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. However, the amount of CO2 in the blood has a lot to do with the acid-base balance of the body fluids, which is discussed in Chapter 31. Under normal resting conditions, an average of 4 milliliters of CO2 are transported from the tissues to the lungs in each 100 milliliters of blood.

CHEMICAL FORMS IN WHICH CARBON DIOXIDE IS TRANSPORTED

 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, shown in Figure 1, which are essential for CO2 transport.

Fig1. Transport of carbon dioxide in the blood. 

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 Ion

Reaction of Carbon Dioxide with Water in the Red Blood Cells—Effect of Carbonic Anhydrase. The dis solved 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 anhydrase, 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 trans porting CO2 is by far the most important. Indeed, when a carbonic anhydrase inhibitor (acetazolamide) is administered 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.

Transport of 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.

CARBON DIOXIDE DISSOCIATION CURVE

The curve shown in Figure 2—called the carbon dioxide dissociation curve—depicts the dependence of total blood CO2 in all its forms on PCO2. Note that the normal blood PCO2 ranges between a narrow range of 40 mm Hg in arterial blood and 45 mm Hg in venous blood. Note also that the normal concentration of CO2 in the blood in all its different forms is about 50 volumes percent, but only 4 volumes percent of this is exchanged during normal transport of CO2 from the tissues to the lungs. That is, the concentration rises to about 52 volumes percent as the blood passes through the tissues and falls to about 48 volumes percent as it passes through the lungs.

Fig2. Carbon dioxide dissociation curve.

WHEN OXYGEN BINDS WITH HEMOGLOBIN, CARBON DIOXIDE IS RELEASED (THE HALDANE EFFECT) TO INCREASE CARBON DIOXIDE TRANSPORT

 Earlier in the chapter, we pointed out that an increase in CO2 in the blood causes O2 to be displaced from the hemoglobin (the Bohr effect), which is an important factor in increasing O2 transport. The reverse is also true: binding of O2 with hemoglobin tends to displace CO2 from the blood. Indeed, this effect, called the Haldane effect, is quantitatively far more important in promoting CO2 transport than is the Bohr effect in promoting O2 transport.

The Haldane effect results from the simple fact that the combination of O2 with hemoglobin in the lungs causes the hemoglobin to become a stronger acid. This displaces CO2 from the blood and into the alveoli in two ways. First, the more highly acidic hemoglobin has less tendency to combine with CO2 to form carbaminohemoglobin, thus displacing much of the CO2 that is present in the carb amino form from the blood. Second, the increased acidity of the hemoglobin also causes it to release an excess of hydrogen ions, and these ions bind with bicarbonate ions to form carbonic acid, which then dissociates into water and CO2, and the CO2 is released from the blood into the alveoli and, finally, into the air.

Figure 3 demonstrates quantitatively the significance of the Haldane effect on the transport of CO2 from the tissues to the lungs. This figure shows small portions of two CO2 dissociation curves: (1) when the PO2 is 100 mm Hg, which is the case in the blood capillaries of the lungs, and (2) when the PO2 is 40 mm Hg, which is the case in the tissue capillaries. Point A shows that the normal PCO2 of 45 mm Hg in the tissues causes 52 volumes percent of CO2 to combine with the blood. Upon entering the lungs, the PCO2 falls to 40 mm Hg and the PO2 rises to 100 mm Hg. If the CO2 dissociation curve did not shift because of the Haldane effect, the CO2 content of the blood would fall only to 50 volumes percent, which would be a loss of only 2 volumes percent of CO2. However, the increase in PO2 in the lungs lowers the CO2 dissociation curve from the top curve to the lower curve of the figure, so the CO2 content falls to 48 volumes percent (point B). This represents an additional two volumes percent loss of CO2. Thus, the Haldane effect approximately doubles the amount of CO2 released from the blood in the lungs and approximately doubles the pickup of CO2 in the tissues.

Fig3. Portions of the carbon dioxide dissociation curve  when the PO2 is 100 mm Hg or 40 mm Hg. The arrow represents the  Haldane effect on the transport of carbon dioxide. 

Change in Blood Acidity During CO2 Transport

The carbonic acid formed when CO2 enters the blood in the peripheral tissues decreases the blood pH. However, reaction of this acid with the acid-base buffers of the blood prevents the H+ concentration from rising greatly (and the pH from falling greatly). Ordinarily, arterial blood has a pH of about 7.41, and as the blood acquires CO2 in the tissue capillaries, the pH falls to a venous value of about 7.37. In other words, a pH change of 0.04 unit takes place. The reverse occurs when CO2 is released from the blood in the lungs, with the pH rising to the arterial value of 7.41 once again. In heavy exercise or other conditions of high metabolic activity, or when blood flow through the tissues is sluggish, the decrease in pH in the tissue blood (and in the tissues themselves) can be as much as 0.50, about 12 times normal, thus causing significant tissue acidosis.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم رعاية الصحن الشريف ينشر لافتات الحزن بذكرى شهادة الإمام الجواد (عليه السلام)

قسم العلاقات العامة يقيم مجلس عزائه الأسبوعي لاستذكار فضائل أهل البيت (عليهم السلام)

متحف الكفيل يستعرض درعًا هنديًّا يعود إلى القرن التاسع عشر

ضمن محفلها الأسبوعيّ.. دار علوم نهج البلاغة تقدّم محاضرةً عن تحرير الإنسان من قيود المنصب

المزيد

الآخبار الصحية

طريقة مبتكرة للكشف المبكر عن أمراض القلب والشرايين

منظمة الصحة تحذر من خطر إدمان الشباب لأكياس النيكوتين

خبراء يحذرون من عادة شائعة تهدد صحة الملايين

خبير يكشف 5 حقائق طبية صادمة وغير معروفة

المزيد

الآخبار التكنلوجية

الأولى في العالم.. الصين تجري تجربة رائدة في الفضاء باستخدام "جنين اصطناعي"

دراسة حديثة تفند ربط التستوستيرون بالمخاطرة لدى الرجال

ميتا تضيف وضع "المحادثة المخفية" إلى تطبيق "واتساب"

فوبيا المرتفعات ليست من الدماغ.. دراسة تكشف السبب

المزيد

مواضيع ذات صلة

Respiratory Center

Respiratory Exchange Ratio

Factors that Shift the Oxygen-Hemoglobin Dissociation Curve—Their Importance for Oxygen Transport

Hemoglobin “Buffers” Tissue PO2

Reversible Combination of O2 with Hemoglobin

Diffusion of Carbon Dioxide From Peripheral Tissue Cells Into the Capillaries and From the Pulmonary Capillaries Into Alveoli

Diffusion of Oxygen from the Peripheral Capillaries into the Tissue Fluid

Diffusion of Oxygen From the Alveoli to the Pulmonary Capillary Blood

Factors that Affect the Rate of Gas Diffusion Through the Respiratory Membrane

Compositions of Alveolar Air and Atmospheric Air are Different

Fluid in the Pleural Cavity

Blood Volume of the Lungs