To live and thrive - oxygen transportation mechanisms within us must work flawlessly. The human body and its many intricate processes even outside of oxygenation are truly something. Our ability to adapt to physiologic changes and heal ourselves during times of disease is pretty remarkable too.
In this blog we will unpack hemoglobin and its affinity for oxygen -- the phenomenon known as the Bohr Effect. As you may recall red blood cells (RBC's) are the vehicle for oxygen transport. Inside each RBC is a large multi-globular protein called hemoglobin. The heme molecule has a centrally located Iron (Fe) molecule. One molecule of oxygen can bind to the iron atom of a heme group, giving each hemoglobin the ability to transport four oxygen molecules. This gives each hemoglobin the maximum capacity to transport four oxygen molecules. This ability to sequentially bind oxygen to each subunit results in the unique sigmoidal shape of the oxyhemoglobin dissociation curve.
We will talk more about this curve later. Oxygen delivery to both the tissues (unloading) and the lungs (loading) is thankfully a pretty efficient process, but at times environmental changes, like acidosis or alkalosis to name a few alter how well this mechanism can work. The shape and conformity of the proteins can change and heme attraction to oxygen can improve or sharply decline based on internal/external conditions.
In the lungs the concentration of oxygen is high and the concentration of both hydrogen and CO2 is low. In peripheral issues the opposite is true, oxygen is lower, while hydrogen and CO2 (bicarbonate) are higher.
Christian Bohr discovered that pH differences existed between the lungs and the peripheral tissues. The tissues were slightly more acidic (pH=7.2) when compared to the lungs (pH=7.6). To a lesser extent (< 10%) hemoglobin also has the ability to carry carbon dioxide, albeit at different sites. The Bohr effect enables the body to adapt to changing conditions and makes it possible to supply extra oxygen to tissues that need it the most. Think about marathon runner or think about your firefighter colleague inside a working structure fire. Tissue demands increase as do the formation of waste products. The waste products lower the pH of the blood, which increases oxygen delivery (offloading) to the active muscles.
The oxyhemoglobin curve is represented by two variables. On the x-axis (horizonal) we have the partial pressure of oxygen in mmHg. On the y-axis (vertical) we have the percentage of binding sites that are filled expressed as a percentage. In other words, how much hemoglobin is saturated or occupied by oxygen. Historically the independent variable is known as the manipulated variable and resides on the x-axis. The dependent or responding variable or what is actually being measured is located on the y-axis. The oxygen saturation value explicitly DEPENDS on how you alter or change the PO2 in a given environment.
The curve is dynamic and not set in stone. Physiologic changes can make the above curve shift to the right or the left. I have always remembered this helpful mnemonic to keep things straight.
CADET - face right!! (Rightward shift means oxygen gets unloaded to the tissues and its affinity for hemoglobin subsequently decreases)
Increase in CO2
Acidosis (Decrease in pH)
DPG increase (common in anemia and other conditions)
Exercise
Increase is Temp
Logic tells us the curve will have a leftward shift (higher affinity of Hgb for oxygen) with the values above are switched as in alkalosis or when there is a decrease in temperature, as well as in cases of septic shock).
The Bohr effect is crucial in how our bodies operate, and it carries with it obvious clinical relevance, especially in the intensive care units as well as in the respiratory and exercise science space. The Bohr effect is important because it improves oxygen supply to muscles and tissues where metabolism and carbon dioxide production occur. This aids in the delivery of oxygen to the areas where it is most needed. Kind of a big deal!!
October 14, 2024
Author: Joshua Ishmael, MBA, MLS(ASCP)CM, NRP
Pass with PASS, LLC
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