Hemoglobin, a protein in red blood cells, delivers oxygen throughout the body. It picks up oxygen in the lungs and releases it to various tissues. This transport mechanism is fundamental for health and metabolic processes. Without it, the body cannot meet its oxygen demands, which is essential for survival.
Hemoglobin’s Structure
Hemoglobin is a protein with four subunits (two alpha and two beta chains in adults). Each subunit contains a heme group. At the center of each heme group lies an iron atom, the site where oxygen binds.
Oxygen binds to the iron atom (Fe2+) within each heme group. Each iron atom reversibly binds one oxygen molecule, allowing a hemoglobin molecule to carry up to four. This arrangement enables oxygen transport.
Acquiring Oxygen in the Lungs
In the lungs, where oxygen concentration is high, hemoglobin binds to oxygen. This involves cooperative binding: one oxygen molecule binding increases the affinity of others. This changes hemoglobin from a low-affinity T-state to a high-affinity R-state.
As oxygen binds, hemoglobin adjusts, making it easier for more oxygen to attach. Cooperative binding results in a sigmoidal, or S-shaped, oxygen dissociation curve, showing hemoglobin becomes nearly saturated. High oxygen pressure in the alveoli drives this binding, saturating about 98% of hemoglobin before it leaves the lungs.
Releasing Oxygen to Tissues
In tissues, where oxygen levels are lower due to metabolism, hemoglobin releases oxygen. Lower oxygen pressure in these active areas triggers this release. As oxygen dissociates, hemoglobin shifts from the high-affinity R-state back to the low-affinity T-state.
This conformational change reduces hemoglobin’s attraction to oxygen, facilitating its unloading into tissue cells. The oxygen dissociation curve shifts right, indicating decreased affinity and greater release. This ensures oxygen is available for active tissues’ energy needs.
Regulating Oxygen Delivery
Several physiological factors fine-tune hemoglobin’s oxygen-binding affinity, ensuring oxygen delivery where needed. The Bohr effect is one mechanism: increased acidity (lower pH) and higher carbon dioxide (CO2) decrease hemoglobin’s oxygen affinity. Active tissues produce more CO2, forming carbonic acid and lowering local pH. This acidic environment and elevated CO2 stabilize hemoglobin’s T-state, promoting oxygen release.
Temperature also influences oxygen delivery; increased body temperature, common with active muscles, reduces hemoglobin’s oxygen affinity. This shifts the oxygen dissociation curve right, meaning more oxygen unloads to warmer tissues. Another factor is 2,3-bisphosphoglycerate (2,3-BPG), an organic phosphate in red blood cells. 2,3-BPG binds to deoxyhemoglobin, stabilizing its low-affinity T-state and promoting oxygen release, especially in low oxygen.