circulatory system

The circulatory system is the body's whole delivery-and-collection service: a closed loop of vessels through which blood travels, carrying supplies out and waste back. Like a city's road network with a central distribution hub, nothing in the body stays fresh for long without it.

Blood does not flow in a straight line but in a continuous circuit. The heart pushes oxygen-rich blood out through arteries, which branch into ever-smaller arterioles and finally capillaries, where oxygen and nutrients pass into the tissues. The now oxygen-poor blood collects into veins and returns to the heart, which sends it to the lungs to reload with oxygen before the cycle repeats.

In humans the system is sometimes described more broadly to include the lymphatic vessels, which return fluid that leaks out of capillaries back into the bloodstream. The core cardiovascular circuit — heart, arteries, veins, capillaries — is what cardiology is mainly concerned with.