system
Given the following post, respond as if you're commenting on Reddit:
human
If there is a considerable amount of blood loss, is there now empty space in your veins?

Hopefully I can make this question more clear. 

If someone loses an amount of blood, would your veins be 'half-empty' as less blood is pumping through them? Or would your blood vessels constrict and become smaller to deal with the lesser volume?
gpt
They would constrict. Veins have soft walls and would easily collapse under pressure simply due to lost volume. Furthermore part of the body's response to blood loss is to constrict small blood vessels such as arterioles to maintain adequate pressure. This is how the body can maintain a healthy blood pressure after considerable blood loss, until the "decompensated" stage of shock. 


[Edit] Interesting side fact: Arteries have more rigid walls and appear empty after death. As a result the ancient Greeks thought that veins carried blood and arteries carried air.