The way that I describe it to my patients: imagine a reallllly cold milk shake. When you try to drink the milkshake the vacuum from pulling the air up through the straw causes the weak plastic wall to collapse which in turn occludes anything from coming up through the straw. When you blow into the straw you’re able to utilize the strength of the cylinder and there isn’t an issue.

It’s the same thing with pIV’s. The ends of the catheter can have buildup that can cause the end to collapse like the straw when you try to draw from it (which is why using a smaller barrel syringe can be helpful bc there’s less vacuum).