The heart is like a seesaw

When you sense something from inside, it reduces the processing of external signals. When your heartbeat is going, it's loading up the seesaw on one side.

An experiment showed that when people were given a faint electrical stimulus to their finger, they were more likely to notice it during diastole and miss it during systole. When the heart pushes blood through your body during systole, it's possible to feel your pulse in your fingertips.