Hemoglobin was the compound in red blood cells that gave them their red color and served to convey oxygen to the body tissues.
Human hemoglobin was iron-based, while Vulcan hemoglobin was copper-based and green in color. (TOS: "The Man Trap", "Obsession"; TAS: "The Pirates of Orion"; ENT: "The Communicator")
In 2268, Spock suggested he ought to set a trap for the dikironium cloud creature, given the low level of hemoglobin in his blood. (TOS: "Obsession")
In 2268, after Leonard McCoy had received treatment for his xenopolycythemia, Spock reported that the doctor's hemoglobin count was back to normal. (TOS: "For the World is Hollow and I Have Touched the Sky")
Terran leeches consumed hemoglobin. In 2370, Doctor Beverly Crusher compared the interphasic organism's consumption of cellular peptides to leeches consu,ming hemoglobin. (TNG: "Phantasms")
In 2374, a shrunken Julian Bashir dissuaded a shrunken Miles O'Brien from leaving the miniature USS Rubicon, as the oxygen molecules outside were nearly 2,000 times too big for his hemoglobin to take in. (DS9: "One Little Ship")