What is blood pressure?
What is blood pressure?
A garden hose
Without a pump or water tank, no water will flow. Hose pipe properties also affect water pressure. Similar principles apply for blood flow.
Blood pressure is the force that moves blood through our circulatory system.
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It is an important force because oxygen and nutrients would not be pushed around our circulatory system to nourish tissues and organs without blood pressure.
Blood pressure is also vital because it delivers white blood cells and antibodies for immunity, and hormones such as insulin.
Just as important as providing oxygen and nutrients, the fresh blood that gets delivered is able to pick up the toxic waste products of metabolism, including the carbon dioxide we exhale with every breath, and the toxins we clear through our liver and kidneys.
Blood itself carries a number of other properties, including its temperature. It also carries one of our defenses against tissue damage, the clotting platelets that prevent blood loss following injury.
But what exactly is it that causes blood to exert a pressure in our arteries? Part of the answer is simple - the heart creates blood pressure by forcing out blood when it contracts with every heartbeat. Blood pressure, however, cannot be created solely by the pumping heart.
Function
Our circulation is similar to a highly sophisticated form of plumbing - blood has 'flow' and arteries are 'pipes.' A basic law of physics gives rise to our blood flow, and this law also applies in a garden hose pipe.
Blood flows through our body because of a difference in pressure.
Our blood pressure is highest at the start of its journey from our heart - when it enters the aorta - and it is lowest at the end of its journey along progressively smaller branches of arteries. That pressure difference is what causes blood to flow around our bodies.
Arteries affect blood pressure in a similar way to the physical properties of a garden hose pipe affecting water pressure. Constricting the pipe increases pressure at the point of constriction.
Without the elastic nature of the artery walls, for example, the pressure of the blood would fall away more quickly as it is pumped from the heart.
While the heart creates the maximum pressure, the properties of the arteries are just as important to maintaining it and allowing blood to flow throughout the body.
The condition of the arteries affects blood pressure and flow, and narrowing of the arteries can eventually block the supply altogether, leading to dangerous conditions including stroke and heart attack.
Measurement
Lady having her blood pressure checked by a doctor
When the pressure from the arm cuff stops the pulse briefly, it gives the top figure of arterial blood pressure that we are familiar with from medical dramas - for example, "140 over 90"
The device used to measure blood pressure is a sphygmomanometer, it consists of a rubber armband - the cuff that is inflated by hand or machine pump.
Once the cuff is inflated enough to stop the pulse, a reading is taken, either electronically or on an analogue dial.
The reading is expressed in terms of the pressure it takes to move mercury round a tube against gravity. This is the reason for pressure being measured using the unit millimeters of mercury, abbreviated to mmHg.
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