Breathing and Exchange of Gases - Exchange of gases, transport of gases and regulation of respiration

Exchange of gases occurs primarily in the alveoli and tissues via simple diffusion, driven by a partial pressure gradient. The solubility of gases (where $CO_2$ is $20-25$ times more soluble than $O_2$) and the thickness of the diffusion membrane are critical factors.

The diffusion membrane is composed of three layers: the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries, and the basement substance between them. Its total thickness is less than $1$ mm.

Partial pressure of gases (in $mmHg$): In Alveoli, $pO_2 = 104$ and $pCO_2 = 40$. In deoxygenated blood, $pO_2 = 40$ and $pCO_2 = 45$. In oxygenated blood, $pO_2 = 95$ and $pCO_2 = 40$. In tissues, $pO_2 = 40$ and $pCO_2 = 45$.

Transport of $O_2$: $97\%$ is transported by Red Blood Cells (RBCs) as oxyhaemoglobin, and $3\%$ is carried in a dissolved state through plasma. Each Haemoglobin ($Hb$) molecule can carry a maximum of $4$ molecules of $O_2$.

The Oxygen-Haemoglobin dissociation curve is sigmoid. Factors like high $pO_2$, low $pCO_2$, lesser $H^+$ concentration, and lower temperature in the alveoli favour the formation of oxyhaemoglobin. Opposite conditions in tissues (low $pO_2$, high $pCO_2$, high $H^+$, high temperature) favour dissociation.

Transport of $CO_2$: Nearly $20-25\%$ is transported as carbamino-haemoglobin, $7\%$ in dissolved state in plasma, and the majority $70\%$ as bicarbonate ($HCO_3^-$) ions, facilitated by the enzyme Carbonic Anhydrase.

Regulation of Respiration: The 'Respiratory Rhythm Centre' is located in the medulla oblongata. The 'Pneumotaxic Centre' in the pons can moderate its functions by reducing the duration of inspiration. A chemosensitive area near the rhythm centre is highly sensitive to $CO_2$ and $H^+$ concentrations.

Receptors in the aortic arch and carotid artery also recognize changes in $CO_2$ and $H^+$ concentration and send signals to the rhythm centre for remedial actions. The role of $O_2$ in the regulation of respiratory rhythm is quite insignificant.