# 3.4 TRANSPORT OF GASES The transport of respiratory gases, oxygen ($O_2$) and carbon dioxide ($CO_2$), is a critical function of the circulatory system, facilitated by the blood. Blood carries $O_2$ from the lungs to the body's tissues and transports $CO_2$ from the tissues back to the lungs for exhalation. ### 3.4.1 Transport of Oxygen in Blood Oxygen is transported from the lungs to the tissues in two main ways: * **As Oxyhaemoglobin (97%):** The vast majority of $O_2$ binds reversibly with haemoglobin (Hb), a protein found in red blood cells (RBCs), to form oxyhaemoglobin ($Hb_4O_2$). * **Dissolved in Plasma (3%):** A small fraction of $O_2$ dissolves directly into the blood plasma. #### Formation of Oxyhaemoglobin * In the lungs, where the partial pressure of oxygen ($pO_2$) is high, $O_2$ readily binds to the iron-containing heme groups of haemoglobin. * This reaction is reversible. * Each haemoglobin molecule can bind with up to four oxygen molecules. $$ \mathrm{Hb} + 4\mathrm{O}_2 \rightleftharpoons \mathrm{Hb}_4\mathrm{O}_2 $$ #### Oxygen-Carrying Capacity of Blood * This is defined as the maximum amount of $O_2$ that can be transported by the blood. * It is directly proportional to the partial pressure of oxygen ($pO_2$). * **Factors affecting binding:** * High $pO_2$ (in lungs) promotes $O_2$ binding. * Low $pO_2$, low pH (high $H^+$), high $pCO_2$, and high temperature (in tissues) promote the release of $O_2$ from haemoglobin. This phenomenon where high $CO_2$ and low pH decrease hemoglobin's affinity for oxygen is known as the **Bohr Effect**. | Blood Type | $pO_2$ (mmHg) | $O_2$ Content (ml/100ml) | Hb Saturation | | :--- | :---: | :---: | :---: | | **Arterial Blood** | 95 | 19.4 | 97% | | **Venous Blood** | 40 | 14.4 | 75% | * *Note:* The maximum capacity is **20 ml $O_2$ / 100 ml** of blood (100% saturation at 100 mmHg). On average, **5 ml of $O_2$** is delivered to the tissues by every 100 ml of blood under normal conditions. ### 3.4.2 Transport of Carbon Dioxide in Blood Carbon dioxide, a waste product of cellular metabolism, is transported from the tissues to the lungs in three forms: | Method of Transport | Percentage | | :--- | :--- | | **Bicarbonate ions ($HCO_3^-$)** | ~70% | | **Carbaminohaemoglobin** | ~23% | | **Dissolved in Plasma** | ~7% | #### (I) As Bicarbonate Ions ($HCO_3^-$) This is the primary method for $CO_2$ transport. 1. **Diffusion:** $CO_2$ diffuses from tissue cells into the blood and enters Red Blood Cells (RBCs). 2. **Carbonic Acid Formation:** Inside the RBC, $CO_2$ rapidly combines with water ($H_2O$) to form carbonic acid ($H_2CO_3$). This reaction is catalyzed by the enzyme **carbonic anhydrase**. $$ \mathrm{CO}_2 + \mathrm{H}_2\mathrm{O} \xrightleftharpoons[\text{Carbonic anhydrase}]{} \mathrm{H}_2\mathrm{CO}_3 $$ 3. **Dissociation:** Carbonic acid is unstable and quickly dissociates into hydrogen ions ($H^+$) and bicarbonate ions ($HCO_3^-$). $$ \mathrm{H}_2\mathrm{CO}_3 \xrightleftharpoons{} \mathrm{H}^+ + \mathrm{HCO}_3^- $$ 4. **Buffering:** The released $H^+$ ions are buffered by haemoglobin, which binds them to form haemoglobinic acid (HHb). This process also facilitates the release of oxygen to the tissues (Bohr effect). $$ \mathrm{Hb}_4\mathrm{O}_2 + \mathrm{H}^+ \longrightarrow \mathrm{HHb} + 4\mathrm{O}_2 $$ 5. **Chloride Shift (Hamburger's Phenomenon):** To maintain electrical neutrality, as bicarbonate ions ($HCO_3^-$) move out of the RBC into the plasma, chloride ions ($Cl^-$) move into the RBC from the plasma. 6. **In the Lungs:** The entire process is reversed. $HCO_3^-$ re-enters the RBC, combines with $H^+$ to form carbonic acid, which then breaks down into $CO_2$ and $H_2O$. The $CO_2$ diffuses into the alveoli to be exhaled. #### (II) As Carbaminohaemoglobin * $CO_2$ binds directly to the globin (protein) part of haemoglobin, not the heme group. * This binding is reversible and dependent on the partial pressure of $CO_2$ ($pCO_2$). * Formation occurs in the tissues where $pCO_2$ is high. * Dissociation occurs in the lungs where $pCO_2$ is low. #### (III) As Dissolved $CO_2$ in Plasma * A small amount of $CO_2$ dissolves directly in the blood plasma. This is the least efficient transport mechanism.