How the Human Body Is Organised

Your body is organised in levels. Cells are organised into tissues, tissues combine to form organs, and organs work together in systems. This hierarchy gives you a simple map for understanding how the body is built.

Before you learn individual organs and body systems, it helps to have a map of how the body is put together. For this course, we begin at the cell and use five levels: cell, tissue, organ, organ system and organism . Anatomy textbooks may also describe a chemical level below the cell. We start with cells because they are the smallest living units in this map. Cells: the smallest living units A cell is the smallest living unit of the body. An adult human body contains tens of trillions of human cells, although the exact number varies between people. Different cell types are specialised for different jobs. Muscle cells can contract, nerve cells carry signals, and cells in the skin help form a protective barrier. Their shapes and structures differ because their jobs differ. This introduces an important principle: structure and function are closely linked . How a body part is built helps it do its work. Tissues: cells organised for related work A tissue is an organised group of cells, often with supporting material around them, that carries out related functions. Muscle tissue can contract. Epithelial tissue forms coverings and linings. Connective tissue supports, links and protects structures. Tissues are not always made from one identical type of cell. Different cells and surrounding material can contribute to the same tissue. Organs: different tissues working together An organ is a distinct body structure made from two or more tissue types working together. Take the heart as an example. Cardiac muscle provides the force for contraction, connective tissue supports its structure, and an inner lining helps blood move smoothly through its chambers. Together, these tissues allow the heart to pump blood. Your lungs, stomach, skin and kidneys are organs too. Each contains several tissue types contributing to its functions. Organ systems: organs working together An organ system is a group of organs that work together to perform major functions or meet the body's needs. The digestive system is one example. The mouth, oesophagus, stomach, intestines, liver and pancreas each contribute to digestion, absorption or the processing of nutrients. Many anatomy textbooks describe about eleven major organ systems, although the exact grouping can vary because some organs contribute to more than one system. The organism: the whole person All the organ systems together make the organism, the whole living person. The levels are connected, but a change at one level does not automatically affect every other level. The effect depends on what changed, where it happened and how much function was affected. A heart attack gives one example. It happens when blood flow to part of the heart is blocked. The affected heart muscle is deprived of oxygen and cells may become injured or die. If enough heart muscle is affected, the heart may pump less effectively or develop a dangerous rhythm. A problem that begins with disrupted blood flow can therefore damage cells and tissue, affect an organ and disturb the wider circulation. This hierarchy is one useful way to describe what is happening, but clinicians combine it with many other kinds of information.

For this course, we start with five levels: cell, tissue, organ, organ system and organism. Lower levels contribute to higher ones, and changes at one level can sometimes affect the levels above.