Breathing and Exchange of Gases

Respiratory Organs

All animals need to exchange gases ($O_2$ from the atmosphere and $CO_2$ produced during metabolism) with their environment. Different animals have evolved various structures for this purpose, depending on their habitat and level of organisation.

Respiratory Organs in Different Animals:

• General body surface: Simple diffusion across the moist body surface in lower invertebrates like sponges, coelenterates, and flatworms. Earthworms use their moist cuticle.
• Tracheal system: A network of tubes (tracheae) that transport air directly to the cells in insects. Spiracles are external openings.
• Gills: Specialised vascularised structures for aquatic respiration in fish and some arthropods and molluscs.
• Lungs: Vascularised bags used for terrestrial respiration in amphibians, reptiles, birds, and mammals.

Frogs (Amphibia) respire through their moist skin (cutaneous respiration), lungs (pulmonary respiration), and buccal cavity (buccopharyngeal respiration).

Human Respiratory System

The human respiratory system is responsible for facilitating the exchange of gases ($O_2$ and $CO_2$) between the atmosphere and the blood.

Components of the Human Respiratory System:

The respiratory system consists of the respiratory tract and the lungs.

1. Respiratory Tract: The passage for air, extending from the nostrils to the bronchioles.

• External nostrils: Openings at the tip of the nose, leading to the nasal chamber.
• Nasal chamber: Divided into two by a septum. Lined with hair and mucus to filter and warm the air.
• Nasal cavity: The space within the nose.
• Pharynx: A common passageway for air and food. Located at the back of the mouth.
• Larynx (Voice box): A cartilaginous box that produces sound. Located in the neck. During swallowing, the glottis (opening of the larynx into the pharynx) is covered by the epiglottis (a cartilaginous flap) to prevent food from entering the trachea.
• Trachea (Windpipe): A straight tube extending up to the mid-thoracic cavity. Supported by C-shaped cartilaginous rings, which prevent it from collapsing.
• Bronchi: The trachea divides into two primary bronchi (left and right) at the level of the 5th thoracic vertebra.
• Bronchioles: Each primary bronchus subdivides into secondary and tertiary bronchi, and then into very fine tubes called bronchioles. The initial bronchioles are supported by cartilaginous rings, but later bronchioles lack cartilage.
• Terminal bronchioles: The finest bronchioles, which lead to the respiratory bronchioles and then to the alveoli.

2. Lungs: The primary organs of respiration.

• Humans have a pair of lungs, located in the thoracic cavity, surrounded by the rib cage, vertebral column, and diaphragm.
• Lungs are covered by a double-layered membrane called the pleura. The outer pleural membrane is in close contact with the thoracic wall, and the inner pleural membrane is in contact with the lung surface. The space between the two pleural layers is filled with pleural fluid, which reduces friction on the lung surface during breathing.
• The lungs are highly vascularised and contain millions of tiny air sacs called alveoli.
• The conducting part of the respiratory system (from external nostrils up to the terminal bronchioles) transports atmospheric air to the alveoli, clears it from foreign particles, humidifies, and warms it.
• The respiratory or exchange part (alveoli and their ducts) is the actual site of gas exchange ($O_2$ and $CO_2$) between blood and atmospheric air.

*(Image shows a diagram of the human respiratory system from the nasal cavity/mouth down to the lungs, showing trachea, bronchi, bronchioles, and the structure of the lungs within the thoracic cavity)*

Structure of Alveolus:

• Alveoli are very thin-walled air sacs (about $0.1 \:\text{mm}$ in diameter).
• Their walls are composed of a single layer of squamous epithelium.
• The alveolar walls are surrounded by a dense network of blood capillaries.
• The total surface area provided by the alveoli for gas exchange is enormous (estimated $70-100 \text{ square metres}$ in an adult human).
• The thinness of the alveolar and capillary walls, along with the large surface area, facilitates efficient gas exchange by diffusion.

*(Image shows a diagram illustrating an alveolus and its surrounding capillaries, indicating the diffusion of O2 from alveolus to blood and CO2 from blood to alveolus across the thin membrane)*

Mechanism of Breathing

Breathing (also called ventilation) is the physical process of inhaling atmospheric air into the lungs and exhaling carbon dioxide-rich air out. This process is driven by pressure gradients created by changes in the volume of the thoracic cavity.

Steps of Breathing:

1. Inhalation (Inspiration): The process of drawing air into the lungs.
   • It is initiated by the contraction of the diaphragm (a dome-shaped muscle below the lungs). The diaphragm flattens and moves downwards, increasing the volume of the thoracic cavity.
   • The contraction of the external intercostal muscles (muscles between the ribs) lifts the ribs and sternum upwards and outwards, further increasing the volume of the thoracic cavity.
   • The increase in thoracic volume leads to an increase in pulmonary volume (volume of lungs).
   • This increase in pulmonary volume decreases the intrapulmonary pressure (pressure inside the lungs) to less than the atmospheric pressure.
   • The pressure gradient from higher atmospheric pressure to lower intrapulmonary pressure causes air to move into the lungs.
   • Inhalation is an active process.
2. Exhalation (Expiration): The process of expelling air out of the lungs.
   • It is usually a passive process during normal breathing.
   • The diaphragm relaxes and returns to its dome shape.
   • The external intercostal muscles relax, allowing the ribs and sternum to return to their original position.
   • This decreases the thoracic volume, which in turn decreases the pulmonary volume.
   • The decrease in pulmonary volume increases the intrapulmonary pressure to slightly above the atmospheric pressure.
   • The pressure gradient from higher intrapulmonary pressure to lower atmospheric pressure causes air to be expelled out of the lungs.

During forceful breathing (e.g., during exercise), the internal intercostal muscles and abdominal muscles also contract, increasing the rate and volume of air movement.

*(Image shows two diagrams side-by-side: one for inhalation (diaphragm down, rib cage up/out) and one for exhalation (diaphragm up, rib cage down/in), illustrating volume and pressure changes)*

The capacity to inflate the lungs is based on the surface tension created by a substance called surfactant, secreted by some alveolar cells. Surfactant reduces surface tension and prevents the collapse of alveoli.

Respiratory Volumes And Capacities

The volume of air involved in breathing movements can be estimated using a spirometer. These volumes and capacities are important for clinical assessment of respiratory function.

Respiratory Volumes:

• Tidal Volume (TV): Volume of air inhaled or exhaled during a normal quiet breath.

  Approximate value: $500 \text{ mL}$ (in a healthy human adult).

• Inspiratory Reserve Volume (IRV): Additional volume of air that can be inhaled by a forceful inspiration after a normal tidal inspiration.

  Approximate value: $2500 - 3000 \text{ mL}$.

• Expiratory Reserve Volume (ERV): Additional volume of air that can be exhaled by a forceful expiration after a normal tidal expiration.

  Approximate value: $1000 - 1100 \text{ mL}$.

• Residual Volume (RV): Volume of air remaining in the lungs even after a forceful expiration. This volume cannot be measured by spirometer.

  Approximate value: $1100 - 1200 \text{ mL}$.

Respiratory Capacities (Combinations of two or more volumes):

• Inspiratory Capacity (IC): Total volume of air a person can inhale after a normal expiration.

  $ IC = TV + IRV \approx 500 + (2500-3000) = 3000 - 3500 \text{ mL} $

• Expiratory Capacity (EC): Total volume of air a person can exhale after a normal inspiration.

  $ EC = TV + ERV \approx 500 + (1000-1100) = 1500 - 1600 \text{ mL} $

• Functional Residual Capacity (FRC): Volume of air remaining in the lungs after a normal expiration.

  $ FRC = ERV + RV \approx (1000-1100) + (1100-1200) = 2100 - 2300 \text{ mL} $

• Vital Capacity (VC): The maximum volume of air a person can breathe in after a forceful expiration OR the maximum volume of air a person can breathe out after a forceful inspiration.

  $ VC = ERV + TV + IRV \approx (1000-1100) + 500 + (2500-3000) = 4000 - 4600 \text{ mL} $

• Total Lung Capacity (TLC): Total volume of air that can be accommodated in the lungs at the end of a forceful inspiration.

  $ TLC = RV + ERV + TV + IRV = RV + VC \approx (1100-1200) + (4000-4600) = 5100 - 5800 \text{ mL} $

Knowledge of respiratory volumes and capacities is helpful in diagnosing respiratory disorders.

Exchange Of Gases

Exchange of gases ($O_2$ and $CO_2$) occurs at two main sites in the human body: between the alveoli and the blood in the lungs, and between the blood and the tissues throughout the body.

Mechanism of Gas Exchange:

Gas exchange occurs by simple diffusion. The rate of diffusion is influenced by several factors:

• Partial pressure of gases: Gases diffuse from a region of higher partial pressure to a region of lower partial pressure.
• Solubility of gases: $CO_2$ is about 20-25 times more soluble than $O_2$.
• Thickness of the diffusion membrane: The diffusion membrane is very thin.
• Surface area available for diffusion: The alveolar surface area is very large.

Partial Pressures of O$_2$ and CO$_2$:

The partial pressure ($P_x$) of a gas is the pressure contributed by that gas in a mixture of gases.

*(Values are approximate and in mm Hg or Torr)*

Exchange at the Alveoli:

• Between alveolar air and deoxygenated blood in pulmonary capillaries.
• $PO_2$ is high in alveoli (104 mm Hg) and low in deoxygenated blood (40 mm Hg). Thus, $O_2$ diffuses from alveoli into the blood.
• $PCO_2$ is high in deoxygenated blood (45 mm Hg) and low in alveoli (40 mm Hg). Thus, $CO_2$ diffuses from blood into the alveoli.
• The diffusion membrane is very thin (around $0.5 \:\mu\text{m}$), consisting of the squamous epithelium of alveoli, the endothelium of pulmonary capillaries, and the basement membrane between them.
• This diffusion leads to the oxygenation of blood in the pulmonary capillaries.

*(Image shows an alveolus and a capillary, with arrows indicating O2 movement into blood and CO2 movement out of blood, labelled with partial pressures in both)*

---

Exchange at the Tissues:

• Between oxygenated blood in systemic capillaries and tissue cells.
• $PO_2$ is high in oxygenated blood (95 mm Hg) and low in tissues (40 mm Hg). Thus, $O_2$ diffuses from blood into the tissues.
• $PCO_2$ is high in tissues (45 mm Hg) and low in oxygenated blood (40 mm Hg). Thus, $CO_2$ diffuses from tissues into the blood.
• This diffusion leads to the deoxygenation of blood in the systemic capillaries and supplies oxygen to the tissues for cellular respiration.

*(Image shows a tissue capillary and surrounding cells, with arrows indicating O2 movement out of blood and CO2 movement into blood, labelled with partial pressures)*

---

The partial pressure gradients are the main driving force for gas exchange at both the alveolar and tissue levels.

Transport Of Gases

Oxygen and carbon dioxide are transported by the blood between the lungs and the tissues.

Transport Of Oxygen

Oxygen is transported in the blood in two main ways:

1. As dissolved oxygen in plasma: A very small amount (about 3%) of $O_2$ is transported dissolved in the plasma.
2. Bound to Haemoglobin: The majority (about 97%) of $O_2$ is transported bound to the respiratory pigment haemoglobin, which is present in red blood cells (RBCs).

Haemoglobin:

• Haemoglobin (Hb) is a red-coloured iron-containing protein.
• Each haemoglobin molecule can bind to a maximum of four molecules of $O_2$.
• The binding of oxygen to haemoglobin is reversible and forms oxyhaemoglobin ($HbO_2$).

  $ Hb + O_2 \rightleftharpoons HbO_2 $

• Binding of $O_2$ to Hb is primarily dependent on the partial pressure of $O_2$ ($PO_2$).
• The relationship between $PO_2$ and the percentage saturation of haemoglobin with $O_2$ is plotted as the oxygen dissociation curve. This curve is typically sigmoid (S-shaped).

*(Image shows a sigmoid graph with % Saturation of Haemoglobin on Y-axis and Partial Pressure of Oxygen ($PO_2$) on X-axis)*

---

Factors Affecting Oxygen Binding to Haemoglobin:

• $PO_2$: Higher $PO_2$ (as in alveoli) favours the formation of oxyhaemoglobin. Lower $PO_2$ (as in tissues) favours the dissociation of oxygen from haemoglobin.
• $PCO_2$: Higher $PCO_2$ (as in tissues) shifts the curve to the right, favouring dissociation of $O_2$ from Hb (Bohr effect). Lower $PCO_2$ (as in alveoli) shifts the curve to the left, favouring formation of $HbO_2$.
• $H^+$ concentration (pH): Higher $H^+$ concentration (lower pH, as in tissues with high $CO_2$) shifts the curve to the right, favouring dissociation of $O_2$. Lower $H^+$ concentration (higher pH, as in alveoli) shifts the curve to the left, favouring formation of $HbO_2$.
• Temperature: Higher temperature shifts the curve to the right, favouring dissociation of $O_2$. Lower temperature shifts the curve to the left, favouring formation of $HbO_2$.

In the alveoli ($PO_2$ high, $PCO_2$ low, pH high, temperature low), oxygen readily binds to haemoglobin. In the tissues ($PO_2$ low, $PCO_2$ high, pH low, temperature high), oxygen dissociates from haemoglobin and diffuses into the cells.

---

Transport Of Carbon Dioxide

Carbon dioxide is transported by the blood in three main ways:

1. As dissolved $CO_2$ in plasma: A small amount (about 7%) of $CO_2$ is transported dissolved in the plasma.
2. Bound to Haemoglobin: About 20-25% of $CO_2$ is transported bound to the amino groups of haemoglobin, forming carbamino-haemoglobin. The binding is reversible and is influenced by $PCO_2$. Higher $PCO_2$ (in tissues) favours the formation of carbamino-haemoglobin. Lower $PCO_2$ (in alveoli) favours its dissociation (Haldane effect - binding of $O_2$ to Hb facilitates the release of $CO_2$ from blood).
3. As Bicarbonate ions ($HCO_3^-$): The majority (about 70%) of $CO_2$ is transported in the form of bicarbonate ions. This process involves the enzyme carbonic anhydrase, which is present in high concentration in RBCs and in small amount in plasma.
   • In the tissues (high $PCO_2$), $CO_2$ diffuses into the blood (plasma and RBCs).
   • In RBCs, $CO_2$ reacts with water to form carbonic acid ($H_2CO_3$), catalysed by carbonic anhydrase.

     $ CO_2 + H_2O \xrightarrow{\text{Carbonic Anhydrase}} H_2CO_3 $

   • Carbonic acid is unstable and dissociates into $H^+$ and $HCO_3^-$.

     $ H_2CO_3 \rightarrow H^+ + HCO_3^- $

   • Most $HCO_3^-$ ions diffuse out of the RBCs into the plasma. To maintain electrical neutrality, chloride ions ($Cl^-$) move from the plasma into the RBCs (Chloride shift or Hamburger phenomenon).
   • $H^+$ ions released in this process bind to haemoglobin (buffering effect).
   • In the alveoli (low $PCO_2$), the process is reversed. $CO_2$ diffuses from blood to alveoli. $HCO_3^-$ ions from plasma enter RBCs (chloride ions move out). $HCO_3^-$ combines with $H^+$ to form carbonic acid, which is then broken down into $CO_2$ and $H_2O$ by carbonic anhydrase. $CO_2$ diffuses out into the alveoli.

*(Image shows a diagram illustrating CO2 transport in blood, highlighting its conversion to bicarbonate in RBCs, chloride shift, binding to Hb, and dissolved in plasma)*

Efficient transport of gases is crucial for delivering oxygen to tissues and removing carbon dioxide, thereby maintaining cellular respiration and homeostasis.

Regulation Of Respiration

The process of breathing is regulated by the nervous system to maintain the appropriate rate and depth of respiration, ensuring that the body's needs for oxygen and carbon dioxide exchange are met.

Neural Regulation:

• Respiration is controlled by the respiratory rhythm centre located in the medulla oblongata of the brain. This centre is the primary controller of the breathing rhythm.
• Another centre, the pneumotaxic centre, is located in the pons region of the brain. It can modify the function of the respiratory rhythm centre. A signal from the pneumotaxic centre can limit the duration of inspiration and reduce the respiratory rate.
• The apneustic centre (also in the pons) can prolong inspiration.

The respiratory rhythm centre generates the basic rhythm of breathing. Signals from the pneumotaxic centre fine-tune this rhythm.

Chemical Regulation:

The respiratory rhythm can also be influenced by chemical stimuli, specifically the concentration of $CO_2$ and $H^+$ ions in the blood and cerebrospinal fluid.

• A chemosensitive area is located adjacent to the respiratory rhythm centre in the medulla. This area is highly sensitive to $CO_2$ and $H^+$ ions (but not directly to $O_2$).
• An increase in $CO_2$ concentration in the blood leads to an increase in $H^+$ concentration (due to the formation of carbonic acid). This increase is sensed by the chemosensitive area, which signals the respiratory rhythm centre to increase the respiratory rate and depth, thus removing excess $CO_2$.
• Receptors are also located in the aortic arch and carotid artery (peripheral chemoreceptors). These receptors are also sensitive to changes in $CO_2$ and $H^+$ concentrations, and a significant decrease in $PO_2$. They send signals to the respiratory rhythm centre to adjust breathing.

While a decrease in $PO_2$ can also stimulate breathing, the receptors are much more sensitive to changes in $PCO_2$ and $H^+$ concentration. Therefore, the primary stimulus for regulating breathing is the level of $CO_2$, not $O_2$ (under normal conditions).

*(Image shows a simplified diagram of the brainstem highlighting the medulla and pons with respiratory centres, and the locations of peripheral chemoreceptors in the aortic arch and carotid bodies)*

The nervous and chemical regulation mechanisms work together to maintain optimal levels of $O_2$ and $CO_2$ in the blood, ensuring efficient gas exchange and supporting the body's metabolic demands.

Disorders Of Respiratory System

Various factors can impair the normal functioning of the respiratory system, leading to respiratory disorders.

Common Respiratory Disorders:

• Asthma: A chronic inflammatory disease of the airways, causing bronchospasm (constriction of bronchioles) and swelling of the airway lining. This leads to difficulty in breathing (wheezing), shortness of breath, coughing, and chest tightness. Often triggered by allergies, exercise, or irritants.
• Emphysema: A chronic progressive lung disease that primarily affects the alveoli. The walls of the alveoli are damaged and lose their elasticity, reducing the surface area for gas exchange. This leads to permanent enlargement of air spaces and shortness of breath. A major cause is cigarette smoking.
• Bronchitis: Inflammation of the lining of the bronchial tubes, which carry air to and from the lungs. Causes coughing, mucus production, fatigue, and shortness of breath. Can be acute (short-term) or chronic (long-term).
• Occupational Respiratory Disorders: Certain industries involve grinding or stone-breaking, generating dust. Long-term exposure to such dust can cause inflammation and fibrosis (proliferation of fibrous tissue) in the lungs, leading to severe lung damage. Examples include silicosis (from silica dust) and asbestosis (from asbestos fibres). Workers in such industries need to wear protective masks.
• Pneumonia: An infection that inflames the air sacs in one or both lungs. The air sacs may fill with fluid or pus, causing cough with phlegm or pus, fever, chills, and difficulty breathing. Caused by bacteria, viruses, or fungi.
• Tuberculosis (TB): An infectious disease caused by the bacterium Mycobacterium tuberculosis. It usually affects the lungs but can also affect other parts of the body. Symptoms include chronic cough, chest pain, fever, weight loss, and coughing up blood.
• Acute Respiratory Distress Syndrome (ARDS): A severe, life-threatening lung condition that prevents enough oxygen from getting into the body. Fluid builds up in the alveoli.

Prevention often involves avoiding triggers (in asthma), quitting smoking (emphysema), vaccination (pneumonia, flu), and using protective equipment in hazardous environments (occupational disorders).