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You are watching: Which of the following is the dominant method of carbon dioxide transport

StatPearls . Sweetheart Island (FL): StatPearls Publishing; 2021 Jan-.


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Introduction

Carbon dioxide is an essential side product of both glycolysis and the citric mountain cycle (Krebs cycle). This oxidized carbon to represent an end product of metabolism that, ultimately, demands to be eliminated using deliver to the lungs and also subsequent expiration out into the bordering environment. Together with renal regulation, this facility process that carbon dioxide production, transport, and elimination is the principal means by i m sorry the human body regulates the blood’s pH. Obstacle in this delicate procedure can an outcome in acid-base derangements and also may it is in acute or chronic.


Cellular

Carbon dioxide production occurs in cells, mainly during glycolysis and also the citric acid cycle in the cytoplasm and mitochondria, respectively. During these succeeding biochemical reactions, the power stored in the reduced carbon bonds of fats, sugars, and also proteins is slowly liberated in a collection of stepwise reactions until the carbon atom is completely oxidized and bound to 2 oxygen atoms. This final product is carbon dioxide. Like other molecules, carbon dioxide always moves under its concentration gradient, from sites of manufacturing in the mitochondria and cytosol through the phospholipid membrane and into the extracellular space. However, carbon dioxide diffuses readily, much quicker than oxygen. Together the cells create carbon dioxide, the dissolves right into the water of the cytoplasm and also continues to build up until it reaches a partial pressure greater than 40 come 45 mmHg. This buildup sets increase a concentration gradient under which carbon dioxide have the right to diffuse. From that extracellular space, carbon dioxide molecules easily diffuse through the capillary walls, rapidly equilibrating and increasing the partial press of carbon dioxide in the blood from about 40 mmHg on the arterial next of a capillary come 45 come 48 mmHg on the venous side.<1> 

Once the venous blood returns to the lungs, the carbon dioxide diffuses the end of the bloodstream, with the capillaries, and into the alveoli from whereby it is expelled, during which time oxygen at the same time binds with hemoglobin to be carried back to the tissues.


Mechanism

There are three way by which carbon dioxide is transported in the bloodstream indigenous peripheral organization and back to the lungs: (1) dissolved gas, (2) bicarbonate, and also (3) carbaminohemoglobin bound to hemoglobin (and other proteins). Together carbon dioxide diffuses into the bloodstream native peripheral tissues, roughly 10% of it remains dissolved either in plasma or the blood"s extracellular liquid matrix, to a partial pressure of about 45 mmHg.<2> Most of the carbon dioxide diffusing through the capillaries and also ultimately into the red blood cells combines v water via a chemical reaction catalyzed by the enzyme carbonic anhydrase catalyzes, developing carbonic acid. Carbonic acid nearly immediately dissociates into a bicarbonate anion (HCO3-) and a proton. Thus, bicarbonate is the primary way by which carbon dioxide is transport occurs throughout the bloodstream follow to the equation CO2 + H2O --> H2CO3 --> H+ + HCO3-.  

As carbon dioxide proceeds to be produced by tissues, this reaction is continuous driven forward in the periphery, according to Le Chatelier"s principle. The proton created by this reaction is buffered through hemoglobin, while the bicarbonate anion diffuses the end of the red blood cell and also into the serum in exchange for a chloride anion through a unique HCO3-/Cl- transporter. Thus, venous blood has both a higher concentration the bicarbonate and also a reduced concentration of chloride thanks to this so-called chloride shift. In the lungs, this process reverses together both the HCO3-/Cl- exchanger and also carbonic anhydrase enzyme reverse directions; this results in an flow of bicarbonate into red blood cells, one efflux the chloride ions, and the generation of first carbonic acid and then carbon dioxide. The carbon dioxide diffuses the end of the red blood cells, with the capillary walls, and also into the alveolar spaces f exhaled.<1> Finally, the remaining 10% of the carbon dioxide that diffuses into the bloodstream and, subsequently, right into the red blood cells, binding to the amino terminus that proteins, predominantly hemoglobin, to kind carbaminohemoglobin.<2> Of note, this site is different from the one come which oxygen binds. Multiple physiologic phenomena ensure that this constant cycle runs through maximal efficiency.

Oxygen delivery and carbon dioxide removal intrinsically link with one one more through processes explained by the Bohr and Haldane effects. If not thorough here, the Bohr effect states that the boost of carbon dioxide in the blood in peripheral tissues reasons a right change in the oxygen-hemoglobin dissociation curve and, consequently, increased oxygenation that the tissues. Once the carbon dioxide-enriched blood reaches the lungs, however, the turning back of this reaction will likewise occur. Together the influx of oxygen rises hemoglobin saturation, the carbon dioxide is more likely to come to be detached and diffused into the alveoli for exhalation; this is called the Haldane effect.<3>

Specifically, the Haldane effect explains the difference in carbon dioxide transporting capacity in oxygenated blood contrasted with deoxygenated blood. In ~ a constant partial push of carbon dioxide, the Haldane effect states the oxygenated (arterial) blood will carry less carbon dioxide than deoxygenated (venous) blood as result of a combination of an impaired ability of hemoglobin to buffer the overabundance carbon dioxide as fine as a decreased capacity for carbamino carriage.<2> together oxygen binds to hemoglobin, the hemoglobin becomes an ext acidic, which has two effects. First, it reduces the binding affinity the the hemoglobin because that carbon dioxide, making the carbon dioxide more likely come dissociate indigenous the hemoglobin and also diffuse out of the red blood cell right into the alveolar space. Second, acidic hemoglobin can release a proton the will integrate with bicarbonate to form carbonic acid. Again, Le Chatelier"s principle cd driver the adhering to reaction forward as blood passes through the alveoli: H+ + HCO3- --> H2CO3 --> CO2 + H2O. The carbon dioxide produced here continuous diffuses into the alveoli and is exhaled, ensuring favorable kinetics because that the reaction to proceed. Thus, the Haldane impact increases the amount of carbon dioxide that can be eliminated during a given timeframe. Graphically, the Haldane effect is represented by a right transition that wake up in the carbon dioxide dissociation curve (see graph).

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In peripheral tissues, whereby oxygen contents is low, carbon dioxide binds to hemoglobin to form carbaminohemoglobin. As blood return to the lungs and also the partial push of oxygen increases, the carbon dioxide dissociation curve shifts best (seen by the arrow showing the offloading of carbon dioxide as oxygenation increases), lowering the full carbon dioxide content in the bloodstream. Thus, return the partial press of carbon dioxide just decreases native 45 or 46 mmHg on the venous next to 40 mmHg on the arterial side, the complete amount of carbon dioxide in the bloodstream decreases by a much higher percentage.