Is this negative feedback or positive feedback? When a human increases physical activity, the amount of fuel burned in its cells also increases, which in turn increases the concentration of dissolved
CO2 in the blood. The CO2 reacts with water in the blood to make a weak acid,
which lowers the pH of the blood, Sensory cells in the medulla of the brain
register this drop in pH and send signals to the diaphragm and heart to
increase respiration. This will clear the CO2 from the bloodstream.