Breathing and Exchange of Gases Class 11 Notes

Key Terminology for Breathing and Gas Exchange

Breathing (Pulmonary Ventilation)

The mechanical process of moving air into and out of the lungs. It involves two stages: inspiration (inhalation) and expiration (exhalation).

Respiration

A broader biochemical process that includes breathing, gaseous exchange, and cellular respiration (oxidation of food to release energy).

Partial Pressure

The pressure contributed by an individual gas in a mixture of gases. It is crucial for understanding the diffusion of gases across respiratory surfaces.

Tidal Volume (TV)

The volume of air inspired or expired during a normal, resting breath. Approximately 500 ml for a healthy adult.

Vital Capacity (VC)

The maximum volume of air a person can breathe out after a forced inspiration. It is the sum of IRV, TV, and ERV.

Oxygen-Haemoglobin Dissociation Curve

A sigmoid (S-shaped) curve that plots the proportion of haemoglobin saturated with oxygen against the partial pressure of oxygen. It illustrates the affinity of haemoglobin for oxygen.

Bohr Effect

A phenomenon where an increase in carbon dioxide concentration (or a decrease in blood pH) leads to a decrease in haemoglobin's affinity for oxygen, facilitating oxygen release in tissues.

Haldane Effect

A phenomenon where deoxygenation of blood (release of O₂) increases its ability to carry carbon dioxide. This facilitates CO₂ uptake in tissues and its release in the lungs.

The Human Respiratory System: Pathway of Air

The human respiratory system is divided into two main parts: the conducting part and the respiratory or exchange part. The conducting part transports atmospheric air to the alveoli, clears it of foreign particles, humidifies it, and brings it to body temperature. The pathway starts with the external nostrils leading to the nasal passage and then the nasal chamber. The air then moves through the pharynx, which is a common passage for food and air. From the pharynx, air enters the larynx (sound box) through a slit-like opening called the glottis, which is guarded by a cartilaginous flap called the epiglottis to prevent food entry during swallowing. The larynx leads to the trachea (windpipe), a straight tube supported by C-shaped cartilaginous rings that prevent it from collapsing. The trachea divides into a right and a left primary bronchus at the level of the 5th thoracic vertebra. Each bronchus undergoes repeated divisions to form secondary and tertiary bronchi, and finally, very thin terminal bronchioles. This entire network up to the terminal bronchioles constitutes the conducting part.

The respiratory part is where the actual diffusion of O₂ and CO₂ occurs. It consists of the alveoli and their ducts. The bronchioles open into clusters of tiny, thin-walled, vascularized bag-like structures called alveoli. The membrane of the alveoli and the surrounding blood capillaries form the respiratory membrane, which is extremely thin to facilitate efficient gas exchange.

Mechanism of Breathing

1. Inspiration (Active Process) —
2. Expiration (Passive Process) —

Key Respiratory Volumes and Capacities

• Tidal Volume (TV): Volume of air moved during normal breathing (~500 mL).
• Inspiratory Reserve Volume (IRV): Additional volume of air that can be forcibly inhaled after a normal inspiration (~2500-3000 mL).
• Expiratory Reserve Volume (ERV): Additional volume of air that can be forcibly exhaled after a normal expiration (~1000-1100 mL).
• Residual Volume (RV): Volume of air remaining in the lungs even after a forcible expiration (~1100-1200 mL). This air prevents the lungs from collapsing.
• Inspiratory Capacity (IC): Total volume of air a person can inhale after a normal expiration. IC = TV + IRV.
• Expiratory Capacity (EC): Total volume of air a person can exhale 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, or vice versa. VC = ERV + TV + IRV.
• Total Lung Capacity (TLC): Total volume of air accommodated in the lungs at the end of a forced inspiration. TLC = VC + RV.

Partial Pressures (in mm Hg) Driving Gas Exchange

Transport of Gases in Blood

1. Transport of Oxygen (O₂):
Oxygen is transported in two ways:

• Dissolved in Plasma (≈3%): A very small amount of O₂ is carried in the dissolved state in blood plasma due to its low solubility.
• As Oxyhaemoglobin (≈97%): The vast majority of oxygen binds reversibly with haemoglobin (Hb) in Red Blood Cells (RBCs) to form oxyhaemoglobin (HbO₂). Each Hb molecule can carry a maximum of four O₂ molecules. The binding is dependent on the partial pressure of O₂ (pO₂). In the alveoli, high pO₂ favors the formation of oxyhaemoglobin. In the tissues, low pO₂ favors the dissociation of O₂ from haemoglobin for cellular use. This relationship is depicted by the Oxygen Dissociation Curve, which is S-shaped (sigmoid). Factors like high pCO₂, high H⁺ concentration (low pH), and high temperature shift the curve to the right, promoting O₂ release (Bohr effect).

2. Transport of Carbon Dioxide (CO₂):
Carbon dioxide, being more soluble than oxygen, is transported in three ways:

• Dissolved in Plasma (≈7%): Carried in a dissolved state.
• As Carbamino-haemoglobin (≈20-25%): CO₂ binds to the amino groups of globin chains in haemoglobin to form carbamino-haemoglobin (HbCO₂). This binding is related to the pCO₂.
• As Bicarbonate Ions (≈70%): This is the primary method. CO₂ from tissues enters RBCs, where it combines with water in the presence of the enzyme carbonic anhydrase to form carbonic acid (H₂CO₃). This acid rapidly dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻). The HCO₃⁻ ions are then transported out of the RBCs into the plasma in exchange for chloride ions (Cl⁻), a process known as the chloride shift or Hamburger's phenomenon.

Common Exam Traps and Tips

Oxygen Dissociation Curve: Remember the mnemonic 'CADET, face Right!' for factors that shift the curve to the right (facilitating O₂ release): CO₂, Acid (low pH), 2,3-DPG, Exercise, and high Temperature. A left shift means increased affinity and occurs under opposite conditions (e.g., in the lungs).

Primary Respiratory Driver: A common mistake is thinking low O₂ is the main trigger for breathing. In reality, the respiratory rhythm is primarily regulated by the concentration of CO₂ and H⁺ ions in the blood. Even a small increase in pCO₂ is a potent stimulus for the chemosensitive area in the medulla.

Breathing vs. Respiration: Do not use these terms interchangeably. Breathing is the physical act of ventilation. Respiration is the entire process including gas exchange and cellular energy production. Be precise in your answers.

Enzyme Location: The enzyme carbonic anhydrase is present in very high concentrations in RBCs and in minute quantities in the plasma. This is why the conversion of CO₂ to bicarbonate happens primarily inside the red blood cells.

Worked Examples

• {"title":"Calculating Vital Capacity (VC)","content":"Problem: A person has a Tidal Volume (TV) of 500 mL, an Inspiratory Reserve Volume (IRV) of 3000 mL, and an Expiratory Reserve Volume (ERV) of 1100 mL. Calculate their Vital Capacity.\n\nSolution:\nVital Capacity (VC) is the maximum amount of air that can be moved in or out of the lungs.\nThe formula is: VC = TV + IRV + ERV\nVC = 500 mL + 3000 mL + 1100 mL\nVC = 4600 mL"}
• {"title":"Conceptual Application: The Bohr Effect","content":"Scenario: Why is more oxygen delivered to actively contracting muscle tissues than to resting tissues?\n\nExplanation:\nActively contracting muscles produce more CO₂ and lactic acid as byproducts of metabolism. This increases the pCO₂ and H⁺ concentration (lowers the pH) in the blood surrounding the muscle. According to the Bohr effect, these conditions decrease haemoglobin's affinity for oxygen. This causes the oxygen dissociation curve to shift to the right, promoting the release (dissociation) of O₂ from oxyhaemoglobin to meet the high metabolic demand of the muscle cells."}

Practice Questions with Solutions

• What is the role of the diaphragm and external intercostal muscles in inspiration? They contract. The diaphragm flattens and the external intercostal muscles lift the ribs, increasing thoracic volume and causing air to rush in.
• Why is the oxygen-haemoglobin dissociation curve sigmoid (S-shaped)? It reflects the cooperative binding of oxygen to haemoglobin. The binding of one oxygen molecule increases the affinity of haemoglobin for the next, causing a sharp rise in saturation at typical alveolar pO₂ levels.
• Name the enzyme essential for the rapid conversion of CO₂ to bicarbonate ions in RBCs. Carbonic anhydrase.
• What is the primary chemical stimulus detected by the chemosensitive area near the respiratory rhythm centre? Increased concentrations of carbon dioxide (CO₂) and hydrogen ions (H⁺) in the blood and cerebrospinal fluid.

Frequently Asked Questions

Frequently Asked Questions