POULTRY PRODUCTION MANUAL
The respiratory system is involved in the absorption of oxygen, release of carbon dioxide, release of heat (temperature regulation), detoxification of certain chemicals, rapid adjustments of acid-base balance, and vocalization.
While the function of the avian respiratory system is comparable to that of mammals, the two are quite different anatomically. Birds don’t breathe the same way mammals do. Like mammals, birds have two symmetrical lungs that are connected to a trachea (windpipe). But here the similarity ends. Mammalian lungs contain many bronchi (tubes), which lead to small sacs called alveoli. Because alveoli have only one opening, air can flow into and out of them, but it can not flow through them to the outside of a lung. In comparison, the avian lung has parabronchi which are continuous tubes allowing air to pass through the lung in one direction. They are laced with blood capillaries and it is here that gas exchange occurs.
The trachea divides into two smaller tubes called bronchi (plural form; singular = bronchus). In some respiratory diseases tracheal ‘plugs’ are often formed and they physically block the respiratory tract at the junction of the bronchi. As a result, the chickens suffocate. Excessive dust in the air is also believed to result in the formation of caseous tracheal plugs and adversely affect the health of the chickens.
The avian respiratory tract (Figure 3.5 and Figure 3.6 above) starts with the glottis which closes when feed is passing down the throat so that feed does not enter the lungs. The trachea is made up of cartilaginous rings that prevent its collapse from the negative pressure caused by inspiration of air. The syrinx is the voice box.
The chicken ‘voice’ is produced by air pressure on a sound valve and modified by muscle tension. It is not possible to remove the syrinx to prevent roosters from crowing. Both roosters and hens are able to ‘crow.’ The reason hens don’t normally crow is because they ‘don’t feel like it’ due to female hormone effects and the absence of sufficient levels of the male hormone. When the ovaries become diseased and the level of female hormones decrease, many hens will start to show male characteristics, including crowing.
The lungs are relatively small and do not expand. Instead, they are firmly attached to ribs. Birds have an incomplete diaphragm and the arrangements of the chest musculature and the sternum do not lend themselves to expansion in the same way that the chest of mammals does.
Consequently they can’t inflate and deflate lungs in the same way as mammals do. Instead, birds pass air through the lungs by means of air sacs, a uniquely avian anatomical feature. The air sacs are balloon-like structures at the ‘ends’ of the airway system. In the chicken there are nine such sacs: an unpaired one in the cervical region; two interclavicular air sacs, two abdominal air sacs, two anterior thoracic air sacs and two posterior thoracic air sacs (see Figure 3.7 below). The avian respiratory system is described as non-tidal. The mammalian respiratory system, in contrast, is tidal.
The lungs are relatively small and do not expand. Instead, they are firmly attached to ribs. Birds have an incomplete diaphragm and the arrangements of the chest musculature and the sternum do not lend themselves to expansion in the same way that the chest of mammals does. Consequently they can’t inflate and deflate lungs in the same way as mammals do. Instead, birds pass air through the lungs by means of air sacs, a uniquely avian anatomical feature.
The air sacs are balloon-like structures at the ‘ends’ of the airway system. In the chicken there are nine such sacs: an unpaired one in the cervical region; two interclavicular air sacs, two abdominal air sacs, two anterior thoracic air sacs and two posterior thoracic air sacs (see Figure 3.7 above). The avian respiratory system is described as non-tidal. The mammalian respiratory system, in contrast, is tidal.
The key to the avian respiratory system is that distention and compression of the air sacs, not the lungs, moves air in and out. At any given moment air may be flowing into and out of the lung and being ‘parked’ in the air sacs (see Figure 3.8 below). The lungs are stiff and fixed, not at all like the distensible lungs of mammals. The air sacs act as ‘bellow’s to suck air in and blow it out and also to hold part of the total volume. The air sacs fill a large proportion of the chest and abdominal cavity of birds, and also connect to the air spaces in the bones.
Since birds do not have a diaphragm, they depend on the movement of the sternum (keel) and rib cage in order to breathe (see Figure 3.9 below). Holding a bird too tight will restrict movement of the rib cage and suffocate the bird. This often happens when young children hold baby chicks.
A. Inspiration; B. Expiration; C. Sternum (keel)
With each breath, the chicken’s respiratory tract is exposed to the inside environment of a poultry house. Poor environments normally do not cause disease directly but they do reduce a chicken’s defenses, making them more susceptible to existing viruses and pathogens.
The air of poultry houses can contain aerosol particles or ‘dust’ originating from the floor litter, feed, dried manure, and the skin and feathers of the chickens. These aerosol particles can have a range of adverse effects on poultry. They act as an irritant to the respiratory system and coughing is a physiological response designed to remove them. Excessive coughing lowers the chicken’s resistance to disease. Aerosol particles often collect inside the chicken and can increase carcass condemnation at the processing plant.
The chicken’s respiratory tract is normally equipped with defense mechanisms to prevent or limit infection by airborne disease agents; to remove inhaled particles; and to keep the airways clean. Chicken health is affected by the function of three defensive elements: the cilia; the mucus secretions; and the presence of scavenging cells which consume bacteria.
Cilia are tiny hair-like structures in the trachea. Cilia are responsible for propelling the entrapped particles for disposal. Mucus is produced in the trachea. Mucus secretion and movement of cilia are well developed in chickens. The consistency of the mucus produced is important for the efficiency of the ciliary activity. Cilia cannot function when the mucus is too thick.
Scavenging cells in the lungs actively ‘scavenge’ inhaled particles and bacteria that gain entrance to the lower respiratory tract. These cells consume bacteria and kill them, thus preventing their further spread.
It is the integrated function of cilia, mucus and scavenging cells that keeps broiler airways free of disease-producing organisms. The impairment of even one of these components permits an accumulation of disease agents in the respiratory tract and may result in disease.
Gases are generated from decomposing poultry waste; emissions from the chickens; and from improperly maintained or installed equipment, such as gas burners. Harmful gases most often found in poultry housing are ammonia (NH3) and carbon dioxide (CO2). Research has shown that as little as 10 ppm ammonia will cause excessive mucus production and damage to the cilia. Research has also revealed that ammonia levels of 10-40 ppm reduce the clearance of E. coli from air sacs, lungs, and tracheas in chickens.