My cat keeps on panting when fight with younger kitten

My cat is 1 and half year old. Lately I noticed that my cat keeps on panting after some running or whenever he's fighting with his younger brother(who is 4 months old). Anyway, I was so worried last week that we made a visit to the vet. We had both X-Ray and Ultrasound but nothing suspicious. His appetite is good(in fact he gained few weight in a week) and no coughing, so the vet didnt prescribed any medicine and just told me to watch out. I had made an appointment for blood checkup next week. Should I be worried? Could it be something serious?

Any animal will pant after exerting themselves Humans do it, too, like after a run or heavy excercising. Panting helps cool the body down and help the heart rate slow down to normal. It usually lasts a few minutes or so. I wouldn't worry about your cat's panting after running and playing unless he's also showing signs of serious problems like stumbling around or twitching.