Types of alveolarcapillary unit

When considering ventilation (V) and perfusion (Q) Normal ventilation and perfusion: normal unit Ventilation without perfusion: dead space (high V/Q) Perfusion without ventilation: right-to-left shunt (low V/Q) No ventilation or perfusion: silent unit

Normal venous-arterial admixture is around 3%-5% of the cardiac output, and is due to drainage of bronchial venous deoxygenated blood into pulmonary venous oxygenated blood. Passage of blood from systemic venous to arterial system without going through gas-exchange areas of the lung constitutes a right-to-left shunt.

Figure 7.2 Lung volumes. TLC, total lung capacity; VT, tidal volume; IC, inspiratory capacity; VC, vital capacity; FRC, functional residual capacity; ERV, expiratory reserve volume; RV, residual volume.

Figure 7.2 Lung volumes. TLC, total lung capacity; VT, tidal volume; IC, inspiratory capacity; VC, vital capacity; FRC, functional residual capacity; ERV, expiratory reserve volume; RV, residual volume.

* Tidal volume (VT): normal volume of gas inspired and expired with each normal breath. It is the volume of a normal breath beginning at the functional residual capacity. 400-600ml (5-7ml/kg).

* Inspiratory reserve volume (IRV): maximum volume of gas that can be inspired above the end of a normal spontaneous inspiration. 2500-3000 ml.

* Expiratory reserve volume (ERV): maximum volume of gas that can be forcibly exhaled after a normal expiration (i.e., below functional residual capacity). 900-1300ml.

* Vital capacity (VC): maximum volume of gas that can be exhaled by forceful effort after a maximal inspiration. It is the sum of the VT, IRV, and ERV. 4000-5000 ml.

* Residual volume (RV): volume of gas in the lungs after a maximal expiratory effort. It normally comprises 20 %-25 % of the total lung capacity, or 25-30 ml/kg body weight, and increases with age. The RV is the smallest amount of gas possible in the lungs.

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