Diagram of three blood vessels. Labels at the top of the diagram read "Amount of blood in vessel," "Diameter of blood vessel," and "Blood pressure." In each drawing, blood is represented as an arrow traveling through a tubelike vessel. The top drawing shows a normal amount of blood in a vessel of normal diameter, resulting in normal blood pressure. The middle drawing shows too much blood in a vessel of normal diameter, resulting in high blood pressure. The bottom drawing shows a normal amount of blood flowing through a narrow blood vessel, resulting in high blood pressure. To the right of each blood vessel is a gauge that looks like a thermometer. The gauge to the right of the top drawing is shaded only in the lower part, indicating normal blood pressure. The gauges to the right of the middle and bottom drawings are shaded nearly to the top, indicating high blood pressure.