Why is hemoglobin in mammals confined with a red blood cell instead of being dissolved plasma?

Hemoglobin is confined within red blood cells in mammals for several important reasons:

High Oxygen Concentration: Red blood cells allow for a higher concentration of hemoglobin compared to plasma. Hemoglobin can bind and transport a large amount of oxygen, increasing the overall oxygen-carrying capacity of the blood. By confining hemoglobin within red blood cells, mammals can achieve a more efficient oxygen transport system.

Protection: Red blood cells provide a protective environment for hemoglobin. The plasma is exposed to various physical and chemical changes, including pH fluctuations, temperature variations, and enzymatic activities. Hemoglobin is more susceptible to denaturation and loss of function under these conditions. By encapsulating hemoglobin within red blood cells, it is protected from these external factors, ensuring its stability and functionality.

Plasma Viscosity: The presence of a high concentration of dissolved hemoglobin in plasma would increase its viscosity. This would hinder the flow of blood through the narrow blood vessels, especially in the microcirculation. Confining hemoglobin within red blood cells allows for a lower plasma viscosity, maintaining the fluidity of blood and facilitating efficient circulation throughout the body.

Cellular Respiration: The confinement of hemoglobin within red blood cells facilitates cellular respiration. Red blood cells lack mitochondria and, therefore, rely on anaerobic glycolysis for energy production. By limiting hemoglobin to the red blood cells, the oxygen carried by hemoglobin is primarily delivered to tissues for cellular respiration rather than being consumed by the red blood cells themselves.

Filtration: In the kidneys, the glomerular filtration process involves the separation of waste products from the blood. If hemoglobin were freely dissolved in the plasma, it would be filtered out by the kidneys, leading to its loss. By encapsulating hemoglobin within red blood cells, it is prevented from being filtered out, ensuring its retention in the bloodstream.

In summary, confining hemoglobin within red blood cells allows mammals to achieve efficient oxygen transport, protection of hemoglobin, maintenance of plasma viscosity, facilitation of cellular respiration, and prevention of hemoglobin loss through filtration in the kidneys.