Explain the process of inspiration under normal conditions.

1. Respiratory Organs:

(i) The respiratory system allows the passage movement of oxygen to enter the blood while carbon dioxide moves out of the blood.

(ii) The blood vascular system transports respiratory gases between the lungs and the body's cells.

(iii) The conducting portion of the respiratory tract begins at the entrance to the nasal cavity. It extends through many passageways like the pharynx, larynx, trachea, bronchi, bronchioles, and terminal bronchioles.

(iv)The nasal chamber opens into the pharynx, which leads to the larynx. The larynx contains a voice box (sound box) that helps the production of sound.

(v) The respiratory portion of the tract includes the respiratory bronchioles and the sites of gas exchange, the alveoli.

(vi) The alveoli represent the lung's ultimate structural and physiological units.

(vii) Alveoli provide great surface area for gaseous exchange by diffusion.

(viii) The lungs are the principal organs of respiration.

(ix) Each lung is enclosed in two membranes called pleurae (Sing. Pleura).

(x) The diaphragm, a dome-shaped muscular structure separating the thoracic and abdominal cavities, is characteristic of mammals.

(xi) The most important function of the diaphragm in mammals is to aid in respiration.

2. Mechanism of Breathing:

(i) Breathing is the process of inhalation of air (oxygen) from the nose or mouth into the lungs due to muscle contraction and exhaling it due to muscle relaxation.

(ii) Breathing is a physical process involving inhalation and exhalation.

(iii) In normal breathing of a resting adult, there are about $12-18$ complete respiratory cycles per minute.

(iii) Inspiration and expiration are carried out by creating pressure gradients between the atmosphere and the alveoli with the help of specialised intercostal muscles and the diaphragm.

(iv) Volumes of air involved in breathing activities can be estimated with the help of the spirometer and are of clinical significance.

(v) The recording of breathing is termed a spirogram.

(vi) Four measurements are called pulmonary volumes: Tidal Volume (TV or VT), Inspiratory Reserve Volume (IRV), Expiratory Reserve Volume (ERV) and Residual Volume (RV).

3. Exchange and Transport of Gases:

(i) Exchange of ${\mathrm{O}}_2$ and ${\mathrm{CO}}_2$ at the alveoli and tissues occur by diffusion.

(ii) Oxygen does not dissolve easily in water and only $1.5\%$ of the ${\mathrm{O}}_2$ is dissolved in blood plasma.

(iii) $98.5\%$ of ${\mathrm{O}}_2$ is transported by the blood with the help of the respiratory pigment haemoglobin in erythrocytes.

(iv) The structure of foetal haemoglobin which differs from that of adult haemoglobin, gives it much higher affinity for oxygen.

(v) Body tissues obtain oxygen from oxyhaemoglobin because of its dissociation caused by low ${\mathrm{O}}_2$ and high ${\mathrm{CO}}_2$concentration.

(vi) Bohr's effect is the effect of $\mathrm{pH}$ or ${\mathrm{P}}_{\mathrm{CO}2}$ on oxyhaemoglobin.

(vii) Transport of carbon dioxide by blood is much easier than that of oxygen due to high solubility of ${\mathrm{CO}}_2$ in water.

(viii) Carbon monoxide, a poisonous gas, combines with haemoglobin more rapidly than oxygen to form carboxyhaemoglobin.

(ix) Carbon monoxide has $200-250$ times more affinity for haemoglobin as compared to oxygen.

4. Regulation of Respiration:

(i) A specialised centre located in the medulla region of the hindbrain called the respiratory rhythm centre. The pneumotaxic centre in the pons moderates the function of the respiratory rhythm centre.

(ii) Chemo-sensitive area near the rhythm centre is highly sensitive to CO₂ and hydrogen ions.

(iii) Oxygen does not play a major role in controlling the rate of respiration.

5. Disorders of the Respiratory System:

(i) Asthma: The difficulty in breathing, especially exhaling, is called asthma. Asthma is a chronic inflammatory disorder. It results in the contraction of smooth muscles of bronchi and bronchioles and excessive mucous production.

(ii) Emphysema: Under continuous stress, the cells of the alveoli secrete an enzyme that destroys the alveoli's elastic wall, leading to the loss of elasticity. The alveoli become permanently expanded. This condition is called emphysema

(iii) Occupational Lung Diseases: Many lung diseases are due to occupational or environmental hazards.

(iv) The lungs are affected by inhaled particles such as metals, coal, silica, and asbestos.