‘The vital systems’ focuses on two of the body’s main systems—the cardiovascular system and the respiratory system. The heart is the most important muscle in the body and its function is essential for life. It works as a pulsatile, four-chamber pump that moves the blood around the body. The lungs are a key part of the respiratory system that provides oxygen to the body. The lymphatic system plays an important role in circulation by draining fluids that accumulate in the extracellular space during capillary exchange and returning them to the blood. It also helps in defence against bacteria and cancerous tumours as well as in transporting fats.