What is the largest artery in the human body?

The aorta, about the diameter of a garden hose where it leaves the heart. It arches over, runs down through the chest and abdomen, and everything the body receives passes through it - the entire cardiac output, roughly 7,500 litres a day, through one tube.

Its walls are built for that. They are thick with elastic fibres, so the aorta stretches with each beat and recoils between them, which smooths a pulsing output into something closer to steady flow by the time it reaches the small vessels - a function engineers would call a capacitor. That elasticity is lost with age and with high blood pressure, which is why arteries stiffen and why the systolic figure tends to rise over a lifetime.

The failure mode is what makes it feared. An aneurysm - a weakened bulge, usually in the abdomen - typically causes no symptoms at all until it tears, and a rupture is often fatal within minutes, because the volume passing through is so large. It is one of the reasons ultrasound screening for men over 65 exists in several countries: the condition is silent, common, and treatable if found before it announces itself.