All living organisms utilize oxygen ($\textsf{O}_2$) to break down simple molecules like glucose, amino acids, and fatty acids indirectly, releasing energy for various life activities. Carbon dioxide ($\textsf{CO}_2$), a by-product of these catabolic reactions, is harmful and needs to be removed.

Thus, there is a continuous need to supply $\textsf{O}_2$ to cells and remove $\textsf{CO}_2$ produced by them. This process of exchanging atmospheric $\textsf{O}_2$ with the $\textsf{CO}_2$ produced by cells is called breathing or respiration.

Respiratory Organs

The mechanisms used for breathing vary widely among different animal groups. These differences are mainly determined by the organism's habitat and its level of structural organization.

• Lower invertebrates (e.g., sponges, coelenterates, flatworms): Gas exchange ($\textsf{O}_2$ for $\textsf{CO}_2$) occurs by simple diffusion over their entire body surface.
• Earthworms: Use their moist cuticle for cutaneous respiration.
• Insects: Have a network of tubes called tracheal tubes that transport atmospheric air directly throughout the body.
• Aquatic arthropods and molluscs: Employ specialized vascularized structures called gills (branchial respiration).
• Terrestrial forms: Utilise vascularized sacs known as lungs (pulmonary respiration).

Among vertebrates:

• Fishes respire through gills.
• Amphibians, reptiles, birds, and mammals respire through lungs.
• Amphibians like frogs can also breathe through their moist skin (cutaneous respiration).

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Human Respiratory System

The human respiratory system includes air passages and a pair of lungs (Figure 17.1).

Pathway of air:

1. External nostrils: Pair of openings above the upper lips, leading inwards.
2. Nasal chamber: Space inside the nose, connected to the pharynx via the nasal passage.
3. Pharynx: A common passage for both food and air.
4. Larynx: Opens into the trachea. A cartilaginous box ("sound box") involved in sound production. During swallowing, the epiglottis (a cartilaginous flap) covers the glottis (opening of larynx/trachea) to prevent food from entering the windpipe.
5. Trachea: A straight tube extending down into the mid-thoracic cavity. Divides at the level of the 5th thoracic vertebra into right and left primary bronchi.
6. Bronchi: Primary bronchi repeatedly divide into secondary and tertiary bronchi, and then into thinner tubes called bronchioles.
7. Terminal bronchioles: The finest bronchioles, ending in alveoli.

Structural support: The trachea, primary, secondary, tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings to prevent collapse.

Alveoli: Terminal bronchioles give rise to numerous thin-walled, irregular-shaped, vascularized, bag-like structures called alveoli. These are the primary sites for gas exchange.

Lungs: The branching network of bronchi, bronchioles, and alveoli form the lungs. Humans have two lungs, located in the thoracic chamber.

Covering of lungs: Lungs are covered by a double-layered pleura, with pleural fluid in between the layers. Pleural fluid reduces friction on the lung surface during breathing movements. The outer pleural membrane is in contact with the thoracic wall, and the inner pleural membrane is in contact with the lung surface.

Divisions of the respiratory system:

• Conducting part: From external nostrils up to the terminal bronchioles. Transports atmospheric air to the alveoli, cleans, humidifies, and warms the air.
• Respiratory (Exchange) part: Composed of the alveoli and their ducts. This is the site where the actual diffusion of $\textsf{O}_2$ and $\textsf{CO}_2$ between blood and air occurs.

Thoracic chamber: The lungs are situated in the thoracic chamber, which is an air-tight compartment. It is formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs, and inferiorly by the dome-shaped diaphragm. The structure of the thoracic chamber is crucial because changes in its volume directly lead to changes in lung (pulmonary) volume, driving breathing.

Steps involved in respiration (overall process):

1. Breathing (Pulmonary ventilation): Inhalation of atmospheric air and exhalation of $\textsf{CO}_2$-rich alveolar air.
2. Diffusion of gases: $\textsf{O}_2$ and $\textsf{CO}_2$ exchange across the alveolar membrane.
3. Transport of gases: $\textsf{O}_2$ and $\textsf{CO}_2$ are carried by the blood throughout the body.
4. Diffusion of gases: $\textsf{O}_2$ and $\textsf{CO}_2$ exchange between blood and body tissues.
5. Cellular respiration: Utilisation of $\textsf{O}_2$ by cells for metabolic reactions (catabolism) and production of $\textsf{CO}_2$.

Mechanism Of Breathing

Breathing, or pulmonary ventilation, involves two main stages: inspiration (inhaling atmospheric air) and expiration (exhaling alveolar air). These movements are driven by creating pressure gradients between the lungs and the atmosphere.

• Inspiration: Occurs when the pressure inside the lungs (intra-pulmonary pressure) is lower than the atmospheric pressure (negative pressure). This pressure gradient causes air to flow into the lungs.
• Expiration: Occurs when the pressure inside the lungs (intra-pulmonary pressure) is higher than the atmospheric pressure. This gradient forces air out of the lungs.

The creation of these pressure gradients is facilitated by the diaphragm and the intercostal muscles (external and internal) located between the ribs (Figure 17.2 a, b).

Steps of Inspiration:

1. Diaphragm contraction: The diaphragm flattens and moves downward, increasing the vertical (antero-posterior) volume of the thoracic chamber.
2. External intercostal muscles contraction: Pulls the ribs and sternum upward and outward, increasing the dorso-ventral volume of the thoracic chamber.
3. Overall increase in thoracic volume.
4. Increase in pulmonary volume (lungs expand with the thoracic chamber).
5. Intra-pulmonary pressure decreases (becomes lower than atmospheric pressure).
6. Air flows into the lungs.

Steps of Expiration:

1. Relaxation of diaphragm and intercostal muscles: Diaphragm returns to its dome shape, ribs and sternum move back to their original positions.
2. Decrease in thoracic volume.
3. Decrease in pulmonary volume.
4. Intra-pulmonary pressure increases (becomes slightly higher than atmospheric pressure).
5. Air is expelled from the lungs.

Forced inspiration or expiration can be achieved with the help of additional muscles in the abdomen.

A healthy human breathes approximately 12-16 times per minute. The volume of air involved in breathing can be measured using a spirometer, a tool for assessing pulmonary function.

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Respiratory Volumes And Capacities

Spirometry measurements define various volumes of air moved during breathing:

• Tidal Volume (TV): Volume of air inspired or expired during a normal, quiet breath. Average is 500 mL. Normal breathing moves 6000-8000 mL of air per minute.
• Inspiratory Reserve Volume (IRV): Additional volume of air that can be inspired by a forceful inhalation after a normal tidal inspiration. Average is 2500 mL to 3000 mL.
• Expiratory Reserve Volume (ERV): Additional volume of air that can be expired by a forceful exhalation after a normal tidal expiration. Average is 1000 mL to 1100 mL.
• Residual Volume (RV): Volume of air that remains in the lungs even after a maximal forced expiration. Average is 1100 mL to 1200 mL. This air is always present in the lungs.

Pulmonary capacities are combinations of these volumes, useful for clinical diagnosis:

• Inspiratory Capacity (IC): Maximum volume of air that can be inspired after a normal expiration. IC = TV + IRV.
• Expiratory Capacity (EC): Maximum volume of air that can be expired after a normal inspiration. EC = TV + ERV.
• Functional Residual Capacity (FRC): Volume of air remaining in the lungs after a normal expiration. FRC = ERV + RV.
• Vital Capacity (VC): Maximum volume of air that can be breathed out after a forced inspiration (or maximum volume breathed in after a forced expiration). VC = ERV + TV + IRV. Represents the maximum amount of air that can be exchanged.
• Total Lung Capacity (TLC): Total volume of air the lungs can accommodate at the end of a forced inspiration. TLC = RV + ERV + TV + IRV or VC + RV.

Exchange Of Gases

Gas exchange ($\textsf{O}_2$ and $\textsf{CO}_2$) primarily occurs at two main sites:

1. Between the alveoli and the blood in the lungs.
2. Between the blood and the body tissues.

Mechanism: Gas exchange at these sites takes place by simple diffusion.

Factors affecting the rate of diffusion:

• Partial pressure gradient: The difference in the partial pressure of the gas across the diffusion membrane. Gases move from a region of higher partial pressure to a region of lower partial pressure.
• Solubility of the gases: $\textsf{CO}_2$ is much more soluble in water than $\textsf{O}_2$ (20-25 times higher).
• Thickness of the diffusion membrane: A thinner membrane facilitates faster diffusion.

Partial pressure: The pressure exerted by an individual gas in a mixture of gases (denoted as pO$_2$ for oxygen and pCO$_2$ for carbon dioxide).

Partial pressures of $\textsf{O}_2$ and $\textsf{CO}_2$ at different sites (Table 17.1):

Respiratory Gas	Atmospheric Air	Alveoli	Blood (Deoxygenated)	Blood (Oxygenated)	Tissues
O$_2$	159	104	40	95	40
CO$_2$	0.3	40	45	40	45

Partial pressure gradients:

• For $\textsf{O}_2$: High in alveoli (104 mmHg) $\rightarrow$ Low in deoxygenated blood (40 mmHg) $\rightarrow$ $\textsf{O}_2$ diffuses from alveoli into blood. High in oxygenated blood (95 mmHg) $\rightarrow$ Low in tissues (40 mmHg) $\rightarrow$ $\textsf{O}_2$ diffuses from blood into tissues.
• For $\textsf{CO}_2$: High in tissues (45 mmHg) $\rightarrow$ Low in oxygenated blood (40 mmHg) $\rightarrow$ $\textsf{CO}_2$ diffuses from tissues into blood. High in deoxygenated blood (45 mmHg) $\rightarrow$ Low in alveoli (40 mmHg) $\rightarrow$ $\textsf{CO}_2$ diffuses from blood into alveoli.

Diffusion membrane: The membrane across which gas exchange occurs in the alveoli is very thin (much less than a millimeter total thickness) and is formed by three main layers (Figure 17.4):

1. Thin squamous epithelium of the alveoli.
2. Endothelium of the alveolar capillaries.
3. Basement substance (fused thin basement membranes) between the epithelium and endothelium.

The factors mentioned (gradients, solubility, thin membrane) make diffusion of $\textsf{O}_2$ from alveoli to tissues and $\textsf{CO}_2$ from tissues to alveoli highly efficient.

Transport Of Gases

Blood is the primary medium for transporting $\textsf{O}_2$ and $\textsf{CO}_2$ between the lungs and body tissues.

Transport of $\textsf{O}_2$:

• $\sim$97% of $\textsf{O}_2$ is transported bound to haemoglobin in Red Blood Cells (RBCs).
• $\sim$3% of $\textsf{O}_2$ is transported in a dissolved state in the plasma.

Transport of $\textsf{CO}_2$:

• $\sim$20-25% of $\textsf{CO}_2$ is transported bound to haemoglobin as carbamino-haemoglobin.
• $\sim$70% of $\textsf{CO}_2$ is transported as bicarbonate ions ($\textsf{HCO}_3^-$).
• $\sim$7% of $\textsf{CO}_2$ is transported in a dissolved state in the plasma.

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Transport Of Oxygen

Haemoglobin is a red-colored, iron-containing pigment in RBCs. $\textsf{O}_2$ binds reversibly to haemoglobin to form oxyhaemoglobin. Each haemoglobin molecule can bind up to four molecules of $\textsf{O}_2$.

Factors influencing binding of $\textsf{O}_2$ to haemoglobin:

• Partial pressure of $\textsf{O}_2$ (pO$_2$): The primary factor. Higher pO$_2$ favors binding.
• Partial pressure of $\textsf{CO}_2$ (pCO$_2$), hydrogen ion concentration (H$^+$), and temperature also influence binding.

Oxygen dissociation curve: A sigmoid (S-shaped) curve obtained by plotting the percentage saturation of haemoglobin with $\textsf{O}_2$ against pO$_2$ (Figure 17.5). This curve illustrates the relationship between pO$_2$ and haemoglobin saturation and helps study the effect of other factors.

In the alveoli: High pO$_2$, low pCO$_2$, lower H$^+$ concentration (higher pH), and lower temperature favor the binding of $\textsf{O}_2$ to haemoglobin (formation of oxyhaemoglobin). The curve shifts to the left.

In the tissues: Low pO$_2$, high pCO$_2$, higher H$^+$ concentration (lower pH), and higher temperature favor the dissociation of $\textsf{O}_2$ from oxyhaemoglobin. The curve shifts to the right (Bohr effect).

This difference in conditions ensures that $\textsf{O}_2$ is picked up by haemoglobin in the lungs and delivered to the tissues.

Under normal physiological conditions, every 100 ml of oxygenated blood delivers approximately 5 ml of $\textsf{O}_2$ to the tissues.

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Transport Of Carbon Dioxide

$\textsf{CO}_2$ is transported in three main forms:

1. Carbamino-haemoglobin: $\textsf{CO}_2$ binds to the amino groups of haemoglobin. About 20-25% of $\textsf{CO}_2$ is transported this way. Binding is related to pCO$_2$ and is affected by pO$_2$. High pCO$_2$ and low pO$_2$ (in tissues) favor binding; low pCO$_2$ and high pO$_2$ (in alveoli) favor dissociation.
2. Bicarbonate ions: About 70% of $\textsf{CO}_2$ is transported as bicarbonate ($\textsf{HCO}_3^-$). This conversion is catalyzed by the enzyme carbonic anhydrase, present in high concentration in RBCs and in small amounts in plasma.

   In tissues (high pCO$_2$): $\textsf{CO}_2$ + H$_2$O $\xrightarrow{\textsf{Carbonic anhydrase}}$ H$_2$CO$_3$ $\rightarrow$ H$^+$ + $\textsf{HCO}_3^-$

   Bicarbonate ions move out of RBCs into the plasma (in exchange for Cl$^-$ ions - chloride shift), and H$^+$ ions are buffered by haemoglobin.

   In alveoli (low pCO$_2$): The reaction reverses. $\textsf{HCO}_3^-$ from plasma enters RBCs (in exchange for Cl$^-$), combines with H$^+$ to form H$_2$CO$_3$, which then breaks down into $\textsf{CO}_2$ and H$_2$O. $\textsf{CO}_2$ diffuses into the alveoli.

3. Dissolved state: About 7% of $\textsf{CO}_2$ is carried dissolved in plasma.

Every 100 ml of deoxygenated blood delivers approximately 4 ml of $\textsf{CO}_2$ to the alveoli.

Regulation Of Respiration

The regulation of respiratory rhythm to meet the body's demands is primarily controlled by the neural system.

Neural centers involved:

• Respiratory rhythm center: Located in the medulla oblongata region of the brain. It is the primary center responsible for setting and maintaining the basic respiratory rhythm.
• Pneumotaxic center: Located in the pons region of the brain. It can moderate the function of the rhythm center. Neural 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. It is highly sensitive to changes in the concentration of $\textsf{CO}_2$ and hydrogen ions (H$^+$). An increase in $\textsf{CO}_2$ or H$^+$ activates this area, signaling the rhythm center to adjust breathing to eliminate these substances.

Other receptors: Receptors in the aortic arch and carotid artery can also detect changes in $\textsf{CO}_2$ and H$^+$ concentrations and send signals to the rhythm center for corrective actions.

The role of oxygen (pO$_2$) in regulating respiratory rhythm is relatively insignificant compared to $\textsf{CO}_2$ and H$^+$.

Disorders Of Respiratory System

Common disorders affecting the respiratory system include:

• Asthma: Difficulty in breathing characterized by wheezing, caused by inflammation and constriction of the bronchi and bronchioles.
• Emphysema: A chronic disorder where the alveolar walls are progressively damaged, leading to a decrease in the respiratory surface area. A major cause is cigarette smoking.
• Occupational Respiratory Disorders: Occur in certain workplaces with exposure to dust (e.g., grinding, stone-breaking). Long-term exposure to dust that the body's defense mechanisms cannot fully clear leads to inflammation, fibrosis (proliferation of fibrous tissue), and serious lung damage. Wearing protective masks is recommended in such industries.

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