Question

what happens to most of the CO2 after they are released into the interstitial fluid. what happens at the venous end of the capillary and how are RBC involved in the post-capillary venules.

Answer 1

Ans:-CO2 is formed continually in the body by intracellular metabolic processes. After it is formed, it diffuses from the cells into the interstitial fluids and blood and the flowing blood transports it to the lungs, where it diffuses into the alveoli and then is transferred to the atmosphere by pulmonary ventilation. About 1.2 mol/L of dissolved CO2 normally are in the extracellular fluid, corresponding to a PCO2 of 40 mm Hg. If the rate of metabolic formation of CO2 increases, the PCO2 of the extracellular fluid is likewise increased. Conversely, a decreased metabolic rate lowers the PCO2. If the rate of pulmonary ventilation is increased, CO2 is blown off from the lungs and the PCO2 in the extracellular fluid decreases. Therefore, changes in either pulmonary ventilation or the rate of CO2 formation by the tissues can change the extracellular fluid PCO2.

TRANSPORT OF CARBON DIOXIDE IN THE BLOOD:- Transport of CO2 by the blood is not nearly as problematical as transport of O2 is because even in the most abnormal conditions, CO2 can usually be transported in far greater quantities than can O2.

To begin the process of CO2 transport, CO2 diffuses out of the tissue cells in the dissolved molecular CO2 form. Upon entering the tissue capillaries, the CO2 initiates a host of almost instantaneous physical and chemical reactions,in which are essential for CO2 transport.

Transport of Carbon Dioxide in the Dissolved State A small portion of the CO2 is transported in the dissolved state to the lungs. Recall that the PCO2 of venous blood is 45 mm Hg and that of arterial blood is 40 mm Hg. The amount of CO2 dissolved in the fluid of the blood at 45 mm Hg is about 2.7 ml/dl (2.7 volumes percent). The amount dissolved at 40 mm Hg is about 2.4 milliliters, or a difference of 0.3 milliliter. Therefore, only about 0.3 milliliter of CO2 is transported in the dissolved form by each 100 milliliters of blood flow. This is about 7 percent of all the CO2 normally transported

• .Transport of Carbon Dioxide in the Form of Bicarbonate IonReaction of Carbon Dioxide with Water in the Red Blood Cells—Effect of Carbonic Anhydrase. The dissolved CO2 in the blood reacts with water to form carbonic acid. This reaction would occur much too slowly to be of importance were it not for the fact that inside the red blood cells is a protein enzyme called carbonic anhy-drase, which catalyzes the reaction between CO2 and water and accelerates its reaction rate about 5000-fold. Therefore, instead of requiring many seconds or minutes to occur, as is true in the plasma, the reaction occurs so rapidly in the red blood cells that it reaches almost complete equilibrium within a small fraction of a second. This phenomenon allows tremendous amounts of CO2 to react with the red blood cell water even before the blood leaves the tissue capillaries.Dissociation of Carbonic Acid Into Bicarbonate and Hydrogen Ions. In another fraction of a second, the carbonic acid formed in the red cells (H2CO3) dissociates into hydrogen and bicarbonate ions (H+ and HCO3−). Most of the H+ then combine with the hemoglobin in the red blood cells because the hemoglobin protein is a powerful acid-base buffer. In turn, many of the HCO3− diffuse from the red blood cells into the plasma, while chloride ions diffuse into the red blood cells to take their place. This diffusion is made possible by the presence of a special bicarbonate-chloride carrier protein in the red blood cell membrane that shuttles these two ions in opposite directions at rapid velocities. Thus, the chloride content of venous red blood cells is greater than that of arterial red blood cells, a phenomenon called the chloride shift.The reversible combination of CO2 with water in the red blood cells under the influence of carbonic anhydrase accounts for about 70 percent of the CO2 transported from the tissues to the lungs. Thus, this means of transporting CO2 is by far the most important. Indeed, when a carbonic anhydrase inhibitor (acetazolamide) is admin-istered to an animal to block the action of carbonic anhydrase in the red blood cells, CO2 transport from the tissues becomes so poor that the tissue PCO2 may rise to 80 mm Hg instead of the normal 45 mm Hg.
• Transportof Carbon Dioxide in Combination with Hemoglobin and Plasma Proteins—Carbaminohemoglobin. In addition to reacting with water, CO2 reacts directly with amine radicals of the hemoglobin molecule to form the compound carbaminohemoglobin (CO2Hgb). This combination of CO2 and hemoglobin is a reversible reaction that occurs with a loose bond, so the CO2 is easily released into the alveoli, where the PCO2 is lower than in the pulmonary capillaries.A small amount of CO2 also reacts in the same way with the plasma proteins in the tissue capillaries. This reaction is much less significant for the transport of CO2 because the quantity of these proteins in the blood is only one fourth as great as the quantity of hemoglobin.The quantity of CO2 that can be carried from the peripheral tissues to the lungs by carbamino combination with hemoglobin and plasma proteins is about 30 percent of the total quantity transported—that is, normally about 1.5 milliliters of CO2 in each 100 milliliters of blood. However, because this reaction is much slower than the reaction of CO2 with water inside the red blood cells, it is doubtful that under normal conditions this carbamino mechanism transports more than 20 percent of the total CO2.