What are Parabronchi? Anatomy, Physiology and In-depth Diagrams

 

Parabronchi Explained: Structure Function And Detailed Diagrams 

Parabronchi are the functional unit of gas exchange in Avian lung. 

They are also called tertiary bronchi as they can originate from the secondary bronchi. 

Most of the parabronchi which are organised as a parallel series of several hundred tubes connecting the medioventral and mediodorsal secondary bronchi, are called paleopulmonic parabronchi and together with the primary, cranial and caudal groups of secondary bronchi they compose the simplest scheme of bronchial branching in Avian lung forming the respiratory system (e.g. Pigeon). ( Study Notes on Pigeon Respiratory System)

Passage of air from dorsal to ventral secondary bronchi via parabronchi in lungs of birds 

 In all birds, except some penguins there are additional parabronchi called neopulmonic parabronchi, which are not organised as regular parallel stacks of tubes but may exhibit irregular branching pattern. 

What is parabronchi?

When a bird inhales, it takes air through its nostrils or external nares, thence into the mouth and pharynx and through glottis, a slit-like opening surrounded by the larynx, which has no vocal cords but helps preventing foreign materials entering lower respiratory tract.

The breathed in air reaches the windpipe or trachea. The trachea passing from the chamber of syrinx, bifurcates into two short bronchi,one for each lung.

The left and right bronchi are called primary bronchi or mesobronchi.

Through vestibulum (small space for entrance into the lung), the primary bronchi after entering the lung, sends three short branches into an air sacs near its entrance into the lung.

• One to the anterior thoracic air sacs,
• Second to the cervical air sacs,
• Third to the interclavicular air sacs.

The primary bronchi decreases in diameter and  further branches into secondary bronchi which are designated as dorsal, ventral and lateral according to their position.

(The longest mesobronchi end at the entrance to the abdominal air sacs.)

The dorsobronchi and ventrobrochi further break up into a network of tertiary brochi or parabronchi.

Structure of parabronchi 

These gas exchanging tubes can be several millimetres long and 0.5 to 2.0 mm in diameter, depending on the size of the bird.

( The number of parabronchi can vary from 400 in domestic hens to approximately 1800 in pigeons and ducks, and are laid parallel to each other, forming the true functional unit of the lung.)

Section of parabronchus showing highly vascularised air capillary network that radiates outward from the parabronchial lumen.

The parabronchi are separated from each other along their length by a boundary of connective tissue and larger pulmonary blood vessels. The parabronchial lumen is lined by a meshwork of smooth muscles, which outline the entrances to atria. 

The atria lead to infundibula and finally to the air capillaries, which are 2-10 μm in diameter and as long as one-quarter of the total parabronchial diameter.

Structure of parabronchus: This section shows the internal structure of a single parabronchus where air flows continuously through the openings in its walls into network of air spaces (air capillaries), surrounded by a network of blood capillaries. In between these two structures the exchange of oxygen and carbon dioxide occurs. For easy explanation, from the left portion of the diagram, the blood capillaries have been removed and on right side the air spaces are obscured by blood capillaries. Pulmonary arterioles supply deoxygenated blood and pulmonary venules trasport oxygenated blood away. Credit: Hand book of bird biology ( Cornell Lab of Ornithology )

The air capillaries intertwine with a similar network pulmonary blood capillaries in the parabronchial mantle, where the air blood capillary interface is the site of gas exchange.

How parabronchi functions?

Much inhaled air directly passes to the air sacs through lungs via the mesobronchi, without taking part in gas exchange.

Gas exchange with this air occurs on Exhalation, as it passes out of the air sacs, back into the lungs, and through the parabronchi.

The complicated and unique Avian lung function can be made easy by understanding the diagram below.

Functioning of parabronchus: (a) structure of bird lungs where parabronchi are present, (b) diagram showing passage of air from dorsal to ventral secondary bronchi through parabronchi, (c) section of parabronchi showing parabronchial tissue and lumen, (d) network of air capillaries radiating outwards from parabronchial lumen, (e) internal structure of single parabronchus showing blood capillaries 

The lungs of a bird is mass of interconnecting air tubes, called the parabronchi. They are too small to be seen with the nacked eyes.

Each air tube has openings in its thick wall that allows the air to pass into small interconnecting spaces that weave together like a labyrinth through the surrounding blood capillary bed.

This openings also allow the air to pass back from the spaces into the air tubes.

The capillary bed surrounding the spaces is supplied by the pulmonary artery and drained by the pulmonary vein.

As blood flows through the capillaries ( generally at right angles to, or in the opposite direction of, the flow- permitting the formation countercurrent exchange system), oxygen in the air spaces dissolve into the blood, and carbon dioxide from the blood into the air to finally exhaled.

Birds does not show presence of bronchioles or dead-end pockets or alveoli ( like mammals), but because of the interconnection of the minute air spaces, birds can achieve continuous flow of air across the surface of the capillary bed, continuously extracting highly efficient oxygen. 

Function of parabronchi 

1. Gas exchange 

It the site of exchange of gases where oxygen from inhaled air is transferred into blood stream and removal of carbon dioxide from blood occurs.

2. The air capillaries present in them maximize surface area for gaseous exchange by bringing  blood closer to the inhaled air.

3. The arrangement of air capillaries and blood vessels allows a cross-current gas exchange such that continuous supply of oxygen occurs making the birds respiratory system highly efficient.

4. Birds show unidirectional airflow through parabronchi ( air flows in one direction both in inhalation and exhalation.

5. It helps in maintaining air flow and ventilation with the help of air sacs.