We all evolved from aquatic life forms of billions of years ago in the sea.

Since then we have been keeping the sea of that period alive in our bloodstream. https://twitter.com/CholericCleric/status/1323438504089653250

Our ancestor species were washed over by seawater from which they got oxygen, nutrition, etc. It also washed away the CO2 and excreta they generated, keep them healthy

The sea had a particular balance of chemicals, minerals, gases, etc. to which they had adapted.

When their progeny evolved into more complex forms, they "wrapped" the ocean within themselves, and have since then maintained that ocean in their blood, matching closely the seawater from that period including the osmolality, potassium, sodium, temperature, acidity, etc.

They evolved body mechanisms to carefully check how to maintain these balances within narrow parameters.

These mechanisms are collectively called "Homeostasis"

This is why when O2 in our blood goes down or CO2 goes up, our breathing is automatically speeded up, for example.

There are dozens of other things similarly kept in control by homeostasis.

This development gave the newer species some independence. It meant now that every time the seawater's chemical balance changed they would not need to die.

The independence also meant they could now venture out of the sea. This allowed our lineage of species to transition from aquatic to terrestrial species.

This fact that we all carry in our bloodstream a replica of the ocean water from billions of years ago, and we continue to maintain it, never ceases to amaze me.

To be sure, not all scientists agree with this line of thinking. Some have pointed out that the sea probably had salinity higher than our blood's salinity. But, this too is not easy to prove. Also, the species could have evolved in brackish water, not the full-blown sea.