The movement of blood in a human body, commonly referred to as blood flow, is determined by the volume of blood that traverses a certain section of the bodily system per unit time. It is the rhythmic contraction of the heart's ventricles that primarily instigates this movement. As the ventricles contract, blood is forced into the prominent arteries, which then flow from areas of greater pressure to lower pressure areas. This movement continues into smaller arteries and arterioles and subsequently into capillaries. The venous system is the final destination where the blood is collected through venules and veins. This section elaborates on the critical variables that have pronounced effects on blood flow and the elements that may impede this flow, a phenomenon termed resistance.
Hydrostatic pressure is the force produced by a fluid due to the pull of gravity. This force is usually exerted against the container's wall. A noteworthy example of hydrostatic pressure is blood pressure, which is defined as the force that the blood exerts on the walls of the blood vessels or the heart's chambers. Blood pressure can be measured in capillaries, veins, and the vessels of the pulmonary circulation. However, without specific descriptors, blood pressure generally refers to the force of blood in the arteries of the systemic circulation. In clinical settings, this pressure is quantified in millimeters of mercury (mm Hg) and is commonly measured using the brachial artery of the arm.
The arterial blood pressure in major vessels comprises several distinctive components: systolic and diastolic pressures, pulse pressure, and mean arterial pressure.
Systemic arterial blood pressure is denoted as a ratio of two numbers; for example, a normal adult's blood pressure is 110/70 or 120/80. This ratio represents systolic pressure over diastolic pressure. The systolic pressure is the higher value (around 120 mm Hg), reflecting the arterial pressure resulting from the ejection of blood during the ventricular contraction or systole. The diastolic pressure is the lower value (approximately 80 mm Hg), symbolizing the arterial pressure of blood during ventricular relaxation or diastole.
The numerical difference in the systolic and the diastolic pressures is the pulse pressure. For example, a person with a systolic pressure of 120 mm Hg and a diastolic pressure of 80 mm Hg would have a pulse pressure of 40 mmHg. Typically, a pulse pressure should be at least 25 percent of the systolic pressure. This may go down in conditions such as congestive heart failure, aortic valve stenosis, or significant blood loss following trauma. Conversely, high or wide pulse pressure is typical in healthy individuals following heavy workouts when their resting pulse pressure is 30–40 mm Hg, which may temporarily increase to 100 mm Hg due to an increase in stroke volume. However, a consistently higher pulse pressure, such as at or above 100 mm Hg, could mean abnormally excessive resistance in the arteries and can be a symptom of various disorders. Chronic high resting pulse pressures can damage the heart, brain, and kidneys and require medical intervention.
Mean arterial pressure (MAP) is a measure of the average pressure in a person's arteries during one cardiac cycle. It is approximately one-third of the way between systolic and diastolic pressures. MAP propels blood to cells and tissues and is considered a better indicator of perfusion to vital organs than systolic blood pressure alone. MAP is important because it helps assess the flow, resistance, and pressure within the arteries. A MAP of at least 60 mmHg is generally considered necessary to adequately perfuse and sustain the vital organs of an average healthy person.
