CLOSING CAPACITY
Monthly Archives: April 2018
FUNCTIONAL RESIDUAL CAPACITY [FRC]
The functional residual capacity of the infant’s lungs is only one half that of an adult in relation to body weight. This difference causes excessive cyclical increases and decreases in the newborn baby’s blood gas concentrations if the respiratory rate becomes slowed because it is the residual air in the lungs that smooths out the blood gas variations.
The functional residual capacity equals the expiratory reserve volume plus the residual volume. This is the amount of air that remains in the lungs at the end of normal expiration (about 2300 milliliters).
INTRA-PLEURAL PRESSURE
- The resting position of the lungs and chest wall occurs at FRC.
- If isolated, the lungs, being elastic, would collapse to a volume <FRC.
- The isolated thoracic cage would normally have a volume >FRC.
- Since the chest wall is coupled to the lung surface by the thin layer of intrapleural fluid between parietal and visceral pleura, opposing lung and chest wall recoil forces are in equilibrium at FRC.
- This produces a pressure of about −0.3 kPa [−2 mmHg ] in the pleural space.
- Normal inspiration reduces intrapleural pressure further to −1.0 kPa [−6 mmHg] but with forced inspiration it can reach negative pressures of −4.0 kPa or more.
- Intrapleural pressure may be measured by an intrapleural catheter or from a balloon catheter placed in the mid-oesophagus [Oesophageal pressures
tend to reflect intrapleural pressures] - In the upright adult the intrapleural pressure at the base of the lung is approximately 0.7 kPa greater than the pressure at the apex
- It will increase during coughing
- -1.0 kPa intrapleural pressure is equivalent to a distending transpulmonary pressure of +1.0 kPa
PULMONARY CIRCULATION
The Lay Medical Man 