Question

As part of a blood drive on campus for the american red cross, you and your friends have just donated 500 ml of blood. you are now relaxing at the student lounge, waiting for a&p lab to begin. unfortunately, even though you are thirsty, you haven't bothered to buy yourself a drink. other than a little soreness of the skin and tissue around your median cubital vein, you feel fine. how has your 500 ml decrease in blood volume most likely affected your cardiac output, heart rate, and stroke volume?

Answer 1

There is really no change in cardiac output, there is increased heart rate and a decrease in systolic volume.Although, it is common that after donating blood, vasovagal syncope occurs.
This is basically that the body reacts to de lack of volume, the brain detects an stimulator effect on the vagus nerve, lowers the heart rate and the person feels dizzy or falls, without major consequences.

Answer 2

Donating blood does not change the resting cardiac output. Although, the diminished volume of the blood does minimize venous return and end diastolic volume. This reduces preload, thus, lowering the volume of stroke.  

In turn, decreased stroke volume would minimize the cardiac output. However, the body still needs a resting blood flow of around 5 liters in a minute. In order to fulfill the need, the homeostatic mechanisms quickly restore cardiac output to the resting levels.  

For example, the rate of the heart is upsurged as the parasympathetic effect is withdrawn and sympathetic activity is enhanced. Even though the volume of blood is slightly low, the body possesses no issue in compensating to meet the resting metabolic requirements.