Lung Mechanics Pneumothorax Notes

Semester 2

Case 1: The Stabbing

• What are the mechanics of breathing?

[1] Movement of air into and out of the lungs takes place because of pressure differences caused by changes in lung volumes. Air flows from a high-pressure area to a low-pressure area. The changes in volume in the lungs occur through the contraction of skeletal muscles – those that insert on the ribcage & the diaphragm. When the muscles elevate the ribs, they swing outward, increasing the depth of the thoracic cavity.

Normal breathing at rest Uses diaphragm & external intercostals muscles.

Increased depth & frequency of breathing In addition, uses accessory respiratory muscles, which are: internal intercostals, sternocleidomastoid, serratus anterior, pectoralis minor, scalene, transversus thoracis, transversus abdominis, external & internal oblique, and rectus abdominis muscles.

Inhalation is ALWAYS ACTIVE. The diaphragm contracts, flattening to the floor of the thoracic cavity & increasing its volume. External intercostal muscles contract, elevating ribs. Accessory respiratory muscles can assist by increasing speed + amount of rib elevation.

Exhalation CAN BE ACTIVE OR PASSIVE. Passive occurs via elastic recoil of lungs. When active, internal intercostals + transversus thoracis muscles depress ribs. Abdominal muscles can assist by compressing the abdomen & forcing the diaphragm upward.

Quiet Breathing (Eupnea)

Inhalation involves muscular contractions, but exhalation is passive. Deep breathing relies more on the diaphragm, whereas shallow breathing relies more on the intercostal muscles. Expansion of lungs stretches their elastic fibers. Elevation of rib cage stretches opposing skeletal muscles & elastic fibers in the connective tissues of the body wall. When muscles of inhalation relax, elastic components recoil – AKA. Elastic rebound!

Forced Breathing (Hyperpnea)

Inspiration and exhalation are both active. Accessory muscles assist with inhalation & exhalation also involves the contraction of internal intercostal muscles. Abdominal muscles are also used at maximum levels of forced breathing.

Normal adult respiratory rate = 12-18 per minute.

Normal child respiratory rate = 18-20 per minute.

• What are the different respiratory volumes?

• Respiratory Minute Volume, V_E = Respiratory rate, ƒ X Tidal Volume, V_T.

Measures pulmonary ventilation. Average at rest = 12 x 500ml = 6.0 litres per minute.

• Alveolar Ventilation, V_A = Respiratory rate, ƒ X(Tidal volume, V_T – Anatomic Dead Space, V_D).

The amount of air reaching the alveoli, and hence participating in gas exchange, each minute. Anatomic dead space is the volume of air left in the conducting passages. Average V_A at rest = 12 x (500ml – 150ml) = 4.2litres per minute.

Pulmonary volumes include:

• Resting Tidal Volume, V_T: The amount of air you move into & out of your lungs during a single respiratory cycle under resting conditions. Average = 500ml.

• Expiratory Reserve Volume, ERV: The amount of air you can voluntarily expel after you have completed a normal, quiet respiratory cycle. (i.e. additional air using accessory muscles. Average in males = Additional 1l, in females = 700ml).

• Residual Volume: Amount of air remaining in lungs after a maximal exhalation. Average in males = 1200ml, and females = 1100ml).

• Minimal Volume: Amount of air that would remain in the lungs if they were allowed to collapse. Ranges from 30-120ml. Cannot be measured in a healthy person. The reason some air is still present even after lung collapse is because surfactant coating the alveolar surfaces prevents their collapse.

• Inspiratory Reserve Volume, IRV: Amount of air you can take in over & above the tidal volume. Average in males = 3300ml, females = 1900ml.

Respiratory capacities can be calculated using the pulmonary volumes:

• Inspiratory Capacity = Tidal volume, V_T + Inspiratory Reserve Volume, IRV. It is the amount of air that can be drawn into the lungs after a quiet respiratory cycle.

• Functional Residual Capacity, FRC = Expiratory Reserve Volume, ERV + Residual Volume. It is the amount of air in the lungs after a complete quiet respiratory cycle.

• Vital Capacity = Expiratory Reserve Volume, ERV + Tidal Volume, V_T + Inspiratory Reserve Volume, IRV. It is the maximum amount of air that can be moved into & out of the lungs in a single respiratory cycle. Average in males = 4800ml, females = 3400ml.

• Total Lung Capacity = Vital Capacity + Residual Volume. Average in males = 6l, in females = 4.2l.

• How is respiration controlled?

Under normal conditions, cellular rates of absorption of O₂ and generation of CO₂ = Capillary rates of delivery of O₂ and removal of CO₂ = Rate of O₂ absorption and CO₂ excretion at lungs.

If this becomes unbalanced, homeostatic mechanisms restore equilibrium by: changes in bloodflow & oxygen delivery at the local level, and changes in depth & rate of respiration via the brain’s respiratory centres.

Changes in respiratory centres are coordinated with changes in cardiovascular function. E.g. -fluctuations in blood pressure & cardiac output.

At Local Level:

When a peripheral tissue becomes more active, interstitial O₂ pressure falls & CO₂ pressure rises. Therefore, more oxygen is delivered and more carbon dioxide is carried away. Rising CO₂ pressure levels causes relaxation of smooth muscles in walls of arterioles & capillaries in the area, increasing local bloodflow.

Local factors also influence blood flow to alveoli (lung perfusion) with alveolar ventilation. Alveolar capillaries constrict when O₂ pressure is low, so directing blood towards high O₂ pressures. Also, when CO₂ pressure increases, bronchioles increase in diameter (bronchodilation), directing airflow to bronchioles that have high CO₂ pressure - improves efficiency of gas transport.

At Respiratory Centres In The Brain:

[1,2] Involuntary centres regulate activity of respiratory muscles & control Respiratory Minute Volume by adjusting...

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