Respiratory Mechanics and Lung Function

Breathing/Ventilation

• Respiration involves air flow in and out for gas exchange in alveoli.

Mechanics of Breathing

• Air movement into and out of the lungs is dictated by pressure differences.
• Lung volume changes lead to pressure differences.
• Air flows from high to low pressure regions.

Respiratory Mechanics

• Inhalation is facilitated by the diaphragm contracting, enlarging the lung volume.
• Exhalation occurs when the diaphragm relaxes, reducing the lung volume.

Measuring Lung Function

• Spirometry is the technique used

Lung Function (Spirometry)

• Tidal volume is the amount of air moved in or out of the lungs during rest.
• Inspiratory capacity refers to the volume amount of air that can be drawn into the lungs.
• Expiratory reserve volume refers to the volume amount of air that can be expelled from the lungs.
• Forced vital capacity represents the maximum air volume exhaled after a deep inhalation, within a single respiratory cycle.
• Forced vital capacity is calculated by summing inspiratory reserve capacity, tidal volume, and expiratory reserve volume.

Lung Function Volumes

• Reference values are used to understand lung function volumes.
• Forced Vital Capacity (FVC) is a key measure.

Calculating FEV1/FVC

• FEV1/FVC is the ratio of Forced Expiratory Volume in 1 second to Forced Vital Capacity.
• Normal FEV1/FVC values typically range above 70-80%, adjusted for age and gender.
• Airflow limitation, is indicated by a decreased FEV1/FVC ratio

Lung Function (Flow-Volume)

• A flow-volume loop illustrates lung function as a graph of flow rate vs volume.
• Peak Expiratory Flow (PEF) is the maximum flow rate during exhalation.
• Forced Vital Capacity (FVC) represents the total volume exhaled.
• Asthma is defined as reversible airflow post-bronchodilator
• COPD as persistent decline in lung function

Control of Breathing

• Breathing control involves the brain, chemoreceptors, respiratory muscles, and reflexes
• Central chemoreceptors detect carbon dioxide levels.
• Peripheral chemoreceptors detect oxygen, carbon dioxide, pH, and hydrogen ion levels.

Why Control Breathing

• Lungs facilitate gas exchange of oxygen and carbon dioxide which helps with cellular respiration and pH balance.
• Poor oxygen control leads to poor aerobic respiration.
• Poor carbon dioxide control leads to acidic blood, cellular damage, and organ failure.

How Breathing is Controlled

• Respiratory centers in the medulla send signals to respiratory muscles to control the rate and depth of breathing.
• Central and peripheral chemoreceptors detect carbon dioxide and oxygen levels, adjusting respiratory rate and depth accordingly.

Control of Breathing in Healthy Individuals

• Carbon dioxide and hydrogen ion levels detected by central chemoreceptors primarily dictate breathing rate and depth in healthy individuals under non-hypoxic conditions.
• Increased carbon dioxide and hydrogen ion levels detected during breath-holding.

Respiratory Muscles Role

• The lungs themselves lack muscles for respiration, with the small muscle amount controlling airway diameter only.
• Respiratory muscles generate pressure differences required for breathing.

Respiratory Muscles in Quiet Breathing

• Inspiration is an active process.
• The diaphragm and external intercostal muscles are used

Respiratory Muscles in Forced Breathing

• Inspiration involves the diaphragm, external intercostal muscles, and accessory muscles like the pectoralis major, minor, and serratus anterior.

Diaphragm During Inspiration

• The phrenic nerves innervate the diaphragm, causing it to flatten and draw air into the chest.
• The central tendon of the diaphragm moves 1-2 cm during rest and less than 10 cm during forced breathing.

Role of External Intercostal Muscles During Inspiration

• Two types of movement: pump-handle, where the anterior end of each rib is elevated, and bucket-handle, where the chest diameter increases.

Expiration During Quiet Breathing

• Expiration is largely passive due to the elastic recoil of the lungs.

Expiration During Forced Breathing

• Expiration is active and involves accessory respiratory muscles such as the anterior abdominal muscles and quadratus lumborum.

Alveolar Ventilation Defined

• Alveolar ventilation represents the portion of total ventilation that reaches the alveoli.
• It participates in gas exchange.
• Hypoventilation or hyperventilation can indicate lung diseases.
• Gases move between air and blood through passive diffusion.
• Movement is dictated by partial pressure gradients.

Airway Ventilation & Perfusion

• Airway ventilation and perfusion increase as you descend the airways, due to gravity.

Anatomical Dead Space

• Volume of air in the mouth, pharynx, trachea, and bronchi up to the terminal bronchioles.
• It's approximately 150 ml.
• Composed of upper respiratory tract, trachea, bronchi, and terminal bronchioles.

Anatomical & Alveolar Dead Space

• Alveolar dead space refers to alveoli with insufficient blood supply, reducing their effectiveness in respiratory membrane.
• Presence of alveolar dead space occurs with age or respiratory disease.

Calculating Alveolar Ventilation

• New air rate reaching alveoli calculated as (Tidal volume – dead space) multiplied by respiratory rate.
• Using 500 mL tidal volume, 150 mL dead space, and 12 breaths/min respiratory rate yields 4.2 L/min alveolar ventilation.

Alveolar Ventilation & Respiratory Exchange

• Alveolar ventilation effectively facilitates oxygen and carbon dioxide exchange.
• Room air contains almost zero carbon dioxide, with alveolar gas at 5.5%, resulting in an output of 231 mL/min.
• Atmospheric air contains about 21% oxygen, and alveolar gas about 14%, for an uptake of 294 mL/min.

Summary of Key Points

• Respiratory system's primary function involves air movement in and out of the lungs.
• Lung function is assessed using spirometry.
• Tests assess tidal volume, forced vital capacity, and forced expiratory volume in 1 second.
• Respiratory and accessory muscles play a key role in controlling breathing.
• Alveolar ventilation stands for he portion of total ventilation reaching the alveoli facilitates gas exchange.