According to the sources, erythrocytes (red blood cells) contain a red-coloured, iron-containing complex protein known as haemoglobin . A normal healthy human adult typically maintains a haemoglobin content of 12-16 g per 100 ml of blood . These molecules are critical for the transport of respiratory gases throughout the body . Mastering these standard physiological values is a high-priority strategy for medical aspirants, as approximately 95% of NEET Biology questions are derived directly from the NCERT textbooks .