pulmonary capillary but the Hb within it becomes saturated with in about 0.3 sec, that is this 1 sec is more than sufficient for full

however, be quickly formed or dissociated The actual exchange of CO2 therefore, occurs more, From this compound. Time required for oxygen uptake At real, an RBC stays for only about 1 sec in a pulmonary capillary but the Hb within it becomes saturated with in about 0.3 sec, that is this 1 sec is more than sufficient for full oiygenation of Hb During exercise, when the circulation speed is high, the transit time of the RBC may be as low as 0.3 sec. but even this Time as shown above, is sufficient SHUNTS Fig. 4. 4.2 shows that PO2 of oxigenated pulmonary capillary blood is practically same (100 mm Hg) as that of alveolar PO2, yet The PO2 of systemic arterial blood (PaO2) is little lower (95 mm Hg) How to explain this fall Answer is (i) various venous blood, that is. deoxygenated blood, gets mixed with the pulmonary venous Wood Such veins are bronchial veins and the bisian venous system, both carrying de oxygenated blood, (ii) Moreover in some areas of the lung, the alveoli are very poorly ventilated but perfusion of These alveoli (by the blood) is alright Pulmonary capllary blood, draining these poorly ventilated alveoli contains poor quantities of O2 and they too drain in the pulmonary veins All these factors lower the P02 of pulmonary venous blood slightly The total reduction of P02, due to these causes, in health, is only 5 mm Hg. All these factors, collectively, constitute the shunt APPLIED PHYSIOLOGY Pulmonary edema, bronchopneumonia and lobar pneumonia increase the resistance lo the alveolo capillary diffusion by bringing fluid into the alveolo capillary membrane Various industrial hazards like asbestosis. pneumoconiosis etc. also increase the thickness of the alveolocapillary membrane to produce alveolocapillary block. To overcome the block. O2 Therapy may be given For further details, see chapter 6 of section IV Gaseous enchange in the peripheral tissus. Some special problems. Diffusion of 02 and C02 at the level of peripheral capillaries are governed, as in the case of that at lung alveolar level, by Fick's law One important difference is that. the distance between, capillary blood and needy tissue (T in the Fick's law, see earlier this chapter] is usually much greater and therefore oxygenation of peripheral tissues, on theoretical grounds, should nol be easy Yet, the tissues normally show no anona. To explain this, it has been, proposed that there is a facilitated diffusion' of 02 (chap 1 sec I) at this level and myoglobin acts as the earner. Gunng euercise owever many capillaries open up( recruitment) and the vlaue of T therefore falls Perfiiaion inefficiency (reduction of puknonary blood flow) can also lead to hypoxemie. Thus pulmonary embolism is an important cause of this arveolocapillary block ultimately leads lo hyponemia (low P02 in the artenal blood) end hypercapnia (excess PC02 In the artenal blood) with their train of aitendent symptoms (see chapter 6 of sec. IV) SUMMARY The transfer of 02 or C02 across the alveolo capillary barrier is governed by the Fick's Law of diffusition of gases across a membrane. The transfer, normally, almost wholly, depends upon the diffusion, biological processes are not involved to any appreciable extent. All conditions ensling between the alveoli and pulmonary capillary favor a diffusion Provisions also exists so that total volume of transfer of gas (TL) can increase sufficiently during exercise in vanous diseases of the lung or heart, the abilrty lo exchange gas deienorales and symptoms develop.