3.2—Organisation in multicellular organisms

In a multicellular organism, specialised cells of the same type work together as a tissue; tissues combine into organs, and organs cooperate in organ systems.

Specialisation makes division of labour possible: a red blood cell transports oxygen, a muscle cell contracts, and an epithelial cell absorbs. Their organisation lets a larger structure perform a function no single cell performs as effectively.

Cardiac muscle, blood-vessel and connective tissues combine to form the heart. The heart then works with blood vessels as part of the circulatory system, so the labels describe nested levels rather than interchangeable names.

A tissue is more than a pile of similar cells: the cells must cooperate in a shared function. A single specialised cell is not itself an organ or an organ system.