Blood pressure is a measure of the force that the heart and blood vessels exert on the walls of the arteries as blood flows through them. It is expressed as two numbers: systolic pressure (the peak pressure when the heart contracts and pumps blood out) over diastolic pressure (the pressure during relaxation between beats), measured in millimetres of mercury (mmHg). A normal adult blood pressure is typically around 120/80 mmHg.
Blood pressure is determined by two main factors: cardiac output (the volume of blood pumped per minute) and peripheral vascular resistance (the resistance offered by the small arteries throughout the body). The autonomic nervous system, the kidneys, and various hormones work continuously to regulate blood pressure, adjusting to demands of activity, posture, and emotion. The left side of the heart and the systemic arteries operate as a high-pressure system; the veins, right heart, and pulmonary circulation operate at much lower pressures.
Persistently elevated blood pressure (hypertension, above 140/90 mmHg) is one of the most important cardiovascular risk factors, contributing to heart attack, stroke, heart failure, and kidney disease. Blood pressure that is too low (hypotension) can impair blood flow to the brain and other organs, causing dizziness, syncope, and in severe cases, circulatory shock and cardiac arrest.
After cardiac arrest, maintaining adequate blood pressure is a core priority of intensive care management. Post-cardiac arrest syndrome often involves haemodynamic instability, and vasopressor medications are frequently needed to sustain blood pressure until the heart recovers its function. Blood pressure targets in the post-arrest period are guided by evidence on optimal perfusion pressure for the injured brain.
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