Levels of organisation

The chemical level: atoms and molecules

The chemical level is the simplest. An atom is the smallest unit of an element, a pure substance made of one kind of atom. Examples in the body include carbon, hydrogen, oxygen, nitrogen, calcium and potassium. Atoms join by chemical bonds to form a molecule. Small molecules include water and oxygen gas; larger ones are called macromolecules, such as proteins, carbohydrates, lipids and nucleic acids. Each molecule has a shape, and that shape determines what it can do. Haemoglobin, for example, is a large protein whose shape lets it bind oxygen. You do not need to master chemistry to study anatomy, but the chemical level explains why the body needs water, minerals and nutrients.

For example: A water molecule is two hydrogen atoms bonded to one oxygen atom. The same three kinds of atom arranged differently would give a different substance with different behaviour.

The cellular level

The cell is the smallest unit that can carry out all the basic activities of life, including using energy, responding to its surroundings and, in many cases, dividing. A cell is not a simple bag: inside it are organelles, small internal structures each with a job. The nucleus holds most of the cell’s genetic material; mitochondria release usable energy from nutrients; the cell membrane controls what enters and leaves. Cells that will do different jobs become specialised in a process called differentiation. A nerve cell develops long extensions to carry signals, while a muscle cell fills with contractile proteins. Once differentiated, a cell’s structure matches its task — the complementarity principle met again at a new level.

For example: A red blood cell differentiates by losing its nucleus and packing in haemoglobin, which leaves more room to carry oxygen.

Common misconception: Differentiation is not the same as damage. A specialised cell has changed which of its genes are active, not lost the ability to function.

The tissue level

A tissue is a group of similar cells, and the material around them, working together for a common function. The body has four main tissue families: epithelial tissue, which covers and lines surfaces and forms glands; connective tissue, which supports, binds, protects and transports; muscle tissue, which produces movement; and nervous tissue, which carries and processes information. An organ usually contains several tissue types, because a complex job needs more than one kind of cell. The stomach wall is a clear example: epithelium lines and secretes, muscle mixes, connective tissue holds the layers together, and nervous tissue controls the activity. Recognising tissues lets you reason about what an organ can do and what goes wrong when a tissue is injured.

For example: Scar tissue after a deep cut is mostly collagen-producing connective tissue, which is strong but cannot contract or carry signals like the tissue it replaced.

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Organs, systems and the organism

An organ is a structure made of two or more tissue types that performs a specific set of functions. Your stomach, heart, liver and kidney are organs. An organ system is a group of organs that cooperate to carry out a major function for the body; the digestive system includes the mouth, oesophagus, stomach, intestines, liver, gallbladder and pancreas. Medical teaching usually divides the body into about eleven systems, such as the cardiovascular, respiratory, nervous and urinary systems. At the top of the sequence is the organism, the single living being formed by all the systems working together. This highest level is not a sum of independent parts: a change in one system usually alters others, which is why a nurse assesses the patient as a whole rather than one organ at a time.

For example: Breathing supplies oxygen, the blood carries it, and muscle cells use it to release energy. One function already crosses three systems.

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New properties appear at each level

The most important idea in this section is that each level has properties its parts do not have alone. A single protein molecule cannot digest a meal; a stomach can. A single neuron cannot produce thought; a brain network can. These new abilities are sometimes called emergent properties, because they emerge from the arrangement and interaction of the parts rather than from any one part. Emergent properties are the reason a drug, a disease or an injury can affect a whole function even when its first target is one molecule or one cell. They are also why you cannot fully understand health by studying only the smallest components: you must keep moving up and down the levels, connecting a molecule to its cell, its tissue, its organ, its system and the person in front of you.

For example: Pain is an emergent experience: receptors, nerve pathways and the brain each provide a part, but none alone produces the feeling the patient reports.

Common misconception: Learning a lower level does not replace learning a higher one. Knowing the molecules of muscle does not by itself explain how a hand grips an object.