Whether you are new to exercise or a die-hard workout fanatic, exercise affects your body the same way. The only real difference between a novice and an elite exerciser is work capacity -- both in terms of duration and intensity. Where an elite exerciser's body is adapted to working out, a novice's body has yet to make the same adaptations. Regardless, there are numerous things going on beneath your skin when you exercise.
Take a Breath
When you exercise, your muscles demand more oxygen. To facilitate this, your breathing rate increases, as does the size of each breath, and this increases oxygen intake by as much as 15 times above normal. The maximum amount of oxygen you can take in, transport and utilize during exercise is called your VO2 max and is a common measure of aerobic fitness. The higher your VO2 max, the higher your aerobic work capacity.
The Heart of the Matter
As your breathing rate increases, so does your heart rate as it works hard to pump oxygenated blood to your working muscles. A heart rate of 180 to 200 beats per minute is not unusual. The amount of blood pumped per beat also increases, and the combination of heart rate and heart output per beat is called your cardiac output. In an effort to supply as much blood as possible to your working muscles, blood flow to your digestive organs and other non-essential systems is diverted. Blood vessels dilate to allow for the increased blood flow, and while blood pressure tends to increase with exercise, this vasodilation helps prevent excessively high blood pressure.
Boning Up on the Skeleton
Anywhere two bones meet in your body, a union or joint is formed. Freely moving joints, such as your knees and shoulders, are called synovial joints, and the end of the bones making up a synovial joint are covered in hyaline cartilage and lubricated with a oil-like substance called synovial fluid. Produced on demand and especially during exercise, synovial fluid nourishes and lubricates your joints, which results in an increased range of motion and easier movements. This is one of the reasons it is important to warm up before strenuous exercise.
As blood flow increases to your muscles, they get warmer and contract, relax and stretch more easily. Muscle fibers are arranged in groups called motor units, and as you warm up, you become better able to innervate or turn on more motor units simultaneously. The more motor units you can turn on simultaneously, the more force you can generate. This is why a weight that feels initially heavy feels less so later in your workout. To allow for an even greater oxygenated blood flow to your muscles, microscopic blood vessels called capillaries dilate. You can see this in a reddening of the skin over your muscles, and your muscles may get slightly bigger as they are engorged with blood. Bodybuilders call this phenomenon the pump.