: Red blood cells: Red blood cells are small , double-sided disc-like. They are nucleusless. Particles look red because of the presence of a pigment called hemoglobin. About 2 million particles are produced in the body every second. The average lifespan of a red blood cell is 4 months. White blood cells: White blood cells have a specific size and nucleus. In a healthy human body, there are 6,000 to 11,000 white blood cells per cubic milliliter of blood. Nucleus: Nucleus is the smallest blood cell. They are round , oval or circular and in clusters. It does not have a nucleus. Their average lifespan is 5 to 10 days.