Referring to the earlier discussion of diffusion of gases in water, one can apply the same principles to diffusion of gases through the respiratory membrane. Thus, the factors that determine how rapidly a gas will pass through the membrane are (1) the thickness of the membrane, (2) the surface area of the membrane, (3) the diffusion coefficient of the gas in the substance of the membrane, and (4) the partial pressure difference of the gas between the two sides of the membrane.

The thickness of the respiratory membrane occasionally increases—for instance, as a result of edema fluid in the interstitial space of the membrane and in the alveoli—so the respiratory gases must then diffuse not only through the membrane but also through this fluid. Also, some pulmonary diseases cause fibrosis of the lungs, which can increase the thickness of some portions of the respiratory membrane. Because the rate of diffusion through the membrane is inversely proportional to the thickness of the membrane, any factor that increases the thickness to more than two to three times normal can interfere significantly with normal respiratory exchange of gases.

The surface area of the respiratory membrane can be greatly decreased by many conditions. For instance, removal of an entire lung decreases the total surface area to one-half normal. Also, in emphysema, many of the alveoli coalesce, with dissolution of many alveolar walls. Therefore, the new alveolar chambers are much larger than the original alveoli, but the total surface area of the respiratory membrane is often decreased as much as fivefold because of loss of the alveolar walls. When the total surface area is decreased to about one-third to one fourth normal, exchange of gases through the membrane is substantially impeded, even under resting conditions, and during competitive sports and other strenuous exercise even the slightest decrease in surface area of the lungs can be a serious detriment to respiratory exchange of gases.

The diffusion coefficient for transfer of each gas through the respiratory membrane depends on the gas’s solubility in the membrane and, inversely, on the square root of the gas’s molecular weight. The rate of diffusion in the respiratory membrane is almost exactly the same as that in water, for reasons explained earlier. Therefore, for a given pressure difference, CO2 diffuses about 20 times as rapidly as O2. Oxygen diffuses about twice as rapidly as nitrogen.

The pressure difference across the respiratory mem brane is the difference between the partial pressure of the gas in the alveoli and the partial pressure of the gas in the pulmonary capillary blood. The partial pressure represents a measure of the total number of molecules of a particular gas striking a unit area of the alveolar surface of the membrane in unit time, and the pressure of the gas in the blood represents the number of molecules that attempt to escape from the blood in the opposite direction. Therefore, the difference between these two pressures is a measure of the net tendency for the gas molecules to move through the membrane.

When the partial pressure of a gas in the alveoli is greater than the pressure of the gas in the blood, as is true for O2, net diffusion from the alveoli into the blood occurs; when the pressure of the gas in the blood is greater than the partial pressure in the alveoli, as is true for CO2, net diffusion from the blood into the alveoli occurs.

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

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

Muscles that Cause Lung Expansion and Contraction

انخفاض تركيز الأكسجين في الدم Hypoxia

أمراض الجهاز التنفسي