Breathing and Exchange of Gases
How organisms take in oxygen, release carbon dioxide, and regulate this vital process to sustain life
In this chapter
Oxygen (O₂) is essential for all living organisms. Cells use oxygen to break down simple molecules like glucose, amino acids, and fatty acids through catabolic reactions to derive energy for various life activities. However, these reactions also produce carbon dioxide (CO₂), which is harmful if allowed to accumulate. Therefore, O₂ must be continuously supplied to the cells and the CO₂ produced must be efficiently removed.
This process of exchanging O₂ from the atmosphere with CO₂ produced by the body's cells is called breathing, commonly known as respiration. Place your hands on your chest and you can feel it moving up and down — this is the rhythmic process of breathing at work. The respiratory organs and the mechanism of breathing are explored in this chapter.
14.1 Respiratory Organs
The mechanisms of breathing vary widely among different groups of animals, depending mainly on their habitats and levels of organisation. Different animal groups have evolved remarkably diverse respiratory strategies.
Simple Diffusion
Lower invertebrates like sponges, coelenterates, and flatworms exchange O₂ with CO₂ by simple diffusion over their entire body surface.
Moist Cuticle
Earthworms use their moist cuticle for gas exchange, while insects have a network of tubes (tracheal tubes) to transport atmospheric air within the body.
Branchial Respiration
Aquatic arthropods and molluscs use vascularised structures called gills for gas exchange — a method called branchial respiration.
Pulmonary Respiration
Terrestrial forms use vascularised bags called lungs. Among vertebrates, fishes use gills while amphibians, reptiles, birds, and mammals respire through lungs.
Notably, amphibians like frogs can also respire through their moist skin — a process known as cutaneous respiration, supplementing their pulmonary breathing.
Insects breathe through a system of tracheal tubes that open to the outside through tiny pores called spiracles. This network delivers oxygen directly to the tissues without relying on the circulatory system for gas transport.
14.1.1 Human Respiratory System
The human respiratory system is a complex and highly efficient apparatus for gas exchange. Air enters through a pair of external nostrils that open out above the upper lip. These lead to a nasal chamber through the nasal passage. The nasal chamber opens into the pharynx, a portion of which serves as a common passage for both food and air.
The pharynx opens through the larynx region into the trachea. The larynx is a cartilaginous box that helps in sound production and is therefore called the sound box. During swallowing, a thin elastic cartilaginous flap called the epiglottis covers the glottis to prevent the entry of food into the larynx.
The trachea is a straight tube extending up to the mid-thoracic cavity, which divides at the level of the 5th thoracic vertebra into a right and left primary bronchi. Each bronchus undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles, ending up in very thin terminal bronchioles.
- The tracheae, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings.
- Each terminal bronchiole gives rise to a number of very thin, irregular-walled and vascularised bag-like structures called alveoli.
- The branching network of bronchi, bronchioles and alveoli together comprise the lungs.
We have two lungs which are covered by a double-layered pleura, with pleural fluid between them. This fluid reduces friction on the lung surface. The outer pleural membrane is in close contact with the thoracic lining, whereas the inner pleural membrane is in contact with the lung surface.
Conducting part — everything from external nostrils up to the terminal bronchioles. It transports atmospheric air to the alveoli, clears it from foreign particles, humidifies and brings it to body temperature.
Respiratory (exchange) part — the alveoli and their ducts. This is the site of actual diffusion of O₂ and CO₂ between blood and atmospheric air.
The lungs are situated in the thoracic chamber, which is anatomically an air-tight chamber. It is formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs, and on the lower side by the dome-shaped diaphragm. Any change in the volume of the thoracic cavity is reflected in the lung (pulmonary) cavity — this arrangement is essential for breathing since we cannot directly alter pulmonary volume.
Steps of Respiration
Respiration involves the following steps:
- Breathing or pulmonary ventilation — atmospheric air is drawn in and CO₂-rich alveolar air is released out.
- Diffusion of gases (O₂ and CO₂) across the alveolar membrane.
- Transport of gases by the blood.
- Diffusion of O₂ and CO₂ between blood and tissues.
- Utilisation of O₂ by the cells for catabolic reactions and resultant release of CO₂ (cellular respiration).
14.2 Mechanism of Breathing
Breathing involves two stages: inspiration, during which atmospheric air is drawn in, and expiration, by which the alveolar air is released out. The movement of air into and out of the lungs is achieved by creating a pressure gradient between the lungs and the atmosphere.
Inspiration
Occurs when intra-pulmonary pressure is less than atmospheric pressure — there is a negative pressure in the lungs with respect to the atmosphere, causing air to rush in.
Expiration
Takes place when intra-pulmonary pressure is higher than atmospheric pressure, forcing air out of the lungs.
The diaphragm and a specialised set of muscles — the external and internal intercostals between the ribs — help in generating these pressure gradients.
Inspiration in Detail
Inspiration is initiated by the contraction of the diaphragm, which increases the volume of the thoracic chamber in the antero-posterior axis. The contraction of the external intercostal muscles lifts the ribs and the sternum, causing an increase in the volume of the thoracic chamber in the dorso-ventral axis.
The overall increase in thoracic volume causes a similar increase in pulmonary volume. This increase in pulmonary volume decreases the intra-pulmonary pressure to less than the atmospheric pressure, which forces air from outside to move into the lungs — inspiration.
Expiration in Detail
Relaxation of the diaphragm and the intercostal muscles returns the diaphragm and sternum to their normal positions, reducing the thoracic volume and thereby the pulmonary volume. This leads to an increase in intra-pulmonary pressure to slightly above the atmospheric pressure, causing the expulsion of air from the lungs — expiration.
We have the ability to increase the strength of both inspiration and expiration with the help of additional muscles in the abdomen. On an average, a healthy human breathes 12–16 times per minute. The volume of air involved in breathing movements can be estimated using a spirometer, which is used for clinical assessment of pulmonary functions.
14.2.1 Respiratory Volumes and Capacities
Several important measurements describe how much air the lungs can hold and move. These are clinically significant for diagnosing respiratory conditions.
| Volume / Capacity | Abbreviation | Description | Average Value |
|---|---|---|---|
| Tidal Volume | TV | Volume of air inspired or expired during a normal respiration | ~500 mL |
| Inspiratory Reserve Volume | IRV | Volume of air that can be inspired by a forcible inspiration beyond normal tidal volume | 2500–3000 mL |
| Expiratory Reserve Volume | ERV | Volume of air that can be expired by a forcible expiration beyond normal tidal volume | 1000–1100 mL |
| Residual Volume | RV | Volume of air remaining in the lungs even after a forcible expiration | 1100–1200 mL |
A healthy man can inspire or expire approximately 6000 to 8000 mL of air per minute. By combining the above volumes, we derive various pulmonary capacities, which are used in clinical diagnosis.
| Capacity | Formula | Description |
|---|---|---|
| Inspiratory Capacity (IC) | TV + IRV | Total volume of air a person can inspire after a normal expiration |
| Expiratory Capacity (EC) | TV + ERV | Total volume of air a person can expire after a normal inspiration |
| Functional Residual Capacity (FRC) | ERV + RV | Volume of air remaining in the lungs after a normal expiration |
| Vital Capacity (VC) | ERV + TV + IRV | Maximum volume of air a person can breathe in after a forced expiration, or breathe out after a forced inspiration |
| Total Lung Capacity (TLC) | RV + ERV + TV + IRV | Total volume of air accommodated in the lungs at the end of a forced inspiration (VC + RV) |
14.3 Exchange of Gases
Alveoli are the primary sites of exchange of gases. Gas exchange also occurs between blood and tissues. O₂ and CO₂ are exchanged at these sites by simple diffusion, mainly driven by pressure or concentration gradients.
Two additional factors affect the rate of diffusion: the solubility of the gases and the thickness of the membranes involved in diffusion.
The pressure contributed by an individual gas in a mixture of gases is called its partial pressure, represented as pO₂ for oxygen and pCO₂ for carbon dioxide.
Table 14.1 — Partial Pressures (in mm Hg) at Different Sites
| Respiratory Gas | Atmospheric Air | Alveoli | Blood (Deoxygenated) | Blood (Oxygenated) | Tissues |
|---|---|---|---|---|---|
| O₂ | 159 | 104 | 40 | 95 | 40 |
| CO₂ | 0.3 | 40 | 45 | 40 | 45 |
This data clearly shows a concentration gradient for oxygen from alveoli to blood and blood to tissues. Similarly, a gradient is present for CO₂ in the opposite direction — from tissues to blood and blood to alveoli.
The solubility of CO₂ is 20–25 times higher than that of O₂. This means the amount of CO₂ that can diffuse through the diffusion membrane per unit difference in partial pressure is much higher compared to O₂ — even though the partial pressure difference for CO₂ is relatively small.
The Diffusion Membrane
The diffusion membrane is made up of three major layers:
- The thin squamous epithelium of alveoli
- The endothelium of alveolar capillaries
- The basement substance between them — composed of a thin basement membrane supporting the squamous epithelium and the basement membrane surrounding the single-layer endothelial cells of capillaries
Despite having three layers, the total thickness of the diffusion membrane is much less than a millimetre. All these factors in our body are therefore favourable for diffusion of O₂ from alveoli to tissues and that of CO₂ from tissues to alveoli.
14.4 Transport of Gases
Blood serves as the medium of transport for O₂ and CO₂. The distribution of these gases across different forms is as follows:
O₂ Transport
About 97% of O₂ is transported by RBCs. The remaining 3% is carried in a dissolved state through the plasma.
CO₂ Transport
Nearly 70% of CO₂ is carried as bicarbonate. About 20–25% is transported by RBCs. About 7% is dissolved in plasma.
14.4.1 Transport of Oxygen
Haemoglobin is a red-coloured iron-containing pigment present in the RBCs. O₂ can bind with haemoglobin in a reversible manner to form oxyhaemoglobin. Each haemoglobin molecule can carry a maximum of four molecules of O₂.
The binding of oxygen with haemoglobin is primarily related to the partial pressure of O₂. Several other factors can also interfere with this binding:
- Partial pressure of CO₂ — higher pCO₂ promotes dissociation
- Hydrogen ion concentration (H⁺) — higher H⁺ promotes dissociation
- Temperature — higher temperature promotes dissociation
The Oxygen Dissociation Curve
When the percentage saturation of haemoglobin with O₂ is plotted against the pO₂, a sigmoid (S-shaped) curve is obtained. This is called the oxygen dissociation curve. It is extremely useful in studying the effect of factors like pCO₂, H⁺ concentration, and temperature on the binding of O₂ with haemoglobin.
In the alveoli: High pO₂, low pCO₂, lesser H⁺ concentration, and lower temperature — all conditions are favourable for the formation of oxyhaemoglobin.
In the tissues: Low pO₂, high pCO₂, high H⁺ concentration, and higher temperature — conditions favour dissociation of oxygen from oxyhaemoglobin.
This means O₂ gets bound to haemoglobin at the lung surface and gets dissociated at the tissues. Every 100 mL of oxygenated blood can deliver around 5 mL of O₂ to the tissues under normal physiological conditions.
14.4.2 Transport of Carbon Dioxide
CO₂ is transported by haemoglobin as carbamino-haemoglobin (about 20–25%). This binding is related to the partial pressure of CO₂, while pO₂ is a major factor that affects this binding.
When pCO₂ is high and pO₂ is low (as in the tissues), more binding of carbon dioxide occurs. When pCO₂ is low and pO₂ is high (as in the alveoli), dissociation of CO₂ from carbamino-haemoglobin takes place — meaning the CO₂ bound to haemoglobin from the tissues is delivered at the alveoli.
The Bicarbonate Buffer System
RBCs contain a very high concentration of the enzyme carbonic anhydrase, and minute quantities of the same are present in the plasma too. This enzyme facilitates the following reversible reaction:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺
Carbonic anhydrase catalyses this reaction in both directions
- At the tissue site (high pCO₂ due to catabolism): CO₂ diffuses into blood (RBCs and plasma) and forms HCO₃⁻ and H⁺.
- At the alveolar site (low pCO₂): The reaction proceeds in the opposite direction, leading to the formation of CO₂ and H₂O.
Thus, CO₂ trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO₂. Every 100 mL of deoxygenated blood delivers approximately 4 mL of CO₂ to the alveoli.
14.5 Regulation of Respiration
Human beings have a remarkable ability to maintain and moderate the respiratory rhythm to suit the demands of body tissues. This regulation is accomplished primarily by the neural system.
Respiratory Rhythm Centre
A specialised centre present in the medulla region of the brain. It is primarily responsible for the regulation of respiration.
Pneumotaxic Centre
Present in the pons region of the brain. It can moderate the functions of the respiratory rhythm centre. Neural signals from this centre can reduce the duration of inspiration and thereby alter the respiratory rate.
A chemosensitive area is situated adjacent to the rhythm centre, which is highly sensitive to CO₂ and hydrogen ions. Increase in these substances can activate this centre, which in turn signals the rhythm centre to make necessary adjustments in the respiratory process by which these substances can be eliminated.
Receptors associated with the aortic arch and carotid artery can also recognise changes in CO₂ and H⁺ concentration and send necessary signals to the rhythm centre for remedial actions.
Interestingly, the role of oxygen in the regulation of respiratory rhythm is quite insignificant. It is primarily CO₂ and H⁺ that drive the neural regulation of breathing under normal conditions.
14.6 Disorders of Respiratory System
Several conditions can impair the normal functioning of the respiratory system, ranging from chronic inflammatory conditions to occupational hazards.
Asthma
A difficulty in breathing causing wheezing, due to inflammation of bronchi and bronchioles. It is characterised by spasm of the smooth muscles of the airways, narrowing the lumen.
Emphysema
A chronic disorder in which alveolar walls are damaged, leading to a decreased respiratory surface. One of the major causes is cigarette smoking. The walls of the alveoli break down, reducing the surface area for gas exchange.
In certain industries, especially those involving grinding or stone-breaking, so much dust is produced that the body's defense mechanism cannot fully cope with the situation. Long exposure can give rise to inflammation leading to fibrosis (proliferation of fibrous tissues) and thus causing serious lung damage. Workers in such industries should wear protective masks.
Other notable respiratory disorders include bronchitis (inflammation of the bronchi), pneumonia (infection of the alveoli, often bacterial), and tuberculosis (caused by Mycobacterium tuberculosis, affecting the lungs). These conditions can range from mild to life-threatening and require appropriate medical treatment.
14.7 Summary
• Cells utilise oxygen for metabolism and produce energy along with carbon dioxide, which is harmful. Animals have evolved different mechanisms for oxygen transport and CO₂ removal.
• We have a well-developed respiratory system comprising two lungs and associated air passages. The first step is breathing — inspiration (air drawn in) and expiration (alveolar air released).
• Exchange of O₂ and CO₂ between deoxygenated blood and alveoli, transport of gases by blood, exchange between oxygenated blood and tissues, and utilisation of O₂ by cells (cellular respiration) are the subsequent steps.
• Inspiration and expiration are carried out by creating pressure gradients between the atmosphere and the alveoli using intercostals and diaphragm. Air volumes can be measured with a spirometer.
• Gas exchange occurs by simple diffusion, dependent on partial pressure gradients, solubility, and membrane thickness. These factors favour O₂ diffusion from alveoli to blood and CO₂ in the opposite direction.
• Oxygen is transported mainly as oxyhaemoglobin (~97%). Nearly 70% of CO₂ is transported as bicarbonate (HCO₃⁻) via carbonic anhydrase, while 20–25% is carried as carbamino-haemoglobin.
• Respiratory rhythm is maintained by the respiratory centre in the medulla, moderated by the pneumotaxic centre in the pons and a chemosensitive area sensitive to CO₂ and H⁺.