Respiratory (Manual)

• In mammals, the ribs, intercostal muscles, abdominal muscles, and diaphragm all aid in respiration.
• Contraction of the diaphragm and the external intercostal muscles enlarges the pleural cavity, and air enters the lungs. Relaxation of the diaphragm and the ribs muscles causes expiration, which is therefore passive.
• In reptiles the force pump method of respiration is replaced by costal respiration, which involves the use of ribs.
• Costal respiration is also used to some extent in birds, where expansion of the thoracic cage is produced by the action of the intercostal muscles, which move the ribs forward and outward, while the sternum is slightly depressed.
• Ventilation of the lungs in amphibians such as the frog largely involves movement of the floor of the buccal cavity. Air is habitually moved into the buccal cavity, when the glottis is closed, by relatively small movements of the buccal floor.
• T or F. Lungs evolved from the swim bladder.

  False. It has evolved in environments that were prone to periods of drought.
• Physostomous: swim bladder has retained connection with the pharynx via pneumatic duct

  Physocleistous: swim bladder is a separate structure
• The gills that form on the pharyngeal pouches are internal gills and are common in aquatic gnathostomes.
• External gills


  Arise as filamentous outgrowths of the superficial epithelium of the gill arches
• External gills. 


  They are found in the larvae of lungfishes and some actinopterygians.
• External gills are also common in amphibian larvae; in some cases they are retained in the adult and as such are regarded as paedomorphic characters- larval characters that persist in an otherwise adult animal.
• When the water is well oxygenated the gills are small and retracted, whereas in water with little oxygen the gills may be large and extended.
• The internal gills or branchiae typically consist of a double row of gill filaments.
• Gills

  The spaces between successive filaments are not simple, since the upper and lower surfaces of the filaments are thrown into a series of folds or secondary lamellae
• The secondary lamellae have thin epithelial walls separated by specialized pillar or pilaster cells.
• The presence of filaments and lamellae greatly increases the gill surface available for gaseous exchange.
• A gill arch with lamellae on both anterior and posterior faces is a holobranch, that with lamellae on one face only is a hemibranch.
• In sharks, which usually have six gill slits, the last gill is a hemibranch since it lacks lamellae on its posterior surface.
• The hyoid gill, which is invariable reduced in size to form a spiracle, is also a hemibranch.
• In addition the spiracle receives oxygenated blood from a vessel that has already passed through a gill; this is therefore not usually considered a "true gill" and is termed a pseudobranch.
• In many teleosts the pseudobranch is totally without lamellae and becomes a glandular structure.
• Pseudobranch is concerned with the production of the enzyme carbonic anhydrase, which is important in the release of gases into the swim bladder
• The pseudobranch may also be involved in salt excretion and the production or activation of a hormone, since experimental removal of the gland may cause the skin to darken.