Question

Compare and contrast the cost and benefits of gills, lungs, and cutaneous respiration. What are the constraints of each? Which form of respiratory organ is used by each Vertebrate clade? How is fluid (air or water) ventilated to these organs in each clade?

How are gills, lungs, and cutaneous respiration used by each vertebrate group?

Answer 1

1. Cutaneous respiration

Cost-

Gas exchange in cutaneous respiration is controlled by three factors and this process is very vulnerable to external changes.

• Ventilation: the rate of delivery of respiratory medium (water or air) to the respiratory surface
• Diffusion: the passage of gases through the skin
• Convection: the carrying of dissolved gases towards or away from the lungs

Benefits- External cutaneous respiration is the ancestral form of respiration found in most protochordates
During external respiration gas exchange occurs at the level of the skin and oxygen and carbon dioxide are passed into and out of tissues.The process still occurs in small vertebrates as long as they have low activity levels and live in cool flowing water or in damp air - frogs meet about half of their needs for gas exchange through their skin

2. Gills

Cost- This system is only efficient in water as the buoyancy of the water helps keep lamellae apart. On land they would stick together drastically reducing the surface area available for gas exchange. The gills would also dry out as fish have no need for systems to keep the gas exchange system mois

By adopting many alternative respiratory organs some Fish from multiple groups can live out of the water for extended time periods. Amphibious fish such as the mudskipper can live and move about on land for up to several days, or live in stagnant or otherwise oxygen depleted water. Many such fish can breathe air via a variety of mechanisms. Breathing air is primarily of use to fish that inhabit shallow, seasonally variable waters where the water's oxygen concentration may seasonally decline. Fish dependent solely on dissolved oxygen, such as perch and cichlids, quickly suffocate, while air-breathers survive for much longer, in some cases in water that is little more than wet mud. At the most extreme, some air-breathing fish are able to survive in damp burrows for weeks without water, entering a state of aestivation (summertime hibernation) until water returns.

Benefits- Ahigh surface area is crucial to the gas exchange of aquatic organisms, as water contains only a small fraction of the dissolved oxygen that air does. The use of sac-like lungs to remove oxygen from water would not be efficient enough to sustain life. Rather than using lungs, "gaseous exchange takes place across the surface of highly vascularised gills over which a one-way current of water is kept flowing by a specialised pumping mechanism. the efficiency of the gills is greatly enhanced by a countercurrent exchange mechanism in which the water passes over the gills in the opposite direction to the flow of blood through them. This mechanism is very efficient and as much as 90% of the dissolved oxygen in the water may be recovered.

3. Lungs-

Cost-

• Does not work under water. Water is denser and more viscous and could not easily be ventilated (forced in and out of the tracheal tubes).
• Air flow is tidal (in / out) and not all of the air can be forced out of the lungs. Some oxygen poor air always remains and mixes with fresh air coming into the lungs, slightly reducing the concentration gradient and making gas exchange less efficient.
• The gas exchange surface is warm, moist and continuously ventilated, making it a site of significant water loss (despite being internal). Humans exhale over a cup of water each day. Most larger mammals rely on reservoirs of fresh water such as rivers and lakes which may limit their range.
• The gas exchange surface is warm and moist making it prone to infection. Respiratory illnesses such as cold and flu can also be easily transmitted when fine droplets containing bacteria / viruses are coughed or sneezed into the air and then breathed in by others.

Benefits

• Having an internal gas exchange system (lungs) helps to reduce water loss and maintain a moist gas exchange surface. This allows mammals to inhabit a greater variety of terrestrial habitats.
• The highly branched structure of the bronchial tree leads to many tiny alveoli. The large number and small size of the alveoli gives the lungs a large surface area to volume ratio. This allows mammals to grow to much larger sizes without the limitations normally imposed by the associated increase in the size and weight of the gas exchange system. The large surface surface area of the lungs also helps to meet the high metabolic demands of these larger, warm blooded animals.
• A transport system (blood vessels) reduces the distance dissolved gasses have to diffuse. This also enables mammals to grow to larger sizes than many other animal groups.

Respiratory organs and their working in different vertebrae clades.

RESPIRATORY SYSTEM IN FISH The fish gill adapted a structure for extraction of oxygen from water that is formed by a large number of filaments spaced out along the gill arches on either side of the pharynx. Each filament has a series of plates projecting at right angles from its upper and lower surfaces, the secondary lamellae, which are extremely numerous, are the site of gaseous exchange and form a fine sieve which ensures that all the water comes into close contact with the blood

The first air-breathing vertebrates were fishes, and a Devonian air-breathing sarcopterygian (lobefin) occupies the basal position in the lineage extending from the Paleozoic fishes to the most derived tetrapods

AMPHIBIANS

Based on paleontological criteria, the first amphibians have arisen by evolution of fish Crossopterígeos ripidistios, extinct in the late Devonian period.

The transition from aquatic to land environment exposed the gas exchange organ to a much richer oxygen ambience, which allowed a drastic reduction in the ventilation require-ments, but at the same time created problems for the disposal of carbon dioxide, because at 20ºC the water solubility of this gas is 28 times greater than that of oxygen. To prevent a severe respiratory acidosis, the Terran animal began to use the skin as an important respiratory organ, designed especially for the removal of carbon dioxide, which required a substantially reduction of the barrier represented by the scales that covered the surface of their aquatic ancestors. At the same time there must have occurred an increased bicarbonate concentration in plasma, in order to compensate the increase of carbon dioxide. These animals are mainly characterized for presenting an aquatic larval form, the tadpole stage, where hematosis takes place through the gills. Next they suffered a metamorphosis that allowed them to reach adulthood in terrestrial habitat and in which the breathing air was carried out by the lungs, skin and mouth. The amount of cutaneous and buccal gas exchange and its percentage in the total gas exchange, varied from species to species and also during seasons. Amphibians have the simplest lungs, rudimentary lungs that are adequate for ectothermic and low aerobic meta-bolism animals. In the various amphibian species the lungs differ greatly in size, their topographic extension and the dimension of exchange surface by the development of interconnected folds with highly varying number of subdivisions and height of their folds. The highly varying extent in lung exchange is due to differences in the amount of gas exchange performed by via lungs in concert with cutaneous and buccal cavity exchange. The remarkable heterogeneity of the morphology of the amphibian gas exchangers matches that of the diversity of the environments in which the animals live, the lifestyle they pursue, and their pattern of interrupted development. The skin is the main pathway for gas transfer in aquatic species while in terrestrial ones, it has been relegated or rendered redundant.

REPTILES

It is assumed that reptiles made their appearance on Earth about 310 million years ago, and their adaptation was so perfect that they dominated the planet for over a hundred million years.

Reptilians are the first vertebrates adequately adapted for terrestrial habitation and utilization of lungs as a sole pathway for acquisition of oxygen. The skin that was no longer necessary for gas exchange, became an armor to protect against dehydration, being waterproof, dry, covered with keratinized epidermal scales or developing dermal bone plates .

The reptilian display great pulmonary structural hetero-geneity and there is no single model of reptilian lung. Based on complexity of internal organization, different classifica-tion suggested that the turtles, monitor lizard, crocodiles and snakes have a profusely subdivided (multicameral) lung, the chameleons and iguanids have a simpler (paucicameral) lung and the teju lizard (Tupinambis nigropunctatus) have a saccular, smooth-walled, transparent (unicameral) lung

The intrapulmonary bronchi of the reptiles that give immediate access to respiratory areas correspond to the mammalian respiratory bronchioles, the tubular chambers, according to their position and morphofunctional structure, are equivalent to the alveolar channels in mammals, and the niche are similar to alveolar sacs. By its position in the respiratory system and anatomical constitution the aedicules are equivalent to the alveoli of mammals, however they have an oblong structure compared with the spherical form of mammal’s alveoli. The intrapulmonary bronchi of turtles that live essentially in aquatic environment have a reinforcement that extends to or near the respiratory areas, characteristic that is similar to the aquatic mammals that have the ability to dive to great depths, such as seals, dolphins and whales. This reinforcement, along with the presence of a smooth muscle, appear to be adaptations that allow these animals to support the high pressures to which they are subjected during the immersion to great depths.  

The role of surfactant in reptiles, which are not highly susceptible to collapse from surface tension forces, is obscure, and may have other important functions such as prevention of transendothelial transudation of blood plasma across the blood-gas barrier, immune suppression and attraction of macrophages.

AVES

Birds’ respiratory system, the lung – air sac system, is the most complex and efficient gas exchanger that has evolved in air-breathing vertebrates. The compact and virtually cons-tant-volume avian lung has been totally uncoupled from the compliant, avascular air sacs. The main properties that impart high respiratory effi-ciency on the lung–air sac system of birds are a cross-current design and inbuilt multicapillary serial arterialisation system; auxiliary counter-current system; large tidal volume; large cardiac output; continuous and unidirectional parabronchial ventilation; short pulmonary circulatory time; superior mor-phometric parameters; a particularly large respiratory surface area and a remarkably thin blood-gas (tissue) barrier.

The respiratory system of birds is separated into lung (the gas exchanging part) and a series of airs sacs (non res-piratory) with anastomosing air capillaries and pneumatized bones, that allow unidirectional flow of air, compared to the blind sac and tidal flow in mammalian lungs.

MAMMALS

Mammals evolved homoiothermy independently from birds, but in a very similar way. For the mandatory increased metabolism, they required a correspondingly increased gas exchange surface, which became available by the develop-ment of the broncho-alveolar lung. The nearest ancestors of mammals appear to have been same group of reptiles and the lung of the mammals derived from a multicameral reptilian lung with three rows of lung chambers. The branched conducting bronchial system originated by stepwise further subdivision of these lung chambers, terminating in the branched respiratory bronchioli and ductus, covered with alveoli.

The strong musculature of the diaphragm does not only act as a forceful inspiratory muscle together with the inter-costals musculature, but is also responsible for maintaining a pressure gradient between the pleural and the peritoneal activity during strong exercise. During respiration at rest, expiration is performed by elastic retractile forces of the extended rib cage and by the retraction forces of the lung itself out of the surface tension of the alveoli together with their extended elastic fibre systems. During exercise, expiratory movement of the intercostals musculature is strongly supported by the muscles of the abdominal wall, which is also the case for all sound productions, speech and singing

In the mammalian lung, the airway and vascular systems form a complex multigenerational dichotomous branching tree-like arrangement. Transported by bulk-flow (con-vention) in the initial (large) parts of the bronchial system and mainly by diffusion in the terminal (fine) sections of the airway system, the inspired air ultimately reaches the alveoli where it is exposed to capillary blood across a thin, extensive tissue barrier.

The alveolar surface is mainly lined by type I and type II cells. Type II cells secrete surfactant. In mammals the capillaries are located in the alveolar walls which are widely separated from each other. Thus the BGB has to withstand the full transmural pressure. The capillary is typically polarized with one side having very thin BGB whereas on the other side the barrier is thicker and contains strands of type collagen which provides support for the alveolar wall and maintains the integrity of the alveoli. In contrast to an uniform thin BGB in the birds, in mammals half of the surface area of the capillaries provides inefficient gas exchange due to its increased thickness.