FAQ on Mechanics of Breathing

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Cough reflex:

This is a long drawn and deep inhalation followed by closure of the rima glottidis. It may be caused by a foreign body lodged below the pharynx below, particularly in the larynx, trachea or epiglottis.

Sneeze reflex:

This reflex occurs when irritants affect the upper parts of the tract up to the nose. This irritation causes impulses to travel via the cranial nerve V to the medulla where the reflex is triggered, the glottis is kept open, uvula is depressed and air is gushed through the nose.

What is Boyle's Law? How does it affect ventilation?

Boyle's Law states that pressure is inversely proportional to volume. When the volume of the chest rises, pressure decreases and air flows in. On the other hand, when the volume decreases, pressure rises and air flows out.

Discuss the basic steps in inspiration. What is the most important muscle involved? What nerve controls this muscle? What other muscles are involved?

During inspiration (breathing in), pressure inside alveoli must be lower than the atmospheric pressure. This is done by increasing lung volume. The most important muscle involved is the diaphragm. This muscle is innervated by the phrenic nerves, which emerge from the spinal cord at cervical levels 3, 4 and 5. The next muscles involved are the intercostals.

What do we mean by intrapleural pressure? How does the intrapleural pressure change during a normal quiet breath?

During quiet inhalations, pressure between two pleural layers is called intrapleural pressure. This pressure is always subatmospheric during quiet inhalations. When the thoracic cavity increases in size, intrapleural pressure decreases.

Describe the process of normal expiration. Why is it considered a "passive process?" What occurs during labored breathing to enhance expiration?

During normal expiration, lungs volume decreases because of muscle recoil, i.e., muscles begin to relax increasing internal pressure and this results in a passive process of expiration. During labored breathing, the abdominals and intercostals contract resulting in air gushing out of the lungs at a higher rate.

What factors influence airflow in the lungs? What happens to airflow during inspiration and what causes this to occur?

Surface tension of alveolar fluid, compliance of the lungs, and airway resistance. During inspiration, air pressure is lower in the lungs compared to that of the mouth, hence, air flows from the mouth inwards. This is caused by greater volume of the lung and a lower pressure in the alveoli.

What happens to airflow during expiration and what causes this to occur? What factors increase airway resistance?

During expiration, air pressure is higher in the lungs compared to that of the mouth, hence, air flows outward. This is caused by lesser volume of the lung and higher in the alveoli. Airway resistance is influenced by pressure and volume of the airway.

What is compliance? Give examples of conditions where compliance is increased and conditions where it is decreased.

Compliance is the ease with which the lungs are inflated. As an analogy, a thin balloon is easier to inflate, thus, has a higher compliance than a thick balloon that is harder to inflate.

Compliance can decrease when:

• Lung has scar tissue.

• Lung tissue is filled with fluid.

• Deficiency in surfactant.

Compliance can increase during emphysema due to destruction of elastic fibers.

What is your tidal volume and minute ventilation? What about alveolar ventilation?

Average tidal volume is 500 mL.

Average minute ventilation is 12 breaths/min x 500 mL = 6 liters/min.

Average Alveolar volume = 350 mL.

Average Alveolar ventilation = 12 breaths/min x 350 mL = 4.2 liters/min

Lung capacity = 6000 mL

Additional Reading:

Basic Pulmonology

1. Lung Mechanics
2. Alveolar-Blood Gas Exchange
3. Gas Transport and Regulation of Respiration
4. Four Causes of Hypoxemia
5. Control of Respiration
6. Systemic vs Pulmonary Circulation FAQ
7. Principles of Gas Exchange in Lungs
8. Hypoxia, Hypoxemia & Hemoglobin-Oxygen Saturation Curve
9. FAQ on Mechanics of Breathing
10. FAQ on Control of Breathing
11. Criteria for Transudate Pleural Effusion
12. Light's Criteria for Exudate Pleural Effusion
13. Notes on Lung Sounds
14. Patient with Acute Respiratory Distress Syndrome (ARDS)
15. Management of Acute Deep Venous Thrombosis
16. Notes on Asthma Treatment

Pulmonology Videos

1. Video of Pulmonology Examination in a Clinical Setting

Related Topics

1. Histology of the Respiratory System
2. Upper and Lower Respiratory Disorders
3. Pulmonary Examination for Internal Medicine

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