Introduction:
Carbon dioxide is generated through cell metabolism that takes place in the mitochondria. Carbon dioxide amount that is produced is dependent on the metabolism rate as well as on the relative carbohydrate, protein and fat amount being metabolized. The amount of carbon dioxide produced at rest is usually about 200 ml per minute eating a mixed diet. This results in the utilization of about 80% of oxygen and gives respiratory quotient of 0.8 (Arthurs & Sudhakar 2005, p. 207). Carbon is one of the main products that are released following metabolism and the level of it in the body is highly regulated. This paper aims at discussing the production and transport of carbon dioxide from the cells through the blood to the lungs.
Carbon Dioxide Production in Cells:
Production of carbon dioxide is through a cellular process called cellular respiration that takes place in the cells. Cellular respiration takes place in three main stages. The first stage is where organic fuel molecules such as glucose, amino acids and fatty acids are oxidized to form two carbon fragments known as. In the second stage, acetyl-coenzyme A is taken to the citric acid cycle and enzymatically oxidized to form carbon dioxide (Nelson & Cox 2005, p. 601). The energy that released is usually conserved in carriers of electron known as NADH and FADH2. The last stage involves the oxidation of the reduced electron carriers giving up their protons as well as the electrons. The released electrons are transferred to the oxygen molecule which is the final electron acceptor producing water. The two main reactions that produce carbon dioxide in the citric acid cycle are the dehydrogenation of isocitrate molecule forming a five-carbon compound known as α-ketoglutarate or oxoglutarate and conversion of α-ketoglutarate into succinate (Nelson & Cox 2005, p. 601).
Transport of Carbon Dioxide in the Blood:
Once the carbon dioxide is generated in the tissues, it diffuses through and enters the circulatory system. In the blood, carbon dioxide is usually transported to the lungs in three different ways (Beckett 1986). The first way is when dissolved in the solution content of the blood buffered with water molecules as carbonic acid. The other way is when the carbon dioxide is bound to plasma proteins mainly hemoglobin, which are in the red blood cells. More than 75% of carbon dioxide that is taken to the lungs is usually through the red blood cells while about 25% of the gas is transported in the plasma. The low amount of carbon dioxide in the plasma results from the fact that there is no carbonic anhydrase enzyme leading to low association of the gas with water. The plasma is also incapable of playing a part in buffering (Arthurs & Sudhakar 2005, p. 207).
There exists a significant difference between the portion of the total carbon dioxide gas that is carried in the different forms and the portion of the gas that is exhaled from the different forms. For instance, only five percent of the total gas is transported in soluble form, but 10% of the exhaled carbon dioxide is taken from the soluble sources. In a similar manner, the amount of the total carbon dioxide that is carried bound in proteins is 10% of the total but this gas accounts for 30% of the total amount of carbon dioxide exhaled (Arthurs & Sudhakar 2005, p. 207).
Dissolved Carbon Dioxide:
One of the ways through which carbon dioxide is transported in the blood is when it is dissolved in the blood plasma. The solubility of carbon dioxide in water is 20 times more than that of oxygen. This fact follows the Henry's law that indicates that the molecule number in a solution has a proportional relationship with the partial pressure existing at the surface of the liquid (Moore, et al., 2009, p. 515). The solubility coefficient of the carbon dioxide gas is about 0.231 mmol litre−1 kPa−1 at a temperature of 37°C. This means that 0.5 ml kPa−1 carbon dioxide is dissolved in 100 ml of blood at 37°C. The partial pressure of CO2 in the artery is about 5.3pKa and 6.1pKa in the venous blood (Baylis &Till 2009, p. 472). This means that the blood in the artery will contain roughly 2.5 ml of dissolved carbon dioxide gas in every 100 ml while, in the venous blood, there will be 3 ml of the gas in every 100 ml. With a cardiac output that is producing 5 liters of blood in every minute, about 150 ml of carbon dioxide that is dissolved will be taken to the lung, and only 25 ml of it will be exhaled. Due to the high solubility, as well as the diffusion capability of the carbon dioxide gas, there is no big difference between the partial pressure of carbon dioxide in the alveolar with the one in the pulmonary end-capillary blood (Kent, 2000, p. 116).
Carbon Dioxide Transported as Carbonic Acid:
The other form through which carbon dioxide is transported is as carbonic acid. The combination of carbon dioxide with water leads to the formation of carbonic acid. This reaction process is catalyzed by an enzyme known as carbonic anhydrase. The formed carbonic acid then dissociates freely forming hydrogen ions and bicarbonate ions. The carbonic anhydrase enzyme is found in a number of body organs such as the eyes, brain, and kidney. Once there is the formation of the carbonic acid, the acid dissociates easily forming a ratio of acid to the bicarbonate of 1:20 (Arthurs & Sudhakar 2005, p. 207).
Water and carbon dioxide move into the red blood cell freely and water diffuse freely into the red blood cell where carbonic acid conversion takes place. The produced hydrogen ions are not able to go through the cell membranes unlike the carbon dioxide that passes readily (Arthurs & Sudhakar 2005, p. 207). The situation is hardly sustained since the concentration of intracellular hydrogen ion as well as the bicarbonate ion may result in cell rupturing. The bicarbonate ions move into the plasma through diffusion where they are exchanged for chloride ions in a process called chloride shift (Westen & Prange 2003, p. 603). This process is facilitated by an ion exchange transporter molecule known as Band 3.
Increased levels of hydrogen ion produced in the red blood cell are known to cause inhibition of further carbonic acid conversion producing bicarbonate ion (Helfman, et al. 2009, p. 64). Hydrogen ions usually bind with ease to the reduced hemoglobin that is made available after oxygen molecule is released in the tissues resulting in the removal of the hydrogen ions from the solution (West 2004, p. 187). Carbon dioxide is more in a deoxygenated than in the oxygenated hemoglobin a situation that is referred to as Haldane effect (Schmidt-Nielsen, 1997, p. 70). These changes in the concentration of ions in the plasma compared to that in the red blood cells lead to the absorption of water by the red blood cells leading to a slight swelling (Arthurs & Sudhakar 2005 p. 208).
Carbon Dioxide Bound To Plasma Proteins:
Carbon dioxide is also transported bound in proteins that are in the plasma. The molecules of carbon dioxide usually combine in a rapid way with the uncharged amino acid terminals forming carbamino compounds (McArdle, et al., 2010, p. 285). Most of the proteins only have the terminal amino acid that can be used in forming carbamino compound. However, hemoglobin is able to form a number of carbamino groups since it has a high amount of the molecules known as histidine. These histidine molecules are 38 in number, in a single molecule of hemoglobin. The hydrogen ions are, therefore, able to attach themselves to the imidazole group contained in the amino acid histidine. There is a variation in the affinity for carbon dioxide, oxygen and carbon monoxide by different hemoglobin molecules (Arthurs & Sudhakar 2005, p. 208). Carbon dioxide is capable of combining with the hemoglobin forming carbamino bond at a much lower partial pressure that that of oxygen. The amount of carbon dioxide carried by the hemoglobin is, however, only less than a quarter of oxygen that it carries (Arthurs & Sudhakar 2005, p. 208).
Carbon Dioxide Transport in the Tissue:
After the carbon dioxide is made in the tissues, it forms carbonic acid by combining with water. The formation of carbonic acid in the plasma is very slow when compared to the same reaction in the red blood cells. The acidic nature in the red blood cells causes the oxygen molecules to be released from the hemoglobin, and more hydrogen ions are taken into the hemoglobin (Arthurs & Sudhakar 2005, p. 208).
Carbon Dioxide Transport in the Lungs:
When the hemoglobin molecules get into the lungs, oxygen combines with the hemoglobin and is facilitated by the basic nature of the histidine groups (Arthurs & Sudhakar 2005, p. 208). The basic nature of the histidine group increases the affinity for the oxygen molecules by the heme group as the carbon dioxide is lost. This is one of the reasons behind Bohr Effect. The basic nature is attained as the hydrogen ions are released which shifts the equilibrium to favor the formation and elimination of carbon dioxide (Beckett, 1986, p. 73).
Conclusion:
The process of transporting carbon dioxide is a highly controlled process making sure that much of carbon dioxide does not accumulate in the tissues. The process also ensures that the movement of the gas through the blood stream does not increase the pH of the blood by buffering the hydrogen ions. The success of carbon dioxide transport determines the success of the oxygen transport and the proper functioning of the body cells.
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