Surfactant see Figure

- lines the alveoli.

- reduces surface tension by disrupting the intermolecular forces between molecules of liquid. This reduction in surface tension prevents small alveoli from collapsing and increases compliance.

Large alveolus Small alveolus Small alveolus

r with surfactant

!r tp t tendency to collapse with surfactant

I tendency to collapse

!r tp t tendency to collapse

I tendency to collapse

Figure 4-4. Effect of alveolar size and surfactant on the pressure that tends to collapse the alveoli. P= pressure; r= radius; T- surface tension.

- is synthesized by type II alveolar cells and consists primarily of the phospholipid dipalmitoyl phosphatidylcholine (DPPC).

- In the fetus, surfactant synthesis is variable. Surfactant may be present as early as gestational week 24 and is almost always present by gestational week 35.

- Generally, a lecithin-.sphingomyelin ratio greater than 2:1 in amniotic fluid reflects mature levels of surfactant.

- Neonatal respiratory distress syndrome can occur in premature infants because of the lack of surfactant. The infant exhibits atelectasis during expiration, difficulty reinflating the lungs (as a result of decreased compliance), and hypoxemia because of the V/Q mismatch.

E. Relationships between pressure, airflow, and resistance 1. Airflow

- is driven by, and is directly proportional to, the pressure difference between the mouth (or nose) and the alveoli.

-is inversely proportional to airway resistance; thus, the higher the airway resistance, the lower the airflow. This inverse relationship is shown in the following equation:

where:

Q = airflow (ml/min or L/min) AP = pressure gradient (mm Hg or cm H20) R = airway resistance (cm H20/L/sec)

2. Resistance of the airways

- is described by Poiseuille's law, as shown in the following equation:

where:

R = resistance t) = viscosity of the inspired gas 1 = length of the airway r = radius of the airway

- Notice the powerful inverse fourth-power relationship between resistance and the size (radius) of the airway.

- For example, if airway radius decreases by a factor of 4, then resistance will increase by a factor of 256 (44), and airflow will decrease by a factor of 256.

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