What causes blood to "spurts" out when an artery is cut?

The reason blood spurts out when an artery is cut is due to the pumping action of the heart. Arteries are high-pressure blood vessels that carry oxygen-rich blood from the heart to various parts of the body. When an artery is severed, the pressure generated by the heart's contractions forces the blood to escape rapidly from the cut, often resulting in a spurt of blood. This is in contrast to veins, which operate under lower pressure and would result in a steady flow if cut, rather than a spurt. The rhythmic contractions of the heart create a strong pressure gradient in the arteries, leading to the characteristic spurting effect when an arterial injury occurs.

Factors such as gravity might influence the direction in which blood flows, while blood viscosity refers to the thickness of the blood, neither of which fundamentally contribute to the spurt associated with arterial bleeding. The pressure from the lungs primarily affects the venous return and is not a direct cause of the high-pressure ejection of blood from arteries. Thus, the pumping action of the heart is the correct explanation for this phenomenon.