How the body is organised

Cells, tissues, organs, systems.

Four levels

Cells group into tissues, tissues into organs, and organs into systems. Naming the level makes anatomy far easier: the heart is an organ, cardiac muscle is a tissue, and circulation is the system.

There are roughly 30 trillion human cells in an adult body.
Most organs belong to more than one system: the pancreas serves digestion and hormones.

Keeping conditions steady

Homeostasis is the constant correction that keeps temperature, blood sugar and acidity inside narrow limits. Almost every system in the body exists to hold something steady.