L9 part 1 physiology

Mechanics of Respiration I

• This presentation covers the mechanics of breathing.
• Learning Objectives include describing the functions of inspiration and expiration muscles, explaining inspiration and expiration processes, defining and describing the importance of intra-alveolar, intrapleural, and transpulmonary pressures.
• Breathing is a mechanical process for moving air in and out of the lungs.
• Respiratory rate is 12-20 breaths per minute.

Breathing Movements

• The body uses thoracic cavity muscles to change the volume of the chest.
• This alters pressure inside the lungs.
• An increase in volume = a decrease in pressure (and vice versa).

Basics of the Respiratory System

• Diagrams show the muscles and membranes involved in respiration. Images include the diaphragm, sternocleidomastoids, scalenes, and intercostals (internal and external).

Muscles of Inspiration

• The diaphragm is the principal inspiratory muscle.
• During inspiration, it contracts and increases the vertical diameter of the thoracic cage.
• External intercostals move the ribs, increasing both the transverse and anteroposterior diameters of the chest.
• Inspiration is an active process.

Muscles of Expiration

• Expiration is a passive process during quiet breathing, brought about by relaxation of inspiratory muscles.
• During forced expiration (coughing, blowing), expiration muscles (abdominal muscles and internal intercostals) contract to reduce thoracic cage volume.
• Contraction of abdominal muscles increases intra-abdominal pressure, pushing the diaphragm upward.
• Internal intercostals contract, pulling ribs downwards, further decreasing the chest size.

Lung Pressures

• Intrapulmonary pressure (intra-alveolar) measures pressure within alveoli.
• Normal range is -1 to +1 mm Hg, with 0 mmHg corresponding to atmospheric pressure.
• Intrapleural pressure is the pressure in the intrapleural space. Typical range is -6 to -3 mmHg.
• Average intrapleural pressure is -4 mmHg, which is always negative.
• Transpulmonary pressure is the pressure difference across the wall of the lung. It's always positive.
• This difference prevents lung collapse.

Importance of Intrapleural Pressure

• Helps inflate the lungs during inspiration.
• Prevents lung collapse.
• Decreases the work required for breathing.
• Aids venous and lymphatic drainage.

Regulation of Respiration

• Learning objectives include explaining the importance of breath control, outlining how the central nervous system controls breathing, listing anatomical locations of ventilation control chemoreceptors, describing the Hering-Breuer reflex, and chronic obstructive pulmonary disease (COPD) effects on lung mechanoreceptors, and defining breathing patterns (Kussmaul, Cheyne-Stokes, and Biot).

Case Presentation

• Patient GB is brought to an emergency department with altered mental status, history of illicit drug use, decreased respiratory rate, irregular breathing and episodes of apnea.
• Given the presentation, the reason for the abnormally deep breathing must be determined.

Why is Breath Control Important?

• Breathing is mostly an involuntary process.
• Respiratory muscles (diaphragm, intercostals, and abdominal muscles) receive signals from the brainstem (pons and medulla).
• Some voluntary control like breathing in activities (swimming, singing, yoga) exists.

How Does the Central Nervous System Control Breathing?

• Each brain region has specific neuronal groups for respiration.
• Medulla and pons stimulate respiratory muscles.
• Diagram of brain regions (medulla oblongata, pons, midbrain) is shown.

Medulla

• Dorsal Respiratory Group (DRG) controls inspiration and sets the normal breathing rhythm.
• Efferent DRG sends impulses via phrenic and intercostal nerves to stimulate inspiration by contracting the diaphragm and external intercostal muscles.
• Ventral Respiratory Group (VRG) is inactive during normal breathing but stimulates forced inhalation and exhalation in intense situations like exercise or illness.

Pons

• Pneumotaxic Center (PNC): located in the upper pons, inhibits inspiration by decreasing tidal volume and respiratory rate.
• Apneustic Center (APC) in lower pons stimulates inspiration, leading to deep gasping inspirations with occasional expirations.

What Chemical Sensors Control Breathing?

• Peripheral chemoreceptors detect decreased PaO2, increased PaCO2, and increased hydrogen ion concentration.
• They send impulses to the solitary nucleus in the brainstem for initiating inspiration.
• Decreased PaO2 is the primary and most important signal detected by peripheral chemoreceptors.

Central Chemoreceptors

• Located in the brain (ventral medulla).
• Sensitive to decreased pH of cerebrospinal fluid (CSF): acidic pH.
• This decreased pH stimulates central chemoreceptors to increase respiratory rate and tidal volume.
• Peripheral chemoreceptors are affected directly by low blood PaO2, high PaCO2, and low blood pH.
• Central chemoreceptors are affected indirectly by high PaCO2.

What Mechanical Sensors Control Breathing?

• Sensory receptors detect mechanical pressure or distortion.
• These include pulmonary stretch receptors, irritant receptors, and J receptors.

Pulmonary Stretch Receptors

• Stimulated by lung distension during inspiration.
• Send impulses via the vagus nerve to inhibit inspiration and prevent overinflation of the lungs.
• Typically not active at low tidal volumes, but importantly affect patients with COPD, a respiratory condition causing trapped air and increased lung volume/distension, therefore the reflex is diminished.

Irritant Receptors

• Found in upper respiratory airways (trachea, bronchi).
• Respond to mechanical and chemical irritants (chemicals, dust, pollens, cold air).
• Initiates coughing and bronchoconstriction for clearing harmful substances.

J Receptors

• Located near capillaries in alveolar septa.
• Sensitive to increased pulmonary capillary pressure.
• Initiates rapid, shallow breathing when stimulated.
• Important in conditions like heart failure with pulmonary edema and pulmonary emboli.

Mechanical Receptors that Control Breathing

• Pulmonary stretch receptors, irritant receptors, and J receptors (summary). Their stimuli and responses are listed.

What Are Some Pathological Breathing Patterns?

• Kussmaul, Cheyne-Stokes, and Biot breathing illustrated in a graph.

Case Connection

• GB's history and Biot breathing pattern are suggestive of opioid overdose.
• Opioid intoxication can cause respiratory arrest, and is reversed with an opioid receptor antagonist (naloxone).