Blood pressure is the force of blood against the artery walls. It depends on how much blood the heart pumps (cardiac output) and how much the small arteries resist flow (systemic vascular resistance).
Systolic, diastolic and MAP
Systolic pressure is the peak during ventricular contraction; diastolic is the lowest point during relaxation. Mean arterial pressure (MAP), about diastolic plus one third of the pulse pressure, reflects organ perfusion, and most organs need at least about 65 mmHg.
Fast control by nerves
Baroreceptors in the carotid sinus and aortic arch sense stretch. When pressure falls, the sympathetic system raises heart rate, contractility and vessel tone within seconds.
Slower control by hormones and kidneys
Low blood flow to the kidneys releases renin, which leads to angiotensin II (a vessel constrictor) and aldosterone (salt and water retention). Antidiuretic hormone also holds water, and over days the kidneys set blood volume.
Vessel size and blood volume
Small changes in arteriole diameter change resistance a lot. Less blood volume, as in bleeding or dehydration, lowers preload and blood pressure.
| Measure | Normal adult | Concern |
|---|---|---|
| Systolic | 90 to 119 mmHg | 130 or higher (hypertension), under 90 (hypotension) |
| Diastolic | 60 to 79 mmHg | 80 or higher (hypertension) |
| MAP | 70 to 100 mmHg | Under 65 mmHg (poor perfusion) |
At the bedside
Use the right cuff size and keep the arm at heart level, since a cuff that is too small reads falsely high. If a patient feels dizzy on standing, check orthostatic vital signs.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).