Respiration process in mammals

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 )