Lecture 14: Pulmonary Ventilation: Muscles & Airflow

Pulmonary Ventilation

• Smooth muscle in the lungs affects blood vessels and bronchioles
• The airway diameter impacts the speed of airflow.
• Muscle recruitment involves the diaphragm and intercostal muscles.
• The diaphragm is the prime mover for 2/3 of airflow
• When stimulated, it pulls down to increase thoracic volume.
• When relaxed it will recoil to decrease volume

Other Muscles (non-diaphragm)

• Intercostals stiffen the thoracic cage during inhalation, preventing collapse.
• Pulling the ribs upwards expands the cavity, increasing transverse and anteroposterior dimensions.
• Intercostals contribute 1/3 of airflow.
• Forced expiration involves pushing the ribs downward.
• Accessory muscles, usually forced, include the erector spinae (arching the back), and pectoralis major and minor (lifting the ribs).

Normal Expiration

• Normal expiration is mostly passive and energy-saving
• It relies on lung elasticity, the bronchial tree, rib attachments, and muscle tendons.
• Braking action during expiration involves gradual muscle relaxation for a smoother transition.

Abdominal Pressure

• Thoracic pressure can influence the abdomen
• Diaphragm depression increases abdominal pressure, which can expel contents of abdominal organs.
• Examples include urination, defecation, childbirth, and vomiting
• The Valsalva maneuver involves a deep breath, breath-holding, and abdominal muscles.

Brain Respiratory Centers

• The ventral respiratory group (VRG) in the medulla oblongata uses a reverberating circuit.
• Inspiratory neurons stimulate the phrenic and costal nerves.
• Expiratory neurons inhibit inspiratory neurons, leading to recoil.
• The I circuit in the spinal cord (SC) integration center affects the phrenic and intercostal nerves.
• Eupnea, or quiet breathing, occurs at a rate of 12 breaths per minute.
• The dorsal respiratory group (DRG) in the medulla oblongata changes the activity of the VRG, also in the medulla
• Integrates sensory information from chemoreceptors (medulla oblongata, arteries) and stretch receptors (airway).
• It takes input from brainstem centers for emotional pathways.
• The pontine respiratory group (PRG) is located on each side of the pons.
• It receives sensory input from the hypothalamus, limbic system, and cerebral cortex.
• Relays information/output to the VRG & DRG centers of the medulla oblongata
• It regulates short/long and shallow/deep breaths and adapts to conditions like sleep, exercise, and emotions.

Input to Respiratory Centers

• Central chemoreceptors in the brainstem neurons monitor CSF pH.
• Peripheral chemoreceptors are located in the carotid and aortic arteries with the glossopharyngeal and vagus nerves
• Monitor oxygen and carbon dioxide levels, and pH.
• Stretch receptors with smooth muscle, visceral pleura, and bronchioles detect the inflation of the lungs via vagus nerves to the DRG
• The inflation reflex prevents excessive inflation by inhibiting inspiratory neurons
• Irritant receptors at nerve endings among epithelial cells sense smoke, dust, pollen, fumes, cold, and mucus levels.
• It signals through the vagus nerve to the DRG, affecting respiratory and bronchial muscles, causing reflexes like breathing patterns and constriction of bronchioles.

Voluntary Control

• The cerebral motor cortex controls voluntary breathing
• The corticospinal tract and SC integration centers can bypass brainstem centers
• Voluntary breath holding is an example of this control.

What Determines Airflow?

• Airflow is determined by pressure and resistance

Pressure, Airflow

• Airflow is affected by atmospheric pressure vs. intrapulmonary pressure
• Boyle's Law states that the pressure of gas is inversely proportional to volume
• Air flow down a pressure gradient
• Intrapulmonary (or intra-alveolar) pressure < atmospheric pressure during inhalation
• Ribs "expand/pulled out" pleura, and alveoli expand
• Pressure decreases (Boyle's Law)
• Transpulmonary pressure is the difference of 4mmHg

Pneumothorax

• Pneumothorax occurs when air enters the pleural cavity, often due to a punctured thoracic wall.
• Inspiration allows air to enter through the wound, causing the pleurae to separate.
• The condition prevents establishing negative interpulmonary pressure, leading to lung collapse.

Resistance to Airflow

• The diameter of bronchioles affects airflow resistance
• It can be controlled by hormones, such as epinephrine and norepinephrine (dilation) and histamine and acetylcholine (constriction), cold temperature, and irritants
• Pulmonary compliance measures how easily the lungs expand.
• High compliance (flexibility) means low resistance.
• Certain diseases, such as tuberculosis and black lung disease, can reduce compliance.

Surface Tension

• The thin layer of water on the respiratory membrane is necessary for gas exchange.
• Water molecules attracted to each other through H bonds can cause the walls of airways to collapse, especially in bronchioles and alveolar ducts.
• Surfactant disrupts with H bonds and therefore prevents alveolar collapse (detergent)
• Deep breathing helps spread to smaller

Alveolar Ventilation

• Anatomical dead space refers to the conducting zone, where no gas exchange occurs.
• Physiological dead space includes the total dead space, including any pathology of alveoli.
• Alveolar Ventilation Rate (AVR)
• Resistance is high at terminal bronchioles.
• Residual volume cannot be exhaled, mixing with fresh incoming air, and requiring ~18 average breaths to be replaced.