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Physiology of Organisms 2/1/15 Compare respiration in birds to respiration in mammals. In comparing mammals and birds with similar ecological niches (insectivore bats and insectivore birds, for example), are there apparent advantages one class of animal has over the other when it comes to respiration? Respiration in physiology is defined as the process of gas exchange to transfer oxygen and carbon dioxide between the atmosphere and metabolically active cells. In both mammals and birds respiration is carried out by the lungs and associated structures, which differ between the two groups. The mammalian lung consists of branching tubes, from the trachea to bronchi to bronchioles, which become increasingly narrow and more numerous as they penetrate deeper into the lung. These do not carry out gas exchange, but acts as airways through which air moves from the exterior to the alveoli; small sacs in which gas exchange occurs. The equivalent sites of gas exchange in birds are 10 μm in diameter air capillaries that stem from the parabronchi, tubes that branch between dorsobronchi and ventrobronchi, which are connected to the larger mesobroncus. These tubes comprise the bird lung, and are supplied with air through the trachea. The differences in structure of the lung and associated elements have implications for the mechanism of ventilation and for the efficiency of gas exchange. Despite differences in the structures in which gas exchange between the atmosphere and the blood occurs in mammals and birds, this element of respiration is fairly similar in both groups. In both mammals and birds oxygen and carbon dioxide move between the air and the blood by simple diffusion, the driving force for this movement being the partial pressure gradients between the blood and the air. Fick’s law of diffusion states that diffusion of gas across a sheet of tissue is proportional to the area of the sheet and inversely proportional to the thickness of the tissue, therefore efficiency of this transfer is increased by greater surface area and decreased thickness of the gas exchange surfaces. In mammals the alveoli make up a very large surface area. For example; a human adult lung contains 300-500 million alveoli, with a surface area of 50-100 m2 over which gas exchange can occur. Additionally there are very short distances for diffusion of gases between the blood and the air. The pulmonary circulation is closely associated with the alveoli; a network of capillaries wrap around the alveoli minimising the distance for diffusion of oxygen into the blood and carbon dioxide out into the alveoli. This diffusion distance can be as small as 0.5μm. The large surface area and short diffusion distances improve the efficiency of gas exchange in the mammalian lung, following Fick’s law. The same is true in birds. The air capillaries in the parabronchi where gas exchange occurs have a large surface area, and are closely associated with blood capillaries to create short diffusion distances. Additionally the air capillaries and blood capillaries are arranged so that flow is crosscurrent; air passing through the air capillaries in the parabronchi and blood through the blood capillaries travel at right angles to one another. This increases the efficiency of gas exchange, since oxygen and carbon dioxide pressure gradients are maintained. In both birds and mammals, the pulmonary circulation receives the entire cardiac output, so flow is high despite low pressure, meaning red blood cells can pass through the circulation in less than a second while passing numerous alveoli or air capillaries, maximising gas Physiology of Organisms 2/1/15 exchange. This also ensures that gas exchange occurs with the blood every time that it is circulated through the body. Both mammalian and avian lungs have large surface areas and small diffusion distances resulting in efficient gas exchange, and despite different structures (gas exchange occurs across the walls of tubes in birds, and sacs in mammals) gas exchange takes place by the same process of diffusion. However, this gas exchange is more efficiently in birds due to the crosscurrent system, creating an advantage in birds occupying a similar ecological niche to certain mammals. The way in which gas exchange occurs is far more similar in birds and mammals than their mechanisms of ventilation. The mechanism of ventilating the lung is different in birds and mammals, largely due to differences in lung structure, although the underlying principles involved in ventilation are the same. Flow of air is defined by the equation: Flow = Δ Pressure/Resistance. The pressures differences between any two regions causes flow between those regions, therefore to have flow of air between the atmosphere and the alveoli, and the atmosphere and the parabronchi there must be a pressure gradient generated between the two. Both mammalian and avian lungs are contained in the airtight thoracic cavity, but in mammals are separated from the abdomen by the diaphragm, which is not present in birds. In the mammalian lung the pressure gradient required to induce air flow from the atmosphere to the alveoli relies on the low volume intrapleural space between the parietal pleura lining the thoracic cage and the visceral pleura encasing the lungs. These are separated by a 10μm thick layer of fluid, the interactive forces in which hold the surfaces together. When mammals inspire, the diaphragm contracts, lowering and compressing the contents of the abdomen to expand the thoracic cavity vertically. The intercostal muscles that lie between the ribs contract to raise the ribcage. The volume of the thoracic cavity increases, but since it is airtight subatmospheric pressure is generated in the intrapleural space. This causes the lungs to expand, alveolar volume increases, until the negative pressure is equal to the outward elastic recoil forces of the chest wall, and inward elastic recoil forces of the lungs. This leads to the generation of a pressure differential, since the increased volume causes a fall in pressure inside the lungs to below atmospheric pressure, resulting in flow of air into the lungs, followed by a return to atmospheric pressure. The same principle of flow of air = Δ Pressure/Resistance is important for ventilation of the avian lung, although birds’ lungs are ventilated by a different mechanism to mammals’. Although the bird’s lung is also located inside the thoracic cavity, the ribs move forward only very slightly during inspiration, and the volume of the lung does not change greatly during breathing. Instead volume changes predominately occur in the associated air sacs, which penetrate between organs and into bones; the cranial air sacs that are connected to ventrobronchi, and the caudal air sacs connected to the mesobronchus. The air sacs are expanded during inspiration by lowering of the sternum and lateral movement of the posterior ribs, increasing the volume and creating subatmospheric pressure, resulting in airflow into the lungs as in the mammalian lung. During inspiration air flows into the air sacs, passing through the parabronchi when flowing to the cranial air sacs. Then during expiration compression of the air sacs occurs by motion of the sternum dorsally, which forces air from the caudal air sacs through the parabronchi, and from the cranial air sacs through the ventrobronchi to the trachea. Airflow is directed in this manner through the parabronchi, Physiology of Organisms 2/1/15 unidirectionally, due to the openings of the ventrobronchi and dorsobronchi into the mesobronchus having variable resistance to airflow dependent on its direction. This mechanism results in oxygenated air passing through the parabronchi continually during both inspiration and expiration. It maximises the amount of oxygen extracted from and carbon dioxide lost to the air, as there is no mixing of oxygenated air with air that has already undergone gas exchange, therefore only oxygenated air undergoes gas exchange, meaning large partial pressure gradients are maintained. In comparison, in mammals there is bidirectional airflow; fresh air moving into the lungs is mixed with air that has undergone gas exchange, meaning the air in the alveoli has a lower partial pressure of oxygen. Therefore gas exchange is even more efficient in birds than in mammals, creating an even more significant advantage in terms of their respiration to birds occupying the same ecological niche as mammals. In conclusion the main difference between birds and mammals in respiration is their mechanism of ventilation of the lungs, where birds utilise air sacs while mammals use movement of the diaphragm and ribcage to create pressure differences to induce airflow into the lungs. The process of gas exchange occurs by diffusion in both groups, both mammals and birds maximise gas exchange through use of large surface areas and short diffusion distances. However, gas exchange occurs far more efficiently in birds due to the cross-current system and greater partial pressure differences from the unidirectional flow of air through the parabronchi. The implication of this in birds and mammals with similar ecological niches, such as insectivore birds and insectivore bats, is that the birds have an advantage in respiration over the mammals. With more efficient respiration, the birds, for example during hunting where a greater level of respiration is required due to an increased level of activity, will be able to oxygenate their blood and remove carbon dioxide at a faster rate. The mammals at the same level of activity would need to increase their ventilation rate in order to achieve the same rate of gas exchange, resulting in greater energy costs. Bibliography ‘Respiration in air and water’ Lecture notes Michael J. Mason, Ph.D. Randall, D. Burggren, W. & French, K. (2002) Eckert Animal Physiology, 5th ed. New York: W.H. Freeman & Co "respiratory system". Encyclopædia Britannica. Encyclopædia Britannica Online. Encyclopædia Britannica Inc.,2014 <http://www.britannica.com/EBchecked/topic/499584/respiratorysystem/66213/Birds>. http://en.wikivet.net/Avian_Respiration_-_Anatomy_%26_Physiology "pulmonary circulation". Encyclopædia Britannica. Encyclopædia Britannica Online. Encyclopædia Britannica Inc., 2014. <http://www.britannica.com/EBchecked/topic/483161/pulmonarycirculation>. http://people.eku.edu/ritchisong/RITCHISO/birdrespiration.html