Breathing And Exchange Of Gases

Mechanism of Breathing: Inspiration and Expiration

Breathing, or pulmonary ventilation, is the physical process of moving air into and out of the lungs. It is driven entirely by pressure gradients established between the atmosphere and the intra-pulmonary space. During inspiration, the active contraction of the diaphragm (which flattens) and the external intercostal muscles (which lift the ribs and sternum) expands the thoracic cavity volume. This expansion reduces the intra-pulmonary pressure below atmospheric level, forcing external air to rush into the lungs. Conversely, expiration is mostly a passive phase during relaxed breathing. It occurs when these muscles relax, restoring the thoracic cavity to its original volume. This action compresses the lungs, raises the internal pressure above atmospheric pressure, and expels the air out.

Comparison between Inspiration and Expiration

Key Respiratory Volumes and Capacities

• Tidal Volume (TV): Volume of air inspired or expired during a normal respiration. It is approximately 500 mL in a healthy adult.
• Inspiratory Reserve Volume (IRV): Additional volume of air a person can inspire by a forceful inspiration. Ranges from 2500 mL to 3000 mL.
• Expiratory Reserve Volume (ERV): Additional volume of air a person can expire by a forceful expiration. Ranges from 1000 mL to 1100 mL.
• Residual Volume (RV): Volume of air remaining in the lungs even after a forceful expiration. Ranges from 1100 mL to 1200 mL.
• Inspiratory Capacity (IC): Total volume of air a person can inspire after a normal expiration ($IC = TV + IRV$).
• Expiratory Capacity (EC): Total volume of air a person can expire after a normal inspiration ($EC = TV + ERV$).
• Functional Residual Capacity (FRC): Volume of air that will remain in the lungs after a normal expiration ($FRC = ERV + RV$).
• Vital Capacity (VC): The maximum volume of air a person can breathe in after a forced expiration ($VC = ERV + TV + IRV$).
• Total Lung Capacity (TLC): Total volume of air accommodated in the lungs at the end of a forced inspiration ($TLC = RV + ERV + TV + IRV$ or $VC + RV$).

Crucial Definitions for Board Exams

Partial Pressure

The individual pressure contributed by a specific gas in a mixture of gases, represented as $pO_2$ for oxygen and $pCO_2$ for carbon dioxide.

Oxyhaemoglobin

A highly unstable chemical complex formed by the reversible binding of oxygen molecules to the iron-containing haem group of haemoglobin under high $pO_2$ conditions.

Carbaminohemoglobin

The chemical compound formed when carbon dioxide directly binds to the amine group of the globin part of haemoglobin (responsible for about 20-25% of $CO_2$ transport).

Chloride Shift (Hamburger's Phenomenon)

The exchange of chloride ions ($Cl^-$) from plasma into erythrocytes (RBCs) in exchange for bicarbonate ions ($HCO_3^-$) to maintain electrical neutrality.

Bohr's Effect

The physiological phenomenon where an increase in carbon dioxide concentration or a decrease in pH (acidic conditions) decreases haemoglobin's affinity for oxygen.

Pneumotaxic Centre

A specialized neural center located in the pons region of the brainstem that moderates the functions of the respiratory rhythm center and limits the duration of inspiration.

Step-by-Step Mechanisms of Gas Transport

1. Oxygen Binding in Alveoli — In the alveoli, high $pO_2$, low $pCO_2$, low $H^+$ concentration, and lower temperature favor the binding of $O_2$ to haemoglobin to form Oxyhaemoglobin. Each haemoglobin molecule can carry up to 4 oxygen molecules.
2. Oxygen Dissociation in Tissues — At the tissue site, low $pO_2$, high $pCO_2$, high $H^+$ concentration (acidic pH), and higher temperature favor the dissociation of oxygen from oxyhaemoglobin, releasing $O_2$ into the tissues.
3. Carbon Dioxide Transport as Bicarbonate (70%) — At the tissues, $CO_2$ diffuses into RBCs where it reacts with water to form carbonic acid ($H_2CO_3$), catalyzed by carbonic anhydrase. This dissociates into $HCO_3^-$ and $H^+$. Bicarbonate then diffuses into the plasma.
4. Release of CO2 in Alveoli — At the alveoli, where $pCO_2$ is low, the reverse reaction occurs: $HCO_3^-$ and $H^+$ recombine to form $H_2CO_3$, which is cleaved into $H_2O$ and $CO_2$. The $CO_2$ is then exhaled.

Numerical Calculations and Partial Pressure Profiles

• {"title":"Calculating Vital Capacity (VC)","description":"Question: A patient has a Tidal Volume (TV) of 500 mL, an Inspiratory Reserve Volume (IRV) of 2800 mL, and an Expiratory Reserve Volume (ERV) of 1100 mL. Calculate their Vital Capacity (VC).\nFormula: $VC = TV + IRV + ERV$\nCalculation: $VC = 500 + 2800 + 1100 = 4400$ mL."}
• {"title":"Alveolar vs Tissue Partial Pressure Gradient","description":"Compare partial pressures ($pO_2$ and $pCO_2$) to understand passive diffusion:\n- Alveoli: $pO_2 = 104$ mm Hg, $pCO_2 = 40$ mm Hg\n- Deoxygenated Blood: $pO_2 = 40$ mm Hg, $pCO_2 = 45$ mm Hg\n- Oxygenated Blood: $pO_2 = 95$ mm Hg, $pCO_2 = 40$ mm Hg\n- Tissues: $pO_2 = 40$ mm Hg, $pCO_2 = 45$ mm Hg. This steep gradient drives continuous gas exchange."}

High-Yield Board Exam Traps

1. Oxygen Dissociation Curve Shifts: Remember the acronym CADET, face Right! (CO2, Acid/H+, DPG, Exercise, Temp shift the curve to the Right, meaning lower oxygen affinity).
2. The Carbonic Anhydrase Trap: Students often forget that carbonic anhydrase is present in extremely high concentrations inside RBCs, but only in minute quantities in the plasma. Do not state that this reaction occurs primarily in the plasma!
3. Volume Calculations: Be careful not to include Residual Volume (RV) in the Vital Capacity formula. RV can never be expired, so it is only part of Functional Residual Capacity (FRC) and Total Lung Capacity (TLC).

Quick Chapter Revision Check

• Why does carbon dioxide diffuse faster than oxygen across the respiratory membrane? The solubility of carbon dioxide ($CO_2$) is 20 to 25 times higher than that of oxygen ($O_2$). Therefore, the rate of diffusion of $CO_2$ per unit difference in partial pressure is much higher.
• What is the role of the pneumotaxic centre in breathing regulation? The pneumotaxic centre, located in the pons of the brain, acts as a switch-off point for inspiration. It sends inhibitory signals to the respiratory rhythm centre, limiting the duration of inspiration and thereby altering the respiratory rate.
• State the conditions that favor the formation of oxyhaemoglobin in the lungs. The formation of oxyhaemoglobin is favored by high $pO_2$, low $pCO_2$, lower $H^+$ concentration (alkaline pH), and lower temperature within the alveoli.
• What is Occupational Respiratory Disorder? Give an example. These are lung diseases caused by the long-term inhalation of dust particles in industrial settings (e.g., stone-breaking or mining). Examples include Silicosis and Asbestosis, which lead to lung fibrosis and serious lung damage.

Frequently Asked Questions