3/25/22, 12:27 PMFunctions of Human Life – Anatomy and Physiology2/8OrganizationA human body consists of trillions of cells organized in a way that maintains distinct internalcompartments. These compartments keep body cells separated from external environmentalthreats and keep the cells moist and nourished. They also separate internal body fluids from thecountless microorganisms that grow on body surfaces, including the lining of certain passagewaysthat connect to the outer surface of the body. The intestinal tract, for example, is home to morebacterial cells than the total of all human cells in the body, yet these bacteria are outside the bodyand cannot be allowed to circulate freely inside the body.Cells, for example, have a cell membrane (also referred to as the plasma membrane) that keepsthe intracellular environment—the fluids and organelles—separate from the extracellular environ‐ment. Blood vessels keep blood inside a closed circulatory system, and nerves and muscles arewrapped in connective tissue sheaths that separate them from surrounding structures. In the chestand abdomen, a variety of internal membranes keep major organs such as the lungs, heart, andkidneys separate from others.The body’s largest organ system is the integumentary system, which includes the skin and its as‐sociated structures, such as hair and nails. The surface tissue of skin is a barrier that protects inter‐nal structures and fluids from potentially harmful microorganisms and other toxins.
