There are also changes in the respiratory response during the first few minutes of exercise. The respiratory system is responsible for getting oxygen into the body and getting carbon dioxide out of the body. Firstly, there is an increase in breathing rate. This increase in breathing rate means that there is more oxygen inhaled into the body so that the body can keep on going without fatiguing if the exercise goes on for longer than the first two minutes. Because when you start to exercise, you need to take in more oxygen for it to be used up to help produce energy, tidal volume increases. Tidal volume is the amount of air inhaled and exhaled with each breath. Intercostal muscles are used to help with breathing during exercise. External intercostal muscles help with inspiration and internal intercostal muscles help with expiration. Therefore with training, it is possible to increase the efficiency of your breathing during exercise. Also, during anaerobic exercises, it is possible to perform the valsalva manoeuvre which is the process of breathing out against a closed mouth and nose. Pulmonary ventilation is a measure of the rate of ventilation, meaning the exchange of air between the lungs and the air. The rate of pulmonary ventilation is defined as the tidal volume multiplied by the number of breaths taken per minute. During the first minute of exercise, breathing rate increases and so does tidal volume. Therefore, the pulmonary ventilation also increases.
When we want our muscles to move for exercise, we must send a message from our brains to our muscles through nerve impulses. These nerve impulses are small electric currents which run through the central nervous system, through the nerves and then into the muscle tissue. These nerves that send the signal are known as motor neurones. This is what happens in the first moments of exercise, so that the brain knows that the muscles need to work. The neuromuscular junction is where the nerve meets the muscle. Here, the nerve transmits its signal to make the muscle contract. Firstly, the presynaptic membrane releases acetycholine. This then diffuses across the gap and produces an electrical signal. If this signal is big enough, the muscle then contracts. After the muscle has completed its job, cholinesterase breaks down the acetycholine so that the process is ready to start again. Motor units are groups of muscle fibres. There is a signal sent from the central nervous system down to the motor unit to tell it whether or not to contract. However, it can only fully contract or not contract at all. During the first minutes of exercise these motor units produce muscle contraction at different rates. Many of these groups all contracting at the same time results in one smooth muscle contracting ready for exercise. Muscle spindles then detect when the muscle is contracted. If they find that the muscle is contracted, the spindle changes in tension to tell the central nervous system that the muscle is contracted. The central nervous system can then either continue the contraction if the exercise is going to last longer than the first minutes or relax the muscle if the exercise ends.
To take part in exercise, our muscles need to be able to contract. For this to happen we need to produce adenosine triphosphate through our energy systems. When the phosphate is broken off the ATP to make adenosine triphosphate energy is released from the chemical bonds being broken. This energy is then used to make the muscles contract for the start of exercise. There are three energy systems in our body that make the ATP we need. As soon as we start exercising, the system that we use to produce ATP is the phosphocreatine system. This produces ATP much quicker than any other system and does not involve oxygen which means it is a anaerobic energy system. This system is used for the first ten seconds of the two minutes of exercise but cannot last for longer than that as it is designed for short bursts of energy such as sprinting. This is where the lactic acid system comes in. In the absence of oxygen, glucose is broken down into pyruvate. This is then converted into lactic acid which produces ATP very quickly. This system will produce the majority of energy for the first two minutes of exercise. The stored ATP will be used on the outset, then for the next ten seconds the phosphocreatine energy system will be used and for the remaining time the lactic acid system is used as it can last up to three minutes.