Summary: Dynamic Understanding of Blood Pressure
Mechanism: Systolic pressure arises from the movement of blood from the heart into the aorta, displacing existing blood and increasing intravascular pressure. Diastolic pressure is influenced by the elasticity of vessels and the resistance in peripheral vessels.
Blood Pressure Propagation: Blood flow continues during diastole due to vessel elasticity, which also determines the true minimum diastolic pressure. Variations in heart rate affect the diastolic minimum pressure.
★ Static Understanding of Blood Pressure
- Mean Blood Pressure
Mean blood pressure is considered to be determined by stroke volume and total peripheral resistance.- Cardiac Output
Cardiac output is controlled inversely according to the body’s oxygen demand, with stroke volume determined by dividing the circulating blood volume by heart rate. In a normal state, stroke volume is approximately 60ml-100ml per beat. - Total Peripheral Resistance
Total peripheral resistance is regulated by the degree of arteriolar constriction just before the transition of arteries to capillaries. If there is no resistance in the arterioles, the pressure in the arteries is directly transferred to the capillaries, causing an increase in capillary pressure similar to arterial pressure, which can lead to edema in tissues and organs as fluid moves from capillaries into the tissues. Thus, arteriolar resistance plays a protective role against edema in tissues. - Mean Blood Pressure
While mean blood pressure is accurately derived from the integral value of the arterial pressure waveform, it can also be roughly estimated as the product of stroke volume and total peripheral resistance. This represents a more static approach to understanding blood pressure.
- Cardiac Output
★ Dynamic Understanding of Blood Pressure
- Systolic and Diastolic Blood Pressure
Systolic blood pressure (generally refers to the peak pressure during systole) and diastolic blood pressure (generally refers to the lowest pressure during diastole). - Dynamic Understanding of Blood Pressure
As systole begins, blood is ejected from the heart into the aorta. The resistance encountered by this blood is due to the blood already present in the aorta. The blood ejected from the heart moves into the arteries by displacing the existing blood. This movement of blood causes an increase in intravascular pressure, which results in systolic blood pressure.
The blood from the heart pushes the blood ahead of it, moving both forward along the vessel and outward to expand the vessel. This continuous movement, where blood pushes forward and expands the vessel, causes pressure waves to propagate through the artery. It’s important to note that the blood does not move through the vessel without resistance; rather, it moves by displacing the existing blood, which in turn propagates the arterial pressure wave. The systolic pressure is highest when the largest amount of blood is stored in the vessel, expanding it the most. As the systolic phase progresses, the blood continues to move forward, eventually reaching the arterioles, where resistance is high, causing a backlog that exerts force in the reverse direction, opposing the forward movement of blood.
Even when systole ends, blood flow does not immediately stop. The blood that was stored in the arteries during systole is gradually pushed forward as the vessels recoil due to their elasticity. This elastic recoil of the vessels continues to drive blood flow during diastole. Blood flow stops when the force of the elastic recoil equals the resistance from the peripheral vessels. If the diastolic phase is prolonged, arterial blood flow stops, and the pressure at this point becomes the true diastolic minimum pressure.
There are two types of diastolic pressure: in cases of bradycardia, sufficient diastolic time allows for blood flow cessation, resulting in a clear minimum diastolic pressure. However, in cases of tachycardia, the next systolic phase begins before blood flow cessation during diastole, leading to a diastolic minimum pressure that is higher than it would otherwise be.
★ Differences Between Juvenile Hypertension and Elderly Hypertension
- Juvenile Hypertension:
In young individuals, diastolic pressure tends to be high while systolic pressure is within the normal high range. This is because the vascular elasticity is well maintained, allowing the arteries to accommodate blood without a significant rise in pressure during systole, resulting in greater blood flow during diastole. The arteries are soft, so systolic pressure does not rise significantly. However, due to increased arteriolar resistance, diastolic pressure is elevated. Thus, young individuals typically have normal-high systolic pressure and elevated diastolic pressure. Furthermore, atherosclerosis is not advanced, and arteriolar resistance may be increased due to factors like the renin-angiotensin system, indicating a potentially reversible condition. - Elderly Hypertension:
In older individuals, arterial elasticity is reduced, making the arteries stiffer. Even small changes in intravascular volume during systole can lead to excessive increases in pressure, resulting in elevated systolic pressure. Additionally, because the arteries cannot store as much blood during systole, the amount of blood flowing during diastole is reduced, leading to lower diastolic pressure.
★ Blood Pressure Variability by Position Due to Vascular Elasticity
- Positional Blood Pressure Variability:
Even when the material is the same, thinner and larger diameter vessels have higher elasticity. Therefore, even without differences between elastic arteries and muscular arteries, systolic pressure is lower at the base of the arteries and higher at the periphery. For arteries of the same muscular type and diameter, blood pressure varies according to the amount of blood flow, which is influenced by the oxygen demand of the organ and the density of blood vessels supplying it. Diastolic pressure, unaffected by these factors, remains relatively consistent across different regions.
★ Clinical Measurement of Blood Pressure
- Blood Pressure Measurement Using a Cuff To measure blood pressure using a standard blood pressure monitor, a cuff is first wrapped around the arm and inflated by compressing the air inside it with a pump. The pressure is increased until it temporarily stops blood flow in the artery. Then, the pressure in the cuff is slowly reduced. When the arterial pressure (blood pressure) exceeds the pressure inside the cuff, blood begins to flow through the artery again. The flow of blood through the artery causes vibrations, known as “Korotkoff sounds,” which can be heard with a stethoscope. The pressure at the point when these sounds first become audible is the systolic blood pressure (maximum pressure). As the cuff pressure is further reduced, the Korotkoff sounds become weaker and eventually disappear. The pressure at the point when the sounds disappear is the diastolic blood pressure (minimum pressure).
