BREATHING AND EXCHANGE OF GASES

1. Introduction To Breathing And Gaseous Exchange

• Oxygen (O₂) is very important for living organisms. It helps in breaking down simple food molecules like glucose, amino acids, and fatty acids. This breakdown gives energy to the body, which is needed for all activities.
• While oxygen helps release energy, it also creates carbon dioxide (CO₂) as a waste product. This gas is harmful and must be removed from the body regularly.
• Since oxygen is used and carbon dioxide is produced all the time, the body must keep taking in oxygen and pushing out carbon dioxide. This constant exchange is necessary for survival.
• The process of taking in oxygen from the air and removing carbon dioxide from the body is called breathing. In simple words, breathing is how our body exchanges gases with the environment.
• If you place your hands on your chest, you can feel your chest moving up and down. This movement is caused by breathing. It shows that air is going in and out of the lungs.
• The body has special parts called respiratory organs that help in breathing. These organs, along with a proper system, help take in oxygen and push out carbon dioxide. The next sections of this chapter explain how breathing works in detail.

---

2. Organs Involved in Breathing

• Mechanisms of breathing are different in various animals. This depends on their habitat (where they live) and how their body is structured.
• Lower invertebrates like sponges, coelenterates, and flatworms exchange gases by simple diffusion. They don’t have special organs for breathing. Instead, oxygen (O₂) and carbon dioxide (CO₂) move across their entire body surface.
• Earthworms breathe through their moist skin (cuticle). This skin helps in the exchange of O₂ and CO₂ directly with the air.
• Insects have a tracheal system. It is made of tiny air tubes that carry atmospheric oxygen directly to body cells and remove carbon dioxide.
• Many aquatic arthropods and molluscs use gills to breathe. These are vascularised structures, meaning they have blood vessels. This process is called branchial respiration.
• Terrestrial animals such as reptiles, birds, and mammals use lungs for breathing. Lungs are vascularised sacs where oxygen and carbon dioxide are exchanged. This is known as pulmonary respiration.
• Fishes, which are also vertebrates, breathe through gills. These gills are specially adapted for life in water.
• Amphibians, like frogs, can breathe through their lungs, but they can also exchange gases through their moist skin. This is called cutaneous respiration.

---

3. Components of Human Respiratory Tract

• We have a pair of external nostrils located above the upper lips. These nostrils open into a nasal passage, which leads to a nasal chamber.
• The nasal chamber opens into the pharynx, a region that serves as a common path for both food and air.
• The pharynx connects to the larynx, which leads into the trachea. The larynx is a cartilaginous box that helps in producing sound, so it’s also known as the sound box.
• During swallowing, a thin elastic cartilage called the epiglottis covers the glottis. This prevents food from entering the larynx.
• The trachea is a straight tube that reaches down to the middle of the chest (thoracic cavity). At the level of the 5th thoracic vertebra, it divides into the right and left primary bronchi.
• Each primary bronchus divides further to form secondary bronchi, tertiary bronchi, and then into smaller tubes called bronchioles, which finally end in terminal bronchioles.
• The trachea, bronchi, and initial bronchioles are supported by incomplete rings of cartilage, which keep the airways open.
• Each terminal bronchiole gives rise to many small, thin-walled, and vascularised structures called alveoli, where gas exchange happens.
• The network formed by the bronchi, bronchioles, and alveoli together makes up the lungs.
• There are two lungs, and each is covered by a double-layered pleura. Between these layers is pleural fluid, which helps reduce friction when the lungs move.
• The outer pleural membrane is attached to the chest wall (thoracic lining), while the inner pleural membrane is attached to the lung surface.
• The part of the respiratory system from the nostrils to the terminal bronchioles is called the conducting part. It transports air, filters foreign particles, adds moisture, and warms the air.
• The alveoli and their ducts form the respiratory or exchange part, where actual gas exchange of oxygen (O₂) and carbon dioxide (CO₂) takes place between air and blood.
• The lungs are located in the thoracic cavity, which is a completely closed, air-tight space.
• The thoracic cavity is formed by the vertebral column at the back, the sternum at the front, the ribs on the sides, and the dome-shaped diaphragm at the bottom.
• Any change in the size of the thoracic cavity leads to a change in lung volume. This link between the chest cavity and lung cavity is essential, because we cannot directly control lung size.

---

Respiration includes the following important steps:-

• Breathing or pulmonary ventilation – Air from outside enters the lungs, and CO₂-rich air is pushed out.
• Gas diffusion across alveolar membrane – Oxygen enters the blood, and CO₂ leaves the blood through the alveoli.
• Transport of gases through blood – Blood carries oxygen and carbon dioxide to and from the tissues.
• Diffusion of gases between blood and body tissues – Oxygen goes into the tissues and carbon dioxide comes back into the blood.
• Cellular respiration – Oxygen is used by cells to break down nutrients (catabolic reactions), and carbon dioxide is released as a waste product.

---

4. Process of Inhalation and Exhalation

• Breathing has two main steps: inspiration and expiration. In inspiration, air from the atmosphere enters the lungs. In expiration, air from the lungs is pushed out.
• Air moves in and out of the lungs due to a pressure difference between the lungs and the outside atmosphere.
• Inspiration happens when the pressure inside the lungs (intra-pulmonary pressure) is lower than the atmospheric pressure. This negative pressure pulls air into the lungs.
• Expiration occurs when the intra-pulmonary pressure becomes higher than the atmospheric pressure, which forces the air out of the lungs.
• This pressure difference is created by the diaphragm and the intercostal muscles (muscles between the ribs), which help in expanding and shrinking the chest cavity.
• Inspiration starts when the diaphragm contracts, making the chest cavity larger in the front-to-back (antero-posterior) direction.
• At the same time, the external intercostal muscles contract and lift the ribs and sternum, which increases the chest size from top to bottom (dorso-ventral direction).
• The increase in chest volume leads to an increase in lung (pulmonary) volume.
• As the lung volume increases, the intra-pulmonary pressure drops below atmospheric pressure, and air flows into the lungs. This is inspiration.
• When the diaphragm and intercostal muscles relax, the chest cavity shrinks, and the lung volume decreases.
• This decrease in pulmonary volume causes the intra-pulmonary pressure to rise slightly above atmospheric pressure, and air is pushed out. This is expiration.
• We can breathe more strongly when needed, using extra muscles in the abdomen to increase the force of inhalation and exhalation.
• A healthy person usually breathes 12 to 16 times per minute under normal conditions.
• The amount of air moved during breathing can be measured using a device called a spirometer. It is useful in checking lung health and respiratory function in medical tests.

---

5. Types of Pulmonary Volumes and Capacities

Some Important Terms Are Below There🥰-

• Tidal Volume (TV) is the amount of air a person breathes in or out during normal breathing. It is about 500 mL. So, in one minute, a healthy person breathes in and out 6000 to 8000 mL of air.
• Expiratory Reserve Volume (ERV) is the extra amount of air a person can breathe out with effort after a normal exhalation. It is about 1000 to 1100 mL.v
• Inspiratory Reserve Volume (IRV) is the extra amount of air a person can breathe in with effort after a normal inhalation. It is around 2500 to 3000 mL.
• Residual Volume (RV) is the amount of air that always remains in the lungs, even after a strong and forceful exhalation. This is about 1100 to 1200 mL.

By combining these basic respiratory volumes, we get different lung capacities, which are useful in medical diagnosis.

• Expiratory Capacity (EC) is the total air a person can breathe out after a normal inhalation. It includes tidal volume (TV) and expiratory reserve volume (ERV).
  • So, EC = TV + ERV.
• Inspiratory Capacity (IC) is the total air a person can breathe in after a normal exhalation. It includes tidal volume (TV) and inspiratory reserve volume (IRV).
  • So, IC = TV + IRV.
• Vital Capacity (VC) is the maximum amount of air a person can inhale after a forceful exhalation, or exhale after a forceful inhalation.
  • It includes ERV, TV, and IRV.
• Functional Residual Capacity (FRC) is the amount of air left in the lungs after a normal exhalation. It includes expiratory reserve volume (ERV) and residual volume (RV).
  • So, FRC = ERV + RV.
• Total Lung Capacity (TLC) is the maximum volume of air the lungs can hold after a strong inhalation.
  • It includes RV, ERV, TV, and IRV, or you can say TLC = VC + RV.

---

6. Mechanism of Oxygen and Carbon Dioxide Exchange

• Exchange of gases is a vital process in human physiology where oxygen (O₂) and carbon dioxide (CO₂) are transferred between the lungs, blood, and tissues. The alveoli, which are tiny sac-like structures in the lungs, are the primary site where this exchange occurs. However, gas exchange is not limited to the alveoli; it also takes place between the blood and tissues throughout the body. This entire process works through simple diffusion, where gases move from regions of higher concentration or partial pressure to regions of lower concentration. The movement is passive and requires no energy.
• One of the most important concepts in gas exchange is partial pressure, which refers to the pressure exerted by an individual gas in a mixture of gases. In this context, pO₂ represents the partial pressure of oxygen and pCO₂ represents that of carbon dioxide. The direction of gas diffusion is determined by the partial pressure gradient, where each gas moves from a region of high partial pressure to low partial pressure. In the case of oxygen, the partial pressure is highest in the alveoli (104 mm Hg), lower in oxygenated blood (95 mm Hg), and lowest in the tissues (40 mm Hg). This creates a clear gradient for oxygen to move from the alveoli → blood → tissues. For carbon dioxide, the reverse happens. Its partial pressure is highest in tissues (45 mm Hg), slightly lower in deoxygenated blood (45 mm Hg), and lowest in alveolar air (40 mm Hg). This allows CO₂ to move from tissues → blood → alveoli where it is expelled through exhalation.
• According to the partial pressure table, atmospheric air contains about 159 mm Hg of O₂ and only 0.3 mm Hg of CO₂, while alveolar air has 104 mm Hg of O₂ and 40 mm Hg of CO₂. Deoxygenated blood arriving at the lungs has only 40 mm Hg of O₂ and 45 mm Hg of CO₂. After oxygenation, this blood leaves the lungs with about 95 mm Hg of O₂ and 40 mm Hg of CO₂, ready to deliver oxygen to tissues and pick up carbon dioxide. These pressure differences across different parts of the body support a continuous and effective exchange of gases.
• Another critical factor in the exchange of gases is the solubility of gases. Carbon dioxide is about 20–25 times more soluble in plasma than oxygen. Because of this, even a small partial pressure difference can cause a large amount of CO₂ to diffuse, making its removal more efficient. In contrast, oxygen, being less soluble, requires a steeper pressure gradient for effective diffusion. The rate of gas exchange also depends on the thickness of the diffusion membrane. In humans, the diffusion membrane is made up of three extremely thin layers – the squamous epithelium of alveoli, the endothelium of alveolar capillaries, and a thin basement membrane lying between them. Despite being made of three layers, the overall thickness of this membrane is less than 1 millimetre, which is ideal for quick gas exchange.
• In summary, the exchange of gases is a highly efficient process, supported by structural and functional adaptations such as thin membranes, partial pressure gradients, and the high solubility of CO₂. These conditions ensure that oxygen is delivered to the tissues effectively while carbon dioxide is rapidly removed from the body through the lungs.

---

7. Transport Mechanism of O₂ and CO₂

Blood serves as the medium for the transport of gases, specifically oxygen (O₂) and carbon dioxide (CO₂). Most of the oxygen, about 97%, is transported through the blood by red blood cells (RBCs). These RBCs carry oxygen by binding it with hemoglobin. The remaining 3% of oxygen is transported in a dissolved form in the blood plasma, which is the fluid component of blood. When it comes to carbon dioxide, around 20–25% is carried by RBCs, while the majority, nearly 70%, is transported in the form of bicarbonate ions in the plasma. Only about 7% of carbon dioxide is transported in a dissolved state through the plasma. This shows that while both gases are transported by the blood, they use different mechanisms and proportions for their movement.

---

7.1 Oxygen Transport Mechanism

Haemoglobin is a red-coloured pigment containing iron and is found in red blood cells (RBCs). Oxygen (O₂) binds to haemoglobin in a reversible way, forming a compound known as oxyhaemoglobin. Each molecule of haemoglobin has the capacity to carry a maximum of four oxygen molecules. The binding of oxygen with haemoglobin depends mainly on the partial pressure of oxygen (pO₂). However, other factors such as the partial pressure of carbon dioxide (pCO₂), the concentration of hydrogen ions (H⁺), and temperature also affect this binding process. When the percentage saturation of haemoglobin with oxygen is plotted against pO₂, a sigmoid-shaped curve is obtained. This curve is known as the oxygen dissociation curve, and it is useful for understanding how factors like pCO₂ and H⁺ concentration affect the oxygen-haemoglobin binding. In the alveoli of the lungs, the pO₂ is high, pCO₂ is low, H⁺ concentration is low, and temperature is also lower – all these conditions support the formation of oxyhaemoglobin. In contrast, in the body tissues, pO₂ is low, pCO₂ is high, H⁺ concentration is high, and the temperature is also higher, making the conditions suitable for oxygen to separate from oxyhaemoglobin. This means oxygen gets bound to haemoglobin in the lungs and is released in the tissues. Under normal physiological conditions, every 100 ml of oxygenated blood delivers around 5 ml of oxygen to the tissues.

---

7.2 Carbon Dioxide Transport Mechanism

About 20–25% of carbon dioxide (CO₂) in the body is carried by haemoglobin in the form of carbamino-haemoglobin. This binding depends on the partial pressure of CO₂ (pCO₂) and oxygen (pO₂).

• In tissues (where pCO₂ is high and pO₂ is low), more CO₂ binds to haemoglobin.
• In alveoli (where pCO₂ is low and pO₂ is high), CO₂ is released from haemoglobin.

Red Blood Cells (RBCs) have a lot of the enzyme carbonic anhydrase, which helps convert:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺
This reaction goes:

• Forward in tissues (CO₂ turns into bicarbonate HCO₃⁻ for transport).
• Backward in alveoli (HCO₃⁻ turns back into CO₂ and is released during exhalation).

So, CO₂ is carried as bicarbonate in blood from tissues to lungs, and about 4 ml of CO₂ is released from every 100 ml of deoxygenated blood at the alveoli.

---

8. How Respiration Is Regulated ?

• Human beings can control and adjust their breathing rate based on the needs of body tissues. This control is managed by the nervous system. A special part of the brain located in the medulla called the respiratory rhythm centre mainly controls this process. Another part of the brain in the pons region, called the pneumotaxic centre, can influence the actions of the respiratory rhythm centre. Signals from the pneumotaxic centre can reduce the length of inspiration and thus change the breathing rate.
• There is also a chemosensitive area near the rhythm centre that is very sensitive to carbon dioxide (CO₂) and hydrogen ions (H⁺). When the levels of these substances increase, this area gets activated. It then signals the rhythm centre to adjust the breathing process in a way that helps remove the excess CO₂ and H⁺ from the body.
• Additionally, receptors in the aortic arch and carotid artery can detect changes in CO₂ and H⁺ concentrations and send signals to the rhythm centre to take corrective action. Interestingly, oxygen does not play a major role in controlling the breathing rate.

---

9. Diseases Related to Breathing System

1. Emphysema:
   It is a chronic (long-term) disease in which the walls of the alveoli (air sacs in lungs) get damaged. This reduces the surface area for gas exchange. Cigarette smoking is a major cause of emphysema.
2. Asthma:
   It is a breathing problem where the person faces difficulty in breathing with a wheezing sound. This happens because of inflammation in the bronchi and bronchioles (airways).
3. Occupational Respiratory Disorders:
   In jobs like grinding or stone-breaking, workers inhale a lot of dust. The body’s defense system fails to handle so much dust, leading to inflammation and fibrosis (formation of extra fibrous tissues). This causes serious lung damage. To avoid this, workers should use protective masks.

---

10. Chapter Overview

• Oxygen is needed for energy:– Cells use oxygen to carry out metabolism, which helps them to produce energy. But during this process, carbon dioxide (CO₂) is also produced, which is harmful for the body.
• Oxygen and carbon dioxide transport system:- Animals have developed different methods to deliver oxygen to all body cells and remove carbon dioxide from them.
• Human respiratory system:– In humans, the respiratory system is well-developed. It includes two lungs and airways like the nose, trachea, bronchi, etc., which help in breathing.
• Breathing is the first step of respiration:- Breathing means taking in air (called inspiration) and releasing air out (called expiration). This is the first basic step in respiration.
• Other steps of respiration:-
  • Exchange of oxygen and carbon dioxide between alveoli (lungs) and blood.
  • Transport of these gases by blood throughout the body.
  • Exchange of gases between blood and body tissues.
  • Cells using oxygen to produce energy. This is called cellular respiration.
• How breathing happens:- Breathing involves creating pressure differences between outside air and alveoli. This is done with the help of intercostal muscles (between ribs) and the diaphragm (a muscle below the lungs).
• Measuring air during breathing:- The amount of air involved in breathing can be measured using a device called a spirometer. These measurements are helpful in medical diagnosis.
• How gases are exchanged in lungs and tissues:-
  • The gases (oxygen and carbon dioxide) move through the process called diffusion. This movement depends on:
    • Partial pressure of oxygen (pO₂) and carbon dioxide (pCO₂),
    • Solubility of the gases,
    • Thickness of the membrane where exchange happens.
• Direction of gas diffusion:-
  • Oxygen moves from alveoli to blood and from blood to tissues.
  • Carbon dioxide moves from tissues to blood and then to alveoli (opposite direction).
• Oxygen transport in the body:–
  • Most of the oxygen is carried by haemoglobin in red blood cells. This combination is called oxyhaemoglobin.
    • In lungs, where pO₂ is high, oxygen binds to haemoglobin.
    • In tissues, where pO₂ is low and pCO₂ is high, oxygen is released.
• Carbon dioxide transport in the body:–
  • Around 70% of CO₂ is transported as bicarbonate ions (HCO₃⁻), with the help of an enzyme called carbonic anhydrase.
  • 20–25% of CO₂ binds with haemoglobin to form carbamino-haemoglobin.
  • In tissues (where CO₂ is high), it enters blood; in alveoli (where CO₂ is low), it comes out of blood.
• Brain controls our breathing rhythm:–
  • The medulla part of the brain contains the respiratory centre which controls the rhythm of breathing.
    • The pons region of the brain has a pneumotaxic centre which can modify the rate of breathing.
    • There is also a chemosensitive area in the medulla that helps adjust breathing based on chemical signals like CO₂ levels.

Thank You🥰

Learn Sufficient Notes💖