Breathing Mechanics and Gas Exchange

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Questions and Answers

Which is not true regarding the work of breathing?

Total work of breathing is minimal at approximately 15 breaths/min.

Work required to overcome airflow plus elastic resistance equals total work. (correct)

Minimum work requires higher rates of breathing with obstructive lung disease.

Minimum work is performed at higher frequencies with stiff lungs.

Under resting metabolic conditions, a healthy adult produces approximately how much CO2 per minute?

200 ml/min

Which of the following does not describe hypoventilation?

Alveolar ventilation is more than CO2 production. (correct)

High CO2 level in the blood.

Low blood pH.

Alveolar ventilation (CO2 removal) is less than CO2 production.

Regarding distribution of ventilation, which is not true?

The lung is a perfect organ for gas exchange with a V/Q ratio of 1. Signup and view all the answers

Which is not correct regarding the distribution of ventilation?

Alveoli at the apexes expand more during inspiration. Signup and view all the answers

A lung unit has a short time constant if compliance or resistance is high.

False Signup and view all the answers

Which of the following constitute the physiologic dead space?

Anatomic dead space plus alveolar dead space Signup and view all the answers

What is the formula for minute ventilation?

Tidal volume x frequency of breathing Signup and view all the answers

For a healthy adult breathing 16 breaths/min and having a VT of 450 ml, what is the minute ventilation?

7.2 L/minute Signup and view all the answers

In a healthy adult with a tidal volume of 500 ml, how much fresh gas goes to the alveoli for gas exchange?

350 ml Signup and view all the answers

The normal dead space ratio is approximately how much?

30% (20-40%) Signup and view all the answers

The physical tendency of an object to return to an initial state after deformation refers to which of the following?

Elasticity Signup and view all the answers

Correct statement regarding resistance:

Total resistance is highest in the smaller airways. Signup and view all the answers

Which is not true regarding the pressure differences during breathing?

During a normal breathing cycle, the glottis remains closed. Signup and view all the answers

At functional residual capacity (FRC), which is not true?

Airway opening and alveolar pressures are both zero. Signup and view all the answers

In patients with increased airflow resistance (obstructive lung disease), to maintain a minimum work of breathing, the patient tends to breathe at this rate:

Slower Signup and view all the answers

To improve oxygenation in a patient with a diseased left lung, the patient should be placed on which of the following?

Lying with the right side down Signup and view all the answers

Which is the formula for minute ventilation?

Rate of breathing (fB) x tidal volume (VT) Signup and view all the answers

Which is the formula for computing the volume of gas reaching the alveoli (alveolar volume) in a healthy adult?

Tidal volume (VT) – anatomic dead space (VD anatomic) Signup and view all the answers

To accurately assess the alveolar ventilation in clinical practice, which of the following is the preferred formula?

Alveolar ventilation = fB (tidal volume - physiologic dead space) Signup and view all the answers

Which statement is NOT true regarding lung compliance?

Obesity and kyphoscoliosis increase the combined compliance. Signup and view all the answers

Regarding the distribution of resistance, which is true?

Resistance is highest in smaller bronchi. Signup and view all the answers

Study Notes

Work of Breathing

With obstructive lung disease (increased airflow resistance), minimum work requires higher rates of breathing
In normal lungs, total work of breathing is minimal at approximately 15 breaths/min
The work required to overcome airflow plus elastic resistance equals total work
With stiff lungs (increased elastic resistance), minimum work is performed at higher frequencies
In patients with increased airflow resistance (obstructive lung disease), to maintain a minimum work of breathing the patient’s tend to breath at this rate: slower

CO2 Production

Under resting metabolic conditions, a healthy adult produces approximately 200 ml/min CO2

Hypoventilation

Alveolar ventilation (CO2 removal) is less than CO2 production
Low blood pH
High CO2 level in the blood

Distribution of Ventilation

The lungs have uneven ventilation in proportion to perfusion.
Regional factors causing the uneven distribution are the differences in thoracic expansion and transpulmonary pressure gradients
Expansion of the lower chest is approximately 50% greater than expansion of the upper chest
Alveoli at the bases expand more than alveoli at the apexes
Alveoli at the apexes having a larger resting volume than do alveoli at the bases.
There is greater transpulmonary pressure gradient at the top of the upright lung.
Alveoli at the apexes expand more during inspiration

Time Constant

A lung unit has a short time constant if compliance or resistance is high

Physiologic Dead Space

Anatomic dead space plus alveolar dead space

Minute Ventilation

Tidal volume x frequency of breathing
Rate of breathing ( fB) x tidal volume (VT)

Alveolar Volume

Tidal volume (VT) – anatomic dead space (VD anatomic)

Dead Space Ratio

Approximately 30% (20-40%)

Elasticity

The physical tendency of an object to return to an initial state after deformation

Resistance

Resistance is lowest in the upper airways
Airway resistance decreases as lung volume increases because the airways distend as the lungs inflate
The total resistance is highest in the smaller airways because they are arranged in parallel

Pressure Differences During Breathing

During a normal breathing cycle, the glottis remains close.
The pressure at the surface of the body (PBS) and PAO remain at zero (i.e., atmospheric) throughout the cycle
A zero pressure in the trachea means the pressure in the trachea is equal to the atmospheric pressure
Alveolar pressure (PA) is sub-atmospheric in the beginning of inspiration compared with the pressure at the airway opening (PAO)

Functional Residual Capacity (FRC)

The trans-airway pressure gradient also is zero
Air flows out of the airway opening
Airway opening and alveolar pressures are both zero
The lung is at its resting volume

Lung Compliance

The combined compliance of the system (lung and the chest wall) is reduced to approximately half that of the individual components.
In healthy adults, the compliance of the lungs and chest wall are each equal to approximately 0.2 L/cm H2O.
The lungs and chest wall each have their own compliance or distensibility.

Distribution of Resistance

Considering the number of airways, the majority of frictional resistance comes from the smaller airways, which are arranged in parallel.
The majority of the frictional resistance, however, comes from the tissues moving during breathing.

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Description

This quiz covers the fundamental concepts of breathing mechanics, particularly in relation to obstructive lung diseases and CO2 production. It explores the work of breathing, hypoventilation, and the distribution of ventilation within the lungs. Test your understanding of how these factors affect respiratory efficiency and gas exchange.