Respiratory Physiology

Study Notes

Respiration: The exchange of gases between the atmosphere, blood, and cells, requiring three processes: Ventilation (breathing), External (pulmonary) respiration, and Internal (tissue) respiration.
The cardiovascular system assists the respiratory system by transporting gases.

Average 150ml: The part of the airway not involved in gas exchange (conducting zone).
Alveolar dead space: Alveoli that are ventilated but not perfused, varying depending on circumstances.

Transports air to the lungs: Warms, humidifies, filters, and cleans the air.
Voice production: In the larynx as air passes over the vocal folds.

Main-stem bronchi branch into lobar bronchi: Three on the right and two on the left, which further branch into segmental and subsegmental bronchi.
Trachea divides at the carina into right and left main-stem bronchi: Penetrating the lung parenchyma.

Nasal breathing is preferential: Filters particulate matter and plays a major role in lung defense, humidifies inspired air.
Mouth breathing during exercise: Higher resistance than nasal breathing.

Oxygen and carbon dioxide move via simple diffusion: Through the blood gas barrier, which is extremely thin (0.2-0.3µm) and has a large surface area (50-100m²).
Surface tension: Reduced by surfactant, essential for alveolar expansion.

Primary principle of ventilation: Air moves down its pressure gradient, from high to low pressure.
Boyles Law: The volume of a gas varies inversely with its pressure.
Inhalation and Exhalation: Inspiration is active; expiration is passive at rest.

Diaphragm: The most important muscle of inspiration, supplied by phrenic nerves (C3, 4, 5).
Intercostal muscles: Play a prominent role in respiration.
Abdominal muscles: Contract during exercise to aid expiration.

Diffusion gradient: Affected by surface area, diffusion distance, and diffusion gradient.
Surface area: The large surface area of alveoli (50-100m²) enhances gas exchange.
Diffusion distance: Normally short (0.2-0.4µm) but increased in diseases like pneumonia or cystic fibrosis.

The ratio of ventilation to perfusion: Varies throughout the lungs, with a normal whole-lung ratio of 0.8.
Regional variations: Due to gravity, with more ventilation at the apices and more perfusion at the bases.
Factors that decrease V-Q ratio: Chronic bronchitis, asthma, pulmonary oedema, and pulmonary fibrosis.
Factors that increase V-Q ratio: Pulmonary embolism, COPD, and high altitude.

Brainstem: The normal automatic process of breathing originates in impulses from the brainstem.
Cortex: Can override the automatic process for voluntary control.
Central and peripheral chemoreceptors: Respond to changes in PCO2 and PO2 levels in the blood.
Hering-Breuer inflation reflex: A negative feedback mechanism that slows respiratory frequency in response to lung inflation.### Central and Peripheral Chemoreceptors
Central chemoreceptors are located near the ventral surface of the medulla and are sensitive to PCO2 but not PO2 of blood.
They respond to changes in pH of the ECF/CSF when CO2 diffuses out of cerebral capillaries.
Peripheral chemoreceptors are located in the carotid and aortic bodies and respond to decreased arterial PO2 and increased PCO2 and H+.
Peripheral chemoreceptors are rapidly responding.

Control of respiration involves a basic rhythm generated by the brainstem that is modified by multiple neural inputs.
The pons and medulla generate a normal cyclic pattern of respiration that is altered by homeostatic and adaptive reflexes.
The dorsal respiratory group (DRG) and ventral respiratory group are associated with inspiration and expiration, respectively.
Neural signaling of inspiratory motor neurons involves one inspiratory and two expiratory phases.

The brainstem, specifically the medullary respiratory centre in the reticular formation, is responsible for the automatic process of breathing.
The Pre-Bötzinger Complex, located in the ventrolateral region of the medulla, is essential for respiratory rhythm.
The dorsal respiratory group and ventral respiratory group have the property of intrinsic periodic firing and are responsible for the basic rhythm of ventilation.

Voluntary control from the cortex can override automatic breathing, useful for communication such as speaking, but is limited in extent.
Other CNS areas, such as those involved in emotion, can influence ventilation, leading to transient effects like gasping and sobbing.
Ventilation can be increased or reduced due to emotional responses.