In this lesson, we will review the biological hierarchy of organization. Anatomy is the study of the body’s structures and physiology is a study of how those structures function. The human body is very complex and to truly understand its complexities we need to first understand how it is organized. Think about a house. It’s made from a lot of small parts, like boards, nails, and wiring, for example. Right? When we look at living organisms, we see the same thing: a hierarchy of organization, from the simplest level of organization or simplest parts up to the most complex. We refer to this as the biological hierarchy of organization. Here are the levels in the biological hierarchy of organization, from simplest to most complex: starting with the atom; the molecule or compound, the macromolecule; the organelle; the cell; the tissue; the organ; the organ system; and, finally, the organism. Now let’s go through these starting with the simplest, the atom. Chemistry is a science of matter and all matter is made of chemical elements, such as carbon, nitrogen, and oxygen. The smallest piece of an element is an atom. Atoms can unite together to form molecules or compounds. To understand this, consider water. It is made of two atoms of hydrogen joined to one atom of oxygen to form a molecule of water, H2O. Water molecules are quite small but some molecules, such as fat and DNA for example, are rather large. So these are often referred to instead as macromolecules, with “macro” meaning large. Atoms, molecules and macromolecules provide the nutrients and building materials our bodies need to stay alive and healthy, and they also participate in chemical reactions that do all of the work performed in our bodies. Macromolecules can unite together to build complex structures that carry out various functions inside cells. These are the organelles. You can think of the organelles as being like “mini organs” inside of the cells. Examples of organelles include mitochondria, ribosomes, and centrioles. Cells contain the combination of organelles necessary to sustain life and, for this reason, life begins at the cell. With few exceptions, your body’s cells are alive. In fact, cells are the basic units of all living organisms. Tissues are groups of cells organized together to perform some common function. For example, muscle tissue contracts to provide movement. There are four general categories of tissues. Epithelial tissues form all of the body’s linings and coverings, such as the outer surface of your skin or the lining of your stomach. Connective tissues have many functions and include bone, cartilage, blood, and fibrous connective tissue, which forms very strong ligaments and tendons. Muscle tissue provides movement both of materials through the body as well as movement of the body itself. And nervous tissue allows control of various body processes and also communication throughout the body. Tissues can organize into larger functional units called organs, such as your lungs, your heart, and your liver. Each organ performs at least one specific job. Your lungs, for example, bring in oxygen and clear out carbon dioxide. Multiple organs join together to form organ systems, each with some overall function. For example, the organs in your cardiovascular system are your heart and your blood vessels, and they carry your blood, which is a type of tissue. The heart acts as a pump and the vessels are the pipes, so to speak, through which your blood travels. These structures work together to deliver nutrients and oxygen to your cells to supply their needs and to haul away their waste products. Our organ systems include the integumentary system, which is mostly our skin; the skeletal system, which includes our bones; the muscular system, which includes all of our muscles and their tendons; the nervous system, which includes the brain and all of the nerves in the body; the cardiovascular system that we just spoke about; the lymphatic system, which helps keep us healthy; the respiratory system, that brings in oxygen and clear carbon dioxide; the digestive system, that brings in nutrients; the urinary system, that fine tunes our body fluids and clears our waste products; and the reproductive system, which helps us continue our species. When you put all of these organ systems together, as well as all the levels of organization