Final answer:
Hemoglobin is a protein in red blood cells that carries oxygen to body tissues and returns carbon dioxide to the lungs, with its four subunits each holding one oxygen molecule.
Step-by-step explanation:
Hemoglobin is a complex protein found in red blood cells (RBCs) that is essential for transporting oxygen from the lungs to the rest of the body and returning carbon dioxide from the tissues back to the lungs. It is composed of four protein subunits, two alpha and two beta subunits, with each surrounding a heme group that contains iron. This iron is critical as it binds oxygen molecules, with each hemoglobin molecule having the capacity to carry four oxygen molecules at a time. This property allows our blood to be highly efficient in oxygen transport, resulting in the bright red color of oxygenated blood, while deoxygenated blood is darker due to having less oxygen bound to hemoglobin.
The number of hemoglobin molecules within RBCs also defines a person's ability to transport oxygen efficiently and can be indicative of medical conditions such as anemia when the concentration is low. Standard measurements of hemoglobin are an important diagnostic tool and are typically performed with automated methods although manual methods are available.