What is hemoglobin \((\mathrm{Hb})\) ? What is its function?

Inside every red blood cell, or erythrocyte, there exists a remarkable protein known as hemoglobin (Hb). This protein is quintessential to our survival, as it facilitates a very critical physiological process - the transportation of oxygen. Hemoglobin is a complex protein made up of four polypeptide chains, each surrounding an important structure called a heme group. At its core, each heme group contains an iron atom, which is pivotal for binding oxygen.

The structure of hemoglobin allows it to perform efficiently within the red blood cells. The flexibility of the protein enables it to change shape slightly when oxygen binds, which assists in the oxygen pick-up and release process. This adaptability is crucial because the protein must maintain its function across a wide range of physiological conditions.

Oxygen transport is hemoglobin's most notable role. The journey of oxygen begins in the lungs, where hemoglobin binds with oxygen to form oxyhemoglobin. Upon reaching the body's tissues via the circulatory system, the oxygen is released to facilitate various cellular activities. This release is precisely regulated by factors such as pH and temperature, ensuring that oxygen is delivered where it is most needed.

The cooperative binding of oxygen - a characteristic feature of hemoglobin - means that as one molecule of oxygen binds, it increases the affinity for the next oxygen molecule to bind. Conversely, as one molecule of oxygen is released, the affinity for holding onto remaining oxygen molecules decreases, which aids in efficiently offloading oxygen throughout the tissues.

Aerobic respiration is the process by which cells generate energy in the form of adenosine triphosphate (ATP) through the utilization of oxygen. The oxygen that hemoglobin transports is critical for the completion of this process. When oxygen is released from hemoglobin, it diffuses into cells and is used in the mitochondria to help convert nutrients like glucose into ATP, carbon dioxide, and water.

Aerobic respiration is vastly more efficient at energy production than anaerobic respiration (which occurs in the absence of oxygen) and is thus vital for high-energy-demand activities and for the general sustenance of multicellular organisms. Hemoglobin's role in supplying oxygen is therefore essential not just for survival, but also for an organism to thrive and perform complex functions.

While much attention is given to oxygen transport, hemoglobin's role in carbon dioxide removal from the body is equally significant. Carbon dioxide is a byproduct of aerobic respiration and must be carried back to the lungs for exhalation. Hemoglobin assists in this process by binding to carbon dioxide directly or by facilitating its transport in the form of bicarbonate ions.

This dual function of oxygen delivery and carbon dioxide removal underscores the crucial role hemoglobin plays in maintaining the delicate balance required for the body's respiratory gas exchange. Efficient removal of carbon dioxide ensures that the body's pH levels remain stable, thereby allowing all biochemical processes to occur in their optimal state.