Evolution and Development of the Lungs and Respiration

Hi, in this post we’ll discuss the evolutionary origins of the lungs. We’ll take a look at the differences between the buccal and aspiration pumps, how they work and which animals use them. I’ll also describe the embryological development of the larynx, trachea and lungs.

Two types of ventilation exist in animals: unidirectional (this is seen in fish which have gills) and bidirectional (seen in mammals). It is thought that vertebrate lungs evolved from ventral swim bladders which were originally used for the maintenance of buoyancy and that gas exchange was a later adaptation.

Buccal and Aspiration Pumps

 The buccal and aspiration pumps are two forms of muscular respiratory pumps. The buccal pump involves either a two-stroke or four-stroke mechanism. With the two-stroke mechanism, the buccal cavity expands to bring air from the lungs and compresses to force air into the lungs. With the four-stroke mechanism, the first stroke is an expansion of the buccal cavity which brings air from the lungs into the cavity. The cavity then compresses in the second stroke to force air out into the external environment. In the third stroke the buccal cavity expands and this draws air into the mouth. The fourth stroke involves the compression of the buccal cavity which forces air into the lungs. This cycle is then repeated during breathing. This type of pump is found in frogs and air-breathing fish.

With the aspiration pump, the volume of the buccal cavity remains constant while the volume of the thoracic cavity changes during respiration. During exhalation, the diaphragm moves cranially while the walls of the thoracic cavity move inwards to decrease the volume of the cavity, this changes the air pressure in the lungs and forces air into the external environment. During inhalation, the diaphragm moves caudally while the thoracic walls move outward to decrease the air pressure in the lungs and draw air inwards. Aspiration pumps are found in reptiles and mammals.

Embryological Development

  The lungs have five stages of embryological development:

1. The trachea-bronchial tube grows caudally. This forms from a groove which arises from the ventral foregut. 
2.  This tube divides into two lung buds. 
3.  These buds divide into three bronchi on the right and two on the left. This defines the major lobes and bronchi of the adult lungs. 
4.  The respiratory portion of the lung develops 
5.  The alveoli develop.

In mammals, the coelom (body cavity) is divided by a thin, non-muscular oblique septum called the pulmonary fold. This fold grows from the midline to suspend the lungs and liver from the body wall. Another transverse fold from the dorsal body wall separates the liver and lungs and fuses with the pulmonary fold. This is the diaphragm. Once these developmental stages have finished, each lung is separated into its own plural cavity.

That’s all for this post, if you have any questions please feel free to ask :)