Why is carbon considered tetravalent?

All what the word tetravalent means is that carbon can form 4 chemical bonds, due to the fact that carbon has 4 unpaired valence electrons in its furthest shell, which can be used to bond to other atoms, to form linear and cyclic structures in organic compounds.

Tetravalent also tells us indirectly what the molecular geometry of the atom would be if bonded to those number of bonds, if 4, then it would be tetrahedral and have bond angles of 109.5.

The heart pumps blood to the whole body, and circulates deoxygenated blood back to the lungs.