1. The thin squamous epithelial cells of the alveoli are composed of Type I and Type II cells.
2. Type I cells are very thin so that gases can diffuse rapidly through them.
3. Type II cells are thicker, synthesize and secrete a substance called Surfactant.
4. Surfactants are the thin non-cellular films made of protein and phospholipids covering the alveolar membrane.
5. The surfactant lowers the surface tension in the alveoli and prevents the lungs from collapsing.
6. It also prevents pulmonary oedema.
7. Premature Babies have low levels of surfactant in the alveoli may develop the newborn respiratory distress syndrome (NRDS) because the synthesis of surfactants begins only after the 25th week of gestation.

Transport of Carbon dioxide: Blood transports CO2 from the tissue cells to the lungs in three ways,
(i) Dissolved in plasma: About 7-10% of CO₂ is transported in a dissolved form in the plasma.
(ii) Bound to haemoglobin: About 20-25% of dissolved CO₂ is bound and carried in the RBCs as carbaminohaemoglobin (HbCO₂)
CO₂ + Hb \(\leftrightarrows \) Hb CO₂
(iii) As bicarbonate ions in plasma: About 70% of CO₂ is transported as bicarbonate ions.
1. This is influenced by pCO₂ and the degree of haemoglobin oxygenation. RBCs contain a high concentration of the enzyme, carbonic anhydrase, Whereas small amounts of carbonic anhydrase is present in the plasma.
2. At the tissues the pCO₂ is high due to catabolism and diffuses into the blood to form HCO₃^-and H⁺ ions. When CO₂ diffuses into the RBCs, it combines with water forming carbonic acid (H₂CO₃) catalyzed by carbonic anhydrase. Carbonic acid is unstable and dissociates into hydrogen and bicarbonate ions.
3. Carbonic anhydrase facilitates the reaction in both directions.
CO₂ + H₂O  H₂CO₃
                          HCO₃^-  +  H⁺ 
4. The HCO₃^- moves quickly from the RBCs into the plasma, where it is carried to the lungs. At the alveolar site where pCO₂ is low, the reaction is reversed leading to the formation of CO₂ and water. Thus CO₂ trapped as HCO₃^- at the tissue level it is transported to the alveoli and released out as CO_2.
5. Every 100mL of deoxygenated blood delivers 4mL of CO₂ to the alveoli for elimination.  

Respiratory Volumes - Tidal Volume (TV): Tidal volume is the amount of air inspired or expired wit each normal breath. It is approximately 500 mL., i.e. a normal human adult can inspire or expire approximately 6000 to 8000mL of air per minute. During vigorous exercise, the tidal volume is about 4 -10 times higher.
Inspiratory Reserve volume (RV): Additional volume of air a person can inspire by forceful inspiration is called Inspiratory Reserve Volume. The normal value is 2500 to 3000 mL.
Expiratory Reserve volume (ERV): Additional volume of air a person can forcefully exhale by forceful expiration is called Expiratory Reserve Volume. The normal value is 1000 -1100 mL.
Residual Volume (RV):The volume of air remaining in the lungs after a forceful expiration. It is approximately 1100 - 1200 mL.
Respiratory capacities - Vital capacity (VC): The maximum volume of air that can be moved out during a smgle breath following a maximal inspiration. A person first inspires maximally then expires maximally. VC = ERV + TV + IRV.
Inspiratory capacity (IC): The total volume of air a person can inhale after normal expiration ..
It includes tidal vo ume and inspiratory reserve volume. IC = TV + IRV.
Expiratory capacity (EC): The total volume of air a person can exhale after normal inspiration: It mcludes tidal volume and expiratory reserve volume. EC = TV + ERV.
Total Lung Capacity (TLC): The total volume of air which the lungs can accommodate after forced inspiration is called Total Lung Capacity. This includes the vital capacity and the esidual volume. It is approximately 6000mL. TLC = VC + RV.
Miunte Respiratory Volume: The amount of air that moves into the respiratory passage per minute is called minute respiratoru volume.
Normal TV=500mL; Normal respiratory rate=12 times/minute. Therefore, miute respiratory volume=6 Litres/minute (for a normal healthy man).

(i) Smoking is inhaling the smoke from burning tobacco.
(ii) There are thousands of known chemicals which includes nicotine, tar, carbon monoxide, ammonia, sulphur dioxide and even small quantities of arsenic.
(iii) Carbon monoxide and nicotine damage the cardiovascular system and tar damages the gaseous exchange system.
(iv) Nicotine is the chemical that causes addiction and is a stimulant which makes the heart beat faster and the narrowing of blood vessels results in raised blood pressure and coronary heart diseases.
(v) Presence of carbon monoxide reduces oxygen supply.
(vi) Lung cancer, cancer of the mouth and larynx is more common in smokers than non-smokers.
(vii) Smoking also causes cancer of the stomach, pancreas and bladder and lowers sperm count in men.
(viii) Smoking can cause lung diseases by damaging the airways and alveoli and results in emphysema and chronic bronchitis.
(ix) These two diseases along with asthma are often referred as Chronic Obstructive Pulmonary Disease (COPD).

The movement of air between the atmosphere and the lungs is known as ventilation or breathing. Inspiration and expiration are the two phases of breathing. Inspiration is the movement, of atmospheric air into the lungs and expiration is the movement of alveolar air that diffuse out of the lungs. Lungs do not contain muscle fibres but expands and contracts by the movement of the ribs and diaphragm. The diaphragm is a sheet of tissue which separates the thorax from the abdomen. In a relaxed state, the diaphragm is dome shaped. Ribs are moved by the intercostal muscles. External find internal intercostal muscles found between the ribs and the diaphragm helps in
creating pressure gradients. Inspiration occurs if the pressure inside the lungs (intrapulmonary pressure) is less than the atmospheric pressure; likewise expiration takes place when the pressure within the lungs is higher than the atmospheric pressure. Inspiration is initiated by the contraction of the diaphragm muscles and external intercostal muscles, which pulls the ribs and sternum upwards and outwards and increases the volume of the thoracic chamber in the dorso-ventral axis, forcing the lungs to expand the pulmonary volume. The increase in pulmonary volume and decrease in the intrapulmonary pressure forces the fresh air from outside to enter the air passages into the lungs to equalize the pressure. This process is called inspiration. Relaxation of the diaphragm allows the diaphragm and sternum to return to its dome shape and the internal intercostal muscles contract, pulling the ribs downward reducing the thoracic volume and pulmonary volume. This results in an increase in the intrapulmonary pressure slightly above the atmospheric pressure causing the expulsion of air from the lungs. This process is called expiration.