What does blood pressure measure?

Blood pressure measures the force or pressure that blood exerts against the walls of the blood vessels, primarily the arteries, as the heart pumps it throughout the body. This measurement is essential in assessing cardiovascular health, as it reflects how hard the heart needs to work to circulate blood and can indicate conditions such as hypertension or heart disease. When the heart beats, it creates a peak pressure called systolic pressure, and when it relaxes between beats, the pressure drops to a lower level known as diastolic pressure.

The other options pertain to different physiological parameters. Oxygen levels in the blood relate to respiratory function and how well oxygen is transported throughout the body. The rate of blood flow in veins pertains to venous return and circulation but does not directly measure pressure. Viscosity refers to the thickness or stickiness of the blood, which can affect circulation but is not a measurement of blood pressure itself. By focusing on the force exerted by the heart, the correct choice provides a clear and direct understanding of what blood pressure indicates in the context of cardiovascular functioning.