The Heart
In the right fore-chamber, it collects blood that's coming from the veins, and with the right main-chamber pumps it into the lungs.
In the lungs, the blood is oxygenated and then flows into the left fore-chamber wich presses it into the left main-chamber, the strongest chamber of the heart.
This chamber then pumps the blood into the aorta and the arteries that circulate it through the whole body.
When the heart-rate is too high, the problem is simply that the chambers can not fill fast enough and the heart loses its efficacy.
When the heart stops completely, it can be pumped externally. Isn't it fascinating that this works? My colleague Per, a former surgeon, told me that he has held hearts in his hand and pumped them manually. What a strange image.
The heart is simply a pump.