Systolic and Diastolic Pressures

Systemic Arterial BP

We define blood pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange., or BP, as the force per unit area. It is exerted on a blood vessel wall by the blood within the wall.  This pressure is expressed as millimeters of mercury. A BP of one hundred twenty millimeters of mercury is the pressure needed to raise a mercury column 120 millimeters high. It maintains it at that height.  To really understand what these two numbers mean, we have to understand how blood pressure is taken

To inflate the blood pressure cuff, we cut off circulation in the artery. This happens only if we inflate the cuff to a pressure higher than the systolic pressure. The systolic pressure is the pressure created in systole.  If you have high blood pressure, you have to inflate the cuff a lot.  As you release the pressure in the cuff, you will hear the blood surge back into the artery.  The pressure at which it fills the artery again is systolic pressure.  You will continue to hear sounds until the vessel is completely full.  The pressure at which you last hear the sounds is the diastolic pressure.

We take blood pressure at the brachial artery with the cuff on the arm. We’re only measuring the systemic circuit’s pressure. We are not measuring the pulmonary circuitThe circulation of blood between the heart and the lungs, where blood is oxygenated.. We’re only taking pressure of an artery, not veinsBlood vessels that return deoxygenated blood to the heart (except pulmonary veins, which carry oxyge. Looking back at the cardiac cycle, we know that there’s systolic pressure. That’s when the heart is at its maximum pump action. It creates the most pressure. We’ll default to calling this 120 millimeters of mercury. We also know that when the heart is at rest, there’s the lowest pressure, which is 80 millimeters of mercury. This pressure, the diastolic pressure, results from afterload or the back pressure on the aortic valve.

Systolic and Diastolic BP

The pressure generated by the heart is a function of systole. The heart’s maximum pumping action creates the pressure to push blood into the aorta. Pressure relies on how much blood is pushed into the aorta. This is known as cardiac output and refers to the amount of blood per minute. It also depends on the heart’s contractilityThe ability of muscle tissue to shorten with force., or the force it generates to push blood into the aorta. Additionally, it depends on the elasticity of your arteriesBlood vessels that carry oxygenated blood away from the heart (except pulmonary arteries, which carr. If you’re young and healthy, your aorta and elastic arteries expand to allow pressure to dissipate through the systemic circuitThe part of the circulatory system that carries oxygenated blood from the heart to the body and retu. However, if your aorta has calcifications and cannot expand when blood enters, the pressure increases due to reduced elasticity.

When the contraction phaseThe period of increasing muscle tension during a twitch. is complete, the ventricles relax. The elastic recoil of the aorta keeps the blood flowing. Other major arteries near the heart also help maintain flow. However, it flows at much lower pressures. Typically diastolic pressure—the pressure as measured when the ventricles are relaxed—is measured at eighty millimeters of mercury.

Mean Arterial Pressure

The accompanying diagram is about blood pressure and shows some things we already know. The x-axis represents the sequence of vessels from the aorta to the venae cavae. This sequence is encountered when traveling from the heart and back to the heart in the systemic circuit. It shows that blood pressure is higher near the heart, with arteries having higher pressure. It also illustrates that blood pressure is pulsatile, increasing with systole and decreasing with diastole. However, we can calculate an average pressure, the mean arterial pressureThe average pressure in the arteries during one cardiac cycle, ensuring adequate blood flow to organ. Interestingly, this mean pressure is not halfway between systole and diastole. It is slightly skewed toward diastole because diastole lasts longer.

When your heart beats faster, it also beats more forcefully. This results in more blood being pushed out into the aorta and pulmonary trunk. Therefore, your arterial blood pressure changes in relation to your heartbeat, as a function of your activity. Mean arterial pressure or M.A.P. is an important pressure determinant. M.A.P. is the pressure that pushes your blood to the tissues. The farther away from the heart you get, the lower the M.A.P. This is due to increasing resistanceThe opposition to airflow in the respiratory tract, influenced by airway diameter. and the loss of elastic recoil in the smaller arterial blood vessels. Take an athlete for example. Their M.A.P. will be greater than in someone who doesn’t work out. This is because the athlete’s heart, muscles, and arterial blood vessels will be able to carry the blood farther. If there is a problem with arterial circulation, the M.A.P. will drop significantly. In addition, the limbs may appear pale and feel cold to the touch. By monitoring M.A.P., a clinician can assess the physical condition of a person. They can also determine whether there may be a problem with peripheral circulation.

Trends in MAP

Blood pressure refers to the arterial pressure in the largest arteries closest to the heart. The pressure initially established in the aorta after ventricle contraction begins a pressure gradient. This pressure gradient is highest near the heart and lowest in the tissues. The blood flows down the gradientMovement from an area of higher concentration to lower concentration. because there is less resistance. By the time the blood reaches the veins, the pressure is very low. You can see this in the graph above. Take a look at how the pressure in the aorta is 120 mm Hg and pulsatile.  Down here in the capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues. it is much lower.  This is a pressure gradient, and blood flows down it form high to low. Arterial blood pressure pulsates with each ventricular contraction. Venous blood pressure is steady. That is why you cannot take your pulse on a vein. The pressure gradient in the veins is only about fifteen millimeters of mercury (the same as in the capillaries).

By the time the blood reaches the peripheral tissues, the blood pressure has decreased to thirty-five millimeters of mercury. Once it exits the capillary beds, the blood pressure is down to fifteen millimeters of mercury. This low pressure is beneficial. If the pressure were too high and blood flow too fast, nutrients would not diffuse out into the tissues. Consequently, cellsThe basic structural and functional units of life. would die. In addition, the capillaries are only one cell layer thick. So pressures that are higher could easily rupture these vessels.

By the time the blood reaches the venous system, the pressure is insufficient to return the blood to the heart. Veins have several modifications that help them get the blood to flow back to the heart. First, veins have valves, which prevent blood from flowing backward. Secondly, more skeletal muscle activity increases the efficiency of venous return. As the skeletal muscles surrounding the deepAway from the surface of the body. veins contract and relax, they move blood toward the heart. This is called the muscular pump. In addition to this, the pressure changes in the thoracic cavityThe body cavity housing the heart and lungs. during breathing help facilitate venous return. This is called the respiratory pump. Of the two, the skeletal muscle pump is the more important.

We can calculate a mean between the systolic and diastolic pressures. To do this, we calculate the pulse pressureThe difference between systolic and diastolic blood pressure, indicating the force of each heartbeat, which is the difference between systolic and diastolic pressures. In this example, it would be 40 millimeters of mercury.