Blood Pressure, Flow, and Resistance

Arterial blood pressure rises and falls in a pattern corresponding to the phases of the cycles of the heart, the cardiac cycle. That is, when the ventricles contract (ventricular systole), their walls squeeze the blood inside their chambers and force it into the pulmonary artery and aorta. As a result, the pressures in these arteries rise sharply. The maximum pressure achieved during such a ventricular contraction is the systolic pressure. When the ventricles relax (ventricular diastole), they begin to fill with blood again to prepare for the next contraction and the arterial pressure drops. The lowest pressure that remains in the arteries before the next ventricular contraction is termed the diastolic pressure.

Arterial blood pressure is usually only measured indirectly in healthy people. The method used to measure blood pressure involves placing a pressurized cuff around the arm to detect the force of blood pulsing through the arm's blood vessels. Blood pressure is normally expressed in units of millimeters of mercury (mm Hg). A blood pressure of 100 mm Hg means that the force exerted by the blood is sufficient to push a column of mercury up to a height of 100 mm (Figure 9). An actual column of mercury is rarely used; instead, an analog pressure scale is used that reflects the pressure measurement in similar units of mm Hg.A

When you have your blood pressure measured at the doctor's office, you will normally be told that your blood pressure is something like 120/80 mm Hg. Although the doctor calls it your blood pressure, it is actually your arterial pressure. It represents how hard your blood is being forced out by your left ventricle (systolic pressure at 120 mm Hg) and how the ventricles are preparing for the next contraction (diastolic pressure at 80 mm Hg).

Figure 9. The original measurement scale used to measure blood pressure was the column of mercury. A less cumbersome analog scale is used today.

The radial pulse rate is normally equal to the rate at which the left ventricle is contracting, which is why it is used to determine heart rate (how fast your heart is beating) quickly and easily. A pulse also can reveal something about blood pressure, because an elevated pressure produces a pulse that feels full, while a low pressure is accompanied by a pulse that is easily compressed.

Flow through a blood vessel is determined by two factors: (1 ) the force that pushes the blood through the vessel, and (2) the resistance of the vessel to the blood flow. Ordinarily, the rate of blood flow is measured in milliliters or liters per minute (ml/min or l/min). The blood flow in the entire human circulation is about 5000 ml/min at rest in an average sized adult, but may be 5-6 times as great during heavy exercise when the body needs more oxygen to fuel that exercise. The amount of blood pumped by the heart in one minute is called the cardiac output.

It is important to note that the flow of blood in the body is directly influenced by gravity. When a person is standing, gravity helps pull the blood downward to the lower extremities. Without gravity, blood tends to remain closer to the heart. The force of gravity also makes it more difficult for the blood to flow upward to return to the heart and lungs for more oxygen. Our bodies have evolved to deal with the ever-present downward force of gravity; our leg muscles function as secondary pumps to help in the process of venous return which is blood flow back to the heart, also referred to as cardiac input). During walking or other leg movements, the muscles contract, forcing blood up through the veins of the calf toward the heart. The valves in the veins are arranged so that blood flows only in one direction (Figure 10). This mechanism effectively counteracts the force of gravity.

Figure 10. Contraction of skeletal muscles in the legs helps to pump blood toward the heart, but is prevented from pushing blood away from the heart by closure of the venous valves.