i) Mechanisms of breathing vary among different groups of animals depending mainly on their habitats and levels of organisation.
ii) Lower invertebrates like sponges, coelenterates, flatworms, etc., exchange O₂ with CO₂ by simple diffusion over their entire body surface.
iii) Earthworms use their moist cuticle and insects have a network of tubes (tracheal tubes) to transport atmospheric air within the body.
iv) Special vascularised structures called gills are used by most of the aquatic arthropods and mollusks.
v) Vascularised bags called lungs are used by the terrestrial forms for the exchange of gases. Among vertebrates, fishes use gills whereas reptiles, birds and mammals respire through lungs. Amphibians like frogs can respire through their moist skin also. Mammals have a well developed respiratory system

Structure of Respiratory System

Pharynx. We have a pair of external nostrils opening out above the upper lips. It leads to a nasal chamber through the nasal passage. The nasal chamber opens into nasopharynx, which is a portion of pharynx, the common passage for food and air.
Larynx. Nasopharynx opens through glottis of the larynx region into the trachea. Larynx is a cartilaginous box which helps in sound production and hence called the sound box. During swallowing glottis can be covered by a thin elastic cartilaginous flap called epiglottis to prevent the entry of food into the larynx.
Trachea. Trachea is a straight tube extending up to the mid-thoracic cavity, which divides at the level of 5th thoracic vertebra into a right and left primary bronchi.
Bronchi. Each bronchi undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles. The tracheae, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings. Each terminal bronchiole gives rise to a number of very thin, irregular-walled and vascularised bag-like structures called alveoli.
Lungs. The branching network of bronchi, bronchioles and alveoli comprise the lungs. We have two lungs which are covered by a double layered pleura. Pleura is filled with pleural fluid. It reduces friction on the lung surface. The outer pleural membrane is in close contact with the thoracic lining whereas the inner pleural membrane is in contact with the lung surface.

The diaphragm and a specialised set of muscles - external and internal intercostals between the ribs, help in generation of such gradients. Inspiration 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, i.e., inspiration.

Relaxation of the diaphragm and the intercostal muscles returns the diaphragm and sternum to their normal positions and reduce the thoracic volume and thereby the pulmonary volume. This leads to an increase in intrapulmonary pressure to slightly above the atmospheric pressure causing the expulsion of air from the lungs, i.e., expiration.

Part (a): IRV vs ERV

Inspiratory Reserve Volume(IRV)	Expiratory Reserve Volume (ERV)
1. The maximum volume of air that can be inhaled after normal inspiration is called inspiratory reserve volume.	1. The maximum volume of air that can be forcefully exhaled after normal expiration is called expiratory reserve volume.
2. The IRV of the human lungs is about 2500 - 3500 mL.	2. The ERV of the human lungs is about 1000-1100 mL.

Part (b): Inspiratory Capacity vs Expiratory Capacity

Inspiratory capacity (IC)	Expiratory capacity (EC)
1. It is the volume air that can be inhaled after normal expiration.	1. It is the volume of air that can be exhaled after a normal inspiration.
2. It includes tidal volume and inspiratory reserve volume	2. It includes the tidal volume along with the expiratory reserve volume
3. IC = TV + IRV	3. EC = TV + ERV

Part (c): Vital capacity vs Total lung capacity

Vital capacity (VC)	Total lung capacity (TLC)
1. It is the maximum volume of air that can be exhaled after maximum inspiration.	1. It is the maximum volume of air in the lungs after maximum inspiration.
2. IC + ERV = VC = 4000 mL in humans.	2. TLC = IC + ERV + RV = 5000-6000 mL in humans.

Oxygen dissociation curve shows the relationship between percentage saturation of haemoglobin with oxygen and partial pressure of oxygen. Each haemoglobin can bind with 4 molecules of oxygen at the most. The first molecule of oxygen binds with he most difficulty. But each subsequent moleculeases the binding of nest oxygen molecule. This results in graph rising rapidly. When all the 4 oxygen molecules bind, no further binding is possible. The graph plateaus at this stage. This explains the sigmoid curve.

Transport of Carbon dioxide
CO₂ is carried by haemoglobin as carbamino-haemoglobin (about 20-25 per cent). This binding is related to the partial pressure of CO₂, pO₂ 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, i.e., CO₂which is 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.
\({ CO }_{ 2 }+{ H }_{ 2 }O\overset { Carbonic\\ anhydrase }{ \rightleftarrows } { H }_{ 2 }{ CO }_{ 3 }\overset { Carbonic\\ anhydrase }{ \rightleftarrows } { HCO }_{ 3 }^{ - }+{ H }^{ + }\)
At the tissue site where partial pressure of CO₂is high due to catabolism, CO₂ diffuses into blood (RBCs and plasma) and forms HCO₃^- and H⁺. At the alveolar site where pCO₂ is low, the reaction proceeds in the opposite direction leading to the formation of CO₂and H₂O. Thus, CO₂trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO₂, Every 100 ml of deoxygenated blood delivers approximately 4 ml of CO₂to the alveoli.

Tidal volume is the volume of air inspired or expired during normal respiration. It is about 6000 to 8000 mL of air per minute.
The hourly tidal volume for a healthy human can be calculated as:
Tidal volume = 6000 to 8000 mL/minute
Tidal volume in an hour = 6000 to 8000 mL × (60 min)
= 3.6 × 10⁵ mL to 4.8 × 10⁵ mL
Therefore, the hourly tidal volume for a healthy human is approximately 3.6 × 10⁵ mL to 4.8 × 10⁵ mL.

Partial Pressure of Gases Compared to Atmospheric Pressure
Respiratory Gases	Atmospheric Air	Alveoli	Blood (Deoxygenated)	Blood (Oxygenated)	Tissue
O2	159	104	40	95	40
CO	0.3	40	45	40	45

Alveoli are the primary sites of exchange of gases. Exchange of gases also occur between blood and tissues. O₂ and CO₂ are exchanged in these sites by simple diffusion mainly based on pressure/ concentration gradient. Solubility of the gases as well as the thickness of the membranes involved in diffusion are also some of the 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.