The human bloodstream
Our veins are a branched system and reach every point in the body. Together the largest to the smallest blood vessels are around 100,000 kilometers (62,000 miles) in length and can hold 4.5 to 6 liters (1.2 to 1.5 gallons) of blood.
The heart takes on the important task of transporting fresh blood to the most remote corners of the bloodstream. To do this, the blood is first pumped through the pulmonary circulation, which runs through the lungs and supplies the blood with oxygen.
The oxygen-rich blood then starts the long journey through the circulatory system, which requires the heart to pump sufficiently hard. This pumping process creates pressure on the blood vessels, which are stretched and stressed with every heartbeat.
Why are there two blood pressure numbers?
Blood pressure is given by two numbers, for example, 130/85 mmHg. The numbers are usually given in the unit millimeter of mercury since earlier measurements were made with mercury manometers to measure blood pressure.
The highest pressure is created when the heart contracts while pumping blood through the circuit. This is measured as the upper number and referred to as systolic blood pressure.
The heart then relaxes again to prepare for the next beat. This results in lower pressure on the blood vessels and the lower number, known as diastolic blood pressure, is reached.
Which blood pressure numbers are healthy?
The optimal blood pressure for adults is considered to be below 120/80 mmHg. Starting at 140/90 mmHg the numbers are considered as high blood pressure, the technical term for this is hypertension.
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