Blood Pressure & Cardiac Output

CO = Stroke Volume × Heart Rate · Week 9
The blood the heart moves each minute is its cardiac output. It is just how much blood it pushes with each beat (stroke volume) times how fast it beats (heart rate). Stroke volume has three dials of its own: preload, afterload, and contractility. Play with them and watch cardiac output and blood pressure respond.

The pump, live

🫀

The red blob on the left is the left ventricle at the end of filling. Preload sets how big it gets. The pale wedge inside is the blood left behind after the squeeze, so the red crescent around it is the stroke volume that leaves. The share that leaves each beat is the ejection fraction. At rest it runs about 55 to 70 percent. The tube on the right is an artery. Afterload thickens its wall and narrows the opening, which makes the heart push harder for less flow. The wave inside the tube is the pulse. Its height is the gap between the top and bottom numbers, and its spacing is the heart rate.

Stroke Volume
70
mL / beat
×
Heart Rate
72
beats / min
=
Cardiac Output
5.0
L / min

The blood pressure it makes

116/74
Normal
Normal <120/80ElevatedStage 1 ≥130/80Stage 2 ≥140/90

Move any dial and the whole picture moves with it. Faster beats raise output until the ventricle runs out of filling time. Then stroke volume falls, and output falls with it.

Systolic (the top number) is the pressure while the ventricles contract. Diastolic (the bottom number) is the pressure while they relax and refill. Pressure that stays high is hypertension. It is driven by extra volume, by stiff or tight arteries (afterload), or by a heart that is working too hard.

The four dials

❤️ Heart Rate72 bpm
🫗 Preloadnormal
🧱 Afterloadnormal
💪 Contractilitynormal

How the body keeps BP steady

↑ RAAS, the system that raises BP
When blood flow to the kidney drops, the kidney releases renin → angiotensin II (a strong vasoconstrictor, so afterload rises) → aldosterone (the kidneys keep salt & water, so preload and volume rise). ADH holds on to water too. Net effect: BP up.
↓ ANP and BNP, the system that lowers BP
When too much volume over-stretches the heart chambers, they release ANP & BNP → the arteries widen and the kidneys dump salt & water. Net effect: BP down. (BNP is also the classic heart-failure blood test.)