What Influence Your Blood Pressure

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.

Physical factors

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.

  • Blood vessel radius: Larger radius would offer less resistance. Narrowing of arteries is one primary cause of high blood pressure.
  • Blood vessel elasticity: As heart contracts and pumps out blood, blood vessel expands to accommodate the additional flow; when heart relaxes, blood vessel relaxes and contracts also. This action would assist the blood circulation. If the blood vessel starts to lose its capability to expand and contract, the heart has to work harder to deliver blood to the body.
  • Blood vessel walls: Blood vessel walls should be smooth and have no bumps. Fatty deposits can build up on the arterial walls thereby increase the friction and reduce the vessel radius.
  • Blood viscosity: As blood gets thicker or more dense, circulating the blood throughout the circulatory system would become more difficult and higher pressure is required. Some medical conditions may affect the blood viscosity. For example, anemia would reduce blood viscosity; higher red blood cell count would increase viscosity. Blood thinner drugs (e.g. aspirin) would have the same effect.

Many medications for hypertension are designed to address some of the factors listed above.