What changes occur in the human body when we enter an anaerobic state?

When there body enters an state of aerobic exercise, the body is working in a zone of heart rate usually above 120bpm, however this is subjective to each individual. Due to the contractions of the muscles, and the call for red blood cells (which carry o2 around the body) the heart begins to pump quicker and stronger, with the lungs calling for more o2 causing quickened breath. The o2 is transferred to the blood from the alveoli to capillaries, and then carried to the muscles. You're body will also heat up due to the increased heart rate and generation f energy in your cells (as heat is a biproduct of this chemical reaction) and so the capillaries on the surface of your skin will expand, so as to have a greater surface area, and so heat can be realised quicker. In addition, you may sweat, helping the heat dispense even quicker with the heat energy being used to evaporate the heat. However, the muscles are working in an O2 deficit, meaning they re not getting enough o2 in comparison to how much they are using. This is anaerobic. Without sufficient o2, they (the muscles) are unable to create the energy they need to fire off for the exercise. As a result, they begin to create lactic acid, as a short term method of energy (creating ATP in the absence of oxygen). This can cause a burning sensation in the muscles if a build up occurs, until eventually the individual will need to stop the exercise, so the body can clear out the lactic acid from he cells when the o2 deficiency is met.