Human Physiology - Breathing and Exchange of Gases

Mechanism of Breathing: It involves two stages—Inspiration, where the diaphragm and external inter-costal muscles contract creating a negative pressure in the lungs relative to the atmosphere, and Expiration, which is a passive process during normal breathing where muscles relax.

Respiratory Volumes: These are critical clinical diagnostic tools including Tidal Volume ($TV \approx 500\ mL$), Inspiratory Reserve Volume ($IRV \approx 2500-3000\ mL$), and Expiratory Reserve Volume ($ERV \approx 1000-1100\ mL$).

Exchange of Gases: Occurs at the alveolar-capillary membrane via simple diffusion based on partial pressure gradients. For instance, $pO_2$ in alveoli is $104\ mmHg$ while in deoxygenated blood it is $40\ mmHg$.

Transport of Oxygen: About $97\%$ of $O_2$ is transported by $RBCs$ as Oxyhaemoglobin ($HbO_2$). Each haemoglobin molecule can carry a maximum of four molecules of $O_2$.

Transport of Carbon dioxide: $CO_2$ is transported in three forms: dissolved state ($7\%$), as carbamino-haemoglobin ($20-25\%$), and as bicarbonate ions ($70\%$).

Chloride Shift (Hamburger's Phenomenon): To maintain ionic balance, chloride ions ($Cl^-$) move from plasma into $RBCs$ when bicarbonate ions ($HCO_3^-$) diffuse out into the plasma.

Oxygen Dissociation Curve: A sigmoid curve representing the relationship between the partial pressure of oxygen ($pO_2$) and the percentage saturation of haemoglobin. Factors like high $pCO_2$, high $H^+$ concentration, and high temperature shift the curve to the right (Bohr effect).

Regulation of Respiration: The respiratory rhythm center is located in the medulla oblongata, while the pneumotaxic center is in the pons varolii which can moderate the functions of the rhythm center.