Your heart rate will increase when activity is done in the heat which creates increased cardiovascular activity. Additionally, the heat increases metabolism levels and speeds up the breakdown of glucose and fatty acids. The result is the body will increase the ability to burn fat and increase metabolism.
Though the core temperature may not rise more than 1/2 a degree, the soft tissue structures, including the fascia, ligaments, muscles and tendons will warm up quickly, allowing you to move further, and producing a fluid-like stretch allowing for greater range of movement. Your muscles and connective tissue increase extensibility as temperature increases, helping you go deeper into a stretch as the body becomes more flexible with less chance of injury.
The heat thins down the blood, allowing it to disperse deeply into body tissues and organs to facilitate healing. Stretching your muscles in heat stimulates blood flow, and encourages circulation to different parts of the body, improving the functioning of the circulatory system.
The heated environment demands a sharpening of mental focus, and concentration during training.