Breathing and Exchange of Gases

Vital capacity (VC) is the maximum volume of air a person can breathe in after a forced expiration, or the maximum volume of air a person can breathe out after a forced inspiration. It includes IRV + TV + ERV.

Significance: It shows the maximum breathing ability of the lungs and is useful in clinical diagnosis of lung function.

After a normal breathing or normal expiration, the air remaining in the lungs is called functional residual capacity. It consists of ERV + RV.

Diffusion of gases occurs mainly in the alveolar region because alveoli are specially adapted for exchange of gases:

- They have a very large surface area.
- Their walls are very thin and irregular.
- They are highly vascularised, so blood is close to the air.
- The diffusion membrane is extremely thin and allows rapid diffusion.

The other parts of the respiratory system are mainly the conducting part. Their function is to carry air, clean it, humidify it, and warm it, not to exchange gases.

CO₂ is transported in the blood in three major forms:

1. **As bicarbonate ions ($HCO_3^-$) — about 70% of CO₂ is carried this way. In RBCs, CO₂ combines with water in the presence of carbonic anhydrase** to form carbonic acid, which dissociates into $HCO_3^-$ and $H^+$.
2. As carbaminohaemoglobin — about 20–25% of CO₂ binds with haemoglobin.
3. In dissolved form in plasma — about 7% of CO₂ is transported this way.

In tissues, where $pCO_2$ is high, CO₂ enters blood and is mostly converted to bicarbonate. In the alveoli, where $pCO_2$ is low, the reaction reverses and CO₂ is released for exhalation.

Atmospheric air has more oxygen and less carbon dioxide than alveolar air. From the chapter data:

- Atmospheric air: $pO_2 = 159$ mm Hg, $pCO_2 = 0.3$ mm Hg
- Alveolar air: $pO_2 = 104$ mm Hg, $pCO_2 = 40$ mm Hg

So compared to alveolar air, atmospheric air has **higher $pO_2$ and lower $pCO_2$**.

Under normal conditions, inspiration occurs as follows:

- The diaphragm contracts and becomes flatter, increasing the volume of the thoracic cavity in the antero-posterior axis.
- The external intercostal muscles contract, lifting the ribs and sternum upward and outward, increasing thoracic volume in the dorso-ventral axis.
- Because thoracic volume increases, pulmonary volume also increases.
- This lowers intra-pulmonary pressure below atmospheric pressure.
- Air then moves from outside into the lungs.

Thus, inspiration is caused by an increase in thoracic and pulmonary volume that creates a pressure gradient.

Respiration is regulated by the neural system.

- The main control centre is the respiratory rhythm centre in the medulla.
- The pneumotaxic centre in the pons can modify the rhythm centre and reduce the duration of inspiration.
- A chemosensitive area near the rhythm centre is sensitive to increased CO₂ and H⁺ concentration. When these rise, it stimulates the rhythm centre to adjust breathing.
- Aortic arch and carotid artery receptors also detect changes in CO₂ and H⁺ and send signals to the rhythm centre.

So, respiration is mainly controlled by centres in the brainstem in response to CO₂ and H⁺ levels.