RESPIRATORY SYSTEM

• Ancestral chordates and ostracoderms were filter feeders.
• Ancestral chordates and ostracoderms were filter feeders.
• A less energy-consuming procedure evolved as a result of further adaptations of the skull and hyoid arches.
• The modifications enabled some teleosts

  to approach close to organisms that are small enough to be swallowed, extend protrusible jaws, create suction, close the mouth, retract the jaws, and swallow.
• External respiration precedes internal respiration, which is usually defined as the exchange of oxygen and carbon dioxide between capillary blood and tissue fluids.
• List the chief organs of external respiration in adult vertebrates:

  The chief organs of external respiration in adult vertebrates are external and internal gills, the oropharyngeal mucosa, swim bladders or lungs, and the skin.
• Embryos employ a variety of respiratory surfaces including extraembryonic membranes.
• In both water and air, respiration through the skin is employed extensively by modern amphibians
• It is also used by some fishes, especially those

  that lack scales and therefore have capillary beds close to the surface.


  respiration through skin exhibit by fishes that lack scales
• Cutaneous respiration is not significant in amniotes because the thick stratum corneum insulates the capillaries from the atmosphere.
• The internal gills of fishes arise in the walls of pharyngeal pouches, which are paired evaginations of the embryonic pharynx
• The pharyngeal pouches meet ectodermal grooves, which are invaginations of the ectoderm
• The buccal cavity of lampreys is pressed tightly into the

  flesh of a host, and the duct from the naris (nasohypophyseal duct) does not open into the pharynx but ends blindly in a nasohypophyseal sac.
• External respiration is the exchange of respiratory gases between organism and environment. It takes place via highly vascular membranes

  with thin moist epithelia.
• Internal respiration is the exchange of gases between capillary blood

  and the tissues.
• The chief adult organs of respiration are pharyngeal gills, oropharyngeal

  mucosa, swim bladders or lungs, and skin.
• Cutaneous respiration is the chief method of respiration in aquatic

  urodeles and some scaleless fishes.
• Internal gills arise in the walls of pharyngeal pouches and are supported

  by the skeleton of the pharyngeal arches. The pouches acquire an opening to the exterior, except in some agnathans.
• A typical gill, or holobranch, consists of two demibranchs and an interbranchial septum, the latter attached to a skeletal component of a pharyngeal arch. The septum is shorter in fishes that have an operculum.
• Respiratory water usually enters via the mouth, but it enters the spiracle

  in some elasmobranchs, the naris in hagfishes, and external gill slits in

  lampreys.
• Some features of the gill systems of lampreys and hagfishes are unlike

  those of jawed fishes. They exhibit adaptations correlated with their parasitic mode of feeding.
• Elasmobranchs have naked gill slits that are visible on the surface. The

  slits of chimaeras, bony fishes, and larval anurans are covered by an

  operculum attached to the hyoid arch.
• A branchiostegal membrane is attached to the ventral edge of each

  operculum in bony fishes, supported by gular bones or branchiostegal

  rays. The paired membranes enclose an opercular chamber, which receives water that has passed over the gills. The water is then expelled to

  the exterior.
• The first embryonic pharyngeal slit persists as a spiracle in adult elasmobranchs and chondrosteans. In some species it houses a pseudobranch on its anterior wall.
• Functional hyoid demibranchs tend to disappear in teleosts, and the

  number of demibranchs is reduced still further in dipnoans.
• Larval gills may be external or internal. They are external in dipnoans, a

  few primitive ray-finned fishes, and amphibians. Anuran larvae later

  develop internal gills. Filamentous internal gills project to the exterior in

  larval elasmobranchs and some bony fishes.
• Gills in fishes function also in maintaining salt homeostasis and in excretion of nitrogenous wastes and carbon dioxide.
• External nares lead to blind olfactory sacs in fishes other than dipnoans.

  In dipnoans and tetrapods they are connected with the oropharyngeal

  cavity or pharynx via nasal canals. They are used for respiration in tetrapods only. Hagfishes have a nasopharyngeal duct that carries respiratory water. A nasohypophyseal duct in lampreys ends blindly.
• Internal nares (choanae) open far forward in the oral cavity of dipnoans

  and amphibians, farther caudad when there is a secondary palate. Despite the presence of internal nares, dipnoans and aquatic amphibians

  take in respiratory air by gulping it.
• Pneumatic sacs arise from the floor of the foregut in nearly all vertebrates. They are called swim bladders in fishes and lungs in tetrapods.
• Swim bladders and the lungs of aquatic urodeles are chiefly hydrostatic

  organs. They are used for respiration in dipnoans, African lungfishes,

  and a few other physostomous fishes. In some teleosts they transmit

  sound waves to the inner ear or produce sounds characteristic of the

  species.
• The glottis is the entrance to the larynx. It is protected against the entrance of fluids and solids by a fleshy valve in crocodilians, by its ability

  to lock into the nasopharynx when necessary in many mammals, and

  by an epiglottis in most mammals.
• The larynx is supported by a single pair of lateral cartilages in urodeles

  and by arytenoid and cricoid cartilages in anurans, reptiles, and birds. A

  thyroid cartilage is added in mammals. Other small cartilages develop in

  mammals.
• Vocal folds (cords) are chiefly mammalian but are found also in anurans

  and some lizards.
• Tracheal walls are reinforced by cartilaginous or bony plates, rings, or

  half rings. The trachea bifurcates into two primary bronchi. At the base

  of the trachea in most birds is an avian voice box, the syrinx.
• Paired lungs arise as an unpaired evagination from the embryonic pharyngeal floor. They occupy the pleuroperitoneal cavity in amphibians

  and most reptiles. Each lung occupies its own pleural cavity in crocodilians, birds, and mammals.
• Fibrous oblique septa separate the pleural cavities from the rest of the

  coelom in crocodilians and birds. In mammals a muscular diaphragm

  does so, and a mediastinum separates one pleural cavity from the other.
• Lungs are relatively simple sacs with shallow pocketed linings in amphibians and most snakes, more compartmentalized by septa in other

  reptiles, highly spongy because of millions of blind air pockets (alveoli)

  at the end of a branchial tree in mammals. One lung in limbless tetrapods is often rudimentary.
• Saccular diverticula of the lungs in some lizards and in birds extend

  among the viscera. In birds they also extend into hollow bones.
• Bird lungs are unlike those of other tetrapods. Inhaled air flows nonstop

  through the lungs and into air sacs. It then returns to the lungs via recurrent bronchi, flows uninterruptedly through open-ended air capillaries where gaseous exchange occurs, and is then vented.