Red blood cells, or erythrocytes, are the body’s primary couriers of oxygen. Making up about 40% of blood’s volume, these cells pick up oxygen in the lungs and deliver it to every tissue and organ. This oxygen supply powers cellular functions that sustain life. The cells also transport the waste product, carbon dioxide, back to the lungs to be exhaled.
The Role of Hemoglobin
The ability of a red blood cell to transport oxygen lies with a specialized protein called hemoglobin. Each red blood cell is packed with approximately 270 million of these molecules. The structure of a single hemoglobin molecule is an assembly of four protein chains: two alpha-globin and two beta-globin chains in adult hemoglobin.
At the center of each globin chain is a non-protein structure called a heme group. The heme group contains a single iron atom (Fe2+) that acts as a binding site for an oxygen molecule. This iron allows oxygen to be picked up and released. Because each hemoglobin molecule has four heme groups, it can transport four oxygen molecules at once.
When an oxygen molecule binds to one iron atom, it induces a slight change in the shape of the hemoglobin molecule. This structural shift makes it easier for the other three heme groups to bind their own oxygen molecules. This cooperative binding ensures that hemoglobin can quickly become saturated with oxygen in the lungs, maximizing transport efficiency.
The Gas Exchange Cycle
The journey of a red blood cell is a continuous loop of gas exchange between the lungs and the body’s tissues. This cycle begins when deoxygenated blood is pumped from the heart to the lungs. In the lungs, blood flows through capillaries surrounding millions of small air sacs called alveoli.
Within the alveoli, the high concentration of oxygen after inhalation drives oxygen across the thin walls of the alveoli and capillaries to bind to hemoglobin. This process, called oxygenation, transforms dark red, deoxygenated blood into the bright red color of oxygen-rich blood.
This oxygenated blood travels from the lungs back to the heart, which then pumps it to the rest of the body. When red blood cells reach tissues where oxygen levels are low, the reverse process happens. The lower oxygen concentration prompts hemoglobin to release its bound oxygen for use in energy production. After releasing oxygen, the deoxygenated hemoglobin picks up waste carbon dioxide for the return trip to the lungs to be exhaled.
Conditions Affecting Oxygen Transport
Several conditions can impair oxygen transport by disrupting the function of red blood cells or hemoglobin. Anemia is a widespread condition characterized by a deficiency in the number of red blood cells or a lower-than-normal amount of hemoglobin within those cells.
Another serious impairment is carbon monoxide (CO) poisoning. Carbon monoxide is a gas that, when inhaled, binds to the same iron site on hemoglobin that oxygen uses. The bond it forms is over 200 times stronger than the bond hemoglobin forms with oxygen.
This process blocks oxygen from being picked up in the lungs, leading to a state where the blood cannot deliver sufficient oxygen to tissues. The binding of CO also changes the hemoglobin molecule’s shape, causing it to hold on more tightly to any oxygen it is still carrying, preventing its release. This dual effect makes carbon monoxide dangerous, as it starves vital organs of oxygen.