Theory:

Blood pressure is not a disease but is essential for life as it is the force that the heart uses to pump oxygenated blood.
Blood pressure is the force exerted during the flow of blood against the lateral walls of arteries.
The blood pressure in the arteries is high, then progressively drops in the arterioles and capillaries and become very low in the veins.
 
Blood pressure is expressed in terms of systolic pressure and diastolic pressure.
 
Systolic pressure:
During ventricular systole, the left ventricle contracts and forces blood into the aorta. The pressure rises to a peak which is referred to as systolic pressure.
Diastolic pressure:
During diastole, the ventricles relax, and the pressure falls to the lowest value, which is called diastolic pressure.
In a healthy adult during the normal resting condition, the systolic pressure is \(120\) \(mm Hg\) and diastolic pressure is \(80\) \(mm Hg\). It is expressed as \(120/80\) \(mm / Hg).
 
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Measuring the blood pressure using sphygmomanometer
 
Blood pressure varies depending onperson's age and rate of the heartbeat. Blood pressure varies during physical activity, emotions, anxiety, fear, excitement, stress, and sleep.
A prolonged rise in blood pressure is known as high blood pressure or hypertension.
Hypertension is caused due to the constriction of arterioles, which results in increased resistance to blood flow. This leads to the rupture of an artery as well as internal bleeding. High blood pressure can increase the risk of heart attack and stroke.
A decrease in blood pressure is called hypotension or low blood pressure.
Stethoscope:
A stethoscope is used to detect the sound produced by the internal organs of the human body. The heart sound is heard by placing the stethoscope on the chest. It is a helpful diagnostic tool for diagnosing and locating health problems and diseases. Modern electronic stethoscopes are of high precision and accuracy.
 
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Stethoscope
Sphygmomanometer:
A sphygmomanometer is a clinical instrument that measures blood pressure when a person is in a relaxed and resting condition. The pressure of the brachial artery is measured. It helps to determine the state of blood circulation and cardiac function
A sphygmomanometer helps to diagnose the conditions such as hypertension and hypotension. Monometric and modern digital types are the apparatus used to measure blood pressure.
 
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The different types of sphygmomanometers
  
Important!
Dr Scipione Riva-Rocci invented the sphygmomanometer in the year \(1896\).
Reference:
https://commons.wikimedia.org/wiki/File:Stethoscope.svg
https://pixahive.com/photo/a-bp-apparatus/