text: why is the normal rate of breathing of the people is decreasing when growing old?   respiratory rate is in part controlled by the person's overall metabolic rate.  all of the body metabolic processes impact each other.  breathing can and does change if you body becomes acidic or basic.  both breathing and metabolism slow with age as the body goes through normal changes. this is the same reason that a 20 year old can eat 3500 calories of food and not gain weight.  the same person at age 45 would likely have a slower metabolism and if they could eat that much would likely gain a lot of weight.