Lung volumes and capacities

Lung volumes – there are four lung volumes, which when added together, equal the maximal volume of the lungs. Tidal volume is the volume of one inspired or expected normal breath (average human = 0,5 L per breath). Inspiratory reserve volume is the volume of air that can be inspired in excess of the tidal volume. Expiratory reserve volume is the extra an that can be expired after a normal tidal expiration.

Residual volume is the volume of gas that re lungs after maximal expiration (average human = 1,2 L).

Total lung capacity is the volume of gas that can be con tained within the maximally inflated lungs (average human = 6 L).

Vital capacity is the maximal volume that can be expelled after maximal inspiration (average human = 4,8 L).

Functional residual capacity is the volume remaining in the lungs at the end of a normal tidal expiration (average luman = 2,2 L).

Inspiratory capacity is the volume that can be taken into the lungs after maximal inspiration following expiration of a normal breath. Helium dilution techniques are used to determine residual volume, FRC and TLC. A forced vital capacity is obtained when a subject inspires maximally and then exhales as forcefully and as completely as possible. The forced expiratory volume (FEV1) is the volume of air exhaled in the first second. Typically, the FEV1 is approximate 80 % of the FVC.

GAS LAWS AS APPLIED TO RESPIRATORY PHYSIOLOGY: Dalton's Law: In a gas mixture, the pressure exerted by each gas is independent of the pressure exerted by the other gases.

A consequence of this is as follows: partial pressure = total pressure x fractional concentration. This equation can be used to determine the partial pressure of oxygen in the atmosphere. Assuming that the total pressure (or barometric pressure, PB) is atmospheric pressure at sea level (760 mmHg) and the fractional concentration of O₂is 21 %, or 0,21: P02 = 760 mmHg Ч 0,21 = 160 mmHg. As air moves into the airways, the partial pressures of the va-ri ous gases in atmospheric air are reduced because of the addi tion of water vapor (47 mmHg). Henry's Law states that the concentration of a gas dissolved in liquid is proportional to its partial pressure and its solubility coef fi-cient (Ks). Thus, for gas X, [X] = Ks Ч Px

Fick's Law states that the volume of gas that diffuses across a barrier per unit time is given by:

Vgas = Y x D x (P1 – P2)

where A and T are the area and thickness of the barrier, P1 and P2 are the partial pressures of the gas on either side of the barrier and D is the diffusion constant of the gas. D is directly proportional to the solubility of the gas and inversely proportional to the square root of its molecular weight.

Date: 2016-02-19; view: 395; Нарушение авторских прав