Additional information | Respiratory system | Physiology

Dead space: The volume of air contained in the conducting airways cannot participate in gas exchange. Hence it is known as anatomic “dead space.” This includes air from the nasal passages to the terminal bronchioles. The average dead space is 150 ml. Physiologic dead space is the total volume of air that does not participate in gas exchange - it includes the anatomic dead space plus functional or alveolar dead space of unperfused alveoli. Normally, it is close to zero and increases in V/Q defects. The ratio of physiologic dead space to TV is ⅓.
Bronchopulmonary segments: These are segments of the lung that are supplied by their own segmental or tertiary bronchus and pulmonary artery branch. This unique arrangement allows individual segments to be resected separately. There are 10 segments of the right lung and 8 segments of the left lung.
TLC corrected for age remains almost constant throughout life. However, the gradual increase in FRC and RV with age results in simultaneous decreases in IC and VC.

Factors that increase FRC	Factors that decrease FRC
PEEP, CPAP, tall stature, male sex, obstructive lung disease.	Supine position; Induction of anesthesia, female sex, pregnancy, ascites, post-op period, restrictive lung disease,

Closing capacity and volume: The volume of air that remains in the lungs when small airways start to close is the closing capacity. Comparatively, residual volume is the volume that remains when all the small airways are closed. The volume of air expired between CC and RV is called the closing volume
Hysteresis of the lung: Hysteresis is the observation that the dynamic pressure-volume loop for inspiration is different from expiration. It is related to differing surface tensions and small airway resistance in inspiration and expiration. Surfactant is known to reduce the surface tension in the alveoli. During inspiration, as the alveoli expand, pulmonary surfactant is dispersed over a larger surface area, leading to an increase in the surface tension. The opposite effect is seen in expiration. Progressively larger number of smaller airways are recruited during inspiration while progressively larger number of smaller airways close down during expiration, also causing hysteresis.
Hering-Breuer reflex: Stretch receptors in the smooth muscle of the airways are activated by distension of the lung and airways, causing a reflex decrease in the respiratory rate. The length of expiration is prolonged, allowing more air to be breathed out.
Oxygen content of blood: It is the total amount of O2 present per volume of blood. It is determined mainly by the hemoglobin concentration and the O2 binding capacity of hemoglobin. Variants of hemoglobin have different O2 binding capacities. The O2 content of blood determines O2 delivery to tissues, as less content will result in less O2 delivery to tissues and vice versa.