The human body is a biological machine made of body systems; groups of organs that work together to produce and sustain life. Sometimes we get lost while studying about cells and molecules and can’t see the forest for the trees. It can be helpful to step back and look at the bigger anatomical picture. The human body is a complex, highly organized structure made up of unique cells that work together to accomplish the specific functions necessary for sustaining life. The biology of the human body includes Physiology (how the body functions) Anatomy (how the body is structured) Anatomy is organized by levels, from the smallest components of cells to the largest organs and their relationships to other organs. Gross anatomy is the study of the body's organs as seen with the naked eye during visual inspection and when the body is cut open for examination (dissection). Cellular anatomy is the study of cells and their components, which can be observed only with the use of special techniques and special instruments such as microscopes. Molecular anatomy (often called molecular biology) is the study of the smallest components of cells at the biochemical level. The human body is a complex and intricate piece of engineering in which every structure plays a precise role. There are approximately 200 bones, 650 muscles, 79 organs, and enough blood vessels to circle the Earth twice! Anatomy and physiology change remarkably between fertilization and birth. After birth, the rate of anatomic and physiologic changes slows, but childhood is still a time of remarkable growth and development. Some anatomic changes occur past adulthood, but the physiologic changes in the body's cells and organs are what contribute most to what we experience as aging Our bodies consist of a number of biological systems that carry out specific functions necessary for everyday living. The job of the circulatory system is to move blood, nutrients, oxygen, carbon dioxide, and hormones, around the body. It consists of the heart, blood, blood vessels,arteries and veins. The digestive system consists of a series of connected organs that together, allow the body to break down and absorb food, and remove waste. It includes the mouth, esophagus, stomach, small intestine, large intestine, rectum, and anus. The liver and pancreas also play a role in the digestive system because they produce digestive juices. The endocrine system consists of eight major glands that secrete hormones into the blood. These hormones, in turn, travel to different tissues and regulate various bodily functions, such as metabolism, growth and sexual function. The immune system is the body's defense against bacteria, viruses and other pathogens that may be harmful. It includes lymph nodes, the spleen, bone marrow, lymphocytes (including B-cells and T-cells), the thymus and leukocytes, which are white blood cells. The lymphatic system includes lymph nodes, lymph ducts and lymph vessels, and also plays a role in the body's defenses. Its main job is to make is to make and move lymph, a clear fluid that contains white blood cells, which help the body fight infection. The lymphatic system also removes excess lymph fluid from bodily tissues, and returns it to the blood.