Smoking is inhaling the smoke from burning tobacco. There are thousands of known chemicals which includes nicotine, tar, carbon monoxide, ammonia, sulphur- dioxide and even small quantities of arsenic. Carbon monoxide and nicotine damage the cardiovascular system and tar damages the gaseous exchange system. Nicotine is the chemical that causes addiction and is a stimulant which makes the heart beat faster and the narrowing of blood vessel results in raised blood pressure and coronary heart diseases. Presence of carbon monoxide reduces oxygen supply. Lung cancer, Cancer of the mouth and larynx is more common in smokers Smoking also causes cancer of the stomach, pancreas and bladder and lowers sperm count in men. Smoking can cause lung diseases by damaging the airways and alveoli and results in emphysema and chronic bronchitis. These two diseases along with asthma are often referred as Chronic Obstructive Pulmonary Disease (COPD). When a person smokes, nearly 85% of the smoke released is inhaled by the smoker himself and others in the vicinity, called passive smokers, are also affected. Guidance or counselling should be done in such users to withdraw this habit.

A specialised respiratory centre present in the medulla oblongata of the hind brain called respiratory rhythm centre is responsible for this regulation. Pneumotaxic centre present in pons varoli region of the brain moderates the function of the respiratory rhythm centre to ensure normal breathing. The chemosensitive area found close to the rhythm centre is highly sensitive to CO₂ and H+ And H+ are eliminated out by respiratory process. Receptors associated with the aortic arch and carotid artery send necessary signals to the rhythm centre for remedial action. The role of O₂ is insignificant in the regulation of respiratory rhythm.

The primary site for the exchange of gases is the alveoli. The uptake of O₂ and the release of CO₂ occur between the blood and tissues by simple diffusion driven by partial pressure gradient of O₂ and CO₂. Partial pressure is the pressure contributed by an individual gas in a mixture of gases. It is represented as pO₂ for oxygen and pCO₂ for carbon- dioxide. Due to pressure gradients, O₂ from the alveoli enters into the blood and reaches the tissues. CO₂ enters into the blood from the tissues and reaches alveoli for elimination. As the solubility of CO₂ is 20-25 times higher than that of O₂ the partial pressure of CO₂ is much higher than that of O₂.

Some of the inspired air never reaches the gas exchange areas but fills the respiratory passages. where exchange of gases does not occur. This air is called dead space. Dead space is not involved in gaseous exchange. It amounts to approximately 150mL.

The lungs are covered by double layered pleural membrane containing several layers of elastic connective tissues and capillaries. It encloses the pleural fluid which reduces friction when the lungs expand and contract.

The lungs are light spongy tissues enclosed in the thoracic cavity surrounded by an air tight space. It is 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 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.

Surfactants are the thin non-cellular films made of protein and phospholipids covering the alveolar membrane. The surfactant lowers the surface tension in the alveoli and prevents the lungs from collapsing. It also prevents pulmonary oedema.

1. The volume of air remaining in the lungs after a forceful expiration.
2. It is approximately 1100-1200 mL.

(i) Some of the inhaled air never reaches the areas of gaseous exchange but respiratory passages are filled by it.
(ii) Such lodged sites of respiratory tract do not undergo gaseous exchange such sites constitute a dead space.
(iii) It amounts to approximately 150 mL.

(i) Entry of air with high pressure may collapse the tracheal tree.
(ii) The smooth muscles which contract and relax to adjust the diameter of air passage with a typical resistance against airways under normal pressure.

(i) A thin elastic flap called epiglottis is found in the upper respiratory tract.
(ii) It prevents the food from entering into the larynx.
(iii) It avoids choking of food.

RBCs contain an enzyme carbonic anhydrase
(i) It catalyses the carbonic acid (H₂CO₃) which is unstable and breaks into HCO^-₃ and H⁺ ions.
(ii) This enzyme facilitates the reaction in both directions. They are

External nostrils → Nasal cavity → Internal nostrils → Nasopharynx → Larynx → Trachea → Bronchi → Bronchioles → Hilum → Lungs → Bronchioles → Alveoli