How circulation works
A plain map of the system every study on this site is about: one pump, three kinds of pipe, and one thin lining that does more than it looks.
The short version
- Your heart pumps blood out through arteries and gets it back through veins.
- The real work happens in tiny capillaries, where oxygen and food pass into your tissues.
- The thin lining of your blood vessels makes nitric oxide, which tells vessels to relax and widen.
The pump
Your heart is a muscle about the size of your fist. Each beat pushes blood out into your arteries. The right side sends blood to your lungs to pick up oxygen. The left side sends that oxygen-rich blood to the rest of your body.
The pipes
- Arteries carry blood away from the heart. They have thick, muscular walls that can tighten or relax.
- Capillaries are so narrow that red blood cells pass through them single file. This is where oxygen and nutrients move into your tissues.
- Veins carry blood back to the heart. In your legs, they rely on calf muscles squeezing them and on one-way valves to push blood uphill.
The lining that matters most
Every blood vessel is lined with a single layer of cells called the endothelium. When blood flows across it, it releases nitric oxide, a gas that tells the muscle in the vessel wall to relax. A relaxed vessel is wider, so more blood gets through.
When the lining isn’t working well, vessels stay tighter, blood pressure rises, and plaque builds more easily. Many of the habits covered on this site (greens, cardio, cocoa, breathing training) were studied because they seem to help this lining.
Why erections are a blood flow event
An erection happens when nitric oxide relaxes the arteries and spongy tissue in the penis, letting blood rush in and get trapped there. The arteries involved are small, so they tend to show problems early. That is why erectile dysfunction is often treated as an early warning about the heart. See our guide to erections and blood flow.
What blood pressure numbers mean
The top number (systolic) is the pressure in your arteries when your heart beats. The bottom number (diastolic) is the pressure between beats. Under 120/80 is considered normal by the American Heart Association. Many studies on this site report changes in the top number in “points” (millimeters of mercury, or mmHg).
Sources
Educational information, not medical advice. Read our medical disclaimer.