s DNA. When a cell divides, each of the two daughter cells receives half of the marked DNA, allowing researchers to track how often each cell type is replaced. This process ranges from occurring on a weekly basis in some parts of the body, to happening just a handful of times during an entire lifetime in others.
TRACHEA – 1-2 MONTHS: The cells that line the windpipe are replaced less frequently than the cells inside the lungs, around once a month.
WHITE BLOOD CELLS – 1-5 DAYS: Neutrophils are the immune system’s front line soldiers. Stem cells in the bone marrow ensure that they are replaced every few days.
RED BLOOD CELLS – 120 DAYS: After a few months, these cells become old and stiff. They are removed by the spleen and replaced by the bone marrow.
FAT – 8 YEARS: The number of fat cells in the body does not change much, even when we gain or lose weight.
SMALL INTESTINE – 2-4 DAYS: The lining of the small intestine gets a lot of wear, and the cells are constantly being replaced.
BRAIN – THE SAME AGE AS YOU: Many researchers believe that brain cells are never replaced in adults, but there is some animal evidence that suggests otherwise.
LUNGS – 8 DAYS: The cells that line the inside of the lungs are replaced roughly once a week.
HEART – 10-20 YEARS: The muscle cells inside the heart are replaced every decade or so by specialist stem cells known as satellite cells.
LIVER – 6-12 MONTHS: The liver has an incredible capacity for regeneration, and its cells are replaced once or twice a year.
LARGE INTESTINE – 3-4 DAYS: The cells of the large intestine are constantly being shed into the gut, and are swiftly replaced by cells that move upwards from underneath.