System integration and the hierarchy of subsystemsSpec C3.1.1, C3.1.2, C3.1.3
In short
System integration is the coordination of the component parts of a system so that they collectively perform an overall function. In a multicellular organism, cells, tissues, organs and body systems form a hierarchy of subsystems. Nerves and hormones send signals between organs, and the blood transports materials and energy between them, giving emergent properties.
System integration is a necessary process in living systems. Coordination is needed for the component parts of a system to collectively perform an overall function. Without it, organs would work independently and the organism could not respond to its environment as a whole.
A hierarchy of subsystems
A multicellular organism is built as a hierarchy of subsystems: cells form tissues, tissues form organs, and organs form body systems. Each level is integrated with the others. Integration gives emergent properties: abilities of the whole organism that no single part has on its own.
A cheetah becomes an effective predator by integration of its body systems. Its eyes detect prey, the brain processes the information and plans the chase, motor neurons stimulate skeletal muscles to contract in sequence, the skeleton provides levers, and the heart, lungs and blood supply oxygen and glucose to the muscles. No single organ can hunt; the whole integrated organism can.
How organs are integrated in animals
| Feature | Nervous system | Endocrine system |
|---|---|---|
| Signal | Electrical impulses along neurons, chemical (neurotransmitter) across synapses | Hormones secreted by glands into the blood |
| Route | Along neurons to specific cells | Carried by the blood to all parts of the body; only target cells with receptors respond |
| Speed | Very fast (milliseconds) | Slower (seconds to hours) |
| Duration of effect | Usually short | Usually longer lasting |
| Example | Withdrawal of the hand from a painful stimulus | Epinephrine preparing the body for vigorous activity |
The blood system integrates organs by transporting materials and energy between them. For example, glucose absorbed in the small intestine is carried to the liver and to muscles; oxygen from the lungs is carried to every respiring tissue; urea from the liver is carried to the kidneys; heat from active muscles is distributed around the body. The blood also carries hormones from endocrine glands to their target organs.
When asked to distinguish nervous and hormonal signalling, give paired points (nerve impulses vs hormones in blood; fast vs slow; short-lived vs long-lasting; specific route vs reaches all cells).
Written and checked against the IB Biology HL specification · Updated October 2026