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, CO2 diffuses in the direction exactly opposite to the diffusion of O2. Yet there is one major difference between diffusion of CO2 and of O2: CO2 can diffuse about 20 times as rapidly as O2. Therefore, the pressure differences required to cause CO2 diffusion are, in each instance, far less than the pressure differences required to cause O2 diffusion. T he CO2 pressures are approximately the following:

1. Intracellular PCO2, 46 mm Hg; interstitial PCO2, 45 mm Hg. Thus, there is only a 1 mm Hg pressure differential, as shown in Figure 1.

2. PCO2 of the arterial blood entering the tissues, 40 mm Hg; PCO2 of the venous blood leaving the tissues, 45 mm Hg. Thus, as shown in Figure 1, the tissue capillary blood comes almost exactly to equilibrium with the interstitial PCO2 of 45 mm Hg.

3. PCO2 of the blood entering the pulmonary capillaries at the arterial end, 45 mm Hg; PCO2 of the alveolar air, 40 mm Hg. Thus, only a 5 mm Hg pressure difference causes all the required CO2 diffusion out of the pulmonary capillaries into the alveoli. Furthermore, as shown in Figure2, 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. This is the same effect that was observed earlier for O2 diffusion, except that it is in the opposite direction.

Fig1. Uptake of carbon dioxide by the blood in the tissue  capillaries. (PCO2 in tissue cells = 46 mm Hg, and in interstitial fluid  = 45 mm Hg.) 

Fig2. Diffusion of carbon dioxide from the pulmonary blood  into the alveolus. (Data from Milhorn HT Jr, Pulley PE Jr: A theoretical study of pulmonary capillary gas exchange and venous admixture. Biophys J 8:337, 1968.)

Effect of Rate of Tissue Metabolism and Tissue Blood Flow on Interstitial PCO2. Tissue capillary blood flow and tissue metabolism affect the PCO2 in ways exactly opposite to their effect on tissue PO2. Figure 3 shows these effects, as follows:

 1. A decrease in blood flow from normal (point A) to one-quarter normal (point B) increases peripheral tissue PCO2 from the normal value of 45 mm Hg to an elevated level of 60 mm Hg. Conversely, increasing the blood flow to six times normal (point C) decreases the interstitial PCO2 from the normal value of 45 mm Hg to 41 mm Hg, down to a level almost equal to the PCO2 in the arterial blood (40 mm Hg) entering the tissue capillaries.

2. Note also that a 10-fold increase in tissue metabolic rate greatly elevates the interstitial fluid PCO2 at all rates of blood flow, whereas decreasing the metabolism to one-quarter normal causes the interstitial fluid PCO2 to fall to about 41 mm Hg, closely approaching that of the arterial blood, 40 mm Hg.

Fig3. Effect of blood flow and metabolic rate on peripheral  tissue PCO2.

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قسم الشؤون الفكريّة يُهدي إصداراته العلمية إلى مكتبات محافظة كركوك

شعبة فاطمة بنت أسد تطلق مسابقتها الإلكترونية (منارة الجود في مهد الصبا) لكتابة القصة للأطفال

المجمع العلمي يقيم برنامجًا قرآنيًا تعليميًا في النجف الأشرف

قسم الشؤون الدينية يقيم مجلسًا عزائيًّا لاستذكار أهل البيت (عليهم السلام)

المزيد

الآخبار الصحية

ما هي أفضل العصائر لصحة القلب وتقوية المناعة والنوم العميق؟

الأطعمة الغنية بالبروتينات النباتية تقلل خطر الأمراض المزمنة

وجع الأوتار قد يخفي خطرا صحيا مرتبطا بالقلب

أزمة "السفينة الموبوءة"..الأرجنتين تحقق في مصدر تفشي "هانتا"

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الآخبار التكنلوجية

باحثون يحددون تأثير القهوة على الميكروبيوم والمزاج

روسيا.. ابتكار تقنية لمعالجة سبائك التيتانيوم لحماية الطائرات والسيارات

بكين وواشنطن تدرسان إطلاق محادثات رسمية حول الذكاء الاصطناعي

لأول مرة في الولايات المتحدة.. تشغيل مفاعل مصغر نووي يغذي الذكاء الاصطناعي بالطاقة

المزيد

مواضيع ذات صلة

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

Pressures in the Pulmonary System

Physiological Anatomy of the Pulmonary Circulatory System

Abbreviations and Symbols used in Pulmonary Function Studies

Recording Changes in Pulmonary Volume—Spirometry

Effect of the Thoracic Cage on Lung Expansibility

Pressures that Cause the Movement of Air in and out of the Lungs