Hemoglobin is a protein in red blood cells that transports oxygen throughout the body. This iron-containing protein gives blood its red color. Hemoglobin can exist in different forms depending on its oxygen load. Deoxygenated hemoglobin is the state where the molecule has released its oxygen.
Oxygen Release and Hemoglobin’s Transformation
Oxygen release from hemoglobin occurs in tissues and cells, where oxygen demand is high for metabolic processes. As oxygenated blood reaches these areas, hemoglobin unloads its oxygen. This transition to deoxygenated hemoglobin involves a structural change.
Hemoglobin shifts from a relaxed (R) state, with high oxygen affinity, to a tense (T) state, with lower affinity, upon oxygen release. This process, known as the Bohr effect, is influenced by several factors. Increased carbon dioxide, decreased pH, and higher temperatures in active tissues reduce hemoglobin’s oxygen affinity.
These conditions, common in active tissues like exercising muscles, encourage hemoglobin to release oxygen. Hydrogen ions bind to hemoglobin’s amino acid residues, causing a structural change that promotes oxygen dissociation. This mechanism ensures oxygen delivery where needed, facilitating cellular respiration and energy production.
Role in Carbon Dioxide Transport
After releasing oxygen, deoxygenated hemoglobin’s capacity to bind carbon dioxide increases. This allows it to transport carbon dioxide from tissues, a metabolic waste product, back to the lungs for exhalation.
While most carbon dioxide (70-90%) is transported as bicarbonate ions, 10-20% binds directly to deoxygenated hemoglobin, forming carbaminohemoglobin. This binding occurs at the amino groups of the globin chains. Deoxygenated hemoglobin has a greater affinity for carbon dioxide than oxygenated hemoglobin, a phenomenon known as the Haldane effect.
This dual function helps maintain the body’s pH balance and efficiently removes metabolic waste. Carbon dioxide binding to hemoglobin buffers blood pH, preventing excessive acidity. In the lungs, high oxygen levels reduce hemoglobin’s affinity for carbon dioxide, allowing its release and exhalation.
The Distinct Color of Deoxygenated Blood
Deoxygenated blood appears darker, often dark red or bluish-red, unlike bright red oxygenated blood. This color difference results from how light interacts with hemoglobin. Oxygenated hemoglobin absorbs more infrared light and reflects more red light, giving it a bright red appearance.
Deoxygenated hemoglobin absorbs more red light and reflects more infrared light, resulting in a darker, less vibrant red hue. When viewed through the skin, this darker color can manifest as a bluish or grayish discoloration, known as cyanosis. Cyanosis is observed in areas with thin skin, such as the lips, nail beds, and earlobes, indicating inadequate oxygen in the blood.
Veins often look bluish, a common example of this phenomenon. Though the blood inside is dark red, light absorption and scattering by skin and tissues above veins can make them appear blue. Cyanosis becomes clinically apparent when oxygen saturation drops below 85%, signifying higher deoxygenated hemoglobin.
Assessing Blood Oxygen Levels
Blood oxygen saturation is measured by the proportion of deoxygenated to oxygenated hemoglobin. Pulse oximetry is a common non-invasive method for this assessment. A pulse oximeter, a small clip-like device placed on a fingertip or earlobe, emits two wavelengths of light: red and infrared.
Oxygenated and deoxygenated hemoglobin absorb these wavelengths differently. The oximeter measures light absorbed and transmitted, calculating the percentage of oxygen-saturated hemoglobin.
Normal oxygen saturation for healthy individuals is 95-100% at sea level. A reading of 92% or lower indicates hypoxemia, a significantly low blood oxygen level. Pulse oximetry quickly monitors oxygen delivery to tissues, alerting medical professionals to potential issues.