To most people being fit means having a greater endurance capacity or being able to keep going for longer.At the core of this process is the cardiovascular system - your heart,lungs and blood.

When you exercise,your muscles need more oxygen in order to convert their fat and carbohydrates stores into usable energy.As you get fitter your CV system improves its ability to deliver oxygen to the working muscles.At the same time your muscles adapt and become more efficient in their use of oxygen,as well as increasing their carbohydrate stores.

So regular biking trips can cause all these things to happen....

LUNGS..
----------
The bodyneeds oxygen to produce energy to keep you biking.Frequent exercise develops the body`s ability to both take in and then deliver oxygen to the exercising muscles.Which means you can cycle harder for longer periods of time.

BLOOD AND VEINS
-----------------
Exercising muscles demand more oxygen and nutrients to be delivered to them.To do this blood flow increases with more capillaries developing in the muscles.In addition there`s a greater opening of existing capillaries and more effective blood distribution.Plus your resting blood pressure decreases.

BRAIN
-------
Exercise causes the releases of endorphins (natural pain killers) in the brain.This is why after exercising you feel great,almost as if you`re on a high.This helps to combat the effects of stress.

MUSCLES
---------
Your muscles become more efficient at generating energy and combatting tiredness,so they can keep working for longer periods of time.
This happens in a number of ways....
* They can tolerate greater levels of lactic acid,the substance which causes that tying up feeling in your muscles,this is known as the lactate threshold.
*they can store more carbohydrate (the muscle fuel).
*there is an increase in the substances in the muscle which enable it to work longer and harder b
efore tiring.

HEART
-------
Your heart is a type of muscle,and it develops in response to exercise in a similar way to the rest of the muscles in your body.To provide for the demand for greater bloodflow around the body the following happens....
*it gets bigger and stronger.
*it becomes more efficient and can pump more blood with each beat.When you`re exercising flat-out this means you can exercise even harder.
*consequently your heart rate decreases,both while exercising and resting.

BONES
-------
Exercise increases both bone and muscle density.This means there is less risk of developing osteoporosis (brittle bone disease).

BODY WEIGHT
-------------
To fuel your activities your body burns more calories.This results in both body weight and the amount of body fat decreasing.

So all in all,cycling has to be a great all round,healthy pastime.