We are currently studying cardiovascular physiology in school, and my professor told an interesting story about compliance of blood vessels in relation to the ancient understanding of heart and soul.
So blood vessels such as veins are highly compliant vessels. This means that they are very distensible. In fact, most of our blood volume is stored in our veins- this can be considered as a reserve blood volume. When we exercise, then we can utilize this 'reserve' volume to increase our cardiac output.
Arteries, which carry the blood away from the heart, are less compliant. You can consider these vessels to be elastic since they are able to resist a high pressure of blood coming out of the heart.
Now, in the olden days when people died and physicians opened their bodies to make observations they noticed an interesting phenomenon. When one is dead, the heart is no longer acting as a pump. This means that it can no longer create a pressure difference to drive blood around the body and move from arteries to veins. As a result, the pressure in the system is quite low, say around 5mm/Hg. This causes most of the blood in the body to pool in the veins - the most compliant blood vessels in the body. Since the veins are able to distend to accommodate the volume of blood, there is nothing in the arteries. When the physicians saw this, they thought that the veins contained the blood of the heart. But then, they had to consider this seemingly completely different system-- the arteries. Here were these vessels, running throughout the body, devoid of any substance within. What could this be? They hypothesized that this system contained our ether or life force. Since the patient was dead, then the life force had left the body, so these vessels were empty.
So there you go- heart and soul related to physiology :)
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