Breathing and exchange

• Oxygen (O2) is used by organisms to break down nutrient molecules like glucose for energy
• Carbon dioxide (CO2) is released during catabolic reactions and needs to be removed
• The process of exchanging O2 from the atmosphere with CO2 produced by cells is called breathing or respiration
• Different groups of animals have varying respiratory organs and breathing mechanisms based on their habitats and levels of organization
• Lower invertebrates exchange O2 and CO2 through simple diffusion over their entire body surface
• Earthworms use their moist cuticle, insects have tracheal tubes, aquatic arthropods and molluscs use gills, and terrestrial forms use lungs for gas exchange
• Among vertebrates, fishes respire through gills, while reptiles, birds, and mammals respire through lungs
• Amphibians like frogs can respire through their moist skin
• Mammals have a well-developed respiratory system
• The human respiratory system includes external nostrils, nasal chamber, pharynx, larynx, trachea, bronchi, bronchioles, and alveoli
• The conducting part of the respiratory system transports air to the alveoli, while the alveoli are the site of gas exchange between blood and atmospheric air
• Respiration involves breathing, diffusion of gases across alveolar membrane, transport of gases by the blood, and diffusion of O2 and CO2 between blood and tissues
• Breathing involves inspiration (drawing in air) and expiration (releasing alveolar air) by creating pressure gradients between the lungs and the atmosphere
• Inspiration occurs when intra-pulmonary pressure is less than atmospheric pressure, while expiration occurs when intra-pulmonary pressure is higher than atmospheric pressure
• The diaphragm and intercostal muscles help in creating pressure gradients for breathing
• Respiratory volumes and capacities include Tidal Volume (TV), Inspiratory Reserve Volume (IRV), Expiratory Reserve Volume (ERV), Residual Volume (RV), Inspiratory Capacity (IC), Expiratory Capacity (EC), Functional Residual Capacity (FRC), Vital Capacity (VC), and Total Lung Capacity
• Alveoli are the primary sites of gas exchange, where O2 and CO2 are exchanged between blood and tissues through simple diffusion based on pressure/concentration gradients
• Solubility of gases and membrane thickness affect the rate of diffusion, and partial pressures of O2 and CO2 play a role in gas exchange
• The diffusion membrane for gases in the respiratory system is made up of three major layers: thin squamous epithelium of alveoli, endothelium of alveolar capillaries, and the basement substance in between them
• The total thickness of the diffusion membrane is much less than a millimetre
• Blood is the medium of transport for O2 and CO2
• About 97% of O2 is transported by RBCs in the blood
• The remaining 3% of O2 is carried in a dissolved state through the plasma
• Nearly 20-25% of CO2 is transported by RBCs, whereas 70% of it is carried as bicarbonate
• About 7% of CO2 is carried in a dissolved state through plasma
• Haemoglobin is a red-colored iron-containing pigment present in RBCs
• O2 can bind with haemoglobin in a reversible manner to form oxyhaemoglobin
• Each haemoglobin molecule can carry a maximum of four molecules of O2
• A sigmoid curve is obtained when percentage saturation of haemoglobin with O2 is plotted against the pO2, known as the Oxygen dissociation curve
• In the alveoli, where there is high pO2, low pCO2, lesser H+ concentration, and lower temperature, the factors are all favorable for the formation of oxyhaemoglobin
• CO2 is carried by haemoglobin as carbamino-haemoglobin (about 20-25%)
• RBCs contain a very high concentration of the enzyme carbonic anhydrase
• At the tissue site where partial pressure of CO2 is high, CO2 diffuses into blood (RBCs and plasma) and forms HCO3- and H+
• At the alveolar site where pCO2 is low, the reaction proceeds in the opposite direction leading to the formation of CO2 and H2O
• Respiratory rhythm is maintained by the respiratory center in the medulla region of the brain
• A pneumotaxic center in the pons region of the brain and a chemosensitive area in the medulla can alter the respiratory mechanism
• In certain industries, long exposure to dust can lead to inflammation and serious lung damage, workers should wear protective masks