system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does a deep scrape on elbows/knees/shins stay white for a period of time before bleeding?

Say when you dive into a pool and scrape your knee on the bottom, why does it stay white without much blood for around 30 seconds before properly bleeding?
gpt
This is because initially after an injury, blood vessels contract in order to minimise blood loss. They increase again in diameter later to facilitate healing and the movement of specialised cells and materials towards the site of injury. You can see this contraction and subsequent dilation of blood vessels anywhere; try scratching your arm with your nail (don't cut yourself!) and you'll see that your skin will become pale initially, then redden.