What does the blood symbolize in the story Through the Tunnel by Doris Lessing?

The blood serves as a symbol of fear and doubt.... the boy's nosebleeds amke him question his ability.... fear the outcome.

But two days before they were to leave, a day of triumph when he increased his count by fifteen, his nose bled so badly that he turned dizzy and had lie limply over the big rock like a bit of seaweed, watching the thick red blood flow on to the rock and trickle slowly down to the sea. He was frightened. Supposing he turned dizzy in the tunnel? Supposing he died there, trapped?