Unit Seven: Respiration

Chapter 37: Pulmonary Ventilation

Guyton and Hall, Textbook of Medical Physiology, 12 edition

Respiration

• Four Major Functions of Respiration

1. Pulmonary ventilation

2. Diffusion of oxygen and carbon dioxide between the alveoli and the blood

3. Transport of oxygen and carbon dioxide in the blood and body fluids

4. Regulation of ventilation

Mechanics of Pulmonary Ventilation

• Muscles That Cause Lung Expansion and Contraction

Lungs can be expanded and contracted in two ways:

1. By downward and upward movement of the diaphragmto lengthen and shorten the chest cavity

2. By elevation and depression of the ribs to increase anddecrease the anteroposterior diameter of the chestcavity

Mechanics of Pulmonary Ventilation

Fig. 37.1 Contraction and expansion of the thoracic cage during expiration and inspiration

Mechanics of Pulmonary Ventilation (cont.)

• Normal quiet breathing is accomplished almostentirely by movement of the diaphragm

a. During inspiration, contraction of the diaphragm pulls the lungs downward

b. During expiration, the diaphragm simply relaxes, and the elastic recoil compresses the lungs and expels the air

Mechanics of Pulmonary Ventilation (cont.)

• During heavy breathing the elastic forces are not powerful enough, so extra force is generated bythe abdominal muscles

a. Muscles that raise the rib cage are the externalintercostals (most important), sternocleidomastoid,anterior serratus, and scalenes

b. Muscles that pull the rib cage downward during expiration are the abominal rectus, and the internalintercostals

Mechanics of Pulmonary Ventilation (cont.)

• Pressures That Cause the Movement of Air In andOut of the Lungs

a. Pleural pressure- pressure of the fluid in the thinspace between the lung pleura and the chest pleura;there is a slight suction so the pressure is negative

b. Alveolar pressure- pressure of the air inside the lungalveoli

c. Transpulmonary pressure- difference between the pleural and alveolar pressures (recoil pressure)

Fig. 37.2 Changes in lung volume, alveolar pressure, pleural pressure, and transpulmonary pressure during normal breathing

Mechanics (cont.)

• Compliance of the Lungs- the extent to which the

lungs will expand for each unit increase intranspulmonary pressure

• Compliance Diagram of the Lungs

a. Inspiratory compliance curveb. Expiratory compliance curvec. Characteristics of the diagrams are

dependent on

1. Elastic forces of the lung tissue2. Elastic forces caused by surface tension

of the fluid that lines the inside walls of the alveoli

Mechanics (cont.)

Fig. 37.3 Compliance diagram in a healthy person Fig. 37.4 Comparison of the compliance of saline and air-filled lungs when the

alveolar pressure is maintained at atmospheric pressure and pleural pressure is changed

Mechanics (cont.)

• Surfactant, Surface Tension, and Collapse of the Alveoli

a. Principle of Surface Tension-when water forms a surfacewith air, the water molecules on the surface of the water have an especially strong attraction for one another. As aresult, the water surface is always trying to contract.

b. Surfactant-surface active agent that reduces the surfacetension of water; secreted by Type II alveolar cells

Mechanics (cont.)

• Effect of the Thoracic Cage on Lung Expansibility

a. Compliance of the thorax and the lungs

b. Work of breathing (3 fractions)

1. Compliance or elastic work2. Tissue resistance work3. Airway resistance work

Pulmonary Volumes and Capacities

• Recording Changes in Pulmonary Volume-Spirometry

Fig. 37.5 Spirometer

Pulmonary Volumes and Capacities

• Pulmonary Volumes

a. Tidal Volume (TV)-amount of air inspired or expired with each normal breath; usually about 500 ml

b. Inspiratory Reserve Volume (IRV)-extra volume that can be inspired over and above the TV; usually about 3000 ml

c. Expiratory Reserve Volume (ERV)-max extra volume that can be expired at the end of a normal TV; usually about 1100 ml

Pulmonary Volumes and Capacities

• Pulmonary Volumes (cont.)

d. Residual Volume (RV)-amount of air remaining in the lungs after the most forceful expiration; usually about 1200 ml

• Pulmonary Capacities

a. Inspiratory Capacity- (TV + IRV); about 3500 ml

b. Functional Residual Capacity- (ERV + RV); about 2300 ml

Pulmonary Volumes and Capacities

• Pulmonary Capacities (cont.)

c. Vital Capacity (VC)- (IRV + TV + ERV); usually about4600 ml

d. Total Lung Capacityi (VC + RV); usually about 5800 ml

Pulmonary Volumes and Capacities

Fig. 37.6 Diagram showing pulmonary volumes and pulmonary capacities

Minute Respiratory Volume

• Minute Respiratory Volume- total amount of new air moved into the respiratory

passages each minute

1. Equal to the TV x Respiratory Rate

2. Usually 500ml x 12 bpm = 6 L/min

Alveolar Ventilation

• Effect of “Dead Space” – air that does not reach the gas exchange areas but simply fill respiratory passages where gas exchange does not occur (dead air space)

• Rate of Alveolar Ventilation- alveolar ventilation per minute is the total volume of new air entering the alveoli each minute

Functions of the Respiratory Passages

Fig. 37.8 Respiratory passages