Hemoglobin is a protein found in human blood that gives our blood cells their characteristic red color.
Hemocyanin, a substance found in some animals like octopuses that contain copper instead of iron, causes the blood of these creatures to appear blue.
Human blood:
Blood is a constantly streaming liquid that gives sustenance, oxygen, and removes waste from the body.
Although blood is inherently fluid, it contains cells and proteins that make it "thicker" than pure water. The typical individual's blood volume is around 5 liters.
Human blood is red because hemoglobin, a protein that circulates in the blood and controls oxygen delivery, is iron-rich and red.
The majority of species, including humans, have red blood.
There are, however, a few exceptions. Blue blood is found in octopuses, squid, and crustaceans. The copper levels in their blood are abnormally high. Copper gives their blood its blue tone.