There are two important blood pressure levels in a human body – a maximum or systolic blood pressure, the pressure in the moment of heart beat, when blood is ejected into aorta, and minimum or diastolic blood pressure when heart muscule is relaxing and ventricules are getting filled with „new“ blood. Normal levels of sysolic and diastolic blood pressures are 120 mm Hg and 80 mm Hg, usually expressed as 120/80 mm Hg, or 120 above 80. Blood level shouldnt be much higher than 120/80 mm Hg.
Blood pressure, the same as any pressure in physics, is a force that blood is creating on walls of blood vessels. It is usually expressed in milimeters of mercury, previously mentioned as mm Hg, and sometimes in medicine in kPa. In a common language using „mm Hg“ scale is widely spread.
Levels of blood pressure are usually the same during the day, and they shouldnt change much. In morning, and while a sleep, blood pressure is usually lower.
Body controls pressure with a heart rate – when heart rate increases, pressure rises, and when heart rate decreases blood pressure falls. BP is also controled with stroke volume or the amount of blood heart ejects in one heartbeat, which depends on different hormones, drugs, blood volume in general, phsyical state – while in rest the stroke volume is less than while being physically active. Third major way that BP is controled is by controling the resistance of a blood flow in a body. It is easier for a blood to flow through flexible and wider than through hardened and narrow arteries. Body controls resistance level by hormones, nervous system stimulation (caffeine, stress etc.), various diseases.
Hormones system renin-angiotensin-aldosterone (RAAS) play a big role in BP control by regulating the width of the arteries, the amount of fluid in the blood, and the sodium and water concentrations by their influence on the kidneys and blood vessels.
The kidneys play the most important role in a long term blood pressure control by regulating the amount of water and sodium, they excrete extra water or when there is water sufficiency in an organism they reabsorb the water making the blood volume bigger which directly creates bigger blood pressure.