Question

1. How does the brain "know" when to increase the breathing rate? What gas is the gas that "drives" our breathing rate when this gas goes up our breathing rate goes up

2. Why did lan creatures, like us, have to develop a respiratory system in the first place?

Answer 1

1. The chemoreceptors in the body act like sensor and detect the level of carbon dioxide in the blood by monitoring the concentrations of hydrogen ions in the blood. Any change or dysregulation in the pH of the blood results in the sending of signals tot he medulla which then signals the respiratory muscles to decrease/increase the ventilation rate so carbon dioxide levels and pH can return to normal levels.

This is how the brain actually controls the breathing rate.

A rise in carbon dioxide means that the oxygen level is declining in the blood. This stimulates the brain to increase the respiratory rate of the body to overcome the decline in the oxygen levels.  

2. Land animals are very large in size and are complex so they cannot maintain gas exchange by diffusion across their outer surface. Therefore, respiratory surface or system needs to be developed in such animals to increase the surface area for exchange.