Respiratory Volumes and Capacities

Learning Objectives

• After this session, it will be possible to outline respiratory volumes and capacities
• You'll be able to explain the local and central control of respiration
• Describing the factors that influence breathing rate and depth will be possible

Respiratory Volumes

• Tidal volume (TV) occurs during normal quiet breathing
• Tidal volume is air volume moving in and out of lungs
• Inspiratory reserve volume (IRV) is the volume that can be forcibly inspired beyond tidal volume
• Expiratory reserve volume (ERV) is volume evacuated from lungs through tidal expiration
• Residual volume (RV) remains in the lung after strenuous expiration
• Residual volume prevents lung collapse

Respiratory Capacities

• Inspiratory capacity (IC) is the total volume that can be inspired
  • IC = TV + IRV
• Functional residual capacity (FRC) is amount of air remaining in lungs after tidal expiration
  • FRC = RV + ERV
• Vital capacity (VC) is the total amount of exchangeable air
  • VC = TV + IRC + ERV
• Total lung capacity (TLC) is the sum of all lung volumes, about 6000 ml

Dead Space

• Some inspired air fills the conducting respiratory passageways
• Anatomical dead space is not involved in gas exchange in the alveoli
• Anatomical dead space is about 150 ml; if TV is 500 ml, only 350 ml is involved in alveolar ventilation
• Alveolar dead space occurs if some alveoli are no longer involved in gas exchange
• Total dead space (physiological dead space) = anatomical dead space + alveolar dead space

Other Measurements of Pulmonary Function

• Minute ventilation is the total amount of gas moving in or out of respiratory tract
  • Normal value is 6 L/min
• Alveolar ventilation rate (AVR) assesses respiratory efficiency and takes dead space into account
  • AVR (ml/min) = frequency (breaths/min) X (TV-dead space) (ml/min)
• Forced vital capacity (FVC) is volume expelled following a deep breath and forceful expiration
• Forced expiratory volume (FEV) is the amount of air expelled during specific time interval
  • The amout of air expelled during the first second is FEV1
  • FEV1/FVC Ratio is normally ~ 80%
  • Is reduced with airway obstruction

Respiratory Control Centers

• The Medullary Respiratory Center has two groups
  • Dorsal respiratory group (DRG) integrates sensory input and allows communication with VRG
  • Ventral respiratory group (VRG) generates rhythm and allows Integrative centers
• Pontine Respiratory Centre influences and modifies medullary neurons for a smoothing effect
• The Pontine Respiratory Centre is active during sleep, exercise, and vocalization

Breathing Control Rate and Depth

• Inspiratory depth is determined by how actively the respiratory control center stimulates motor neurons serving respiratory muscles
• Respiratory rate depends on how long the inspiratory center is active
• Changing body demands impacts breathing
• Chemical stimuli like CO2, O2, and H+ impact breathing

Chemical Influences

• Blood PCO2 is the primary controller of breathing rate and depth
  • Normal arterial PCO2 is 40 mmHg, maintained with ± 3 mmHg
• Specialized cells located in the medulla monitor the blood's chemical composition
• CO2 diffuses from blood into cerebrospinal fluid, becoming hydrated and forming carbonic acid
• Acid dissociates and H+ is liberated
• This excites chemoreceptors and stimulates the respiratory center
• Depth and rate of breathing increases
• CO2 in blood decreases, and pH increases
• Peripheral chemoreceptors are in carotid and aortic bodies
• Peripheral chemoreceptors monitor and respond to changes in PO2, PCO2, and pH in the arterial blood
• Information is transmitted to the respiratory control center
• Peripheral chemoreceptors mediate 30% of ventilatory response to CO2
• O2 levels must decrease below 60mmHg before O2 becomes a major stimulus
• Arterial pH is monitored by peripheral chemoreceptors
• Decreased blood pH may reflect CO2 retention, and metabolic causes like lactic acid accumulation during exercise, and fatty acid metabolites
• The respiratory system attempts to compensate by increasing respiratory rate and depth, and eliminating CO2

Pulmonary Mechano- and Sensory Receptors

• The Hering-Breuer inspiratory-inhibitory reflex is stimulated by increased lung volume
• The Hering-Breuer inspiratory-inhibitory reflex has a stretch reflex mediated by vagal fibers, it causes cessation of inspiration
  • It is inactive during normal quiet breathing
• Irritant receptors are present in the trachea and large airways
• Irritant receptors respond to inhaled dust, noxious gases, and cigarette smoke
• Irritant receptors cause increased airway resistance and cessation of breathing