The river of blood carries oxygen. Breathe deep and feel oxygen fill your lungs and fill your body. Imagine your blood circulating from heart to lung to muscle, delivering oxygen to hungry cells. Oxygen shuns water, so blood must be thicker.
Blood is thick with hemoglobin, carrier of oxygen.
Hemoglobin grabs oxygen when it is plentiful, as your blood rushes through lungs, and releases oxygen when scarce, in straining muscles and tissues. The hemoglobin in liter of blood carries the oxygen from a liter of air.