1. Introduction

What will you do first if you see a person becoming unconscious? You may check whether he is breathing, or you may check his pulse or for any body movements. If you see any of these movements (breathing or pulse or body movements), you can decide that the person is alive. So, we have a notion that a living organism is alive if it has a movement.

 

Seed germinates and grows into a plant by pushing the soil aside. If there is no growth of the plant, there will be no movement of it. In plants, movement indicates the plant's growth, whereas, in animals, movement is not solely for growth.

 

Imagine you are touching a hot pot with bare hands.  You will immediately remove your hands from it. If a dog chases a cat, the cat will run fast to save its life. So, in animals, movement will happen due to environmental changes. These environmental changes may be favourable or unfavourable.

Similarly, all living organisms respond to different kinds of stimuli like light, heat, cold, sound, smell, taste, touch, pressure, pain or the force of gravity etc. Thus, on receiving a stimulus, the body responds in a manner that is most appropriate for its survival and functioning.

 

Have you ever seen the movement of a tiger during a hunt? It carefully controls every step of the movement so that the prey won't notice the tiger's approach towards it. Similarly, if you run fast, your heart will beat more quickly, and lung muscles expand and contracts faster and if you walk slowly, your heart will beat normal and lung expand and contract at normal rate.

 

From the above examples, we can say, controlling movements is essential for any living organism, both voluntarily and involuntarily, to make an appropriate movement if there is a change in the environmental condition.

If you run fast, your muscles need more energy, and to get more energy, blood flow in the muscle has to be high. So, the heart will beat faster,  and the lungs will contract and expand fast to diffuse more oxygen in the blood.