Chapter Notes

Breathing and Exchange of Gases

All living organisms need energy to carry out various activities. This energy is derived from the breakdown of simple molecules like glucose, amino acids, and fatty acids. This process, known as catabolism, requires a continuous supply of oxygen ($O_2$) and produces a harmful waste product, carbon dioxide ($CO_2$).

Breathing is the process of exchanging oxygen from the atmosphere with the carbon dioxide produced by the body's cells. While we often use the term "respiration" for breathing, in biology, respiration is a broader term that includes breathing as well as the cellular processes that use oxygen to produce energy.

The main steps involved in respiration are:

1. Breathing (Pulmonary Ventilation): The process of drawing atmospheric air into the lungs (inspiration) and releasing carbon dioxide-rich air out (expiration).
2. Diffusion across Alveolar Membrane: The exchange of gases ($O_2$ and $CO_2$) between the air in the lungs and the blood.
3. Transport of Gases: The circulation of these gases throughout the body by the blood.
4. Diffusion between Blood and Tissues: The exchange of $O_2$ and $CO_2$ between the blood and the body's tissues.
5. Cellular Respiration: The utilization of $O_2$ by cells to break down nutrients and release energy, with the resulting production of $CO_2$.

Respiratory Organs

Different animals have evolved various mechanisms for breathing, depending on their habitat and complexity.

• Simple Invertebrates (sponges, coelenterates, flatworms): Exchange gases by simple diffusion across their entire body surface.
• Earthworms: Use their moist skin, or cuticle, for gas exchange (cutaneous respiration).
• Insects: Have a network of tracheal tubes that transport atmospheric air directly to the cells within the body.
• Aquatic Arthropods and Molluscs: Use special vascularized structures called gills (branchial respiration).
• Terrestrial Animals (reptiles, birds, mammals): Use vascularized bags called lungs (pulmonary respiration).
• Vertebrates:
  • Fishes: Use gills.
  • Amphibians (e.g., frogs): Can respire through both their moist skin and lungs.
  • Reptiles, Birds, and Mammals: Respire through lungs.

Human Respiratory System

The human respiratory system consists of the air passages and the lungs. The path of air is as follows:

1. External Nostrils: A pair of openings above the upper lips.
2. Nasal Passage & Nasal Chamber: Air enters here and is filtered, warmed, and humidified.
3. Pharynx: A common passage for both food and air.
4. Larynx (Sound Box): A cartilaginous box that opens into the trachea and helps in sound production. The epiglottis, a thin cartilaginous flap, covers the opening of the larynx (glottis) during swallowing to prevent food from entering the windpipe.
5. Trachea (Windpipe): A straight tube extending to the mid-thoracic cavity.
6. Bronchi: The trachea divides into a right and left primary bronchi at the level of the 5th thoracic vertebra. Each bronchus further divides into secondary and tertiary bronchi.
7. Bronchioles: The bronchi continue to divide into smaller tubes called bronchioles, ending in very thin terminal bronchioles.
8. Alveoli: Each terminal bronchiole gives rise to tiny, thin-walled, vascularized sacs called alveoli. This is where gas exchange occurs.

The trachea, bronchi, and initial bronchioles are supported by incomplete cartilaginous rings, which prevent them from collapsing.

Lungs

Humans have two lungs, which are protected by a double-layered membrane called the pleura. The space between these layers is filled with pleural fluid, which lubricates the lungs and reduces friction during breathing movements.

• The outer pleural membrane is in close contact with the thoracic lining.
• The inner pleural membrane is in contact with the lung surface.

Conducting and Exchange Parts

The respiratory system is divided into two main parts based on function:

• Conducting Part: Includes the external nostrils, pharynx, larynx, trachea, bronchi, and bronchioles up to the terminal bronchioles. Its functions are to:
  • Transport atmospheric air to the alveoli.
  • Clear the air of foreign particles.
  • Humidify the air.
  • Bring the air to body temperature.
• Respiratory or Exchange Part: Includes the alveoli and their ducts. This is the actual site where the diffusion of $O_2$ and $CO_2$ occurs between blood and atmospheric air.

The lungs are located in the thoracic chamber, an air-tight cavity formed by the vertebral column (dorsally), the sternum (ventrally), the ribs (laterally), and the dome-shaped diaphragm (on the lower side). This setup ensures that any change in the volume of the thoracic cavity is reflected in the lung (pulmonary) cavity, which is essential for breathing.

Mechanism of Breathing

Breathing is the process of moving air into and out of the lungs. It involves two stages:

• Inspiration (Inhalation): Atmospheric air is drawn in.
• Expiration (Exhalation): Alveolar air is released out.

This movement of air is driven by creating a pressure gradient between the lungs and the atmosphere.

Inspiration

Inspiration is an active process initiated by the contraction of two sets of muscles:

1. Diaphragm: When it contracts, it flattens, increasing the volume of the thoracic chamber in the front-to-back (antero-posterior) axis.
2. External Intercostal Muscles: These muscles, located between the ribs, contract to lift the ribs and the sternum upwards and outwards. This increases the volume of the thoracic chamber in the side-to-side (dorso-ventral) axis.

The overall increase in thoracic volume causes the lungs to expand, increasing the pulmonary volume. According to Boyle's law, an increase in volume leads to a decrease in pressure. The intra-pulmonary pressure drops below the atmospheric pressure, creating a negative pressure that forces air from the outside into the lungs.

Expiration

Under normal conditions, expiration is a passive process:

1. The diaphragm and external intercostal muscles relax.
2. The diaphragm returns to its dome shape, and the ribs and sternum return to their normal positions.
3. This decreases the thoracic volume, which in turn decreases the pulmonary volume.
4. The decrease in volume increases the intra-pulmonary pressure to slightly above atmospheric pressure.
5. This pressure difference forces the air out of the lungs.

For forceful breathing (e.g., during exercise), additional abdominal muscles can be used to increase the strength of both inspiration and expiration. A healthy human breathes about 12-16 times per minute. The volume of air involved in breathing can be measured using a spirometer.

Respiratory Volumes and Capacities

These are measurements of lung function used in clinical diagnosis.

Respiratory Volumes

• Tidal Volume (TV): The volume of air inspired or expired during a normal, quiet breath. It is approximately 500 mL. This means a healthy person moves about 6000 to 8000 mL (6 to 8 litres) of air per minute.
• Inspiratory Reserve Volume (IRV): The additional volume of air a person can forcibly inhale after a normal inspiration. It is about 2500 mL to 3000 mL.
• Expiratory Reserve Volume (ERV): The additional volume of air a person can forcibly exhale after a normal expiration. It is about 1000 mL to 1100 mL.
• Residual Volume (RV): The volume of air that remains in the lungs even after a forceful expiration. This volume cannot be exhaled. It is about 1100 mL to 1200 mL.

Respiratory Capacities

These are derived by adding two or more respiratory volumes.

• Inspiratory Capacity (IC): The total volume of air a person can inhale after a normal exhalation.
  • IC = TV + IRV
• Expiratory Capacity (EC): The total volume of air a person can exhale after a normal inhalation.
  • EC = TV + ERV
• Functional Residual Capacity (FRC): The volume of air that remains in the lungs after a normal exhalation.
  • FRC = ERV + RV
• Vital Capacity (VC): The maximum volume of air a person can breathe out after a forced inhalation (or breathe in after a forced exhalation).
  • VC = ERV + TV + IRV
• Total Lung Capacity (TLC): The total volume of air the lungs can hold after a maximum forced inspiration.
  • TLC = RV + ERV + TV + IRV or TLC = VC + RV

Exchange of Gases

Gas exchange occurs in the alveoli of the lungs and at the body tissues. The process is driven by simple diffusion, based on differences in partial pressure.

Partial pressure is the pressure contributed by an individual gas in a mixture of gases. It is represented as $pO_2$ for oxygen and $pCO_2$ for carbon dioxide.

As the table shows:

• Oxygen ($O_2$) Gradient: The $pO_2$ is highest in the atmosphere, lower in the alveoli, even lower in oxygenated blood, and lowest in the tissues. This gradient drives $O_2$ from the alveoli into the blood, and from the blood into the tissues.
• Carbon Dioxide ($CO_2$) Gradient: The $pCO_2$ is highest in the tissues, lower in the blood, and lowest in the alveoli. This gradient drives $CO_2$ from the tissues into the blood, and from the blood into the alveoli to be exhaled.

The diffusion membrane in the alveoli is extremely thin (much less than a millimeter) and is made up of three layers:

1. The thin squamous epithelium of the alveoli.
2. The endothelium of the alveolar capillaries.
3. The basement substance between these two layers.

These factors—partial pressure gradients, high gas solubility, and a thin diffusion membrane—make our bodies highly efficient at gas exchange.

Transport of Gases

Blood is the medium for transporting $O_2$ and $CO_2$ throughout the body.

Transport of Oxygen

• 97% of $O_2$ is transported by Red Blood Cells (RBCs).
• 3% of $O_2$ is carried in a dissolved state in the plasma.

In RBCs, $O_2$ binds reversibly with haemoglobin, a red-colored, iron-containing pigment, to form oxyhaemoglobin. Each haemoglobin molecule can carry a maximum of four $O_2$ molecules.

The binding of oxygen to haemoglobin is influenced by several factors:

• Partial pressure of $O_2$ ($pO_2$)
• Partial pressure of $CO_2$ ($pCO_2$)
• Hydrogen ion concentration ($H^+$)
• Temperature

Oxygen Dissociation Curve

This is a sigmoid (S-shaped) curve obtained by plotting the percentage saturation of haemoglobin with $O_2$ against the $pO_2$.

• In the Alveoli (Lungs): Conditions are favourable for the formation of oxyhaemoglobin.
  • High $pO_2$
  • Low $pCO_2$
  • Lesser $H^+$ concentration (higher pH)
  • Lower temperature
• In the Tissues: Conditions are favourable for the dissociation (release) of oxygen from oxyhaemoglobin.
  • Low $pO_2$
  • High $pCO_2$
  • High $H^+$ concentration (lower pH)
  • Higher temperature

This ensures that haemoglobin picks up oxygen in the lungs and efficiently delivers it to the tissues that need it most. Every 100 mL of oxygenated blood delivers around 5 mL of $O_2$ to the tissues under normal physiological conditions.

Transport of Carbon dioxide

$CO_2$ is transported in the blood in three ways:

• 70% as bicarbonate ($HCO_3^-$): This is the primary method.
• 20-25% as carbamino-haemoglobin: $CO_2$ binds to haemoglobin.
• 7% dissolved in plasma.

Transport as Bicarbonate

RBCs contain a very high concentration of the enzyme carbonic anhydrase, which facilitates the following reversible reaction:

$CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$

• At the tissues: Where $pCO_2$ is high due to metabolism, $CO_2$ diffuses into the blood and RBCs. The reaction proceeds to the right, forming bicarbonate ions ($HCO_3^-$) and hydrogen ions ($H^+$).
• At the alveoli: Where $pCO_2$ is low, the reaction proceeds in the opposite direction (to the left). Bicarbonate ions are converted back into $CO_2$ and water. The $CO_2$ then diffuses into the alveoli to be exhaled.

Transport as Carbamino-haemoglobin

The binding of $CO_2$ to haemoglobin is affected by $pCO_2$ and $pO_2$.

• In the tissues: High $pCO_2$ and low $pO_2$ favour the binding of $CO_2$ to haemoglobin.
• In the alveoli: Low $pCO_2$ and high $pO_2$ favour the dissociation of $CO_2$ from carbamino-haemoglobin.

Every 100 mL of deoxygenated blood delivers approximately 4 mL of $CO_2$ to the alveoli.

Regulation of Respiration

The body can maintain and adjust the respiratory rhythm to meet the demands of the tissues. This is controlled by the neural system.

• Respiratory Rhythm Centre: A specialized center in the medulla region of the brain. It is primarily responsible for generating and maintaining the basic breathing rhythm.
• Pneumotaxic Centre: Located in the pons region of the brain, this center can moderate the functions of the rhythm center. Signals from this center can reduce the duration of inspiration, thereby altering the respiratory rate.
• Chemosensitive Area: Situated adjacent to the rhythm center in the medulla, this area is highly sensitive to changes in blood $CO_2$ and hydrogen ion ($H^+$) concentration. An increase in these substances activates this center, which then signals the rhythm center to make adjustments to increase breathing rate and eliminate the excess $CO_2$.
• Peripheral Receptors: Receptors in the aortic arch and carotid artery can also detect changes in $CO_2$ and $H^+$ concentration and send signals to the rhythm center for corrective action.

Disorders of Respiratory System

• Asthma: A condition causing difficulty in breathing and wheezing. It is due to the inflammation of the bronchi and bronchioles.
• Emphysema: A chronic disorder where the alveolar walls are damaged, leading to a decrease in the respiratory surface area available for gas exchange. One of the major causes is cigarette smoking.
• Occupational Respiratory Disorders: These occur in certain industries, like those involving grinding or stone-breaking, where large amounts of dust are produced. Long-term exposure to this dust can overwhelm the body's defense mechanisms, leading to inflammation and fibrosis (the growth of fibrous tissue), which causes serious lung damage. Workers in such industries should always wear protective masks.