Oxygen Transport in the Blood

Hemoglobin, a protein in red blood cells composed of four polypeptide chains with iron-rich heme groups, binds to four oxygen molecules. Most oxygen travels bound to hemoglobin, while a small percentage dissolves directly in plasma due to oxygen's low solubility. In oxygen-rich arterial blood, hemoglobin reaches 98% saturation, delivering approximately 20 ml of oxygen per 100 ml of blood.

The Bohr effect occurs when elevated carbon dioxide and hydrogen ion levels lower blood pH, weakening the hemoglobin-oxygen bond and promoting oxygen unloading at tissues. This mechanism ensures efficient oxygen delivery where it is needed most. Active tissues produce carbon dioxide and heat, creating conditions that favor oxygen release from hemoglobin.

Hemoglobin saturation depends directly on partial pressure of oxygen (PO2). As PO2 increases, more oxygen binds to hemoglobin; as PO2 decreases, oxygen is released. In systemic capillaries where PO2 is low, hemoglobin saturation drops from 98% to approximately 75%, releasing about 5 ml of oxygen per 100 ml of blood to tissues.

Temperature, blood pH, carbon dioxide levels, and 2,3-bisphosphoglycerate (2,3-BPG) all regulate hemoglobin's affinity for oxygen. Increased temperature, elevated carbon dioxide, lower pH, and higher 2,3-BPG levels decrease oxygen affinity, promoting release. Red blood cells produce 2,3-BPG during glucose metabolism, with levels rising during chronic oxygen deficiency.

Venous blood maintains approximately 75% hemoglobin saturation with 15 ml of oxygen per 100 ml of blood, representing a substantial oxygen reserve. This reserve ensures adequate oxygen availability during increased metabolic demands or emergency situations. The body releases only about 5 ml of oxygen per 100 ml of blood during normal resting conditions, preserving this critical safety margin.

When hemoglobin binds oxygen, its three-dimensional structure changes, making it more efficient at picking up additional oxygen molecules. This structural adaptation, called cooperative binding, ensures rapid and reversible oxygen loading and unloading. Fully oxygenated hemoglobin is called oxyhemoglobin, while oxygen-depleted hemoglobin is called deoxyhemoglobin or reduced hemoglobin.

Active tissues create optimal conditions for oxygen release through multiple mechanisms. Cellular metabolism produces carbon dioxide and heat, raising local PO2, carbon dioxide levels, and temperature. These factors collectively weaken hemoglobin's oxygen affinity, ensuring efficient oxygen delivery precisely where metabolic activity is highest and oxygen demand is greatest.