Aeons ago, life on earth consisted of single cells suspended in the primordial sea. They took their nourishment directly from the surrounding waters, and excreted their waste products to be carried away, automatically, by the currents.
As more complicated animals evolved, the sea became private and internal in the form of blood, but it retained the original salts and the chemically useful pH, or acid-alkaline balance, of roughly neutral. When the first creatures slithered onto land, they took that inner sea with them.
The balances have changed slightly in the millions of years since then, but the individual cells of the human body are as dependent on the blood’s constancy as the first cells were on the never-changing sea.
From Shock-Trauma (1980), by Jon Franklin and Alan Doelp