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Define vital capacity. What is its significance?

The vital Capacity (VC) of a person during respiration is the maximum volume of air a person can breathe in after a forced expiration or the maximum volume of air a person can breathe out after a forced inspiration. Vital capacity includes ERV (Expiratory Reserve volume), TV (total volume), IRV (Inspiratory Reserve Volume).


State the volume of air remaining in the lungs after a normal breathing.

The average volume of air remaining in the lungs after a normal breathing is called Functional Residual Volume. This includes ERV (expiratory reserve volume) + (RV) reserve volume.


Diffusion of gases occurs in the alveolar region only and not in the other parts of respiratory system. Why?

Diffusion of gases mainly takes in the alveoli, as it is the primary site of exchange of gases. Exchange of gases also occurs between blood and tissues. O2 and CO2 are exchanged in these sites by simple diffusion mainly based on pressure or concentration gradient. Solubility of the gases as well as the thickness of the membranes involved in diffusion are also some important factors that can affect the rate of diffusion.

Pressure contributed by an individual gas in a mixture of gases is called partial pressure and is represented as pO2 for oxygen and pCO2 for carbon dioxide.

Number blood capillaries cover each alveolus. The capillaries that come from the pulmonary artery bring deoxygenated blood and blood capillaries, which leave the alveoli to form the pulmonary vein, carry oxygenated blood.

The partial pressure of O2 in the alveoli is 104 and in oxygenated blood it is 95 due to this variation diffusion of oxygen takes place from the alveoli to the capillaries. The partial pressure of CO2 in the alveoli is 40 and deoxygenated blood is 45 due to this variation in pressure diffusion of CO2 takes place from the capillaries to the alveoli.

These diffusions take place in the alveoli because of the presence diffusion membrane. The diffusion membrane is made up of three major layers namely, the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries and the basement substance in between them. However, its total thickness is much less than a millimetre. Since all the factors in our body are favourable for diffusion of O2 from alveoli to tissues through the blood capillaries and that of CO2 from tissues to alveoli through the blood capillaries diffusion takes place only in the alveoli of the respiratory system.


What are the major transport mechanisms for CO2? Explain.

The transport mechanism of CO2 takes place mainly by the haemoglobin. The carbon dioxide dissolved in the blood combines with the haemoglobin to form carbamino-haemoglobin (about 20-25 per cent), which is carried to the alveoli from the tissue. This binding is related to the partial pressure of CO2. Partial pressure of O2 is a major factor, which could affect this binding. When pCO2 is high and pO2 is low as in the tissues, more binding of carbon dioxide occurs whereas, when the pCO2 is low and pO2 is high as in the alveoli, dissociation of CO2 from carbamino-haemoglobin takes place. During the dissociation of carbamino-haemoglobin the CO2 bound to haemoglobin from the tissues is delivered at the alveoli.

RBCs contain a very high concentration of the enzyme, carbonic anhydrase and minute quantities of the same is present in the plasma too. This enzyme facilitates the following reaction in both directions.

CO2 + H2O H2CO3 HCO-3 + H+

In the above reaction CO2 combines with H2O in the presence of carbonic anhydrase to form H2CO3 this is futher split into to HCO3 - +H+ .

At the alveolar site where pCO2 is low, the reaction proceeds in the opposite direction leading to the formation of CO2 and H2O. Thus, CO2 trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO2.

By these methods every 100 ml of blood transports about 4ml of carbon dioxide to the alveoli to be exhaled.


What will be the pO2 and pCO2 in the atmospheric air compared to those in the alveolar air ?


(i) pO2 lesser, pCO2 higher

(ii) pO2 higher, pCO2 lesser

(iii) pO2 higher, pCO2 higher

(iv) pO2 lesser, pCO2 lesser


(ii) The partial pressure of oxygen higher, and carbon dioxide lesser in the atmospheric pressure when compared with the alveolar air.


Explain the process of inspiration under normal conditions.

The process of inspiration under normal conditions is initiated by the contraction of diaphragm, which increases the volume of thoracic chamber in the antero-posterior axis. The contraction of external inter-costal muscles lifts up the ribs and the sternum causing an increase in the volume of the thoracic chamber in the dorso-ventral axis. The overall increase in the thoracic volume causes a similar increase in pulmonary volume. An increase in pulmonary volume decreases the intra-pulmonary pressure to less than the atmospheric pressure, which forces the air from outside to move into the lungs, this process is called inspiration.


How is respiration regulated?

In the brain the medulla oblongata contains a respiratory center, which controls the breathing mechanisim. The respiratory center consists of a specialized inspiratory center and an expiratory center. The centre is bilateral and its two halves which are connected together by commiseural neurons. The sides of this centre are connected with motor respiratory neurons. The nerve cells of the centre are connected with the breathing apparatus forming a reflex arc. These nerve cells are sensitive to chemical composition of blood. There is another center present in the pons region of the brain called pneumotaxic center this functions in moderating the functions of the respiratory rhythm center. The axons from the nerve cells of these centers lead to the intercostals muscles through the intercostals nerves and to the diaphragm via the phrenic nerves. These nerve fibers transmit impulses to the external intercostals muscels and internal intercostals muscles alternately. The walls of the alveoli have sensory endings, which are stimulated by changes in the tension of the alveolar walls. The streaching of alveoli send stimuli to the expiratory center of the medulla through the vagus nerve, which inhibits further respiration. This sequence of events is called Herring – Breuer reflex.


What is the effect of pCO2 on oxygen transport?

The partial pressure of carbon dioxide plays a major role in the transport of oxygen. In the alveoli, where there is high pO2, low pCO2, lesser H+ concentration and lower temperature, the factors are all favourable for the formation of oxyhaemoglobin, with the formation of oxyhaemoglobin the oxygen molecule is transported to the tissue whereas in the tissues, where low pO2, high pCO2, high H+ concentration and higher temperature exist, the conditions are favourable for dissociation of oxygen from the oxyhaemoglobin. Thus a decrease in the partial pressure of carbon dioxide helps in the oxygen transport.


What happens to the respiratory process in a man going up a hill?

On mountains about 8000ft. from the sea level one feels difficulty in breathing due to considerable decrease in the oxygen in the air. Consequently, the alveolar pO2 reduces and oxygen cannot be diffused into the blood. Thus, the process of oxygenation of blood is progressively gets decreased to such an extent that the person feels mountain sickness. It produces a number of symptoms like breathlessness, headache, dizziness, nausea, vomiting and a bluishness of skin, nails and lips. This is because of lack of adequate oxygen. People suffering from heart ailment or blood pressure should not go to such a height.


What is the site of gaseous exchange in an insect?

In insects the site of gaseous exchange is a network of tubes called tracheal tubes, which transport atmospheric air within the body.


Define the oxygen dissociation curve. Can you suggest any reason for its sigmoidal pattern?

The relationship between oxygen tension and its absorption by haemoglobin produces a graph called the oxygen dissociation curve. At about 100mm Hg O2 tension Hb is 98% saturated. As it falls, the saturation of Hb decreases slowly. When O2 tension is about 40 mm Hg, Oxyhaemoglobin dissociates and oxygen is available to the tissues.


Have you heard about hypoxia? Try to gather information about it, and discuss with your friends.

Hypoxia is a shortage of oxygen in the body. It can be reduction of oxygen specifically in the blood. Environmental hypoxia is a condition where in high altitudes such as mountains the partial pressure of oxygen is less which leads to hypoxia.


Distinguish between


(a) IRV and ERV

(b) Inspiratory capacity and Expiratory capacity.

(c) Vital capacity and Total lung capacity.






The additional volume of air, which a person can forcibly inspire, is called inspiratory reserve volume.

The additional volume of air, which a person can forcibly expire, is called expiratory reserve volume.


Inspiratory capacity

Expiratory capacity

Total volume of air a person can inspire after a normal expiration. Total volume of air a person can expire after a normal inspiration.
This includes tidal volume and inspiratory reserve volume (TV + IRV). This includes tidal volume and expiratory reserve volume (TV + ERV).

Vital capacity

Total lung capacity

The maximum volume of air a person can breathe in after a forced expiration. Total volume of air accommodated in the lungs at the end of a forced inspiration. 
This includes ERV, TV and IRV or the maximum volume of air a person can breathe out after a forced inspiration. This includes RV, ERV, TV and IRV or vital capacity + residual volume.



What is Tidal volume? Find out the Tidal volume (approximate value) for a healthy human in an hour.

Tidal is the volume volume of air breathed in and out during effortless respiration. The tidal volume of a healthy man in an hour is approximately 360000 to 480000 ml.

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