There may be areas where the vitreous is very strongly attached to the retina, usually in the peripheral retina. If the vitreous pulls away from the retina in an area where the retina is weak, the retina may tear. Imagine a piece of adhesive tape attached to tissue paper. If you try to pull the tape off the tissue paper, you will tear the paper. A tear of the retina works very much the same way. If the retina tears across a retinal blood vessel, blood will enter the vitreous. This is called a vitreous hemorrhage.
When there is a little bleed, red blood cells floating and moving in the vitreous create the sensation of walking through a swarm of flies. When a larger vessel is torn, more blood enters the vitreous and it looks like a spiderweb or swirling mass of black or red lines. If there is a great deal of bleeding into the vitreous cavity, vision may be reduced significantly, or even become very dark. When a retinal tear occurs, it is a potentially serious problem. If a vitreous hemorrhage also occurs, it is even more serious.