Alveolar air does not have the same concentrations of gases as atmospheric air (Table 1). There are several reasons for the differences. First, alveolar air is only partially replaced by atmospheric air with each breath. Second, O2 is constantly being absorbed into the pulmonary blood from the alveolar air. Third, CO2 is constantly diffusing from the pulmonary blood into the alveoli. And fourth, dry atmospheric air that enters the respiratory passages is humidified even before it reaches the alveoli.

Table1. Partial Pressures of Respiratory Gases (in mm Hg) as They Enter and Leave the Lungs (at Sea Level)

HUMIDIFICATION OF THE AIR IN THE RESPIRATORY PASSAGES

Table 1 shows that atmospheric air is composed almost entirely of nitrogen and O2; it normally contains almost no CO2 and little water vapor. However, as soon as the atmospheric air enters the respiratory passages, it is exposed to the fluids that cover the respiratory surfaces. Even before the air enters the alveoli, it becomes almost totally humidified.

The partial pressure of water vapor at a normal body temperature of 37°C is 47 mm Hg, which is there fore the partial pressure of water vapor in the alveolar air. Because the total pressure in the alveoli cannot rise to more than the atmospheric pressure (760 mm Hg at sea level), this water vapor simply dilutes all the other gases in the inspired air. Table 1 also shows that humidification of the air dilutes the oxygen partial pressure at sea level from an average of 159 mm Hg in atmospheric air to 149 mm Hg in the humidified air, and it dilutes the nitrogen partial pressure from 597 to 563 mm Hg.

ALVEOLAR AIR IS SLOWLY RENEWED BY ATMOSPHERIC AIR

we pointed out that the average male functional residual capacity of the lungs (the volume of air remaining in the lungs at the end of normal expiration) measures about 2300 milliliters. Yet only 350 milli liters of new air is brought into the alveoli with each normal inspiration, and this same amount of old alveolar air is expired. Therefore, the volume of alveolar air replaced by new atmospheric air with each breath is only one seventh of the total, so multiple breaths are required to exchange most of the alveolar air. Figure 1 shows this slow rate of renewal of the alveolar air. In the first alveolus of the figure, excess gas is present in the alveoli but note that even at the end of 16 breaths the excess gas still has not been completely removed from the alveoli.

Fig1.  Expiration of a gas from an alveolus with successive  breaths.

Figure 2 demonstrates graphically the rate at which excess gas in the alveoli is normally removed, showing that with normal alveolar ventilation, about one half the gas is removed in 17 seconds. When a person’s rate of alveolar ventilation is only one-half normal, one half the gas is removed in 34 seconds, and when the rate of ventilation is twice normal, one half is removed in about 8 seconds.

Fig2. Rate of removal of excess gas from alveoli. 

Importance of the Slow Replacement of Alveolar Air. The slow replacement of alveolar air is of particular importance in preventing sudden changes in gas concentrations in the blood. This makes the respiratory control mechanism much more stable than it would be otherwise, and it helps prevent excessive increases and decreases in tissue oxygenation, tissue CO2 concentration, and tissue pH when respiration is temporarily interrupted.

OXYGEN CONCENTRATION AND PARTIAL PRESSURE IN THE ALVEOLI

Oxygen is continually being absorbed from the alveoli into the blood of the lungs, and new O2 is continually being breathed into the alveoli from the atmosphere. The more rapidly O2 is absorbed, the lower its concentration in the alveoli becomes; conversely, the more rapidly new O2 is breathed into the alveoli from the atmosphere, the higher its concentration becomes. Therefore, O2 concentration in the alveoli, as well as its partial pressure, is controlled by (1) the rate of absorption of O2 into the blood and (2) the rate of entry of new O2 into the lungs by the ventilatory process.

Figure3 shows the effect of alveolar ventilation and rate of O2 absorption into the blood on the alveolar partial pressure of O2 (PO2). One curve represents O2 absorption at a rate of 250 ml/min, and the other curve represents a rate of 1000 ml/min. At a normal ventilatory rate of 4.2 L/min and an O2 consumption of 250 ml/min, the normal operating point in Figure 3 is point A. The figure also shows that when 1000 milliliters of O2 are being absorbed each minute, as occurs during moderate exercise, the rate of alveolar ventilation must increase fourfold to maintain the alveolar Po2 at the normal value of 104 mm Hg.

Fig3. Effect of alveolar ventilation on the alveolar partial  pressure of oxygen (Po2) at two rates of oxygen absorption from the  alveoli—250 ml/min and 1000 ml/min. Point A is the normal operating point.

Another effect shown in Figure 3 is that even an extreme increase in alveolar ventilation can never increase the alveolar PO2 above 149 mm Hg as long as the person is breathing normal atmospheric air at sea level pressure, because 149 mm Hg is the maximum PO2 in humidified air at this pressure. If the person breathes gases that contain partial pressures of O2 higher than 149 mm Hg, the alveolar PO2 can approach these higher pressures at high rates of ventilation.

CO2 CONCENTRATION AND PARTIAL PRESSURE IN THE ALVEOLI

Carbon dioxide is continually formed in the body and then carried in the blood to the alveoli, and it is continually removed from the alveoli by ventilation. Figure 4 shows the effects on the alveolar partial pressure of CO2 (PCO2) of both alveolar ventilation and two rates of CO2 excretion, 200 and 800 ml/min. One curve represents a normal rate of CO2 excretion of 200 ml/min. At the normal rate of alveolar ventilation of 4.2 L/min, the operating point for alveolar PCO2 is at point A in Figure 4 (i.e., 40 mm Hg).

Fig4. Effect of alveolar ventilation on the alveolar partial  pressure of carbon dioxide (Pco2) at two rates of carbon dioxide excretion from the blood—800 ml/min and 200 ml/min. Point A is the  normal operating point.

Two other facts are also evident from Figure 4: First, the alveolar PCO2 increases directly in proportion to the rate of CO2 excretion, as represented by the fourfold elevation of the curve (when 800 milliliters of CO2 are excreted per minute). Second, the alveolar PCO2 decreases in inverse proportion to alveolar ventilation. Therefore, the concentrations and partial pressures of both O2 and CO2 in the alveoli are determined by the rates of absorption or excretion of the two gases and by the amount of alveolar ventilation.

Expired Air Is a Combination of Dead Space Air and Alveolar Air

 The overall composition of expired air is determined by (1) the amount of the expired air that is dead space air and (2) the amount that is alveolar air. Figure 5 shows the progressive changes in O2 and CO2 partial pressures in the expired air during the course of expiration. The first portion of this air, the dead space air from the respiratory passage ways, is typical humidified air, as shown in Table 1. T hen, progressively more and more alveolar air becomes mixed with the dead space air until all the dead space air has finally been washed out and nothing but alveolar air is expired at the end of expiration. Therefore, the method of collecting alveolar air for study is simply to collect a sample of the last portion of the expired air after forceful expiration has removed all the dead space air.

Fig5. Oxygen and carbon dioxide partial pressures (Po2 and  Pco2) in the various portions of normal expired air. 

Normal expired air, containing both dead space air and alveolar air, has gas concentrations and partial pressures approximately as shown in Table 1 (i.e., concentrations between those of alveolar air and humidified atmospheric air).

اخر الاخبار

اخبار العتبة العباسية المقدسة

شعبة الخطابة النسوية تطلق دورة تخصصية لقارئات المنبر الحسيني وخطيباته

المجمع العلمي ينظّم سفرة دينية لطلبة مشاريعه القرآنية في كربلاء

عبر مجلّة (تراث الجنوب) المحكّمة.. قسم شؤون المعارف يوثق إرث مدن جنوب العراق

في ذكرى تأسيسه.. متحف الكفيل يستعرض مسيرته في حفظ التراث الإنساني والإسلامي

المزيد

الآخبار الصحية

ماذا يحدث إذا تناولنا الطعام مرة واحدة فقط في اليوم

اكتشاف عوامل خطر غير متوقعة لمرض الكبد الدهني

فيروس هانتا والسفينة الموبوءة.. تحديث من الصحة العالمية

طبيب يحسم الجدل السجائر الإلكترونية وعلاقتها بالسرطان

المزيد

الآخبار التكنلوجية

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

الكشف عن تفاصيل أول عملية ناجحة لاستعادة انتاج الحيوانات المنوية

أطلس يكشف خفايا الجسم البشري بدقة أرفع بـ50 مرة من شعر الإنسان

المتحف المصري الكبير يتحول إلى "أيقونة خضراء" بدعم ياباني

المزيد

مواضيع ذات صلة

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

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

Muscles that Cause Lung Expansion and Contraction

انخفاض تركيز الأكسجين في الدم Hypoxia

أمراض الجهاز التنفسي