The Circulatory System Explained Simply
The human circulatory system consists of two loops connected in series that share one pump. The heart first pushes blood through the lungs, where it picks up oxygen, and then through the rest of the body, where it gives that oxygen away.
Two loops, one heart
A solid wall divides the heart into two halves that work at the same time but separately. The right half receives used, oxygen-poor blood from the body and sends it to the lungs, where it releases carbon dioxide and takes on oxygen. The refreshed blood returns to the left half, which pumps it around the whole body at a much higher pressure. That is why the left ventricle is far more muscular — it works against roughly five times the resistance.
Right heart to the lungs, left heart to the body. Red marks oxygen-rich blood, blue oxygen-poor blood.
This is where the vocabulary trips people up. Arteries and veins are not named after oxygen content but after direction of flow. The pulmonary artery therefore carries oxygen-poor blood because it leads away from the heart, and the pulmonary vein carries oxygen-rich blood because it leads back to it.
Where the work happens
The large vessels are only transport. The real purpose is fulfilled in the capillaries, whose walls are a single cell layer thick. Oxygen diffuses out into the tissue and carbon dioxide back into the blood. All the capillaries of one adult together would stretch for tens of thousands of kilometres and offer an exchange area the size of several tennis courts.
| Type | Direction | Wall | Pressure |
|---|---|---|---|
| Artery | away from the heart | thick, muscular, elastic | high |
| Capillary | within the tissue | one cell layer | low |
| Vein | back to the heart | thin, with valves | very low |
stroke volume 70 ml × heart rate 70 /min = 4900 ml/min ≈ 5 litres per minute
Under load: 110 ml × 180 /min = 19 800 ml/min ≈ 20 litres per minute
Equating "artery" with "oxygen-rich". The pulmonary artery is the
exception.
Drawing the two loops side by side. They run in series; all blood
passes through both.
Expecting exchange in the large vessels. It happens only in
capillaries.
An athlete has a stroke volume of 120 ml and a resting pulse of 45. Work out the cardiac output and explain the low pulse.
Show solution
120 ml × 45 /min = 5.4 litres per minute — the same demand an untrained person meets with 70 ml and about 77 beats. Endurance training enlarges the heart and improves filling, so each beat moves more blood and fewer beats are needed.