Physiological mechanisms and influencing factors of blood pressure formation

Blood pressure, generally referring to systemic arterial blood pressure, is simply the pressure exerted by blood against the walls of blood vessels as it flows through them; it is the driving force propelling blood flow. During ventricular contraction, blood flows from the ventricles into the arteries, causing a sharp rise in aortic pressure. The highest value reached is called systolic pressure, also known as high pressure. During ventricular relaxation, the arteries elastically recoil, and blood continues to flow slowly, but aortic pressure decreases. The lowest value reached is called diastolic pressure, also known as low pressure. The difference between systolic and diastolic pressure is called pulse pressure. Generally, arterial blood pressure refers to aortic blood pressure. Because the drop in blood pressure is small in larger arteries, in actual measurements, the brachial artery blood pressure measured in the upper arm often represents the aortic blood pressure. For a long time, mercury sphygmomanometers were used to measure blood pressure, so the height of the mercury column is conventionally used to express blood pressure values, i.e., millimeters of mercury (mmHg). Arterial blood pressure is an important indicator of cardiovascular function and an important measure of overall functional status. Sufficient circulating blood volume, normal cardiac function, and peripheral resistance are the three factors that maintain stable overall blood pressure. Stable blood pressure is one of the important conditions for promoting blood circulation and ensuring sufficient blood perfusion to various tissues and organs. Only when all tissues and organs of the body receive sufficient blood perfusion can their biochemical metabolism and physiological functions proceed normally. Systolic blood pressure is mainly affected by ventricular ejection volume, while diastolic blood pressure is mainly affected by peripheral resistance. Under physiological conditions, the factors that frequently affect blood pressure are mainly stroke volume, heart rate, and peripheral resistance. Any factor that can affect cardiac output and peripheral vascular resistance can affect arterial blood pressure. (1) Heart rate: An increase in heart rate can lead to an increase in cardiac output and a shortening of diastolic period, resulting in an increase in diastolic blood pressure and a decrease in pulse pressure. (2) Cardiac contractility: Increased cardiac contractility leads to an increase in stroke volume, a significant increase in systolic blood pressure, a slight increase in diastolic blood pressure, and thus a larger pulse pressure. Systolic blood pressure objectively reflects the amount of cardiac stroke volume. (3) Peripheral vascular resistance: Changes in peripheral vascular resistance affect both systolic and diastolic blood pressure, but the effect on diastolic blood pressure is more pronounced. Increased peripheral resistance slows arterial blood flow, leading to increased blood retention in the arteries at the end of diastole, resulting in elevated diastolic pressure and decreased pulse pressure. Therefore, the level of diastolic pressure reflects the magnitude of peripheral resistance. Hypertensive patients experience excessively high peripheral vascular resistance due to arteriosclerosis, leading to a significant increase in arterial blood pressure, especially diastolic pressure. (4) Elasticity of the large artery wall: The elasticity of the large artery wall buffers the increase in arterial blood pressure and can reduce pulse pressure. In the elderly, many vascular elastic fibers and smooth muscle are gradually replaced by collagen fibers, greatly reducing the elasticity of the vascular wall, leading to increased systolic pressure and pulse pressure. (5) Circulating blood volume: Normal circulating blood volume is basically adapted to cardiovascular volume, maintaining a circulating filling pressure of approximately 7 mmHg. When blood loss exceeds 30%, the mean systemic pressure will decrease, potentially causing shock. Correct blood pressure measurement is fundamental to obtaining accurate values; proper measurement techniques are essential. A mercury sphygmomanometer that meets metrological standards should be selected for measurement. The value is expressed by the height of the mercury column. However, this type of sphygmomanometer must be calibrated regularly by a metrological monitoring unit to accurately reflect the blood pressure value of the human body. The operating steps of the most commonly used mercury sphygmomanometer are as follows: (1) Before measuring blood pressure, the person being measured should rest quietly for 5 minutes to eliminate the influence of factors such as fatigue or tension on blood pressure; tea, alcohol and coffee should not be consumed within 30 minutes before measurement. (2) The person being measured can be seated or lying down, but it is best to be seated. Sit on a chair with your back against the chair back, expose your right upper arm, and place your elbow at the same level as your heart. Regardless of the person being measured, the zero point of the sphygmomanometer should be placed at the level of the heart. (3) Use an appropriately sized cuff. Generally, ordinary adults can choose an air cuff with a width of 13-15cm and a length of 30-35cm. Obese people or people with large arm circumferences should use a larger size cuff, and children should use a smaller cuff. (4) Position the cuff with the middle of the brachial artery aligned with the upper arm of the patient, ensuring the lower edge of the cuff is 2-4 cm above the elbow crease. The cuff should not be too tight or too loose, so as not to cause pain due to excessive tightness, nor to allow the cuff to rotate due to excessive looseness. (5) Place the chest piece of the stethoscope on the brachial artery in the cubital fossa, and then inflate the cuff with the small bulb. Inflate quickly, increasing the inflation level by 30 mmHg after the pulsation is no longer audible. Then slowly release the knob of the small bulb to allow the mercury column to descend slowly (ideally at 2-6 mmHg/s) for accurate reading of the result.