We know that construction of a huge structure requires thousands of slabs and other material like steel or stones to make it functional. Similarly billions of cells, tiny or long, subtle or sturdy, construct a living being. All the cells in the human body are not alike. They differ greatly according to the tasks that they have to perform. Muscle cells, for instance, are long and have the ability to contract and shorten the muscle. There are two types of muscle. Some work when our brain tells them to perform. For instances, when we pick up a load, our brain sends signals to muscles in our arms. Other muscles work even when we are asleep, for example, our stomach muscles go on churning the food we have consumed. The cells of the skin are mostly quite flat as their task is generally to cover a surface of the body. It acts as a barrier to infection and prevents the body losing water and heat too rapidly. In fact nature has designed cells to carry out every necessary bodily task.
When many cells of the same or of different types are arranged together they form a tissue. These cells are put together by a substance called matrix. Its function is similar to that of the mortar placed between layers of bricks. The form of a tissue is determined by the nature and amount of the matrix and the type of cells. For example the quantity of matrix in the skin tissues is very small while in bone tissues it is quite large. The matrix is made from calcium phosphate and carbonate and magnesium phosphate and carbonate. They strengthen the bones. Our blood is made up of pale liquid called plasma and millions of cells or corpuscles. The blood cells float in it quite freely.
Different types of tissues arranged together form an organ which performs a particular task. In the human eye, for example, there are many different tissues, all of which assist in the process of seeing. Though eye is a very small organ of our body, it is very complicated in structure. It is made of different types of tissues. Some organs like liver are much bigger but they are largely composed of a single type of tissue.
When several organs work together to perform a special and complicated task, they are usually called a system. Digestive, respiratory, excretory etc. are the different systems which perform different tasks. Every system in the body helps each other and provides energy. For example, many of our activities are controlled by different parts of our brain. The hind brain, which consists of medulla oblongata, controls the heart-lings and digestive system.
All warm blooded animals and human beings have their own system which varies in different degrees in terms of functional behaviours.