The human body is made up of a number of different systems. Each system has a separate function, but some work together.
One system is the skeleton, which serves to support the body and protect the internal organs. The respiratory system enables us to breathe and take oxygen into the blood, which moves around the body by means of the circulatory system. The digestive system enables us to take in food needed for growth. Waste matter is ejected from the body by means of the urinary system. The nervous system controls the other systems and enables human beings to think.
The endocrine system consists of various glands, such as the thyroid, sex and adrenal glands. The function of these glands is to secrete chemicals, known as hormones, into the blood. These hormones control various processes in the body, such as growth, sexual activities and digestion..
Each system is made up of organs. The lungs, for example, are part of the respiratory system. The heart is an organ in the circulatory system. The liver functions as part of the digestive system and other systems.
Every organ is composed of several kinds of tissue. Epithelial tissue, which includes the skin, forms a covering over organs. Connective tissue supports and holds together parts of the body and includes bone and cartilage. Other types of tissue include nerve tissue and blood tissue.
All tissue consists of cells. These are so small that they are measured in thousandths of a millimeter and can only be seen with a microscope. Each cell is covered with a thin membrane which surrounds a nucleus, and a jelly-like substance, called cytoplasm. This in turn contains minute particles, each with its own special function.
