Physiology Notes

Introduction

The major function of the respiratory system is to receive oxygen and provide it to the tissues of the body and remove the carbon dioxide, that is formed in the body due to metabolism, into the environment.

Respiration, as a whole, comprises of the following distinct processes:

1. Pulmonary ventilation: movement of air in and out of the alveoli, the parts of the respiratory system where actual gas exchange takes place.

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

3. Transport of gases in the blood vessels

4. Regulation of ventilation

How do the lungs expand and contract in pulmonary ventilation?

During inspiration, the lungs expand, and the opposite happens in expiration.

The lungs expand mainly through the contraction of the diaphragm, but the external intercostals and other accessory muscles like the sternocleidomastoid, anterior serrati and scaleni also play a role by increasing the anteroposterior thickness of the chest.

During expiration, relaxation of the diaphragm; elastic recoil of the lungs, chest wall; contraction of the internal intercostals and abdominal recti, which depress the rib cage, take place.

Pressures that cause movement in and out of the lungs

There are three kinds of pressure in relation to pulmonary ventilation:

1. Pleural pressure: the pleura is a membrane that surrounds the lungs. It consists of two layers – an inner and an outer one. Between these two layers is the pleural space, which is filled with a fluid called the pleural fluid. The pressure of the pleural fluid in the pleural space is called the pleural pressure.

2. Alveolar pressure: it is the pressure of the air inside the alveoli.

3. Transpulmonary pressure: it is the difference between the alveolar and pleural pressure.

Air moves into the lungs or alveoli during inspiration, as the alveolar pressure is lower (-1 cm of water) than the atmospheric pressure (0 cm of water). Air moves out from the lungs in expiration, as the alveolar pressure rises to about +1 cm of water, which is more than the atmospheric pressure (0 cm of water).

When the glottis is open, there occurs no movement of air; hence, the pressures throughout the respiratory system are same as the atmospheric pressure (0 cm of water).

Pleural pressure at the beginning of inspiration is -5 cm of water and becomes further negative (averaging -7.5 cm of water) during inspiration.

Compliance

Compliance is the extent to which the lungs expand with increase in transpulmonary pressure. It is about 200 ml per cm of water in the average adult.

Lung compliance depends on two factors: elastic forces of the lung tissues, and elastic forces caused by surface tension of the alveolar fluid. If the elastin and collagen in the lung tissue are rigid, compliance reduces. Likewise, if the surface tension of the alveolar fluid increases, compliance reduces (surface tension of a fluid is the tendency of the water molecules in that fluid to stick to one another). Surface tension of alveolar fluid is one-twelfth to one-half of the surface tension of a pure water surface due to secretion of surfactant by the type II alveolar epithelial cells. The surfactant contains dipalmitoyl phosphatidylcholine, surfactant apoproteins, and calcium ions.

As such, the smaller alveoli are more prone to collapse, and the bigger alveoli more prone to expansion, as according the the Laplace Law, collapse pressure is inversely proportional to radius of the alveoli. But, all alveoli fairly remain stable, because they support each other and are also supported by fibrous tissue.