TheAvianRespiration
Birds are an elite taxon of animals that have evolved the ability for powered flight. The respiratory system of birds is markedly different from other vertebrates in having relatively small lungs and the air sacs that are important for respiration.
7.10.1 Basic Design of the Respiratory System in Birds
The avian lung is non-lobulated and deeply marked by the vertebrae. Unlike mammals, there is no dichotomously branched bronchial system, and the airways do not terminate blindly. Avian lungs have a broad dorsal and a thin ventral aspect. The avian lung is rigid and wedge shaped that alters its volume by only 1.4% between the respiratory cycles. Birds have a larynx, which does not produce sounds. Instead, an organ termed the syrinx functions as the voice box. Lung volume in a bird is approximately 26% smaller, compared to animals of equivalent body mass, respiratory surface area (RSA) about 15% greater, harmonic thickness of the bloodgas barrier (τht) around 62% thinner and PCBV approximately 22% greater.
The tracheal rings are complete in birds rather than incomplete as in mammals. In comparison to mammals, the trachea in the bird is 2.7 times longer and 1.29 times wider, resulting in similar air resistance in the trachea as in mammals. Still, the dead-space volume in the trachea is about 4.5 times greater than in mammals. A lower respiratory frequency accompanied by a larger tidal volume compensates for the large dead space in the trachea.
The lungs are the gas-exchange structures, but they do not contract and expand during respiratory cycles and are relatively small and fixed to the ribs. Their ventilation depends on bellows-like extensions from the lungs and the air sacs, which expand and contract during respiratory cycles. The primary bronchus of birds extends from tracheal bifurcation to the ostium of the abdominal air sac and has only two dusters of secondary bronchi.
From the cranial end of the bird’s intrapulmonary primary bronchus (IPPB) arise four ventrobronchi, while its caudal segment gives rise to 7-14 variably sized dorsobronchi and a variable number (the avian lung, the exchange of gases occurs by simple diffusion. Oxygen diffuses into the blood from the “air capillaries”, and carbon dioxide diffuses into the “air capillaries” from the blood. There is a cross-current flow of air in avian species. As the air passes through the parabronchi, blood moving through capillaries travels at right angles. Crosscurrent exchange is a very efficient system, which enables the pressure gradients of oxygen and carbon dioxide to be maintained along the entire length of the parabronchuscapillary “connection”. Like in mammalian lungs, in avian lungs, too, O2 in inspired air diffuses passively into the pulmonary capillary blood, and CO2 diffuses in the opposite direction. However, there are certain special features in the avian parabronchial lung, different from the mammalian bronchoalveolar lung. The oxygen uptake and loss of carbon dioxide are not affected by the direction of gas flow through parabronchi. Air containing oxygen flows into the parabronchial lumen, from which it diffuses radially into the air capillary network, followed by pulmonary capillaries. The CO2 moves in the opposite direction. Blood flow in pulmonary capillaries occurs in the opposite direction to that of O2 diffusion in the air capillaries. The systemic arterial blood is a mixture of blood drawn from all individual air-blood capillary units that can contribute to a greater arterial PO2 as compared to end-expired PO2. This is unique to avian species only and never observed in the mammalian lung.Another countercurrent-like system also exists in the avian respiratory system along with the cross-current system. It is comprised of the inward and outward directional flow of blood in the blood capillaries and air in the air capillaries.
This system is not an efficient system of gas exchange because of complex, tortuous arrangement and short contact points between blood and air, which prevents sufficient gaseous exchanges. The blood-gas barrier (BGB) in birds is formed by the fusion of type I epithelial and endothelial cells over the basement membrane, having an almost uniform thickness.7.10.4 Control OfVentilation in Birds
Like mammals, the central control area of ventilation in birds is located in the pons and medulla oblongata, which are higher brain control centres. The frequency of respiration and the time duration of inspiration and expiration are controlled by feedback received from several receptors located within the lungs and peripheral chemoreceptors, the receptors located within and in the vicinity of the air sacs, viz. the mechanoreceptors and thermoreceptors (in the hypothalamus and spinal cord).
Unlike mammals, birds have CO2 receptors in their lungs (intrapulmonary receptors) that detect carbon dioxide levels in lung air. With a fall in the PCO2 in the lung, the receptors in the lung get stimulated and increase their rate of discharge. As the rate of discharge increases, ventilation decreases and hence fine-tuning occurs.
7.10.5 SpecialAdaptations
There are two types of haemoglobin in adult birds, HbA and HbD, which vary in their affinity for oxygen. It is advantageous for birds that have to cope with large variations in the partial pressure of oxygen as they move from one habitat to another. HbA is more common and has a lesser affinity for oxygen, which can readily deliver oxygen to the tissues. Avian Hb also shows more cooperativity with oxygen than mammalian Hb. The Hills coefficient indicates the degree of cooperativity, which is 2.8 for mammals and 4 for birds.
In mammals, hyperventilation causes a decrease in PaCO2, which causes vasoconstriction and a decrease in cerebral blood flow, leading to cerebral ischaemia. Mammals can tolerate PaCO2 of 20 mmHg, but birds can maintain cerebral blood flow even at 8-10 mmHg. It is important for the survival of birds, which hyperventilate during flight.
Surfactant SP-B is especially important for maintaining airflow through the “tubes” of the respiratory system in birds, which can be ascribed to the phospholipids and proteins present in it. Present only in the mesobronchi, the surfactant SP-A contributes towards innate defence mechanism and regulation of inflammatory responses and has a prime role in the mesobronchi because airflow is slower and small particles could accumulate there.
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