As the temperature inside the car is too high, Baxter lost a lot of water through evaporation when he panted, it causes Baxter to become dehydrated. Baxter has an increase in heart rate because of dehydration. During dehydration, water leave the cell because the cell has a lower salt concentration than the concentration in the body. Red blood cell will have less space to carry oxygen to other cell, which makes Baxter lack of oxygen. In order to provide enough oxygen to it, its heart rate becomes faster so it pumps more blood. This can help Baxter to get more oxygen.