Brain, nerves & spinal cord.
Functions to provide strength and connections to move the arm or leg or to make the facial expressions. Muscles cover almost every bone of the body except fingers and teeth and skull. It also help in Maintaining posture & generate heat by shivering.
Pumps blood to each part of the body. Takes oxygen from the lungs and transports it to the each part of the body and carry carbon dioxide that is released through respiration. Heart, veins and arteries.
Skin has millions of sensory nerves that send information to the brain; the information regarding the environment, temperature, etc. If the brain senses that the temperature is a bit higher than the body temperature, brain sends signal to the sweat gland to open and cool the body. In some cases, the brain sends signal to the muscles connected to skin hair to contract or expand according to the environment or other condition.
At 25 years the body functions seem to be efficiently functional. On the other hand considering the daily working of the 25 year person is moving door to door to distribute the leaflets and take part in the other programs as organized by the charity.
The muscles (muscular system) in the body particularly the leg muscles require much oxygenated blood in order to work efficiently. The oxygenated blood is transported by the cardiovascular system and the heart pumps the blood as faster as the oxygen is required. Oxygen is supplied through the respiratory system. The respiratory system fills the blood with oxygen. It is common fact that the respiratory system and cardiovascular system synchronize in a proper manner. The rate of respiration increases as the demand for oxygen increases. On the other hand, as the muscular system uses more oxygen it produces more carbon-dioxide that is also released through respiratory process. All these functions are controlled by the brain that informs the heats to pump faster