THE INTRODUCTION

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The human body is arguably the most complex organism that exists on this planet. made up of more that 100000 billion cells, each with its own unique identity, the human anatomy is a fabulously synchronized and integrated living machine that can perform a remarkable range of activities and functions.
           A human being consists of a single structure, but this one entity is made up of billions of smaller structures of four major kinds. these are cells, tissues, organ and systems.
        Cells have long been recognized as a simplest unit of living matter that can maintain life and reproduce themselves. the term cell was first applied by Robert Hooke, a 17th century English scientist , who compared the internal structure of a piece of cork to the cell inhabited by monks in a monastery. The human body, which is made up of numerous cells, begins, as a single, newly fertilized cell. the characteristics common to all living cells include the ability to reproduce, breath, move, react to external stimuli and create or utilize energy in order to perform their tasks. as human beings have evolved, many cells of the body have become increasingly more specialized, performing ever more intricate and incredible functions.one example is in the retina of eye, in which there are two kinds of cone cells, some of which react to red light and some to blue or green.

           When similar kinds of cells are grouped together in the body, the form tissue, such as the epithelial cells that make up the protective coverings of body surfaces and the lining of the lungs and the intestines. Tissues are somewhat more complex units than cells. this is because, by their very nature , the are organizations of a great many similar cells with varying amounts and types of non living, intercellular substance between them.
          Organs are yet more complex than tissues. an organ is an organization of several different kinds of tissues arranged together in such away that they can perform a special function. For example, the stomach is an organization of muscles connective epithelial and nervous tissues. muscle are connective tissues form its wall, epithelial and connective tissues form its lining and nervous tissue extends throughout both its wall and its lining.
         Distinct systems comprise the most complex of the component units of the human body. a system is an organization of varying numbers and kinds of organs so arranged that together they can perform complex functions for the body. these systems interact with one another as well as the innumerable other structures within the body to ensure that the human anatomy functions as one incredible, totally synchronized and integrated entity. The ten key systems that make up the human body are the skeletal system, the muscular system, the nervous system, the endocrine system, the cardiovascular system, the lymphatic system, the respiratory system, the digestive system, the urinary system and the reproductive system.                                                             

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