THE NASAL CAVITY

     In most fish, there is no connection between the nasal cavity and the oral cavity.  This is true of lampreys, most cartilaginous fish, and virtually all actinopterygians.  There are a few exceptions.  For example, in hagfish, a nasopharyngeal pouch (present in lampreys) communicates with the pharynx.  In some cartilaginous fish, there are oronasal grooves which connect the nasal cavity with the mouth.  In the teleost genus Astroscopus, there is a channel which connects the nasal and oral cavities (Weichert, 1970).    Sarcopterygians evolved a connection between the nasal cavity and the oral cavity, called an internal naris (Romer, p. 325; Weichert, 1970).  In none of these fish, not even the lungfishes, are the nostrils used to breathe (Weichert, 1970).

     Tetrapods utilize their nasal cavity for the intake of air.  In reptiles, the length of the nasal cavity is typically longer than in amphibians.  Crocodiles, like mammals, developed a secondary palate in which the nasal and oral cavities fuse are separated by shelves of bone (Weichert, 1970). A hard and soft palate separate the air of the nasal cavity from food in the oral cavity (Romer, p. 327).

LUNGFISH

CAT

LARYNX

     Amphibians evolved a intrapulmonary duct and amniotes evolved cartilaginous support of this duct (Perry, 2004).  Amphibians may possess a short trachea and larynx as well as the cricoid and arytenoids cartilages of the larynx.  Mucus membrane folds in the larynx form vocal cords and the pitch of the sounds produced is increased with additional tension on these folds.  These are not considered to be homologs of mammalian vocal cords (Romer, p. 370; Dutta, p. 246). 

     In reptiles the hyoid apparatus became more closely associated with the larynx.  Only mammals and crocodiles possess a thyroid cartilage of the larynx.  In some lizards, mucus membrane folds may represent the early stages the epiglottis of mammals. (Weichert, 1970, p. 230-1; Gauthier, 1988). The thyroid cartilages fuse in marsupials and placentals but not in monotremes (Weichert, 1970, p. 231).  Some mammals possess additional corniculate and cuneiform cartilages.  Vocal cords evolved in mammals (Romer, p. 370).  Elephants lack false vocal cords while hippos lack true vocal cords.   Howler monkeys have a larynx which is modified to form a resonating chamber (Weichert, 1970).

     In human infants, the larynx and hyoid bone are located in a position similar to that observed in other mammals although both structures subsequently descend.  The resulting positions of the larynx, hyoid and tongue and the shape of the pharynx enable human speech.  This change seems to have evolved in two steps.  The first step, which occurred in the last common ancestor of chimps and humans, was the descent of the larynx.  The descent of the hyoid occurred in human ancestors, perhaps in conjunction with the changes in the jaw or cranial flexion which also occurred in the lineage.  While this allowed for speech, it had the disadvantage of increasing the risk of choking on food (Nishimura, 2003; Kardong, p. 496).

TURTLE

OPOSSUM

CAT

SHEEP

GOAT

MONKEY

HUMAN MODEL

TRACHEA

     In amphibians, the trachea is extremely short.  This is especially true in frogs but the trachea may reach a few cm in length in some salamanders.  In amniotes, the trachea is longer.  Partial rings of cartilage exist around the trachea in some amphibians (such as caecilians).  Snakes may have fused tracheal cartilages. (Weichert, 1970, p. 233).  Some lemurs have complete rings in the upper portion of the trachea (Hartman).  Many birds have ossified rings in their trachea and penguins have a double trachea. 

     Bronchi developed in amniotes.  In some snakes, only the right bronchus and lung are present. (Weichert, 1970, p. 233).  In pigs, whales, and other ruminants a third primary bronchus (the apical or eparterial bronchus) forms.  Mammalian bronchi are supported by cartilage. (Weichert, 1970, p. 235).

TURTLE

ALLIGATOR

CHICKEN

SHEEP

PIG

CAT

MONKEY

HUMAN MODEL

PULMONARY VENTILATION AND HEMOGLOBIN

     More active teleost fish generally have a larger surface area in their gills.  Tuna, which are very active and can even perform some thermoregulation, have a respiratory surface area which approaches the lung surface area in mammals.  They also have blood hemoglobin levels (20g/100 ml blood) which are higher than most fish (Hoar, 1970, p. 270-1).  Fish which either swim rapidly or live in rapidly moving water simply allow water to pass over their gills to obtain oxygen.  Other fish must actively ventilate their gills.  Thus, the energy spent in respiration can vary greatly in fish. (Hoar, 1970)

     Hypoxia leads to increase in hematocrit in fish as it does in mammals. O₂ dissociation curves similar in fish and mammals (Hoar, 1970).  Even in an invertebrate (such as in a worm), the hemoglobin saturation curve has a steep slope and flat upper portion as in higher animals (Hoar, 1983, p. 533).  Hagfish hemoglobin shows no Bohr effect and has a lower oxygen affinity than lamprey hemoglobin (Hoar, 1970).

RESPIRATORY MUSCLES

      The contraction of a number of muscles can allow for greater gas exchange at respiratory surfaces.  Fish depend on the muscles which act on the buccal cavity to drive respiration at the gills.  The same muscles of the buccal cavity are used to drive respiration in both lungfishes and amphibians. (Johansen, Kjell from Hoar, 1970).  Lungfish also use skeletal muscle and aspiratory forces help venous blood return to the heart, as in tetrapods (Hoar, 1970).  

     Amphibians do not use the muscles between the ribs (intercostals muscles) to facilitate respiration.  Because the ribs of amphibians do not contact the sternum as in amniotes, ventilating the lungs cannot be accomplished by changing the volume of the thoracic cavity.  The inefficiency of the resultant amphibian respiration is perhaps best illustrated in the loss of the lungs in the very successful family of salamanders Plethodontidae which perform all gas exchange through their skin.   In amniotes, the ribs attach to the sternum and allow the ventilation of the lungs (Romer, p. 366-70).  Intercostal and abdominal muscles are used in breathing to create negative pressure to inflate the lungs (Kardong, p. 421-2).  With a more efficient pulmonary respiration, cutaneous respiration was reduced and the skin of amniotes became less permeable (to limit water loss).

     Although crocodiles do not possess the mammalian diaphragm, they do possess a transverse septum which separates the pleural cavities from the peritoneum.  Crocodiles also possess a muscle, the diaphragmaticus, which can pull on this septum to expand the pleural cavities. (Webster, 1974, p. 381). 

Note the absence of a diaphragm in the following animals

FROG

ALLIGATOR

CHICKEN

While muscles of the body wall can be used in respiration in amphibians, the use of negative pressure for inhalation is unique to amniotes (Perry, 2004).  While only mammals possess a completely muscularized diaphragm which separates the thoracic and abdominopelvic body cavities, septa and the presence of some muscle also exists in at least some crocodiles, birds, turtles, lizards, and snakes (Gauthier, 1988).

     In mammals, the evolution of the diaphragm allowed the thorax to increase its volume further and thus allow for the inhalation of more air.  Mammals also lost their lumbar ribs so that the organs of the abdominal cavity could be pushed outward when the diaphragm pushed downward.  These modifications for greater gas exchange were important as mammals evolved a higher metabolism (Romer, p. 368).

PIG