🔴T̠H̠E̠ B̠A̠S̠I̠C̠S̠ O̠F̠ R̠E̠S̠P̠I̠R̠A̠T̠O̠R̠Y̠ P̠H̠Y̠S̠I̠O̠L̠O̠G̠Y̠ & A̠B̠G̠🔴 A FEW POINTS

⚡️CO2  is the most important stimulus for respiration

😇Receptors for CO2  are found in the medulla of the brain (central chemoreceptors)

❤️Receptors for O2  are found mainly in carotid and aortic bodies 

👑CO2  is the more important gas as the body has more capacity to store CO2  than O2 or hydrogen ions 

1️⃣0️⃣In normal people at sea level, only 10% of the respiratory drive is due to hypoxic stimulation. 

▶️◀️Unlike the central stimulation of hypercapnia, hypoxia causes central depression of the respiratory drive. 

▶️◀️Acidosis (high H + /low blood pH) stimulates respiration; conversely alkalosis depresses it.

😳For gas exchange, the lungs provide an interface of total surface area about 55 m2 via 700 million alveoli

🎯Alveolar ventilation’ is that part of the total ventilation (i.e. all gas entering the lungs) that participates in gas exchange with pulmonary capillary blood; it is equal to total ventilation minus the ventilation of the conducting airways (i.e. dead-space ventilation).The average alveolar ventilation is about 4 L/min.

📥The alveolar–arterial oxygen gradient ( P(A-a)O2 ) is a measure of the oxygen that has reached the arterial blood supply as a ratio of the total oxygen in the alveoli. It is a useful index of pulmonary gas exchange function. 

➡️This requires that three elements are working correctly: 

⏺Circulatory anatomy is normal. Anomalies such as ASD & PDA can cause anatomical shunting,  i.e. venous blood passes through routes that are not exposed to alveolar air 

⏺Ventilation and perfusion are matched 

⏺The respiratory membrane allows sufficient free diffusion of gases between air and blood.  A diffusion defect impairs the alveolar–capillary membrane, e.g. in interstitial lung fibrosis

🔢In a healthy individual breathing room air (at FiO2 21) the PO2  in alveolar air is 104 mmHg and in arterial blood 95 mmHg . PAO2 exceeds PaO2 by 15 mmHg .Thus, at an FiO2 of 21, the P(A–a)O2 is 15 mmHg 

🎭In blood, CO2  is present as: 

✔️Dissolved in blood plasma (5.3% in arterial blood)

✔️Bound to haemoglobin as carbaminohaemoglobin within erythrocytes (4.5%) 

✔️In the form of bicarbonate attached to a base (90%) As CO2 diffuses from peri

Reference:"Understanding ABGs & Lung Function Tests"  Muhunthan Thillai, Keith Hattotuwa