How can the body control where the blood is pumped? I heard that when somebody faints, it is because most of the blood was pumped to the head to save it from death because of some interruption - leaving the legs too weak to keep standing.
But how can blood be directed? I always figured there was just a pump pushing blood into whatever way it can choose…
The arteries and veins can expand and contract to control the pressure. When you stand up too fast and it makes you feel light-headed, it’s because your body hasn’t adjusted the pressure yet and your brain is not getting enough blood.
Also, when someone is trapped in freezing temperatures, the vessels to their extremities (arms & legs) constrict to keep from losing body head out of them and keep it in the thorax and head. This is why fingers and toes are the first to succumb to frostbite.
Also, people usually faint because NOT ENOUGH blood gets to their head. These could be caused by low blood pressure, blood loss, or shock, among others. Your description, while charming, is wrong.
Actually, it’s the opposite. So much blood has gone to the legs that there’s not enough left for the brain. Hence, the faint.
Regulation of blood flow is, like so much in the body, controlled by multiple mechanisms at multiple sites. Things like the heart itself (rate and volume per beat), artery tone (constrict or relax?, everywhere or just in specific areas of the body?), tone in the veins, levels of oxygen, carbon dioxide, adrenalin, angiotensin, prostaglandins, nitric oxide (NO), … the list goes on and on. These factors can act locally or throughout the system.
Does this help?
There are one-way valves in the veins & arteries. The blood goes the only way it can.
Actually, there are not valves in the arteries. But there are valves in the veins, and of course, also within the heart, so blood does only flow one direction. There are hundreds and hundreds of miles of capillaries, however, throughout the body - in fact, it is in these capillaries that the actual work of the blood is done, and the blood flow through these is regulated by tiny sphincter muscles which respond, as a poster above said, to a number of parameters. We don’t even have enough blood to fill all the capillaries to their fullest extent at the same time, though, and at any one time, many of them are constricted and restricting flow. If for some reason these little sphincters in the brain restrict enough blood, the cells get too little oxygen, and, clunk, down we go. Of course, the head rush of getting up too quickly is a little different - reflective of a larger scale decrease in blood flow to the head, and which is regulated by the barostat, discovery of which once got someone the Nobel prize.