Vertebrate respiratory system

Why do animals need to breathe?
Breathing is important to organisms because cells require energy (oxygen) to move, reproduce and function. Breath also expels carbon dioxide, which is a by-product of cellular processes within the bodies of animals.
Respiration is the process of releasing energy from food and this takes place inside the cells of the body.
The process of respiration involves taking in oxygen (of air) into cells, using it for releasing energy by burning food, and then eliminating the waste products (carbon dioxide and water) from the body.
Respiration is essential for life because it provides energy for carrying out all the life processes which are necessary to keep the organisms alive.
The energy produced during respiration is stored in the form of ATP (Adenosine Tri- Phosphate) molecules in the cells of the body and used by the organism as when required.
KEY POINTS
Life started in an anaerobic environment in the so called ‘primodial broth’ (a mixture of organic molecules.
Subsequently, oxygen strangely enough became an crucial factor for aerobic metabolism especially in the higher life forms.
The rise of an oxygenic environment was an important event in the diversification of life.
It evoked a dramatic shift from inefficient to sophisticated oxygen dependent oxidizing ecosystems.
Anaerobic fermentation, the metabolic process that prevailed for the first about 2 billion years of the evolution of life, was a very inefficient way of extracting energy from organic molecules. Ex: A molecule of glucose, e.g., produces only two molecules of ATP (≈ 15 kCal) compared with 36 ATP molecules (≈ 263 kCal) in oxygenic respiration.
Aerobic metabolism must have developed at a critical point when the partial pressure of oxygen rose from an initial level to one adequately high to drive it passively across the cell membrane.
Respiration is a complex and highly integrated biomechanical, physiological, and behavioral processes.
The transfer of O2 occurs through a flow of tissue barriers and compartments by diffusion down a partial pressure gradient, which drops to about zero at the mitochondrial level.
Acquisition of molecular oxygen (O2) from the external fluid media (water and air) and the discharge of carbon dioxide (CO2) into the same milieu is the primary role of respiration.
The respiratory system is a biological system consisting of specific organs and structures.

1. Dr. P.B.Reddy M.Sc,M.Phil,Ph.D, FIMRF,FICER,FSLSc,FISZS,FISQEM PG DEPARTMENT OF ZOOLOGY GOVERTNAMENT PG COLLEGE, RATLAM.M.P [email protected] Comparative anatomy of vertebrate respiratory system

2. Why do animals need to breathe? Breathing is important to organisms because cells require energy (oxygen) to move, reproduce and function. Breath also expels carbon dioxide, which is a by-product of cellular processes within the bodies of animals . Respiration is the process of releasing energy from food and this takes place inside the cells of the body. The process of respiration involves taking in oxygen (of air) into cells, using it for releasing energy by burning food, and then eliminating the waste products (carbon dioxide and water) from the body. Respiration is essential for life because it provides energy for carrying out all the life processes which are necessary to keep the organisms alive. The energy produced during respiration is stored in the form of ATP (Adenosine Tri- Phosphate) molecules in the cells of the body and used by the organism as when required .

3. KEY POINTS Life started in an anaerobic environment in the so called ‘ primodial broth’ (a mixture of organic molecules. Subsequently, oxygen strangely enough became an crucial factor for aerobic metabolism especially in the higher life forms. The rise of an oxygenic environment was an important event in the diversification of life. It evoked a dramatic shift from inefficient to sophisticated oxygen dependent oxidizing ecosystems. Anaerobic fermentation, the metabolic process that prevailed for the first about 2 billion years of the evolution of life, was a very inefficient way of extracting energy from organic molecules. Ex: A molecule of glucose, e.g., produces only two molecules of ATP (≈ 15 kCal ) compared with 36 ATP molecules (≈ 263 kCal ) in oxygenic respiration. Aerobic metabolism must have developed at a critical point when the partial pressure of oxygen rose from an initial level to one adequately high to drive it passively across the cell membrane. Respiration is a complex and highly integrated biomechanical, physiological, and behavioral processes. The transfer of O 2 occurs through a flow of tissue barriers and compartments by diffusion down a partial pressure gradient, which drops to about zero at the mitochondrial level. Acquisition of molecular oxygen (O 2 ) from the external fluid media (water and air) and the discharge of carbon dioxide (CO 2 ) into the same milieu is the primary role of respiration. The respiratory system is a biological system consisting of specific organs and structures.

4. Characteristics of respiratory organs Every respiratory device must conform to the following essentials features: Blood must be separated from the external environment that is air or water by a thin epithelium. The epithelium must be permeable to permit diffusion of gases through it. The respiratory epithelium must always remain moist with a film of fluid to permit osmosis of gases. The area of respiratory surface should be extensive to allow efficient absorption of oxygen. Both the current of air or water outside and blood in capillaries must be made to circulate constantly for quick replacement of gases. All the respiratory organs should have large surface areas to get enough oxygen . All the respiratory organs (like the skin , lungs, gills ) have a rich supply of blood for transporting respiratory gases. The thickness of the water/air–blood (tissue) barrier, the respiratory surface area, and volume of pulmonary capillary blood are the foremost structural parameters which determine the diffusing capacity of a gas exchanger for O 2 . Diffusion of oxygen from the medium into the blood. Transport of oxygen to the tissues and cells of the body. Diffusion of oxygen from the blood into cells. Carbon dioxide follows a reverse path.

5. External and internal respiration Internal Respiration : Oxygen diffuses out from the blood into tissue during internal respiration . Internal respiration, also known as cellular respiration, is the process of energy production via the breakdown of glucose. Therefore, it occurs within the cells. Internal respiration can be aerobic respiration or anaerobic respiration . External Respiration : External respiration is a physical process of taking oxygen from the external environment into the body and expelling carbon dioxide from the body to the external environment. It is a vital process for life as it supplies oxygen to extract energy from food via internal or cellular respiration. Additionally, it removes carbon dioxide, which is a waste product of internal respiration.

6. Respiratory System in Vertebrates GILLS Gills in Protochordates A large and sieve-like pharynx in majority of these animals performs dual function of respiration and trapping food particles which are brought in through the current of water. The primitive pterobranch hemichordates ( Cephalodiscus and Rhabdopleura ) have either no gill slits or have very few and sport tentaculated arms, which other than food gathering, also function as efficient respiratory organs. Balanoglossus possesses a large pharynx having as many as 700 pairs of gill slits, which appears to be a necessity in the burrowing habitat of the animal. The free-living urochordates, such as Salpa and Doliolum do not possess many stigmata or gill slits as their entire body is permeable to oxygen but in the sedentary ascidians pharynx is prominently enlarged and perforated with no less than 200,000 stigmata for filter-feeding. Cephalochordates use pharynx for both filter-feeding and respiration and hence carry 150-200 pairs of gill slits.

7. Respiratory organs of Cyclostomes Agnathans have 6-15 pairs of gill pouches. They are lateral extensions of pharynx and contain gill lamellae within. Cyclostomes are called marsipobranchs , which means “pouched gills”, since the gill lamellae are housed in gill pouches. The hagfish, Myxine has only 6 pairs of gill pouches whose ducts join together and open to the exterior by a single pair of openings, while Bdellostoma carries 6-15 pairs of gill pouches that vary in different species and open to the outside independently. The lamprey, Petromyzon , has 8 embryonic and 7 adult paired gill pouches that open to the exterior by independent openings. Hagfish

8. . In lower aquatic vertebrates the respiratory organs are not connected to the olfactory organs, but in air-breathing vertebrates there is a close association between the two. In Chondrichthyes there is a direct connection between the olfactory and respiratory organs in which the internal nares or choanae open from the nasal cavities into the buccal cavity, but it is only in tetrapoda that air enters through the nasal cavities into the buccal cavity and then into the lungs.

9. Respiration in Fish Respiration in fish takes place with the help of gills. Most fish possess gills on either side of their head. Gills are tissues made up of feathery structures called gill filaments providing a large surface area for exchange of gases. A large surface area is crucial for gas exchange in aquatic organisms as water contains very little amount of dissolved oxygen. The filaments in fish gills are organized in rows in the gill arch.  Each filament comprises  lamellae , which are discs supplied with capillaries . Blood moves in and out of the gills through these small blood vessels. The gills provide extensive respiratory surface for the efficient gas exchange. Fish take in oxygen-rich water via their mouths and pump it over their gills. When water moves over the gill filaments, the blood within the capillary network takes up the dissolved oxygen. Then, the  circulatory system   supplies oxygen to all tissues of the body and finally to the cells while taking up carbon dioxide that is eliminated through the gills from the body.

10. Mechanism of Respiration: 1. Inspiration: The outer opening of the gill-chamber remains tightly closed. The branchial-arches and the opercula swell laterally increasing the volume of bucco-pharyngeal cavity. Thus , the water flows in through the opened mouth to fill the enlarged bucco-pharyngeal cavity. 2 . Expiration: Mouth becomes closed by oral valves. B ucco ­-pharyngeal cavity contracts exerting pressure on the contained water. In the meantime the opercula and branchio-stegal membranes are lifted off. As a result, water from the bucco-pharyngeal cavity passes out through the gill-chamber, bathing the gill-filaments. Contraction and expansion of pharyngeal cavity occurs by alternate retraction and protraction of the hyoid arch which supports the bucco-pharyngeal cavity.

11. Amphibians utilize gills for breathing early in life Most amphibians breathe through lungs and their skin . Their skin has to stay wet in order for them to absorb oxygen so they secrete mucous to keep their skin moist. Cutaneous Respiration: The skin of frog is an important organ of respiration (water). when frog undergoes summer sleep (aestivation) and winter sleep (hibernation), the skin is the only organ of respiration. The skin of frog is very much suited for the respiratory function as it is very thin and richly supplied with blood capillaries and remains moist with the water and also mucus, secreted by mucous glands. During gaseous exchange the oxygen first dissolves in the moisture present over the body and then diffuses into the blood circulating in the blood capillaries, while the resultant carbon dioxide passes out from the blood into the surrounding medium (water) by diffusion. In cutaneous respiration, no movements are needed because skin always remains exposed to air or water. Amphibia

12. Bucco -Pharyngeal Respiration or Buccal Respiration: The mucous lining of the buccal cavity is richly supplied with blood capillaries and remains moist by the mucus. The buccal respiration occurs by lowering and raising of the floor of the buccal cavity, during the course of which the air is constantly sucked into the buccal cavity and is drawn out through the external and internal nares . In this type, the mouth and glottis remain closed. Thus, no air enters or goes out from the lungs. When the floor of the buccal cavity is lowered, the air enters the buccal cavity through the nostrils or the nares . The oxygen of air dissolves in the layer of mucus and then goes into blood. At the same time carbon dioxide is given out into the buccal cavity from the blood which is expelled along with residual air through the nostrils when the floor of the buccal cavity is raised. Pulmonary Respiration: Respiration on land in air with the help of lungs is the pulmonary respiration. In frog, lungs are poorly developed. The intake of oxygen by lungs is not sufficient to the body. Therefore, oxygen intake through moist skin and buccal cavity is needed. The lungs also hydrostatic organs as they enable frog to float in water when they are inflated. The air enters and leaves the lungs through the respiratory fact. (b) Lungs: A pair of lungs are found in the anterior part of the body cavity, one on the either side of the heart. They are ovoid, thin-walled, elastic sacs with shallow internal folds or septa that increase the inner surface to form many chambers called alveoli. These are separated from each other through septa. The inner surface of the aveoli is covered with a single layer of epithelial cells which are very thin and flattened except on the edges of the septa where they are ciliated and cylindrical.

13. On the inner side of the epithelium there is aerolar type of connective tissue comprising blood and lymph vessels and unstriped muscle fibres which give remarkable power of contraction and expansion to the lungs. The outer surface of the lung is coated with coelomic epithelium called peritoneum. Mechanism of Pulmonary Respiration: The incoming and outgoing of the air from the lung is brought about by the action of the floor of the buccal cavity. The actions of the floor of the buccal cavity are brought by two sets of muscles, the sternohyal and the petrohyal muscles, (i) Sternohyal muscles arise from the coracoid and clavicle or sternum and attached to the lower surface of the hyoid apparatus located in the floor of the buccal cavity. (ii) Petrohyal muscles are attached on one end with the squamosal bone above and on the other side with the upper surface of hyoid apparatus .

14. Respiratory organs: Cutaneous respiration Respiration through the skin can take place in air, water, or both Most important among amphibians (especially the family Plethodontidae) Gills Cartilaginous fishes : 5 ‘naked’ gill slits Anterior & posterior walls of the 1st 4 gill chambers have a gill surface (demibranch). Posterior wall of last (5th) chamber has no demibranch. Interbranchial septum lies between 2 demibranchs of a gill arch Gill rakers protrude from gill cartilage & ‘guard’ entrance into gill chamber 2 demibranchs + septum & associated cartilage, blood vessels, muscles, & nerves = holobranch

15. Bony fishes (teleosts):   Usually have 5 gill slits operculum projects backward over gill chambers Interbranchial septa are very short or absent Larval gills: External gills outgrowths from the external surface of 1 or more gill arches found in lungfish & amphibians Filamentous extensions of internal gills project through gill slits occur in early stages of development of elasmobranchs Internal gills - hidden behind larval operculum of late anuran tadpoles

16. Respiratory Structures in Reptilia The respiratory organs include a pair of external nares , nasal chambers, internal nares , glottis, larynx, trachea, bronchi and lungs. The external nares lie a little in front of eyes. They lead into nasal passages or chambers, which open into the roof of the buccal cavity. The glottis is located behind the tongue and it opens posteriorly into a short chamber, the larynx. It is less prominently developed than in many of the Amphibia. Its walls are supported by a cricoid and a pair of arytenoid cartilages. The larynx opens into a narrow, elongated cylindrical tube, the trachea. Its wall is supported by a large number of small cartilaginous rings, the tracheal rings. The trachea is bifurcated into two narrow tubes in the thorax, the bronchi. Each bronchus enters into a lung. the lung.

17. Lungs: The lungs are elastic, elongated sacs, the right lung is slightly larger than the left one. The walls of the lungs are folded into ridges giving the appearance of a honeycomb. These ridges are much closer and more numerous towards the anterior end than towards the posterior end of the lung. These chambers are called alveoli where gaseous exchange occurs. In the distal part of the lungs, such chambers are absent. This posterior part of the lung is considered as reservoir for the residual air. Respiratory movements are performed by the intercostal muscles attached to the ribs. Inspiration is caused by the movement of intercostal muscles, raising the ribs that increases the volume of the thorax and reduces the lung pressure causing the inflow of air into

18. Swim bladder & origin of lungs – Most vertebrates develop an out pocketing of pharynx or esophagus that becomes one or a pair of sacs (swim bladders or lungs) filled with gases derived directly or indirectly from the atmosphere. Similarities between swim bladders & lungs indicate they are the same organs. Vertebrates without swim bladders or lungs include cyclostomes, cartilaginous fish, and a few teleosts (e.g., flounders and other bottom-dwellers). Swim bladders: may be paired or unpaired have, during development, a pneumatic duct that usually connects to the esophagus. The duct remains open (physostomous) in bowfins and lungfish, but closes off (physoclistous) in most teleosts. serve primarily as a hydrostatic organ (regulating a fish's specific gravity) . gain gas by way of a 'red body' (or red gland); gas is reabsorbed via the oval body on posterior part of bladder may also play important roles in: hearing - some freshwater teleosts (e.g., catfish, goldfish, & carp) 'hear' by way of pressure waves transmitted via the swim bladder and small bones called Weberian ossicles. sound production - muscles attached to the swim bladder contract to produce sound. respiration - the swim bladder of lungfish has number subdivisions or septa (to increase surface area) & oxygen and carbon dioxide is exchanged between the bladder & the blood .

19. Lungs & associated structures Larynx Tetrapods besides mammals – 2 pair of cartilages: artytenoid & cricoid Mammals - paired arytenoids + cricoid + thyroid + several other small cartilages including the epiglottis (closes glottis when swallowing) Amphibians, some lizards, & most mammals - also have vocal cords stretched across the laryngeal chamber Trachea & syrinx Trachea usually about as long as a vertebrates neck (except in a few birds such as cranes) reinforced by cartilaginous rings (or c-rings) splits into 2 primary bronchi &, in birds only, forms the syrinx at that point

20. Lungs Amphibian lungs 2 simple sacs internal lining may be smooth or have simple sacculations or pockets air exchanged via positive-pressure ventilation Reptilian lungs simple sacs in Sphenodon & snakes Lizards, crocodilians, & turtles - lining is septate , with lots of chambers & subchambers air exchanged via positive-pressure ventilation Avian Lungs: modified from those of reptiles: air sacs (diverticula of lungs) extensively distributed throughout most of the body arrangement of air ducts in lungs ----> no passageway is a dead-end air flow through lungs ( parabronchi ) is unidirectional. Double respiration

21. Birds are different from other vertebrates, with birds having relatively small lungs and nine air sacs that play an important role in respiration. The lungs of birds also do not have the capacity to inflate as birds lack a diaphragm and a pleural cavity. Gas exchange in birds occurs between air capillaries and blood capillaries, rather than in alveoli . In addition to lungs, birds have air sacs inside their body. Air flows in one direction from the posterior air sacs to the lungs and out of the anterior air sacs. The flow of air is in the opposite direction from blood flow, and gas exchange takes place much more efficiently. This type of breathing enables birds to obtain the requisite oxygen, even at higher altitudes where the oxygen concentration is low. This directionality of airflow requires two cycles of air intake and exhalation to completely get the air out of the lungs.

22. Mammalian lungs: M ulti chambered & usually divided into lobes air flow is bidirectional:   Trachea <---> primary bronchi <---> secondary bronchi <---> tertiary bronchi <---> bronchioles <---> alveoli

23. The respiratory mechanism involves two phases: (a) Inspiration and (b) Expiration. During pulmonary respiration the air is forced into the lungs. This is aided by the movable premaxillae bones of the upper jaw situated just below the external nares and the foresaid muscles. During inspiration the frog closes the glottis and mouth, and the nostrils remain open. On the contraction of the sternohyal muscles the floor of the buccal cavity along with hyoid is lowered increasing the volume of the buccal cavity. Thus, the air enters the cavity through the external nares . (b) Expiration: Before expiration, when lungs are filled with air, the glottis closes and the air is kept in the lungs for a short time. During this period buccal respiration occurs. Soon the glottis becomes opened and the air from the lungs is expelled into the buccal cavity by the contraction of the lungs and the abdominal muscles and by lowering the floor of the buccal cavity. Now the buccal floor is raised again, the glottis closes and external nares are opened, forcing the air out through the external nares .

24. Mechanism of Breathing or Respiration: I nspiration : Done by the contraction of the obliquely arranged external intercostal muscles dragging the ribs and the sternum forwards and downwards. At the same time the radially arranged muscles of the diaphragm contract, thereby it becomes flattened. This results in the increase of the internal capacity of the thoracic cavity, thus, decreasing the pressure within the thoracic cavity. Expiration. It is a passive process. During expiration, the internal intercostal muscles placed at right angles to external intercostal muscles contract and the later muscles also relax, thereby the ribs and sternum attain their normal position. At the same time the muscles of the diaphragm relax, bringing it in its normal dome-shaped position. Thus, the thoracic cavity and its volume decreases, producing a huge pressure on the lungs. Thus, the elastic walls of lungs shrink, expelling the respired air out through the same above path.