Principal systems of the human body include:

• The nervous system.
• The respiratory system.
• The immune system.

And, possibly the most important for everyone on Everything:

• The Hypertext system

Compare:

Shakespeare, Hamlet:

What a piece of work is man! How noble in reason! how infinite in faculties! in form and moving, how express and admirable! in action how like an angel! in apprehension, how like a god! the beauty of the world! the paragon of animals!

Neal Stephenson, Cryptonomicon:

The room contains a few dozen living human bodies, each one a big sack of guts and fluids so highly compressed that it will squirt for a few yards when pierced. Each one is built around an armature of 206 bones connected to each other by notoriously fault-prone joints that are given to obnoxious creaking, grinding, and popping noises when they are in other than pristine condition. This structure is draped with throbbing steak, inflated with clenching air sacks, and pierced by a Gordian sewer filled with burbling acid and compressed gas and asquirt with vile enzymes and solvents produced by the many dark, gamy nuggets of genetically programmed meat strung along its length. Slugs of dissolving food are forced down this sloppy labyrinth by serialized convulsions, decaying into gas, liquid, and solid matter which must all be regularly vented to the outside world lest the owner go toxic and drop dead. Spherical, gel-packed cameras swivel in mucus-greased ball joints. Infinite phalanxes of cilia beat back invading particles, encapsulate them in goo for later disposal. In each body a centrally located muscle flails away at an eternal, circulating torrent of pressurized gravy.

pointed out by Beth