Respiration is the oxidation of nutrients in the living cells to release energy for biological work. Breathing is the exchange of O2 from the atmosphere with CO2 produced by the cells. Conducting part. It transports atmospheric air into the alveoli. It clears air from foreign particles. It humidifies and brings the air to body temperature. External nostrils→ nasal passage → nasal chamber (nasal cavities) → nasopharynx → glottis → larynx → trachea → primary bronchi → sec. bronchi → tertiary bronchi → bronchioles → terminal bronchioles→ respiratory bronchiole → alveolar duct. Larynx (sound box or voice box) is a cartilaginous box which helps in sound production. During swallowing, glottis is closed by epiglottis (a thin elastic cartilaginous flap) to prevent the entry of food into larynx. Trachea, primary, secondary and tertiary bronchi and initial bronchioles are supported by incomplete cartilaginous half rings. Situate in thoracic chamber and rest on diaphragm. Right lung has 3 lobes. Left lung has 2 lobes. Covered by double-layered pleura (outer parietal pleura & inner visceral pleura). In between these 2 layers, pleural fluid present. It lubricates lung surface and prevents friction between membranes. Lungs= Branching network of bronchi + bronchioles + alveoli Alveoli and their ducts form respiratory or exchange part of the respiratory system. Alveoli are the structural and functional units of lungs. Diaphragm contracts (flattens) ↓ Vertical volume (antero-posterior axis) increases. External inter-costal muscles contracts ↓ Ribs & sternum lift up. Volume in dorsoventral axis increases. Thoracic pressure is reduced. ↓ Lungs expand and Pulmonary volume increases. ↓ Intra-pulmonary pressure decreases. ↓ Air moves from outside into the lungs. Inter-costal muscles & diaphragm relax ↓ Thorax regains its original position ↓ Thoracic volume decreases ↓ Pulmonary volume decreases . ↓ Air moves out. Respiratory cycle= an inspiration + an expiration Normal respiratory (breathing) rate: 12-16 times/min Spirometer (respirometer): To measure the respiratory rate. Respiratory volumes Definition Volume in ml Tidal volume (TV) Volume of air inspired or expired during a normal respiration (volume of air renewed in the respiratory system during each breathing). 500 Inspiratory reserve volume (IRV) or complemental air Additional volume of air, that can inspire by forceful inspiration. 2500-3000 Expiratory reserve volume (ERV) or supplemental air Additional volume of air, that can expire by a forceful expiration. 1000-1100 Residual volume (RV) Volume of air remaining in the lungs after a forcible expiration. 1100-1200 Respiratory capacities Definition Volume in ml Inspiratory capacity (IC) [TV + IRV] Volume of air that can inspire after a normal expiration. 3000-3500 Expiratory capacity (EC) [TV + ERV] Volume of air that can expire after a normal inspiration. 1500-1600 Functional residual capacity (FRC) Volume of air that will remain in the lungs after a normal expiration [ERV + RV]. 2100-2300 Vital capacity (VC) [ERV + TV + IRV] Maximum volume of air that can breathe in after a forced expiration or maximum volume of air that can breathe out after a forced inspiration. 3500-4500 Total lung capacity (TLC) Total volume of air in the lungs after a maximum inspiration. [RV + ERV + TV + IRV or VC + RV] 5000-6000 Part of respiratory tract (from nostrils to terminal bronchi) not involved in gaseous exchange is called dead space. Dead air volume is about 150 ml Gas exchange occurs between 1. Alveoli & blood 2. Blood & tissues Alveoli are the primary sites of gas exchange. O2 and CO2 are exchanged by simple diffusion. It depends on following factors: Pressure/ concentration gradient Solubility of gases Thickness of membranes Surface area of respiratory membrane (lungs) The individual pressure of a gas in a gas mixture is called Partial pressure. Partial pressures of O2 and CO2 (pO2 & pCO2) are given below: Respiratory gas Atmospheric air Alveoli Blood (Deoxygenated) Blood (Oxygenated) Tissues pO2 (in mm Hg) 159 104 40 95 40 pCO2 (in mm Hg) 0.3 40 45 40 45 pO2 in the alveolar air is greater (104 mm Hg) than that in the blood capillaries (40 mm Hg). So O2 diffuses into capillary blood. pCO2 in deoxygenated blood is greater (45 mm Hg) than that in the alveolus (40 mm Hg). So CO2 diffuses to alveolus. Solubility of CO2 is 20-25 times higher than that of O2. So the amount of CO2 that can diffuse through diffusion membrane per unit difference in partial pressure is much higher compared to that of O2. Diffusion membrane is made up of 3 layers: Thin squamous epithelium of alveoli Endothelium of alveolar capillaries Basement substance b/w them. Its total thickness is only 0.5 mm. It enables easy gas exchange. Presence of alveoli increases the surface area of lungs. It increases the gas exchange. In physical solution (blood plasma): 3% of O2 is carried by dissolving in plasma. As oxyhaemoglobin: 97% of O2 is transported by RBC. O2 binds with haemoglobin in a reversible manner to form oxyhaemoglobin. This is called oxygenation. Hb has 4 haem units. So each Hb molecule can carry 4 oxygen molecules. Binding of O2 depends upon pO2 & pCO2, H+ ion concentration (pH) & Temperature. In alveoli, there is high pO2, low pCO2, lesser H+ ion concentration & lower temperature. These factors favour the formation of oxyhaemoglobin. In tissues, low pO2, high pCO2, high H+ ion concentration & higher temperature exist. So oxyhaemoglobin dissociates to release O2. Every 100 ml of oxygenated blood can deliver around 5 ml of O2 to the tissues under normal physiological conditions. It is a sigmoid curve obtained when percentage saturation of Hb with O2 is plotted against the pO2. It is used to study the effect of factors like pCO2, H+ concentration etc., on binding of O2 with Hb. As carbonic acid: About 7% of CO2 is carried in a dissolved state (as carbonic acid) through plasma. As carbamino-haemoglobin: About 20-25% of CO2 is transported by Hb as carbaminohaemoglobin. When pCO2 is high and pO2 is low in the tissues, more binding of CO2 occurs whereas, when the pCO2 is low and pO2 is high in the alveoli, dissociation of CO2 from carbaminohaemoglobin takes place. iii. As bicarbonate: About 70% of CO2. RBCs and plasma contain an enzyme, carbonic anhydrase. This facilitates the following reactions. At the tissue site, pCO2 is high due to catabolism. So CO2 diffuses into blood and forms HCO3 &־H+. At the alveolar site pCO2 is low. So the reaction proceeds in opposite direction. It leads to formation of CO2 & H2O. Every 100 ml of deoxygenated blood delivers about 4 ml of CO2 to the alveoli. Respiratory centres Location Functions 1. Respiratory rhythm centres (Inspiratory & Expiratory centres) Medulla oblongata Regulates normal functioning of inspiration and expiration Respiratory centres 2. Pneumotaxic centre Location Functions Pons Moderates functions of respiratory rhythm centre. Neural signal from this centre reduces the duration of inspiration and thereby alter the respiratory rate. Respiratory centres 3. Chemosensitive area Location Functions Adjacent to rhythm centre Increase in the concentration of CO2 and H+ activate this centre, which in turn signals the rhythm centre. Receptors in the aortic arch and carotid artery also recognize changes in CO2 and H+ concentration and send necessary signals to the rhythm centre. Role of oxygen in the regulation of respiratory rhythm is quite insignificant. It is the difficulty in breathing causing wheezing due to inflammation of bronchi and bronchioles. Alveolar walls are damaged due to which respiratory surface is decreased. Major cause is cigarette smoking. Due to exposure of fumes or dust. Silicosis: Due to breathing of silica. Asbestosis: Due to breathing in asbestos particle.
