Resp S3 - Mechanics of Breathing Flashcards
Describe what happens in inspiration
The respiratory muscles contract
This causes thoracic volume (and so lung volume due to pleural seal) to increase
Therefore alveolar pressure falls below atmospheric pressure so air flows into the lungs until the end of inspiration
What is the resting expiratory level?
The state of the lungs after normal quiet expiration
Respiratory muscles are relaxed and the inward recoil of the lung tissue is balanced by the outward recoil of the chest wall
Describe what happens in expiration
The respiratory muscles relax
Lungs recoil inwards so thoracic volume decreases
Therefore alveolar pressure increases above atmospheric pressure so air flows out of the lungs until the pressure inside the lung is equal to atmospheric
What muscles are used during forced inspiration?
Accessory muscles: sternocleidomastoid, serratus anterior, pectoralis major and the scalene muscles
What muscles are used in forced expiration?
Internal intercostal, innermost intercostal and abdominal wall muscles
What is compliance?
The stretchiness of the lungs. Defined as volume change per unit pressure change
When would compliance be reduced?
Pulmonary fibrosis
When would compliance be increased?
Emphysema
How does surface tension affect compliance?
As alveoli stretch, the layer of liquid coating them also has to stretch, which is opposed by the surface tension of that liquid because the area of gas-liquid interfaces tends to a minimum.
What is surfactant?
A complex mixture of phospholipids and proteins which have a detergent properties.
What are the main actions of surfactant?
Increases compliance by decreasing surface tension
Stabilises lungs by preventing small alveoli collapsing into larger alveoli
Prevents surface tension in alveoli creating a suction force and so transudate formation from pulmonary capillaries
How does surfactant reduce surface tension in the lungs?
Hydrophilic end in the alveolar fluid and hydrophobic end in the gas. Surfactant molecules are distributed in the fluid to reduce interaction between fluid molecules and so reduce surface tension
How doe surfactant stabilise the lung?
Smaller bubbles have higher pressure than larger bubbles and air moves from high to low pressure, therefore smaller alveoli in the lung should collapse into larger alveoli. However, in larger alveoli, surfactant molecules are further apart so less efficient at reducing surface tension, so pressures in larger and smaller alveoli are equalised.
Give a clinical example of the effects of reduced surfactant.
Infant respiratory distress syndrome
When there is insufficient surfactant in newborn lungs, leading to decreased compliance and other effects
How does resistance change throughout the airways?
The smaller airways actually have lower resistance than the larger airways because they are connected in parallel. Therefore, although individually the small bronchioles have the highest individual resistance, overall, resistance is higher in larger airways.
