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What Sets the Pressure: Flow, Resistance & Mean Arterial Pressure

Behind the numbers sits a simple idea borrowed from plumbing: pressure depends on how much blood the heart pumps and how hard the vessels push back. Meet cardiac output, vascular resistance and mean arterial pressure — the engine of the circulation.

The plumbing equation

Pressure in any pipe network comes from two things: how much fluid is flowing and how much the pipes resist that flow. In the circulation, flow is the cardiac output — the volume of blood the heart pumps per minute — and resistance is the systemic vascular resistance, set mostly by how narrow the arterioles are. Multiply them and you get the driving pressure. This relationship is the backbone of hemodynamics, the study of blood in motion.

The single best summary of the pressure pushing blood around the body is the mean arterial pressure (MAP) — the average pressure over a full cardiac cycle. Because the heart spends more time relaxing (in diastole) than contracting, MAP sits closer to the diastolic number than the systolic. It is not the simple midpoint of the two.

Working MAP out

A common bedside estimate weights the diastolic number twice and the systolic once, then divides by three. Equivalently: MAP ≈ diastolic + one-third of the pulse pressure. Let us run it for 120/80.

MAP estimate  =  (2 × diastolic  +  systolic) / 3

Reading: 120 / 80 mmHg
  = (2 × 80  +  120) / 3
  = (160 + 120) / 3
  = 280 / 3
  ≈ 93 mmHg

Same answer the other way:
  MAP = diastolic + (1/3 × pulse pressure)
      = 80 + (1/3 × 40)
      = 80 + 13
      ≈ 93 mmHg

Note: 93 is much closer to 80 than to 120 —
because the heart spends more of each cycle in diastole.
Estimating mean arterial pressure from a 120/80 reading.

Output is rate times volume

Cardiac output itself breaks into two factors: the heart rate (beats per minute) and the stroke volume (millilitres ejected per beat). Multiply them and you have litres per minute. A typical resting heart might beat about 70 times a minute and eject roughly 70 mL each time, giving close to 5 litres a minute — nearly the body's entire blood volume circulated every minute.

Why care about MAP at all? Because it is the pressure that actually drives perfusion — the steady delivery of blood to organs like the brain and kidneys. If MAP falls too low, those organs cannot get enough flow no matter how good the peak systolic looks. MAP, not the headline systolic, is what keeps tissue alive.