Vascular resistance refers to the opposition that blood vessels offer to the flow of blood. It is primarily determined by the diameter of the blood vessels; narrower vessels create more resistance, while wider vessels allow for easier blood flow. This resistance is crucial for regulating blood pressure and ensuring that blood reaches all parts of the body efficiently. Factors such as blood viscosity, vessel length, and the overall health of the vascular system can influence vascular resistance. When resistance increases, the heart must work harder to pump blood, which can lead to conditions like hypertension if not managed properly.