Why do people pass out after donating blood when they have fasted? I've worked in a blood bank. Not many people passed out while i worked there, it's quite rare, but for those who do, not eating/drinking enough (normal amounts) was the most common reason besides beein nervous.. If you haven't been drinking enough your blood volume falls. The fluid in your blood (plasma, mostly water) needs refill all the time, if not, it will "drain" (sweat, urine, etc). When the blood volume falls, the pressure falls. To prohibit this, the veins in your body constrict. That makes the volume of your veins to fall and the amount (volume) of blood has the same ratio to vein-volume, and the blood pressure is unchanged. If you're dehydrated the veins have constricted to try to keep the pressure up. If you add blood-donation to that, the volume falls even further. The veins can only constrict so much, and it might not be enough to keep the pressure at a normal level, you...
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