Vertebrate lungs are designed for air
breathing . Lungs are elastic bags that lie within the body. Their volume
expands when air is inhaled and decreases when air is exhaled .
Air
ventilation : Aspiration pump
( Fig – 1 Air
breathing amniotes : Aspiration pump . In most amniotes , the buccal
cavity has little to do with forcing air in or out of the lungs . Instead a rib
cage expands and compress and / or a diaphragm moves forward and back within
the body cavity to create a positive pressure that expels air or negative
pressure that draws air into the lungs . )
The aspiration
pump is a third type , after dual and buccal pumps , that does not push air
into the lung against a resisting force . Rather air is sucked in , or
aspirated , by the low pressure created around the lung ( fig – 1 ). The lungs are
located within the pump so that the force required to ventilate them is applied
directly . The pump includes the rib cage and often a muscular diaphragm . A
movable diaphragm in the thorax causes pressure changes rather than the action
of the buccal cavity . The diaphragm
like a plunger , alters the pressure on the lungs to favor entry or exit of air
.
( Fig – 2 Unidirectional
and bidirectional flow . (a) in fishes and many aquatic
amphibians , water movement is unidirectional because water flow through the
mouth , across the gill curtain , and out the lateral gill chamber . (b) In many air – breathing vertebrates
, air flows into the respiratory organ and then reverses its direction to exit
along the same route, creating a bidirectional or tidal flow.)
The
aspiration pump is bidirectional and moves air tidally . It is found in
amniotes – reptiles , mammals and birds .
Ventilation
mechanism of mammals :
Ventilation
, or breathing , is the active process of moving the respiratory medium , water
or air , across the exchange surface .
An aspiration pump ventilates the lungs of the mammals . Changes in the shape of the rib cage and
piston like action of a muscular diaphragm contributes this pumping mechanism. The
diaphragm consists of crural , costal , and sternal parts , all of which
converge on a central tendon ( Fig – 3 )
(Fig – 3 : (a)
Location of lungs and diaphragm within the rib cage of the dog (lateral view ).
(b) ventral view of the diaphragm ,
which lies behind the lungs and has a dome shape. Notice the opening that allow
anterior – posterior passage of the aorta, esophagus, and postcava .
Superficial (c) and deep (d) muscle of the rib cage
The
diaphragm of mammals lies anterior to the liver , and acts directly on the
pleural cavities in which the lungs reside ( Fig – 3 , (a) , (b). Intercostal muscles
run between the ribs. The transversus abdominies
, serratus , and rectus abdominies that are inserted on the ribs and originate
outside the rib cage ( Fig – 3 (c) , (d) . All aid in the mammalian
ventilation.
Ventilation
through lung :
Mammalian ventilation involves the
rib cage and diaphragm .
Upon inhalation , the external intercostal muscle contract to
rotate the adjacent ribs and medial sternum forward . because the ribs are
bowed in shape , this rotation includes an outward as well as a forward swing
of each arched rib . Thus the rib cage
expand around the lungs . contraction of the dome – shaped diaphragm it to
flatten , further enlarging the thoracic cavity . The elastic lungs expand to
fill the enlarged thoracic cavity , and air is drawn in ( Fig – 4 (a) (b)
During active exhalation , internal
intercostal muscle slant in the opposite direction of the relaxed external intercostal
and pull the rib back . Relaxation of
the diaphragm causes it to recoil and resumes its arched , dome shape . Rib
retraction and diaphragm relaxation decrease chest volume , forcing air from
the lungs
( Fig – 4 Rib
cage movement in mammals . (a) various muscles run between
adjacent ribs at slanted angles. (b) during
inhalation , external intercostal contract , causing adjacent ribs to be drawn
forward, expanding the pleural cavities around the lungs , and aspirating air
into them. (c) Exhalation is often
passive. Gravity pulls the ribs down , compressing the lungs and expelling air.
During vigorous respiration , exhalation
may be active. When this occurs, internal intercostals , slanted in an opposite
direction , contract to compress the rib cage.
Gas
exchange of mammals :
In mammals , the sites of
respiratory exchange are reached via a different route . The respiratory
passageway ( including trachea , bronchi , bronchioles ) repeatedly divides ,
producing smaller and smaller branches until they finally terminate in blind
ended compartments , the alveoli , which characterize the respiratory bronchioles
and air sac (fig – 5) . The trachea, bronchi and terminal bronchioles that
transport gas to and from the alveoli are called the respiratory tree in
recognition of their branching patterns . Gas exchange occurs in the
bronchioles and alveoli .
In mammals ,
the total alveolar area is extensive, perhaps over ten times that of amphibians
of similar mass . such a large exchange area is essential in mammals to sustain
the high rate of oxygen uptake required by an active endotherm
( Fig – 5 : (a) the
trachea leads to the pleural cavities and branches in to the bronchi to supply
left and right lung . Repeated bronchial branching produce smaller and
smaller bronchioles that eventually leads to alveolar sac. (b) Enlarged alveolar sac , arteries
and veins supply the alveoli to accommodate gas exchange within them. (c) internal subdivision of the
alveolar sacs are shown . Each small compartments in an alveolus where actual
respiratory exchange between blood and air occurs . Note the smooth muscle
bands at the opening .
The alveoli are rich with capillaries , called
alveolar capillaries . Here the red blood cells absorb oxygen from the air and
then carry it back in the form of oxyhemoglobin , to nourish the cells . The
red blood cells also carry carbon dioxide away from the cells in the form of carboxyhemoglobin
and releases it into the alveoli through
the alveolar capillaries . When the diaphragm relaxes , a positive pressure is
generated in the thorax and air rushes out of the alveoli expelling the carbon
dioxide. ( Fig – 5 )
(Fig – 5 Gas
exchange between alveoli and capillaries )
