KP222/HN220 Human Physiology - Week 10: Respiratory-3

Respiratory Volumes and Capacities

• Respiratory Mechanism Overview:

• The respiratory system is essential for gas exchange, supporting cellular respiration and maintaining acid-base balance.

• The spirometer is a clinical instrument used to measure respiratory volumes and airflow, providing vital data for assessing lung function and diagnosing respiratory conditions.

• Lung Volumes:

• Total lung volume can be subdivided into distinct lung volumes:

  • Tidal Volume (VT):

  • This is the amount of air breathed in or out during normal respiration, approximately 500 mL. It represents the volume of air exchanged in each breath during quiet breathing.

  • Inspiratory Reserve Volume (IRV):

  • The maximum additional air that can be inhaled after the completion of a normal inspiration, estimated to be around 3000 mL. It's important during physical exertion when deep inhalation is needed.

  • Expiratory Reserve Volume (ERV):

  • The maximum air that can be forcibly exhaled after a normal expiration, about 1000 mL. This volume is significant in increasing the efficiency of gas exchange.

  • Residual Volume (RV):

  • The volume of air that remains in the lungs after maximum exhalation, approximately 1200 mL. This volume cannot be measured by conventional spirometry and is critical in preventing lung collapse by keeping the alveoli open.

• Lung Capacities:

• Lung capacities are calculated by combining different lung volumes, which include:

  • Inspiratory Capacity (IC):

  • This is the maximum volume of air that can be inspired after a normal expiration; calculated as IC = VT + IRV, totaling approximately 3500 mL.

  • Vital Capacity (VC):

  • This represents the maximum amount of air that can be exhaled after a maximal inhalation, calculated as VC = VT + IRV + ERV, equating to roughly 4500 mL. It is a crucial measurement in pulmonary function tests.

  • Functional Residual Capacity (FRC):

  • The volume of air that remains in the lungs at the end of a normal expiration; calculated as FRC = ERV + RV, approximately 2200 mL. It indicates the baseline lung volume for optimal gas exchange.

  • Total Lung Capacity (TLC):

  • The total volume of the lungs when filled to capacity at the end of a maximal inspiration; calculated as TLC = VT + IRV + ERV + RV, approximately equal to 5700 mL. TLC is significant in diagnosing restrictive and obstructive lung diseases.

• Respiratory Rates and Volumes:

• Breathing Rates:

  • The average normal respiratory rate ranges from 12-20 breaths per minute, varying based on age, activity level, and overall health.

• Minute Ventilation (VE):

  • The total volume of air inhaled or exhaled per minute, calculated as VE = f x VT, with normal values around ~6 L/min. This measurement reflects the overall respiratory efficiency.

• Alveolar Ventilation (VA):

  • This focuses on the effective ventilation available for gas exchange; calculated as VA = f x (VT - VD), resulting in approximately 4.2 L/min for a normal adult. This figure helps assess the adequacy of ventilation in the lungs.

• Terminology:

• Hyperpnea: Increased respiratory rate and/or depth in response to increased metabolic demand, such as during heavy exercise.

• Hyperventilation: A condition where the rate and depth of breathing increase substantially without a corresponding increase in metabolism, often causing respiratory alkalosis.

• Hypoventilation: A state characterized by decreased respiratory rate or volume leading to insufficient alveolar ventilation, which can result in increased carbon dioxide levels.

• Dyspnea: A subjective feeling of breathlessness or difficulty breathing, which can be a symptom of underlying respiratory conditions.

• Apnea: The temporary cessation of breathing, which can occur during sleep or in certain medical conditions.

• Gas Exchange and Partial Pressures:

• Gas Exchange Mechanism:

  • Gas exchange occurs primarily in the alveoli, where oxygen diffuses from alveolar air into the blood, while carbon dioxide diffuses from the blood into the alveoli, driven by differences in partial pressures.

• Dalton’s Law of Partial Pressures:

  • States that in a mixture of gases, each gas exerts its own pressure independently of others; for example, at sea level (760 mmHg):

  • PN2 = 600.7 mmHg, PO2 = 159 mmHg, PCO2 = 0.3 mmHg.

• Henry's Law of Solubility:

  • Describes how gas solubility in liquids is proportional to partial pressures; notably, carbon dioxide is 20 times more soluble than oxygen, affecting its transport in the bloodstream.

• Gas Exchange in the Lungs:

• The efficiency of oxygen and carbon dioxide transport across the alveolar-capillary membrane is crucial for maintaining appropriate oxygen levels in tissues and removing carbon dioxide from the body.

Summary and Key Concepts

• Understand and describe various lung volumes and capacities: VT, IRV, ERV, RV, IC, VC, FRC, and TLC, along with their physiological significance.

• Apply Dalton’s Law and Henry’s Law to explain gas exchange dynamics involving oxygen and carbon dioxide.

• Relate the concept of partial pressures to effective respiration, ensuring optimal oxygen delivery and carbon dioxide removal in the body.