Transport Of O2 & Co2 PDF

Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids Transport of Oxygen and in Blood and Tissue Fluids Once oxygen (O2) has diffused from the alveoli into the pulmonary blood, it is transported to the tissue capillaries almost entirely in combination with hemoglobin. The pr...

Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids Transport of Oxygen and in Blood and Tissue Fluids Once oxygen (O2) has diffused from the alveoli into the pulmonary blood, it is transported to the tissue capillaries almost entirely in combination with hemoglobin. The presence of hemoglobin in the red blood cells allows the blood to transport 30 to 100 times as much O2 as could be transported in the form of dissolved O2 in the water of the blood.  TRANSPORT OF OXYGEN FROM THE LUNGS TO THE BODY TISSUES O2 diffuses from the alveoli into the pulmonary capillary blood because the oxygen partial pressure (PO2) in the alveoli is greater than the PO2 in the pulmonary capillary blood. In the other tissues of the body, a higher PO2 in the capillary blood than in the tissues causes O2 to diffuse into the surrounding cells. In the other tissues of the body, a higher PO2 in the capillary blood than in the  TRANSPORT OF OXYGEN FROM THE LUNGS TO THE BODY TISSUES When O2 is metabolized in the cells to form CO2, the intracellular carbon dioxide partial pressure (P CO2) rises, causing CO2 to diffuse into the tissue capillaries. After blood flows to the lungs, the CO2 diffuses out of the blood into the alveoli, because the P CO2 in the pulmonary capillary blood is greater than that in the alveoli. Thus, the transport of O2 and CO2 by the blood depends on both diffusion and the flow of blood. We now consider quantitatively the factors responsible for these effects. DIFFUSION OF OXYGEN FROM THE ALVEOLI TO THE PULMONARY CAPILLARY BLOOD The PO2 of the gaseous O2 in the alveolus averages 104 mm Hg, whereas the PO2 of the venous blood entering the pulmonary capillary at its arterial end averages only 40 mm Hg because a large amount of O2 was removed from this blood as it passed through the peripheral tissues.  DIFFUSION OF OXYGEN FROM THE ALVEOLI TO THE PULMONARY CAPILLARY BLOOD Therefore, the initial pressure diffrence that causes O2 to diffuse into the pulmonary capillary is 104 − 40, or 64 mm Hg. The blood PO2 rises almost to that of the alveolar air by the time the blood has moved a third of the distance through the capillary, becoming almost 104 mm Hg. DIFFUSION OF CARBON DIOXIDE FROM PERIPHERAL TISSUE CELLS INTO THE CAPILLARIES AND FROM THE PULMONARY CAPILLARIES INTO ALVEOLI When O2 is used by the cells, virtually all of it becomes CO2, and this transformation increases the intracellular PCO2; because of this elevated tissue cell PCO2, CO2 diffuses from the cells into the capillaries and is then carried by the blood to the lungs. In the lungs, it diffuses from the pulmonary capillaries into the alveoli and is expired. Thus, at each point in the gas transport chain, CO 2 diffuses in the direction exactly opposite to the diffusion of O2.  DIFFUSION OF CARBON DIOXIDE FROM PERIPHERAL TISSUE CELLS INTO THE CAPILLARIES AND FROM THE PULMONARY CAPILLARIES INTO ALVEOLI There is one major diffrence between diffusion of CO2 and of O2: CO2 can diffuse about 20 times as rapidly as O2. The pressure differences required to cause CO2 diffusion are, in each instance, far less than the pressure differences required to cause O2 diffusion.  The CO2 pressures are approximately the following: Intracellular PCO , 46 mm Hg; interstitial PCO , 45 2 2 mm Hg with only a 1 mm Hg pressure difference. PCO of the arterial blood entering the tissues, 40 2 mm Hg; PCO2 of the venous blood leaving the tissues, 45 mm Hg the tissue capillary blood comes almost exactly to equilibrium with the interstitial PCO2 of 45 mm Hg. PCO of the blood entering the pulmonary 2 capillaries at the arterial end, 45 mm Hg; PCO2 of the alveolar air, 40 mm Hg.  The CO2 pressures are approximately the following: Thus, only a 5 mm Hg pressure diffrence causes all the required CO2 diffusion out of the pulmonary capillaries into the alveoli. Furthermore, the PCO2 of the pulmonary capillary blood falls to almost exactly equal the alveolar PCO2 of 40 mm Hg before it has passed more than about one third the distance through the capillaries. Thus is the same effect that was observed earlier for O2 diffusion, except that it is in the opposite direction. ROLE OF HEMOGLOBIN IN OXYGEN TRANSPORT Normally, about 97 percent of the oxygen carried with haemoglobin in the red blood cells. The remaining 3 percent is transported in the dissolved state in the water of the plasma and blood cells. Thus, under normal conditions, oxygen is carried to the tissues almost entirely by hemoglobin. REVERSIBLE COMBINATION OF O2 WITH HEMOGLOBIN The O2 molecule combines loosely and reversibly with the haem portion of hemoglobin. When PO2 is high, as in the pulmonary capillaries, O2 binds with the hemoglobin, but when PO2 is low, as in the tissue capillaries, O 2 is released from the hemoglobin. Thus is the basis for almost all O 2 transport from the lungs to the tissues. Maximum Amount of Oxygen That Can Combine with the Hemoglobin of the Blood The a normal person contains about 15 grams of haemoglobin in each 100 milliliters of blood. Each gram of haemoglobin can bind with a maximum of 1.34 milliliters of O2 (1.39 milliliters when the hemoglobin is chemically pure, but impurities such as methemoglobin reduce this). Therefore, 15 times 1.34 equals 20.1, which means that, on average, the 15 grams of haemoglobin in 100 milliliter of blood can combine with a total of about 20 milliliters of O2 if the haemoglobin is 100 percent saturated. Amount of Oxygen Released From the Hemoglobin When Systemic Arterial Blood Flows Through the Tissues. The total quantity of O2 bound with hemoglobin in normal systemic arterial blood, which is 97 percent saturated, is about 19.4 milliliters per 100 milliliters of blood. Upon passing through the tissue capillaries, this amount is reduced, on average, to 14.4 milliliters (PO 2 of 40 mm Hg, 75 percent saturated hemoglobin). Thus, under normal conditions, about 5 milliliters of O 2 are transported from the lungs to the tissues by each 100 milliliters of blood flow.  Oxygen-hemoglobin dissociation curve The O2-hemoglobin dissociation curve, which demonstrates a progressive increase in the percentage of hemoglobin bound with O2 as blood PO2 increases, which is called the percent saturation of hemoglobin. Because the blood leaving the lungs and entering the systemic arteries usually has a PO2 of about 95 mm Hg, one can see from the dissociation curve that the usual O2 saturation of systemic arterial blood averages 97 percent. Conversely, in normal venous blood returning from the peripheral tissues, the PO2 is about 40 OXYGEN-HEMOGLOBIN DISSOCIATION CURVE FACTORS THAT SHIFT THE OXYGEN-HEMOGLOBIN DISSOCIATION CURVE  Transport of Oxygen Is Markedly Increased during Strenuous Exercise. During heavy exercise, the muscle cells use O2 at a rapid rate, which, in extreme cases, can cause the muscle interstitial fluid PO2 to fall from the normal 40 mm Hg to as low as 15 mm Hg. At this low pressure, only 4.4 milliliters of O2 remain bound with the haemoglobin in each 100 milliliters of blood.  Transport of Oxygen Is Markedly Increased during Strenuous Exercise. Thus, 19.4 − 4.4, or 15 milliliters, is the quantity of O 2 actually delivered to the tissues by each 100 milliliters of blood flow, meaning that three times as much O2 as normal is delivered in each volume of blood that passes through the tissues. Keep in mind that the cardiac output can increase to six to seven times normal in well-trained marathon runners. Thus, multiplying the increase in cardiac output (6- to 7- fold) by the increase in O2 transport in each volume of blood (3-fold) gives a 20-fold increase in O2 transport to the tissues. Effect of PO2 on O2 binding to Hb  Transport of Oxygen in the Dissolved State At the normal arterial PO2 of 95 mm Hg, about 0.29 milliliter of O2 is dissolved in every 100 milliliters of water in the blood, and when the PO2 of the blood falls to the normal 40 mm Hg in the tissue capillaries, only 0.12 milliliters of O2 remains dissolved. In other words, 0.17 milliliters of O2 is normally transported in the dissolved state to the tissues by each 100 milliliters of arterial blood flow. Thus figure compares with almost 5 milliliters of O 2 transported by the red blood cell hemoglobin. Combination of Hemoglobin with Carbon Monoxide—Displacement of O2 Carbon monoxide (CO) combines with hemoglobin at the same point on the hemoglobin molecule as does O2; it can therefore displace O2 from the hemoglobin, thereby decreasing the O2-carrying capacity of blood. Further, it binds with about 250 times as much tenacity as O2.  Combination of Hemoglobin with Carbon Monoxide— Displacement of O2 Therefore, a CO partial pressure of only 0.4 mm Hg in the alveoli, that of normal alveolar O2 (100 mm Hg PO2), allows the CO to compete equally with the O2 for combination with the hemoglobin and causes half the hemoglobin in the blood to become bound with CO instead of with O2. Therefore, a CO pressure of only 0.6 mm Hg (a volume concentration of less than one part per thousand in air) can be lethal.  Combination of Hemoglobin with Carbon Monoxide— Displacement of O2 Even though the O2 content of blood is greatly reduced in CO poisoning, the PO2 of the blood may be normal. Thus situation makes exposure to CO especially dangerous because the blood is bright red and there are no obvious signs of hypoxemia, such as a bluish color of the fingertips or lips (cyanosis). Also, PO2 is not reduced, and the feedback mechanism that usually stimulates an increased respiration rate in response to lack of O2 (usually reflected by a low PO2) is absent. Because the brain is one of the fist organs affected by lack of oxygen, the person may become disoriented and unconscious before becoming aware of the danger.  Combination of Hemoglobin with Carbon Monoxide— Displacement of O2 A patient severely poisoned with CO can be treated by administering pure O2 because O2 at high alveolar pressure can displace CO rapidly from its combination with hemoglobin. The patient can also benefit from simultaneous administration of 5 percent CO2 because this strongly stimulates the respiratory center, which increases alveolar ventilation and reduces the alveolar CO. With intensive O2 and CO2 therapy, CO can be removed from the blood as much as 10 times as rapidly as without therapy.