(i) It explains the external respiration i.e exchange of air between the atmosphere and the lungs

(ii) It is also called as ventilation or breathing
(iii) Phases of external respiration
(a) Inspiration: It is the movement of atmospheric air into the lungs
(b) Expiration: It is the movement of air of alveoli of lungs which diffuses out of lungs
(iv) No muscle fibers in lungs
(v) The ribs and diaphragm promote the expansion and contraction of lungs
(vi) A diaphragm is a sheet of tissue which separates the abdomen into an upper thoracic and lower abdominal cavities
(vii) Under a relaxed state, the diaphragm is dome shaped
(viii) The movements of ribs are brought by the intercostal muscles
(ix) The creation of pressure gradients is performed by external and internal intercostals muscles found between the ribs. This event increases or decreases the volume of thoracic cavity horizontally
(x) The movements of diaphragm increases or decreases the volume of thoracic cavity vertically or perpendicularly
(i) Occurrence of Inspiration
(i) Inspiration occurs if the pressure inside the lungs called intrapulmonary pressure is lesser than that of atmospheric pressure. So the pressure in thoracic cavity is low but the atmospheric pressure is high
(ii) This condition allows the atmospheric áir to reach the lungs
(ii) Occurrence of Expiration
(i) Expiration occurs when the pressure within the lungs is higher than that of the atmospheric pressure. So the pressure in thoracic cavity is high but the atmospheric pressure is low.
(ii) This condition allows the alveolar air from the lungs to reach the atmosphere
Event in inspiration:
(i) Contraction of the diaphragmatic muscles and external intercostal muscles initiate inspiration
(ii) These events pull the ribs and sternum upwards and outwards to increase the volume of thoracic cavity in the dorso - ventral axis
(iii) The lungs are forced to expand (flattening of lungs) to increase the pulmonary volume
(iv) Increased pulmonary volume and decreased intrapulmonary pressure promote the process of inspiration
(v) Decreased intrapulmonary pressure forces air from outside to enter the air passages into lungs to equalize the pressure. This is called inspiration
Events in Expiration:
(i) Relaxation of diaphragmatic muscles and sternum to return to its dome shape
(ii) Diaphragm reaches its normal position (contacts with the pleura of lungs)
(iii) The contraction of internal intercostal muscles pull the ribs downwards to reduce both the thoracic and pulmonary volumes.
(iv) The intrapulmonary pressure increases slightly than that of atmospheric pressure
(v) Air is expelled out of alveoli and alveoli are emptied (lungs deflate) this process is called as expiration
(vi) On average, a healthy human breathes 12-16 times/minute
(vii) Spirometer is used to measure the volume of air involved in breathing movements for clinical assessment of a person's pulmonary function.

Inspiration

Expiration

Respiratory centre initiates the stimuli during inspiration                                 \(\downarrow\) Impulses are carried to the inspiratory muscles through nerves.                                 \(\downarrow\) Diaphragm and inspiratory muscles contract                                 \(\downarrow\) The intrapulmonary pressure is reduced                                 \(\downarrow\) The alveolar pressure increases than the atmospheric pressure.                                 \(\downarrow\) Air flows into the alveoli until the alveolar pressure equalizes the atmospheric pressure and the alveoli get inflated

Respiratory centre terminates the stimuli during expiration                      \(\downarrow\) The diaphragm and inspiratory muscles relax                      \(\downarrow\) Chest wall contracts and the thoracic volume gets reduced.                      \(\downarrow\) The intrapulmonary pressure is reduced.                      \(\downarrow\) The alveolar pressure increases than the atmospheric pressure.                       \(\downarrow\) Air is sent out due to the contraction of alveoli                       \(\downarrow\) Air flows into the alveoli until the alveolar pressure equalizes the atmospheric pressure and the alveoli get inflated

(i) It is a transportary/chemical/blood respiration.
(ii) It occurs between alveoli and tissue fluid.

(iii) It mainly occurs in two steps
(A) Transport of O₂
(B) Transport of CO₂
(A) Transport of O₂
(i) About 97% of O₂ is transported by respiratory pigment haemoglobin which has more affinity with O₂ to form HbO₂ and poorly soluble in blood plasma.
(ii) HbO₂ is formed firstly in the blood of pulmonary veins.
(iii) About 3% of O₂ is transported by blood plasma under dissolved condition.
(iv) The rate at which haemoglobin binds with O₂ is regulated by the pO₂ (95 mm).
(v) Each haemoglobin contains four haem ions therefore four O₂ molecules are transported.

Oxygen dissociation curve
(i) In the alveoli pO₂ is high (104 mm) and PCo₂ is low (40 mm)
(ii) In the alveoli the temperature is low and concentration of H⁺ is also less. This conditions promote the formation of HbO₂ (Hb+ O₂→ HbO₂
(iii) In the tissues the pO₂ is low (30 mm) and the pCO₂ is high and high temperature promotes the dissociation of O₂ from haemoglobin

(iv) Sigmoid (s-shaped) curve appears in oxygen dissociation curve. It is formed when % saturation of haemoglobin with O₂ is plotted against pO₂. This is called an oxygen haemoglobin dissociation curve.
(v) This curve appears in tissue and it is a steep slope for pO₂ values between 10 and 50 mm Hg and then flattens between 70 and 100 mm Hg.
(vi) Under normal physiological state every 100 mL of oxygenated blood can deliver about 5 mL of O₂ to the tissues.
(B) Transport of CO₂
(i) CO₂ is transported by
(a) Blood plasma: under dissolved state about 7-10% of CO₂ is transported by plasma as CO₂ is more soluble in blood plasma water.
(b) Bound with haemoglobin: about 20-25% of dissolved CO₂ is bound and carried in the RBCs as HbCO₂ (carbaminohaemoglobin).

(ii) Hb + CO₂ occurs in tissues to form HbCO₂
(iii) HbCO₂ → Hb + CO₂ occurs in alveoli of lungs
(C) As bicarbonate (HCO^-₃) in plasma about 70% of CO₂ is transported as HCO^-₃ ions
(iv) The formation of HCO^-₃ ions is influenced by pCO₂ and the degree of haemoglobin oxygenation with ferrous ions.
(v) RBCs contain a high concentration of the enzyme, carbonic anhydrase but a small quantity of this enzyme is also found in plasma of blood.
(vi) At the tissues the pCO₂ is high due to catabolism and diffuses into blood to form HCO^-₃ and H⁺ ions.
(vii) The CO₂ diffuses into RBCs and forms HbCO₃ and into plasma water to form H₂CO₃ (carbonic acid).
(viii) H₂CO₃ is unstable and it is catalysed by carbonic anhydrase.

(ix) The HCO^-₃ moves quickly from the RBCs into plasma. From here it is transported to the lungs.
(x) At the alveolar site where pCO₂ is low, the reaction is reversed leading to the formation of CO₂ and H₂O.
(xi) Thus CO₂ trapped as HCO^-₃ at the tissue level is transported to the alveoli and released out as CO_2.
(xii) Every 100 mL of deoxygenated blood delivers 4 mL of CO₂ to the alveoli for elimination.

(i) Asthma: It is characterized by narrowing and inflammation of bronchi andbronchioles and difficulty in breathing. Common allergens for asthma are dust, drugs, pollen grains, certain food items like fish, prawn and certain fruits etc.
(ii) Emphysema: Emphysema is chronic breathlessness caused by gradual breakdown of the thin walls of the alveoli decreasing the total surface area of a gaseous 'exchange. i.e., widening of the alveoli is called emphysema. The major cause for this disease is cigarette smoking, which reduces the respiratory surface of the alveolar walls.
(iii) Bronchitis: The bronchi when it gets inflated due to pollution smoke and cigarette smoking, causes bronchitis. The symptoms are cough, shortness of breath and sputum in the lungs.
(iv) Pneumonia: Inflammation of the lungs due to infection caused by bacteria or virus is called pneumonia. The common symptoms are sputum production, nasal congestion, shortness of breath, sore throat, etc.
(v) Tuberculosis: Tuberculosis is caused by Mycobacterium tuberculae. This infection mainly occurs in the lungs and bones. Collection of fluid between the lungs and the chest wall is the main complication of this disease.
(vi) Occupational respiratory disorders: The disorders due to one's occupation of working in industries like grinding or stone breaking, construction sites, cotton industries, etc. Dust produced affects the respiratory tracts. Long exposure can give rise to inflammation leading to fibrosis. Silicosis and asbestosis are occupational respiratory diseases resulting from inhalation of particle of silica from sand grinding and asbestos into the respiratory tract. Workers, working in such industries must wear protective masks.

The human respiratory system constitutes the conducting zone and the respiratory zone. The conducting zone includes the external nostril, nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles and the lungs which contain alveoli. The parts from the external nostrils up to the bronchioles conduct the air and hence they are called the conducting zone. This zone humidifies and warms the incoming air. The nasal cavity has fine hairs and mucus which filter dust particles in the incoming air. At the junction of the pharynx and larynx there is a flap called epiglottis. This closes the respiratory tract while swallowing thus presenting the entry of food particles into the  rachea and choking. The ciliated epithelial cells of trachea, bronchi and bronchioles secrete-mucus rich in glycoprotein. Microorganism and dust particles attach in the mucus films and are carried upwards to pass down the gullet during normal swallowing. The trachea is a semi-flexible tube supported by multiple cartilaginous rings which expands upto the mid thoracic cavity. And the level of the 5th theoretic vertebra, it divides into right and left bronchi which divide into secondary and tertiary bronchi and further divide into terminal bronchioles and respiratory bronchioles. Bronchi have 'c' shaped curved Cartilage plates to ensure that the air passage does not collapse as air pressure changes during breathing. The bronchioles do not have cartilaginous rings but they have rigidity to prevent them from collapsing but are surrounded by smooth muscle which contracts or relaxes to adjust the diameter of these airways. The fine respiratory bronchioles terminate into highly vascularised thin-walled-pouch like air sacs called alveoli. It is made up of thin squamous epithelial cells, the endothelium of the alveolar capillaries and the basement substance are found in between them. The thin squamous epithelial cells of the alveoli, the endothelium of the alveolar capillaries and the basement substance found in between them. The thin squamous epithelial cells of the alveol! are composed of Type I and Type II cells. Type I cells are very thin so that gases can diffuse rapidly through them. Type II cells are thicker, synthesize and secrete a substance called Surfactant. The lungs are light spongy tissues endorsed in the thoracic cavity bound dorsally by the vertebral column and ventrally by the sternum, laterally by the ribs and on the lower side by the dome shaped diaphragm. The lungs are covered by double walled pleural membrane containing several layers of elastic connective tissues and capillaries. It encloses the pleural fluid. It reduces friction when the lungs expand and contract.