The basic and functional unit of the human body are cells. The cells in the body unite to form tissue. There are four types of tissues which include epithelial tissue (the tissue lining the organs), connective tissue (the tissue for support and structure to the body), muscle tissue (the tissue which contracts- actin & myosin) and nerve tissue (tissue generating electrical signals- neurons and glial cells).

Two different tissues join to form the organ. Organs include kidney, liver, heart and skin. The skin differentiated into 3 layers-epidermal layer, dermal layer and subcutaneous layer. The epidermis made of epithelial tissue, acts as a barrier. The dermal layer made of connective tissue with blood vessels, help in nourishment of the skin cells. The subcutaneous layer made of connective tissue helps in protection of the body from cold temperatures and maintaining equilibrium within the body.

Two or more different organs performing for a common purpose are called organ systems. The organ systems include skeletal system (for support to the body),  muscular system (for movement), circulatory system (to supply nutrients, gases, hormones), nervous system (to control physiological process like digestion, circulation), respiratory system (to exchange gas from environment), digestive system (to break and absorb nutrients), excretory system (to remove wastes, toxic products), endocrine system (to pass the chemical messages), reproductive system (to produce cells required for reproduction) and immune system (to destroy and remove microbes).

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