When you train a muscle blood is being diverted from other body functions and rerouted to your muscles.  This allows the muscle to perform at its optimum levels.  The blood needs to be oxygenated first before it is delivered to the muscle. This is done in a process of the heart and the lungs. Once this happens it is pumped to the muscle where it pools and gives you the tight swollen feeling you get.  It is the aim of many people in the gym to gegt this pump. 1 weight training mistake that is made is that you lose your pump and then you lose your motivation.