Dynamics of ventilation & airways L21 Flashcards
How does expiration occur at rest?
Recoil force -> ELASTICITY of the lungs + SURFACE TENSION in the lungs
Elasticity of the lungs
Lungs are distensible - inspiration and expiration
Elasticity versus Compliance
As elasticity increases, compliance decreases (and vice versa).
Compliance:
Defined as the inverse of elasticity.
Radial traction of expiration and inspiration
Compliance
Compliance refers to how easily a structure, such as the lungs or a blood vessel, can expand or stretch when pressure is applied. High compliance means the structure can easily stretch with less pressure.
Elasticity
Elasticity refers to the ability of a structure to return to its original shape after being stretched or expanded. It is essentially the opposite of compliance.
Compliance Formula
Compliance is the change in volume over the change in pressure.
In the lungs, for example, higher compliance means that the lungs can expand with a smaller increase in pressure, whereas lower compliance means more pressure is required to achieve the same expansion.
Surface tension in the lungs
Surface tension is the enhanced intermolecular attraction at the surface of a liquid. Molecules inside the liquid experience cohesive forces from all directions, but those at the surface (liquid-gas interface) only interact with neighbors on the surface, resulting in a net inward force. This creates a “film” on the surface, allowing small objects, like water striders, to float despite being denser than water.
Laplace’s Law relates pressure (P) inside a spherical object, like an alveolus or soap bubble, to the surface tension (T) and its radius (R). The law states that the pressure required to keep the sphere stable is directly proportional to the surface tension and inversely proportional to the radius: 𝑃 = 2𝑇/𝑅
This means smaller alveoli or bubbles require higher pressure to maintain stability due to increased surface tension.
Pressure volume relationships in disease states
Normal Lungs: Display a typical compliance curve where a certain amount of intra-pleural pressure results in a proportional inspired volume.
COPD (Chronic Obstructive Pulmonary Disease): In this condition, lung compliance is increased as elastic content is reduced, meaning that the lungs expand more easily with less pressure, but they also have difficulty recoiling. This results in a steeper curve.
Fibrosis: Lung compliance is decreased due to stiffening of the lung tissue, meaning higher pressures are required to achieve the same volume as normal lungs. This results in a flatter curve, indicating reduced lung expansion.
Lung diseases revealed using x-ray
The lobes in the lung airways
3 on the right, 2 on the left to save space for the heart on the left
Properties of the respiratory airway
Air is slow down at the alveolar as it provides time to exchange gases
Physical factors controlling airflow: Airway Resistance and Lung Volume
Airway Resistance: The Y-axis represents airway resistance, which is a measure of how much the airways resist the flow of air. Higher values mean more resistance, making it harder to breathe.
Lung Volume: The X-axis shows different lung volumes:
RV (Residual Volume): The amount of air left in the lungs after a full exhalation.
FRC (Functional Residual Capacity): The volume of air in the lungs at the end of a normal exhalation.
TLC (Total Lung Capacity): The maximum volume of air the lungs can hold after a full inhalation.
Relationship: As lung volume increases (moving from RV to TLC), airway resistance decreases. This is because larger lung volumes pull the airways open, reducing resistance to airflow.
Physical factors controlling airflow:
Airway diameter
Airway Diameter: The Y-axis shows airway diameter as a percentage. As the diameter increases, the airways expand, allowing more air to flow through.
Lung Volume: As lung volume increases (from about 1 to 6 liters), airway diameter also increases. At higher lung volumes, the airways are stretched open, improving airflow.
Bronchiole and how its diameter and resistance to airflow are controlled, focusing on autonomic regulation:
Smooth Muscle:
Surrounds the bronchiole.
The smooth muscle is crucial in regulating the bronchiole’s diameter. It contracts and relaxes in response to signals from the autonomic nervous system.
Bronchoconstriction: When the smooth muscle contracts, the airway narrows, increasing airway resistance, which reduces airflow. This occurs, for example, in response to parasympathetic stimulation or during asthma.
Bronchodilation: When the smooth muscle relaxes, the airway widens, decreasing resistance and allowing more airflow. This happens in response to sympathetic stimulation (e.g., adrenaline release).
Cilia:
These are hair-like structures lining the bronchioles.
They help move mucus and trapped particles out of the respiratory tract, serving as part of the body’s defense mechanism.
Submucosa:
A layer beneath the epithelial lining containing connective tissue, glands, and blood vessels.
The submucosa helps produce mucus, which moistens the airways and traps particles.
Connective Tissue:
Provides structural support to the bronchiole, maintaining its shape and flexibility during breathing.
Hering-Breuer Reflex and its role in controlling airway diameter and resistance through both sensory and motor pathways
- Sensory Receptors (Vagal Afferents):
Lung Stretch Receptors: These are mechanoreceptors located in the bronchioles. They detect lung inflation (stretching of the lungs during inhalation).
Vagal Afferents: When these stretch receptors are activated during lung inflation, they send signals through the vagus nerve to the medulla oblongata’s respiratory centers in the brainstem.
2.Medulla Oblongata (Respiratory Centers):
This part of the brain processes the sensory input from the stretch receptors and regulates the respiratory response, including control over airway smooth muscle and breathing rate.
- Sympathetic Efferents:
Once the brain processes the information, the response is sent out through sympathetic efferent nerves.
Effector/Target Organ: The bronchioles are the target organs. Activation of the beta-adrenoceptors by noradrenaline (NA) leads to bronchodilation—widening the airways to allow easier airflow during exhalation.
- Hering-Breuer Reflex:
Named after Ewald Hering and Josef Breuer, this reflex limits lung overinflation by inhibiting inspiration once the lungs are sufficiently inflated.
When the lungs expand too much, the stretch receptors send inhibitory signals through the vagus nerve to reduce the inspiratory drive, preventing lung damage and helping regulate breathing rhythm.
Summary:
The Hering-Breuer Reflex involves lung stretch receptors that detect inflation, send signals to the brainstem (medulla oblongata), which then modulates airway diameter and resistance by stimulating bronchodilation via sympathetic nerves. This reflex helps control breathing patterns and prevents overinflation of the lungs.
Key Concepts:
Normal Airway:
In healthy individuals, the airway is wide, and the smooth muscles are relaxed, allowing normal airflow in and out of the lungs.
Asthmatic Airway:
In individuals with asthma, the airway walls become inflamed and thickened, leading to partial narrowing of the airway even when not experiencing an attack. This reduces airflow, making breathing more difficult.
Asthmatic Airway During an Attack:
During an asthma attack, the airway is further constricted due to smooth muscle tightening. The airway becomes significantly narrowed, trapping air in the alveoli (air sacs), which can cause symptoms like wheezing and shortness of breath.
The smooth muscle surrounding the airway tightens, contributing to the narrowing of the airway lumen, further reducing airflow.
Salbutamol (β2-adrenoceptor agonist):
Mechanism of Action: Salbutamol is a medication commonly used by asthmatics through an inhaler. It stimulates β2-adrenoceptors in the smooth muscle of the bronchioles. This mimics the body’s sympathetic nervous system response, leading to bronchodilation (widening of the airways).
Effect: The inhalation of salbutamol relaxes the smooth muscles around the bronchioles, reversing bronchoconstriction and improving airflow. This is especially important during an asthma attack, as it quickly opens the narrowed airways.
Sympathetic Mechanism in Asthma:
Salbutamol acts on the sympathetic nervous system by stimulating β2-adrenoceptors, which directly causes the smooth muscles in the airways to relax. This results in bronchodilation, making it easier for individuals with asthma to breathe by increasing the airway diameter and reducing resistance to airflow.
