How the Human Body Is Organised

You are one person made from tens of trillions of living cells. Those cells are organised into tissues, tissues into organs, and organs into systems. Seeing that structure gives you a map for understanding the rest of the body.

The hierarchy is useful because it lets us describe the body at different scales, from microscopic living units to the whole person. For this course, we use five levels: cell, tissue, organ, organ system and organism . They give us a simple way to move from one level of organisation to the next. Cells A cell is the smallest living unit of the body. Different types of cells are specialised for different jobs. Muscle cells can contract, nerve cells carry signals, and skin cells help form a protective barrier. Their structures differ because their jobs differ. How a body part is built helps it do its work. Tissues A tissue is a group of cells organised to carry out related work, often with supporting material around them. Muscle tissue, for example, is organised to contract. A tissue does not have to contain one identical type of cell. Different cells and material between them can contribute to the same tissue. Organs An organ is a distinct body structure made from two or more tissue types working together. The heart is one example. Heart muscle provides the force for contraction, while other tissues support and line the organ. Together, these tissues allow the heart to work as a pump. The lungs, stomach, skin and kidneys are organs too. Each combines different tissues to carry out its functions. Organ systems Organs also work together to carry out larger jobs. A group of organs working in this way is called an organ system . In the digestive system, the mouth, stomach, intestines, liver and pancreas each contribute to handling food and nutrients. These groupings are useful maps rather than perfect boxes. Some organs contribute to more than one system. The whole person All the organ systems together make the whole living person, or organism . A change at one level may stay local or affect higher levels. That depends on what has changed, where it is and how much function is affected. If enough cells in an organ are damaged, the organ may not do its job as well, and other parts of the body may then be affected. Clinicians can describe the same problem at more than one level. The hierarchy helps show where a problem begins and how far its effects reach.

The body can be described at five linked levels: cell, tissue, organ, organ system and organism. Lower levels contribute to the larger ones, and a problem may stay local or affect higher levels depending on what has changed.