Define blood pressure.

• A. The force that circulating blood pushes against blood vessel walls.
• B. The pressure within the heart chambers.
• C. The force of blood viscosity.
• D. The rate of blood flow.

Answer: (A)

Blood pressure is the force that circulating blood pushes against the walls of blood vessels, especially arteries, as the heart pumps. It reflects how hard the heart is working (cardiac output) and how much resistance the vessels offer (vascular resistance). We measure it in millimeters of mercury (mmHg) and typically report systolic pressure during heart contraction and diastolic pressure during relaxation. Pressure inside the heart chambers is intracardiac pressure and not the systemic blood pressure described here. Blood viscosity relates to how thick the blood is and affects resistance, not the direct pressure itself. The rate of blood flow is about how much blood moves per unit time, which depends on pressure and resistance but is not the pressure.