Grade 12 Zoology Note


Respiration can be defined as biochemical process by which foods are oxidized to liberate energy. Overall respiration involves two processes.


Types of Respiration


1. Aerobic 

It occurs in all higher forms (organisms) in this type Oxygen is necessary.

C₆H₁₂O₆ + 6O₂ -------- 6CO₂ + 6H₂O + 2880 K joule .


2. Anaerobic respiration 

C₆H₁₂O₆ ----------- C₂H₅OH(ethyl alcohol) + CO₂ + 210 K joule example yeast 

C₆H₁₂O₆  ----------- C₃H₂O-3 (lactic acid)+ energy example intestinal worms 


Respiratory organs:

There is a pair of spongy conical hollow bags enclosed in pleural cavities called lungs. The plural cavities are lined by tough, flexible and transparent membranes called pleura. The plura is double walled. The outer membrane is parietal and inner membrane is visceral membrane. These membranes protect lungs and stop leaking of air into thoracic cavity. The fluid secreted by pleura reduces the friction during breathing movement. The lungs are capable of great expansion. They are inflated when filled with air. These are found in either side of heart. The right lung has three lobes and left lung has two lobes. The three lobes of right lung are right superior, right inferior and middle lobe. The two lobes of left lung are left superior and left inferior. The left lung has cardiac notch at its anteromedian border to accommodate the heart. There are about 750 millions of alveoli. These large no. of alveoli provide about 100 sq meter of surface. It is about 50 times of skin surface. Alveoli are extremely thin walled and vascular structure surrounded by capillary of network. These are the site for gases exchange.


                               Fig; Internal respiratory system


Mechanism of respiration:


Breathing is simply taking in of fresh air from atmosphere and giving out of used air from lungs. Breathing renews constantly the air present in the lungs. It is accomplished through changes in the volume and air pressure of the thoracic cavity. Change in volume and air pressure is carried out by movement of ribs, internal and external intercostals muscles, diaphragm and abdominal muscles. Breathing can be divided into inhalation and exhalation. One breath includes one inspiration or inhalation and one expiration of exhalation. The respiratory rate is the no. of breaths taken per minute. At rest, for a normal person, it is equal to 12 to 14 breaths per minute. The pulmonary air volume during breathing is measured by an apparatus called Spiro meter. 



                              Fig; Mechanism of respiration


1.Taking in of atmospheric air 

2.Contraction of external intercostal muscle or inspiratory muscle and relaxation of internal intercostals muscle.

3.Rib cage moves forward and outward 

4.Diaphragm contracts and becomes flattened shaped.

5.Increase in the volume of thoracic cavity. 

6.Decease in air pressure(below atmospheric pressure.

7.Rushing in of air through nostril into alveolar sacs causing inflation of lung.



1.Giving out of air from lungs.

2.Relaxation of external intercostals muscle and contraction of intercostal  muscle.

3.Rib cage moves downward and inward.

4.Diaphragm relaxes and become dome 

5.Decrease in volume of thoracic cavity.

6.Increase in the air pressure.

7.Expulsion of air from lungs into atmosphere causing deflation of lungs.



Exchange of gases in lungs:

The air inhaled by inspiration comes to alveoli. All around the alveoli there is a network of blood capillaries. These capillaries are extremely fine tubes with only one layered wall. The deoxygenated blood collected from different parts of the body is at first brought to the heart, and from here pumped out to the lungs. This blood which may also be called venous blood is sent to the lungs where it has to pass through the network of capillaries around the alveoli. The oxygen from the alveolus diffuses out into the blood capillary due to difference in partial pressures of oxygen and is picked up by the hemoglobin molecules present inside the red blood corpuscles. Again, carbon dioxide, which is in greater amount in the venous blood, comes from the capillary into the alveolus.


Physiology of respiration:


Physiology of respiration can be studied under following headings.


External respiration: It is uptake of O₂ and release of CO₂. It takes place in the lungs called breathing.


Transport of O2 by blood:

RBC of blood contain haemoglobin as respiratory pigment. Human blood contains nearly 150 gram of Hb per 100 ml. haemoglobin readily combines with and dissociates to form free Oxygen.

                                            Hb + O₂    →    HbO₂

The maximum carrying capacity of O2 due to hemoglobin expressed in the graph oxygen dissociation curve.




Internal respiration is tissue respiration. Digested food is oxidized to liberate energy.
Transport of CO₂:

Carbon dioxide is one of the poisonous  gas which by any means should be elimate from the body. It is slightly soluble in water convert in to carbonic acid. After the exchange of gases COeliminate mainly by 3 ways;


1.In the form of carbonic acid:

CO₂ dissolve in water of blood plasma to form carbonic acid, about 7 % of total CO₂  carried in this way. 

2.In the form of bicarbonates of Na and K:

CO₂ enters into the RBC to dissolve into water. It is catalysed by carbonic anhydrase enzyme. H₂CO₃ ionises to form bicarbonate and hydrogen. The bicarbonate ions combine with Potassium ion to form Potassium bicarbonate. Upon saturation, Potassium bicarbonate ionizes into Potassium and bicarbonate ion, These ions being at higher conc. within red cells, come out into plasma and combine with available Na ion to form Sodium bicarbonates. The loss of bicarbonate ions is balanced by chloride ions diffusing into RBC from plasma . the exchange of Cl and HCO₃ between plasma and RBC is known as Chloride shift or Hamberger’s phenomenon. In the lungs, these reactions are reversed. The exchange of Cl and HCO₃ is reversed. H₂CO₃ is formed again which break down into water and CO₃. CO₃ in exhaled. In this way about 70 % of CO₂ is transported.

3.By RBC in the form of carbamino compounds:

Amino acid present in RBC get oxidized to release amino group and carboxyl group. Carbon dioxide combines with amino group to form carbamino compounds. About 23 % is transported in this form.

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