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Blood pressure is the pressure exerted on the walls of arteries by blood and the force is generated by the heart's pumping action. The mean arterial pressure (blood pressure) is defined by the cardiac output (CO), systemic vascular resistance (SVR) and central venous pressure (CVP) according to the following equation.
MAP = (CO x SVR) + CVP
The central venous pressure (right atrial pressure) is the blood pressure in the thoracic vena cava and its normal values are 2-8 mHg. Since its value is small, this is normally ignored in the calculation.
MAP(approx) = CO x SVR
Cardiac output is the volume of blood pumped from the heart per minute. It is the heart rate multiplied by stroke volume, the volume of blood pumped out of the heart at each heart beat.
System vascular resistance is the amount of resistance to the blood flow generated by the blood vessels and internal organs. The resistance would increase if there is blockage or narrowing of blood vessels. Loss of elasticity in blood vessels would also increase the resistance.
These three parameters, CO, SVR, and CVP, are all inter-related. Changing one would also affect the other two.
Under certain situations, such as danger, stress and exercising, the body requires additional oxygen and nutrient resulting in higher heart rate and blood pressure. This is the body's way of coping with external environment.
Heart rate. The blood pressure would increase proportionally as the heart beats faster assuming the stroke volume remains the same.
Blood/fluid volume. This is the amount of blood within the body. Dietary salt, kidney disease and other issues can cause the volume to increase resulting in higher blood pressure.
Resistance. The resistance is related to the friction generated by blood vessels to the flow of blood. The level of resistance is affected by some of the following factors.
Many medications for hypertension are designed to address some of the factors listed above.