Oxyhemoglobin is red, and hemoglobin is blue. Use Le Châtelier's principle to explain why venous blood is blue and arterial blood is bright red.

Chemical equilibrium is a state in a chemical reaction where the rate of the forward reaction equals the rate of the reverse reaction. Consequently, the concentrations of the reactants and products remain constant over time, creating a balance. It is a dynamic equilibrium because the reactions actually continue to occur, but since they are happening at the same rate, there is no net change in the concentration of any species.

In the body, the equilibrium between hemoglobin and oxygen is an excellent example where our physiology takes advantage of chemical equilibrium principles. This state is crucial because it ensures that oxygen is carried efficiently throughout the body to where it is needed and that carbon dioxide, a waste product, is removed effectively. Understanding this concept is fundamental to grasp why and how the body responds to changes in oxygen levels and maintains homeostasis.

The human body relies on a protein called hemoglobin within red blood cells to transport oxygen from the lungs to tissues and to carry carbon dioxide, a waste gas, back to the lungs. The reaction that describes the binding of oxygen to hemoglobin to form oxyhemoglobin is a reversible reaction:\[ Hb (blue) + O_2 \rightleftharpoons HbO_2 (red) \]Here, Hb represents deoxyhemoglobin (hemoglobin not bound to oxygen), which appears darker, and HbO_2 represents oxyhemoglobin, which is brighter in color. When hemoglobin picks up oxygen in the lungs, the balance tips in favor of oxyhemoglobin. As blood travels to tissues and oxygen is released, the balance shifts back towards deoxyhemoglobin. This process is vital for sustaining life as it ensures that tissues receive the oxygen they require to function.

Oxygen binding in blood is a cooperative process where the binding of one oxygen molecule to hemoglobin increases the affinity of the remaining sites for oxygen. This means that as more oxygen binds to hemoglobin, it becomes easier for additional oxygen molecules to bind. Conversely, as oxygen is released, the affinity decreases, facilitating the release of remaining oxygen molecules. This cooperative binding is characteristic of hemoglobin and is essential for allowing the protein to bind oxygen efficiently in the high-oxygen environment of the lungs and release it where it is scarce, such as in metabolically active tissues.

This concept is crucial in understanding how oxygen is transported and delivered throughout the body and how changes in conditions, such as pH and temperature, can affect hemoglobin's ability to bind or release oxygen, resulting in critical physiological responses.