What exactly is blood pressure and why must we have it to live?
Blood pressure is the force which blood applies against the walls of a chamber or blood vessel through which it is moving. Think of it like the pressure you feel against your hand as water rushes through your garden hose. In clinical practice the blood pressure is generally understood to be the arterial blood pressurewhich is the force of blood exerted against the walls of the large systemic arteries. It’s usually measured in the brachial artery that’s present in the upper arm.
When it comes to how the blood pressure is measured it’s important to understand the cardiac cycle and how blood flows within the arterial system. Systole makes up one-third of the cardiac cycle during which time the left ventricle contracts and pumps blood through the aortic valve into the aorta and from there into the large arteries. As this blood is added to what is already present within the brachial artery, this causes the blood pressure within it to rise to a maximum and is called the systolic blood pressure. Diastole makes up two-thirds of the cardiac cycle during which time the left ventricle relaxes and fills with blood. Once pumped out of the left ventricle (which now relaxes during diastole) the blood travels down ever narrowing arteries until it meets resistance in the arterioles which have muscles surrounding them that are tightened to varying degrees. Some of the blood flows through to the capillaries into the tissues while most of it rebounds back to the heart where it again meets resistance, this time from the closed aortic valve. During diastole, the blood ping-pongs back and forth between the resistant downstream arterioles and the closed aortic valve, each time allowing some of it to pass into the capillaries. By the time diastole ends and systole is about to begin again, the blood pressure within the brachial artery has dropped to a minimum and is called the diastolic blood pressure. As noted above, when the left ventricle contracts again, the blood entering the brachial artery increases the blood pressure to a maximum called the systolic blood pressure and this cycle continues as long as you live.
Like all matter, the movement of blood is resisted by natural forces such as inertia, friction and gravity, which must be overcome by the pumping heart to get it where it needs to go. Just as friction slows the motion of an object on the ground, blood flow meets its counterpart (vascular resistance) in the blood vessels. Most of the vascular resistance the blood encounters as it flows from the heart to the capillaries comes from the downstream arterioles due the tightened muscles surrounding them.
The three main factors that affect the blood pressure are: 1) how much blood the heart pumps out, 2) the amount of blood within the arteries (as opposed to the veins), and 3) the vascular resistance applied by the downstream arterioles. The harder and faster the heart pumps the more blood enters the arterial system and the higher the blood pressure, and the slower and less vigorous it pumps, the less blood enters the arterial system and the lower the blood pressure. The more blood within the arteries the higher the blood pressure and the less blood within the arteries, the lower the blood pressure. The more vascular resistance the arterioles apply, the more blood stays within the arteries and the higher the blood pressure, and the less vascular resistance they apply, the less blood stays within the arteries and the lower the blood pressure.
It’s important to realize that the blood pressure represents the driving force that moves blood throughout the circulatory system to feed the cells what they need to live and work properly. This means that just like the water pressure in your home determines how fast you can fill up your bath tub or kitchen sink, in general, blood flow is directly related to the blood pressure. If you don’t have enough blood pressure, you may not have enough blood flow to your tissues and organs and they’ll malfunction or even die, and consequently so will you. So, that’s why you can’t live without blood pressure.
Three Questions for Mr. Darwin
- Where did the information come from that tells my body how much blood my heart needs to pump out to have enough blood pressure and blood flow to my organs and tissues for my survival?
- Where did the information come from that tells my body how much of its blood should be in its arteries to have enough blood pressure and blood flow to my organs and tissues for me to stay alive?
- Where did the information come from that tells my body how much vascular resistance my arterioles must apply to have enough blood pressure and blood flow to my organs and tissues for me to live?
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Howard Glicksman M. D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being.
Copyright 2017 Dr. Howard Glicksman. All rights reserved. International copyright secured.
