(a) Flatworm General body surface

(b) Birds Lungs

(c) Frog Lungs and moist skin

(d) Cockroach Tracheal tubes

The diaphragm and a specialised set of external and intercostals muscles between the ribs, help in the generation of pressure gradient during normal respiration.

Carbon dioxide is carried by the blood in three forms

(i) In Dissolved State Under normal temperature and pressure, about $7 \%$ of $\mathrm{CO}_2$ is carried by physical solution.

(ii) As Carbamino Compounds $\mathrm{CO}_2$ binds directly with Hb to form an unstable compound carbaminocompounds $\left(\mathrm{CO}_2 \mathrm{Hb}\right)$ About $23 \% \mathrm{CO}_2$ is transported in this form. When $\mathrm{pCO}_2$ is high and $\mathrm{pO}_2$ is low as in the tissues, more binding of carbon-dioxide occurs whereas, when $p \mathrm{CO}_2$ is low and $p \mathrm{O}_2$ is high as in alveol as tissue dissociation of $\mathrm{CO}_2$ from carbamino-haemoglobin takes place.

$$\mathrm{HbO}_2+\mathrm{CO}_2 \rightleftharpoons \mathrm{HbCO}_2+\mathrm{H}^{+}+\mathrm{O}_2$$

(iii) As Bicarbonate Ions $\mathrm{CO}_2$ reacts with water to form carbonic acid $\left(\mathrm{H}_2 \mathrm{CO}_3\right)$ in the presence of carbonic anhydrase in RBC. $\mathrm{H}_2 \mathrm{CO}_3$ dissociates into hydrogen and bicarbonate ions ( $\mathrm{HCO}_3{ }^{-}$).

The whole reaction proceeds as follows

$\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \underset{\text { Anhydrase }}{\stackrel{\text { Carbonic }}{\rightleftharpoons}} \underset{\text { Carbonic acid }}{\mathrm{H}_2 \mathrm{CO}_3}$

$\underset{\text { Carbonic acid }}{\mathrm{H}_2 \mathrm{CO}_3} \rightleftharpoons \underset{\substack{\text { Hydrogen } \\ \text { ion }}}{\mathrm{H}^{+}}+\underset{\substack{\text { Bicarbonate } \\ \text { ion }}}{\mathrm{HCO}_3^{-}}$

The carbonic anhydrase reaction mainly occur in RBC as it contain high concentration of enzyme carbonic anhydrase and minute quantity of it is present in plasma too.

As, the solubility rate of $\mathrm{CO}_2$ is 20-25 times higher than that of the $\mathrm{O}_2$, the amount of $\mathrm{CO}_2$ that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to that of $\mathrm{O}_2$.

(d) Pulmonary ventilation by which atmospheric air is drawn in and $\mathrm{CO}_2$ rich alveolar air is released out.

(a) Diffusion of gases $\left(\mathrm{O}_2\right.$ and $\left.\mathrm{CO}_2\right)$ across alveolar membrane.

(b) Transport of gases by blood.

(c) Diffusion of $\mathrm{O}_2$ and $\mathrm{CO}_2$ between blood and tissues.

(e) Utilisation of $\mathrm{O}_2$ by the cells for catabolic reactions and resultant release of $\mathrm{CO}_2$.