NEET UG - Breathing and exchange of gases - mechanism of respiration, transport of gases E-Book Study Material

Breathing and Exchange of Gases

Mechanism of Respiration

Respiration is a vital physiological process that involves the exchange of gases between an organism and its environment. In humans, this process is divided into two main stages: ventilation (breathing) and the actual exchange of gases across the respiratory surfaces. Ventilation is the mechanical process of moving air into and out of the lungs, while gas exchange involves the diffusion of oxygen from the lungs into the blood and carbon dioxide from the blood into the lungs.

1. Ventilation (Breathing)

Breathing is a two-step process: inspiration (inhalation) and expiration (exhalation). These processes are brought about by changes in the volume of the thoracic cavity, which in turn causes changes in the pressure within the lungs.

a. Inspiration (Inhalation)

Inspiration is an active process. It involves the contraction of two sets of muscles: the diaphragm and the external intercostal muscles.

Diaphragm: This is a large, dome-shaped muscle located at the base of the thoracic cavity, separating it from the abdominal cavity. When the diaphragm contracts, it flattens and moves downwards. This increases the vertical dimension of the thoracic cavity.
External Intercostal Muscles: These muscles are located between the ribs. When they contract, they pull the ribs upwards and outwards. This increases the anteroposterior (front-to-back) and lateral (side-to-side) dimensions of the thoracic cavity.

The combined action of these muscles leads to an overall increase in the volume of the thoracic cavity. As the thoracic cavity expands, the parietal pleura (the outer lining of the lungs) is pulled outwards. Since the visceral pleura (the inner lining of the lungs) is closely adhered to the parietal pleura via pleural fluid, the lungs also expand. According to Boyle's Law (which states that at a constant temperature, the pressure of a gas is inversely proportional to its volume), an increase in lung volume causes a decrease in the intra-pulmonary pressure (pressure inside the lungs). When the intra-pulmonary pressure falls below the atmospheric pressure, air rushes into the lungs from the atmosphere.

Boyle's Law Reminder: P₁V₁ = P₂V₂. If volume (V) increases, pressure (P) must decrease, and vice-versa, assuming temperature remains constant. This is the fundamental principle behind breathing.

b. Expiration (Exhalation)

Expiration is typically a passive process during normal, quiet breathing. It relies on the elastic recoil of the lungs and chest wall.

The diaphragm and external intercostal muscles relax.
As they relax, the diaphragm returns to its dome shape and moves upwards.
The ribs move downwards and inwards due to gravity and the elastic recoil of the costal cartilages.

These actions lead to a decrease in the volume of the thoracic cavity and, consequently, the lungs. According to Boyle's Law, this decrease in volume causes an increase in intra-pulmonary pressure, making it higher than the atmospheric pressure. As a result, air is forced out of the lungs.

Forced or vigorous expiration, such as during exercise or coughing, is an active process. It involves the contraction of additional muscles:

Internal Intercostal Muscles: These muscles pull the ribs downwards and inwards, further reducing the thoracic volume.
Abdominal Muscles: Contraction of abdominal muscles pushes the abdominal organs upwards against the diaphragm, decreasing the vertical dimension of the thoracic cavity.

2. Lung Volumes and Capacities

Pulmonary function can be assessed by measuring various lung volumes and capacities using a spirometer. These measurements help diagnose respiratory diseases.

Tidal Volume (TV): The volume of air inhaled or exhaled during a normal, quiet breath. Approximately 500 mL.
Inspiratory Reserve Volume (IRV): The additional volume of air that can be inhaled forcefully after a normal inspiration. Approximately 2500-3300 mL.
Expiratory Reserve Volume (ERV): The additional volume of air that can be exhaled forcefully after a normal expiration. Approximately 1000-1200 mL.
Residual Volume (RV): The volume of air remaining in the lungs even after a forceful exhalation. Approximately 1100-1200 mL. This volume ensures that gas exchange can continue even between breaths.

These volumes combine to form lung capacities:

Inspiratory Capacity (IC): The total volume of air a person can inhale after a normal expiration (TV + IRV).
Expiratory Capacity (EC): The total volume of air a person can exhale after a normal inspiration (TV + ERV).
Functional Residual Capacity (FRC): The volume of air remaining in the lungs after a normal expiration (ERV + RV).
Vital Capacity (VC): The maximum volume of air a person can exhale after a maximal inspiration (TV + IRV + ERV). This represents the maximum amount of air that can be moved in and out of the lungs.
Total Lung Capacity (TLC): The total volume of air in the lungs after a maximal inspiration (VC + RV or TV + IRV + ERV + RV). Approximately 6000 mL in adult males.

Shortcut: Think of VC as your "breathing power" – how much air you can actively move. TLC is the total capacity of your lungs. RV is the "emergency reserve" that never leaves.

3. Exchange of Gases

The actual exchange of respiratory gases (oxygen and carbon dioxide) occurs across specialized surfaces through diffusion. This process is driven by differences in the partial pressures of these gases. The primary sites for gas exchange in humans are the alveoli in the lungs and the capillaries surrounding them, and also at the tissue level between blood and body cells.

a. Gas Exchange in the Alveoli

The alveoli provide an enormous surface area (about 70-100 square meters) for gas exchange, with a very thin barrier (0.5 micrometers) consisting of the alveolar epithelium, the capillary endothelium, and their fused basement membrane.

The partial pressures of oxygen (PO₂) and carbon dioxide (PCO₂) in the alveoli and in the blood are crucial:

Inhaled air (Alveoli): PO₂ is high (approx. 104 mmHg), PCO₂ is low (approx. 40 mmHg).
Deoxygenated blood (Pulmonary Artery): PO₂ is low (approx. 40 mmHg), PCO₂ is high (approx. 45 mmHg).
Oxygenated blood (Pulmonary Vein): PO₂ is high (approx. 95 mmHg), PCO₂ is low (approx. 40 mmHg).

Oxygen diffuses from the alveoli, where its partial pressure is high, into the deoxygenated blood in the pulmonary capillaries, where its partial pressure is low. Carbon dioxide diffuses in the opposite direction, from the blood into the alveoli, because its partial pressure is higher in the blood than in the alveoli.

Partial Pressure Gradient: Diffusion always occurs from an area of higher partial pressure to an area of lower partial pressure. This gradient is the driving force for gas exchange.

b. Gas Exchange at the Tissues

A similar exchange occurs at the systemic tissues. Oxygenated blood arrives at the tissues with a high PO₂ (approx. 95 mmHg) and low PCO₂ (approx. 40 mmHg). The tissue cells, constantly using oxygen for cellular respiration and producing carbon dioxide, have a low PO₂ (approx. 40 mmHg) and a high PCO₂ (approx. 45 mmHg).

Oxygen diffuses from the blood into the tissue cells, where it is needed for metabolism. Carbon dioxide diffuses from the tissue cells into the blood to be transported back to the lungs.

The efficiency of gas exchange depends on:

The partial pressure gradients of O₂ and CO₂ across the exchange surfaces.
The solubility of the gases in the respiratory membrane (CO₂ is about 20-25 times more soluble than O₂).
The thickness of the respiratory membrane.
The surface area available for diffusion.

Transport of Gases

After gases are exchanged between the alveoli and blood, they need to be transported throughout the body. The blood, primarily through its plasma and red blood cells, is responsible for this vital function.

1. Transport of Oxygen

Oxygen is transported in the blood in two ways:

Dissolved in plasma: A very small amount of oxygen (about 3%) dissolves directly in the blood plasma. This is insufficient for the body's needs.
Bound to Hemoglobin: The vast majority of oxygen (about 97%) is transported bound to hemoglobin, a protein found within red blood cells. Each hemoglobin molecule contains four heme groups, and each heme group can bind to one molecule of oxygen. Therefore, one hemoglobin molecule can transport up to four molecules of oxygen. The oxygen-bound hemoglobin is called oxyhemoglobin.

The binding of oxygen to hemoglobin is a reversible process and is influenced by the partial pressure of oxygen (PO₂). This relationship is depicted by the oxygen-hemoglobin dissociation curve, which is typically sigmoid (S-shaped).

In the lungs: High PO₂ facilitates the binding of oxygen to hemoglobin, leading to almost complete saturation of hemoglobin.
In the tissues: Low PO₂, higher PCO₂, higher H⁺ concentration (lower pH), and higher temperature shift the dissociation curve to the right. This means hemoglobin has a lower affinity for oxygen, facilitating its release to the tissues.

Oxygen-hemoglobin Dissociation Curve Factors:

High PO₂: Favors O₂ binding (lungs).
Low PO₂: Favors O₂ release (tissues).
Low PCO₂: Favors O₂ binding.
High PCO₂: Favors O₂ release.
Low H⁺ (High pH): Favors O₂ binding (Bohr effect).
High H⁺ (Low pH): Favors O₂ release (Bohr effect).
Low Temperature: Favors O₂ binding.
High Temperature: Favors O₂ release.

Remember these factors as the "Bohr effect" for O₂ release.

2. Transport of Carbon Dioxide

Carbon dioxide is transported in the blood in three forms:

Dissolved in plasma: About 7% of CO₂ dissolves directly in the blood plasma and is transported as carbonic acid (H₂CO₃), which quickly dissociates into H⁺ and HCO₃⁻ ions. However, only a small fraction remains dissolved as CO₂.
Bound to Hemoglobin: About 20-25% of CO₂ is transported bound to the amino groups of hemoglobin molecules, forming carbaminohemoglobin. This binding also occurs in the tissues where CO₂ concentration is high. Hemoglobin can bind both oxygen and carbon dioxide, but they bind to different sites.
As Bicarbonate Ions (HCO₃⁻): This is the most significant form of CO₂ transport, accounting for about 70% of the total. In the red blood cells, CO₂ reacts with water in the presence of an enzyme called carbonic anhydrase to form carbonic acid. Carbonic acid then rapidly dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻).
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

The bicarbonate ions then move out of the red blood cells into the plasma in exchange for chloride ions (Cl⁻). This exchange is known as the chloride shift.

In the lungs, the process is reversed. Bicarbonate ions re-enter the red blood cells, combine with H⁺ ions to form carbonic acid, which is then converted back to CO₂ and water by carbonic anhydrase. The CO₂ then diffuses out of the blood into the alveoli.

Carbonic Anhydrase: This enzyme is crucial. It speeds up the conversion of CO₂ and water to carbonic acid by about 10,000 times. It's found in high concentrations in red blood cells.

CO₂ Transport Summary:

7% dissolved in plasma.
20-25% bound to hemoglobin (carbaminohemoglobin).
70% as bicarbonate ions (HCO₃⁻) via chloride shift.

Think of it as 7-25-70, a quick way to remember the percentages.

3. Regulation of Respiration

Breathing is largely involuntary and is regulated by the respiratory centers in the brainstem.

Medulla Oblongata: Contains the dorsal respiratory group (DRG) and ventral respiratory group (VRG). The DRG primarily controls the rhythm of breathing by sending signals to the diaphragm and external intercostal muscles for inspiration. The VRG is involved in forced breathing.
Pons: Contains the pneumotaxic and apneustic centers, which modulate the activity of the medullary centers to fine-tune breathing rate and depth.

Chemoreceptors, sensitive to changes in CO₂ and H⁺ levels in the blood and cerebrospinal fluid, also play a significant role in regulating respiration. An increase in PCO₂ is the most potent stimulus for increasing the rate and depth of breathing.