3.4 Transport of Gases

The transport of respiratory gases, oxygen ($O_2$) and carbon dioxide ($C{O}_2$), 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 $C{O}_2$ 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 ($H{b}_4{O}_2$).
• 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 ($p{O}_2$) 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 ($p{O}_2$).
• Factors affecting binding:
  • High $p{O}_2$ (in lungs) promotes $O_2$ binding.
  • Low $p{O}_2$, low pH (high $H^{+}$), high $pC{O}_2$, and high temperature (in tissues) promote the release of $O_2$ from haemoglobin. This phenomenon where high $C{O}_2$ and low pH decrease hemoglobin's affinity for oxygen is known as the Bohr Effect.

• 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:

(I) As Bicarbonate Ions ($HC{O}_3^{-}$)

This is the primary method for $C{O}_2$ transport.

1. Diffusion: $C{O}_2$ diffuses from tissue cells into the blood and enters Red Blood Cells (RBCs).
2. Carbonic Acid Formation: Inside the RBC, $C{O}_2$ rapidly combines with water ($H_{2}O$) to form carbonic acid ($H_{2}C{O}_3$). This reaction is catalyzed by the enzyme carbonic anhydrase.

${\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}\underset{\text{Carbonic anhydrase}}{\rightleftharpoons }{\mathrm{H}}_2{\mathrm{CO}}_3$

3. Dissociation: Carbonic acid is unstable and quickly dissociates into hydrogen ions ($H^{+}$) and bicarbonate ions ($HC{O}_3^{-}$).

${\mathrm{H}}_2{\mathrm{CO}}_3\rightleftharpoons {\mathrm{H}}^{+}+{\mathrm{HCO}}_3^{-}$

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).

${\mathrm{Hb}}_4{\mathrm{O}}_2+{\mathrm{H}}^{+}⟶\mathrm{HHb}+4{\mathrm{O}}_2$

5. Chloride Shift (Hamburger's Phenomenon): To maintain electrical neutrality, as bicarbonate ions ($HC{O}_3^{-}$) move out of the RBC into the plasma, chloride ions ($C{l}^{-}$) move into the RBC from the plasma.

6. In the Lungs: The entire process is reversed. $HC{O}_3^{-}$ re-enters the RBC, combines with $H^{+}$ to form carbonic acid, which then breaks down into $C{O}_2$ and $H_{2}O$. The $C{O}_2$ diffuses into the alveoli to be exhaled.

(II) As Carbaminohaemoglobin

• $C{O}_2$ binds directly to the globin (protein) part of haemoglobin, not the heme group.
• This binding is reversible and dependent on the partial pressure of $C{O}_2$ ($pC{O}_2$).
• Formation occurs in the tissues where $pC{O}_2$ is high.
• Dissociation occurs in the lungs where $pC{O}_2$ is low.

(III) As Dissolved $C{O}_2$ in Plasma

• A small amount of $C{O}_2$ dissolves directly in the blood plasma. This is the least efficient transport mechanism.

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