Comprehensive Biology Notes: Breathing and Exchange of Gases

Overview of Gaseous Exchange and Diffusion

• Primary Sites of Gaseous Exchange: The prime site for gaseous diffusion is the alveoli of the lungs. However, diffusion also occurs between blood vessels and tissues.
• Atmospheric Components: The atmosphere consists of multiple gases, primarily:     * $78\% \, N_2$     * $21\% \, O_2$     * Total Atmospheric Pressure = $760 \, mmHg$
• Partial Pressure Definition: In a mixture of gases, the pressure contributed by an individual gas is known as its partial pressure. It is represented as $pO_2$ for Oxygen and $pCO_2$ for Carbon dioxide.     * Calculation for $pO_2$: $20\% \text{ of } 760 \, mmHg = \frac{20 \times 760}{100} = 159 \, mmHg$.     * Calculation for $pCO_2$: $0.03\% \text{ of } 760 \, mmHg = \frac{0.03 \times 760}{100} = 0.3 \, mmHg$.
• Pulmonary Ventilation and Blood Oxygenation:     * Carbon dioxide ($CO_2$) is loaded from the blood into the lungs to be exhaled.     * Oxygen ($O_2$) is diffused into the blood vessel from the alveoli, resulting in oxygenated blood.     * The left side of the heart contains oxygenated blood ("ox-blood").     * Tissues receive oxygen diffused from blood vessels to perform cellular respiration for ATP formation.     * $CO_2$ is diffused into the blood from tissues as a byproduct, creating deoxygenated (impure) blood.

Factors Affecting Gas Exchange Across the Diffusion Boundary

There are three primary factors that determine the efficiency and rate of diffusion across respiratory surfaces:

• 1. Pressure/Concentration Gradient:     * Gradient refers to the difference in concentration or pressure of molecules between two sites ($A$ and $B$).     * Higher gradient leads to increased diffusion.
• 2. Solubility of Particles:     * $CO_2$ is 20-25 times more soluble than $O_2$ in blood plasma.     * Even if the pressure gradient is the same for both gases, $CO_2$ will still diffuse more significantly because of its higher solubility.
• 3. Thickness of the Membrane:     * Diffusion rate increases as the thickness of the membrane decreases.     * The total thickness of the diffusion membrane consists of three layers:         * The alveolar wall (squamous epithelium).         * The endothelium of the blood vessel.         * The basement substance (found between the two epithelial layers).     * The sum of these three layers is less than $1 \, mm$.

Partial Pressures of Respiratory Gases ($mmHg$)

The following data compares the partial pressures of Oxygen ($O_2$) and Carbon dioxide ($CO_2$) across various points of the respiratory and circulatory systems:

• Atmospheric Air: $pO_2 = 159 \, mmHg$; $pCO_2 = 0.3 \, mmHg$.
• Alveoli: $pO_2 = 104 \, mmHg$; $pCO_2 = 40 \, mmHg$.
• Deoxygenated Blood: $pO_2 = 40 \, mmHg$; $pCO_2 = 45 \, mmHg$.
• Oxygenated Blood: $pO_2 = 95 \, mmHg$; $pCO_2 = 40 \, mmHg$.
• Tissues: $pO_2 = 40 \, mmHg$; $pCO_2 = 45 \, mmHg$.

Pathway of Gas Transport and Blood Circulation

• Alveoli: Atmospheric air entering alveoli has $pO_2 = 104$ and $pCO_2 = 40$.
• Pulmonary Vein: Carries oxygenated blood from the lungs to the heart. This is a unique vein because it carries oxygen-rich blood.
• Systemic Arteries: Transport oxygenated blood ($pO_2 = 95$, $pCO_2 = 40$) from the heart to body tissues.
• Tissues: Use $O_2$ for metabolism and produce $CO_2$. Partial pressures at tissue level are $pO_2 = 40$ and $pCO_2 = 45$.
• Systemic Veins: Carry deoxygenated blood ($pO_2 = 40$, $pCO_2 = 45$) back to the heart.
• Pulmonary Artery: Carries deoxygenated blood from the heart to the lungs. It is the only artery that carries deoxygenated blood.

Transport of Oxygen ($O_2$)

• Modes of Transport:     * 97% is transported as oxyhaemoglobin within Red Blood Cells (RBCs).     * 3% is transported in a dissolved state through blood plasma.
• Haemoglobin (Hb) Characteristics:     * Hb is a respiratory pigment in RBCs that gives blood its red color.     * It contains Iron in the ferrous state ($Fe^{2+}$).     * One Haemoglobin molecule can transport 4 Oxygen molecules ($8 \text{ atoms of } O$).
• Oxygen Dissociation Curve:     * Binding of $O_2$ to Hb is reversible and occurs in a cooperative fashion.     * The first $O_2$ binding is difficult; the second is easier than the first; the third is even easier; the fourth is the easiest.     * The relationship results in a Sigmoid (S-shaped) curve when plotting percentage saturation of Hb against partial pressure of oxygen ($pO_2$).
• Factors Leading to Dissociation (Shift to the Right):     * Known by the acronym CADET-RIGHT:         * C: $CO_2$ (increase in $pCO_2$).         * A: Acidity ($H^+$ increase, $pH$ decrease).         * D: DPG (increase in $2,3-diphosphoglycerate$ or $DPG$).         * E: Exercise (increased physical activity).         * T: Temperature (increase in body temperature).     * Graph Shift to the Left: Occurs if all these parameters are reversed (High $pO_2$, low $pCO_2$, low $H^+$, low temperature).
• Bohr's Effect: This is the effect of $CO_2$ and $H^+$ ions on the affinity of Haemoglobin for Oxygen. High $CO_2$ or metabolic acidity causes oxygen to dissociate from Hb.
• Efficiency: Every $100 \, mL$ of oxygenated blood can deliver around $5 \, mL$ of $O_2$ to the tissues under normal physiological conditions.

Transport of Carbon Dioxide ($CO_2$)

• Modes of Transport:     * 70% as bicarbonate ions ($HCO_3^-$).     * 20-25% (approx. $23\%$) as carbamino haemoglobin ($Hb\text{-}NH\text{-}COOH$).     * 7% dissolved in plasma.
• Enzymatic Activity:     * Carbonic Anhydrase (CA) is highly abundant in RBCs, with trace amounts in plasma.     * Reaction: $CO_2 + H_2O \xrightleftharpoons{CA} H_2CO_3 \xrightleftharpoons{CA} H^+ + HCO_3^-$.
• The Chloride Shift Mechanism:     * At the tissue level, $CO_2$ diffuses into RBCs. It reacts with water to form carbonic acid which dissociates into $H^+$ and $HCO_3^-$.     * $HCO_3^-$ diffuses out of the RBC into the plasma. To maintain ionic balance, Chloride ions ($Cl^-$) diffuse from the plasma into the RBC.     * At the alveoli, the process is reversed. $HCO_3^-$ moves back into RBCs, binds with $H^+$ to form $H_2CO_3$, which then breaks down back into $CO_2$ and $H_2O$ for exhalation.
• Efficiency: Every $100 \, mL$ of deoxygenated blood delivers approximately $4 \, mL$ of $CO_2$ to the alveoli.

Regulation of Respiration

• Normal Rate: $12\text{-}16 \text{ times per minute}$. Humans maintain and moderate this rhythm according to the demands of body tissues.
• 1. Neural Regulation:     * Respiratory Rhythm Center (RRC): The main center located in the Medulla (Hindbrain); it regulates normal and forceful breathing.     * Pneumotaxic Center: Located in the Pons region of the Hindbrain. It acts as a "switch off" button for inspiration by sending inhibitory signals to the medulla, thereby decreasing the duration of inspiration and altering the respiratory rate.
• 2. Chemical Regulation:     * Mediated via Chemoreceptors sensitive to increases in $CO_2$ and $H^+$ levels. These receptors are not very sensitive to Oxygen ($O_2$ has little role in respiratory regulation).     * Central Chemoreceptors: Found in the CNS (Brain), specifically near the RRC in the medulla. They sense changes in the Cerebrospinal Fluid (CSF).     * Peripheral Chemoreceptors: Found in the Aortic arch and Carotid artery. They detect arterial gas concentrations and signal the RRC to increase the respiratory rate (especially exhalation) when $CO_2$ rises.

Respiratory Disorders

• Asthma: An allergy caused by allergens like pollen. It results in the inflammation of the bronchi and bronchioles.     * Spasm: The bronchial muscles contract/pipe is constricted.     * Symptoms: Difficulty in breathing and wheezing (a whistling sound during breathing).
• Emphysema: A chronic disorder typically caused by cigarette smoking.     * Alveolar walls are damaged, and alveoli become elongated/enlarged.     * Significant reduction in the respiratory surface area.
• Occupational Lung Disorders: Found in workers in industries involving cement, glass, and metal who are exposed to constant dust.     * Fibrosis: Proliferation of fibrous (non-stretchable) tissues leading to lung damage.     * Silicosis: Caused by inhalation of silica dust.     * Asbestosis: Caused by inhalation of asbestos.     * Prevention: Workers must wear masks.

Questions & Discussion

• Q: What are the primary sites of gaseous exchange?     * A: Alveoli (though tissues are also secondary sites).
• Q: Why do gases diffuse over respiratory surfaces?     * A: Because $pO_2$ is higher in alveoli ($104 \, mmHg$) than in blood ($40 \, mmHg$).
• Q: How is blood oxygen transported beyond the 97% in RBCs?     * A: Remaining 3% is transported in a dissolved state through plasma.
• Q: How many oxygen molecules does one haemoglobin carry?     * A: Four.
• Q: What is the shape of the oxyhaemoglobin dissociation curve?     * A: Sigmoid.
• Q: What effect does a decrease in pH (increase in acidity) have on the oxygen dissociation curve?     * A: It causes a shift to the right, promoting oxygen dissociation.
• Q: Which enzyme facilitates the reaction of $CO_2$ and water in erythrocytes?     * A: Carbonic anhydrase.
• Q: What is the Bohr's effect?     * A: The decrease in Hb-$O_2$ affinity caused by an increase in $CO_2$ and $H^+$ concentrations.
• Q: What is the cause of asthma?     * A: Spasms in the bronchial muscles and inflammation of bronchi/bronchioles.
• Q: What condition is caused by cigarette smoking resulting in damage to alveolar walls?     * A: Emphysema.
• Q: Where is the controlling center for normal breathing located?     * A: Medulla oblongata.
• Q: What substances are the chemoreceptors in the aortic arch and carotid artery primarily sensitive to?     * A: $CO_2$ and $H^+$ ions.