Respiratory System

Why Respiratory System Is an Important LQ Topic

Contents

1. 1. Overview
2. 2. Different Parts of Respiratory System
3. 3. Nose and Nasal Cavity
4. 4. Pharynx
5. 5. Larynx
6. 6. Trachea
7. 7. Bronchi and Bronchioles
8. 8. Gross Anatomy of Lungs
9. 9. Microscopic Anatomy of Respiratory Tract
10. 10. Alveoli and Respiratory Membrane
11. 11. Functions of Respiratory Parts
12. 12. Pulmonary Ventilation
13. 13. Mechanism of Breathing
14. 14. Lung Volumes and Capacities
15. 15. Alveolar Ventilation
16. 16. Pulmonary Circulation and Perfusion
17. 17. Gas Exchange
18. 18. Partial Pressure of O₂ and CO₂
19. 19. Oxygen Transport
20. 20. Carbon Dioxide Transport
21. 21. O₂-Hb Dissociation Curve and Bohr Effect
22. 22. Haldane Effect
23. 23. Ventilation-Perfusion Relationship
24. 24. Regulation of Respiration
25. 25. Complete 10-Mark LQs
26. 26. Separate 5-Mark Preparation
27. 27. Separate 2-Mark Preparation
28. 28. Quick Revision

1. Overview of Respiratory System

The respiratory system is an organ system responsible primarily for exchange of respiratory gases between the atmosphere and blood. It supplies oxygen required for cellular metabolism and removes carbon dioxide produced during cellular respiration.

Respiration is not limited to movement of air into and out of the lungs. It involves several coordinated processes including pulmonary ventilation, diffusion of gases between alveoli and blood, transport of oxygen and carbon dioxide by blood, and exchange of gases between systemic blood and tissues.

Major Processes of Respiration

1. Pulmonary ventilation: movement of air into and out of lungs.
2. External respiration: exchange of O₂ and CO₂ between alveoli and pulmonary blood.
3. Gas transport: transport of O₂ and CO₂ through blood.
4. Internal respiration: exchange of gases between systemic blood and tissues.

2. Different Parts of Respiratory System

Anatomically, the respiratory system can be divided into the upper and lower respiratory tracts.

Functional Division

Conducting Zone

The conducting zone extends from the nose to the terminal bronchioles. It conducts air but normally does not participate directly in gas exchange.

Its major functions include filtration, warming, humidification and distribution of inspired air.

Respiratory Zone

The respiratory zone includes respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli. This is the principal region where gas exchange occurs.

3. Nose and Nasal Cavity

Gross Anatomy

The nose forms the principal entrance for air into the respiratory tract. The nasal cavity is divided into right and left halves by the nasal septum.

Each nasal cavity contains three major conchae: superior, middle and inferior nasal conchae. These structures increase the surface area and create turbulence, helping contact inspired air with the mucosa.

Microscopic Anatomy

Most of the respiratory region is lined by pseudostratified ciliated columnar epithelium containing goblet cells.

• Ciliated columnar cells move mucus toward the pharynx.
• Goblet cells secrete mucus.
• Basal cells act as progenitor cells.
• Numerous blood vessels help warm inspired air.

Functions

• Provides passage for inspired air.
• Filters particles.
• Warms air.
• Humidifies air.
• Contributes to olfaction.
• Provides resonance for speech.

4. Pharynx

The pharynx is a muscular tube that serves as a common passage for respiratory and digestive pathways.

Parts

1. Nasopharynx
2. Oropharynx
3. Laryngopharynx

Functions

• Conducts air from nasal cavity toward larynx.
• Participates in swallowing.
• Contributes to resonance of voice.

Epithelium

The nasopharynx is primarily lined by respiratory epithelium, while regions exposed to greater mechanical stress, particularly the oropharynx and laryngopharynx, are lined predominantly by stratified squamous epithelium.

5. Larynx

The larynx is a cartilaginous organ situated between the pharynx and trachea. It maintains an open airway and plays an important role in phonation.

Major Cartilages

• Thyroid cartilage
• Cricoid cartilage
• Epiglottis
• Arytenoid cartilages

Vocal Cords

The larynx contains vestibular folds and vocal folds. The true vocal folds contain skeletal muscle and elastic tissue and are involved in sound production.

Microscopic Anatomy

Much of the laryngeal lining contains respiratory epithelium. However, areas exposed to significant mechanical stress, especially the true vocal folds, are covered by stratified squamous epithelium.

Functions

• Maintains airway.
• Produces voice.
• Protects lower respiratory tract during swallowing.
• Participates in cough reflex.

6. Trachea

Gross Anatomy

The trachea is a flexible air-conducting tube extending from the larynx to the main bronchi. It is supported by a series of incomplete C-shaped cartilage rings.

The posterior portion of each cartilage ring is completed by trachealis smooth muscle and connective tissue.

Microscopic Anatomy

The tracheal wall contains several layers.

1. Mucosa: pseudostratified ciliated columnar epithelium with goblet cells.
2. Submucosa: connective tissue containing seromucous glands.
3. Cartilage: hyaline cartilage maintains airway patency.
4. Adventitia: outer connective tissue layer.

Functions

• Conducts air.
• Prevents airway collapse.
• Filters particles through mucociliary clearance.
• Humidifies inspired air.

7. Bronchi and Bronchioles

Bronchi

The trachea divides into right and left main bronchi. The right main bronchus is generally shorter, wider and more vertically oriented than the left.

The main bronchi divide repeatedly into lobar and segmental bronchi, which continue into smaller bronchi and bronchioles.

Microscopic Features of Bronchi

• Respiratory epithelium.
• Goblet cells.
• Smooth muscle.
• Seromucous glands.
• Cartilage plates.

Bronchioles

Bronchioles are smaller airways that lack cartilage and submucosal glands. Their epithelium becomes progressively shorter as the airway diameter decreases.

Smooth muscle becomes relatively important in bronchioles and helps regulate airway diameter.

Terminal Bronchioles

Terminal bronchioles represent the final part of the conducting zone. They are lined mainly by simple cuboidal epithelium and contain club cells.

Functions

• Conduct air deeper into the lungs.
• Distribute air to different lung regions.
• Regulate airflow through smooth muscle.

8. Gross Anatomy of the Lungs

The lungs are paired respiratory organs situated within the thoracic cavity. They are separated by the mediastinum and covered by pleura.

Right Lung

The right lung has three lobes: superior, middle and inferior. These are separated by horizontal and oblique fissures.

Left Lung

The left lung has two lobes: superior and inferior. It contains the cardiac notch and lingula. The left lung is smaller than the right because of the position of the heart.

Surfaces

• Costal surface
• Mediastinal surface
• Diaphragmatic surface

Hilum and Root

The hilum is the region on the mediastinal surface through which bronchi, pulmonary vessels, lymphatics and nerves enter or leave the lung.

9. Microscopic Anatomy of Respiratory Tract

General Change in Epithelium

As the conducting airway becomes progressively smaller, the respiratory epithelium becomes shorter and simpler.

10. Alveoli and Respiratory Membrane

Alveoli are microscopic air sacs forming the principal site of gas exchange in the lungs. They provide a very large surface area and have extremely thin walls.

Types of Alveolar Cells

Type I Pneumocytes

Type I pneumocytes are thin squamous cells covering most of the alveolar surface. They form the major diffusion barrier for respiratory gases.

Type II Pneumocytes

Type II pneumocytes are more cuboidal cells that produce pulmonary surfactant. Surfactant reduces surface tension and helps prevent alveolar collapse.

Alveolar Macrophages

Alveolar macrophages remove inhaled particles and microorganisms and help maintain cleanliness of the alveolar spaces.

Respiratory Membrane

The respiratory membrane is the thin barrier across which oxygen and carbon dioxide diffuse between alveolar air and pulmonary capillary blood.

It consists functionally of the alveolar epithelium, fused basement membrane/interstitial components and capillary endothelium.

11. Functions of Different Parts of Respiratory System

12. Pulmonary Ventilation

Pulmonary ventilation is the movement of air between the atmosphere and alveoli. It consists of two phases: inspiration and expiration.

Inspiration

During normal inspiration, the diaphragm contracts and moves downward. The external intercostal muscles contract and elevate the ribs. Expansion of the thoracic cavity increases lung volume.

According to the pressure-volume relationship, an increase in lung volume causes a decrease in intrapulmonary pressure. When alveolar pressure falls below atmospheric pressure, air enters the lungs.

Expiration

Quiet expiration is mainly passive. The diaphragm and external intercostal muscles relax, and elastic recoil of the lungs decreases thoracic and lung volume.

Alveolar pressure then becomes slightly greater than atmospheric pressure and air flows outward.

Forced Breathing

During forced inspiration, accessory inspiratory muscles assist expansion of the thorax. During forced expiration, internal intercostal muscles and abdominal muscles increase thoracic pressure and actively drive air out.

13. Mechanism of Breathing

13.1 Inspiration

1. Diaphragm contracts.
2. Diaphragm moves downward and flattens.
3. External intercostal muscles elevate ribs.
4. Thoracic volume increases.
5. Intrapleural pressure becomes more negative.
6. Lungs expand.
7. Alveolar volume increases.
8. Alveolar pressure falls slightly below atmospheric pressure.
9. Air flows into lungs.

13.2 Expiration

1. Diaphragm relaxes.
2. External intercostal muscles relax.
3. Thoracic volume decreases.
4. Lung elastic recoil reduces lung volume.
5. Alveolar pressure rises slightly above atmospheric pressure.
6. Air flows out of lungs.

Pressure Changes

Role of Pleural Pressure

The lungs are separated from the thoracic wall by the pleural cavity. The intrapleural pressure is normally negative relative to atmospheric pressure. This negative pressure helps keep the lungs expanded.

14. Lung Volumes and Capacities

Lung volumes describe the amount of air moved or remaining within the lungs during different phases of breathing.

Capacities

15. Alveolar Ventilation

Not all inspired air reaches the gas-exchanging regions of the lungs. Some remains in the conducting airways and forms the anatomical dead space.

Minute Ventilation

Minute ventilation is the total volume of air entering or leaving the respiratory tract per minute.

For example, with a tidal volume of 500 mL and respiratory rate of 12 breaths/min:

Alveolar Ventilation

Alveolar ventilation is the volume of fresh air reaching the respiratory zone per minute.

If tidal volume is 500 mL, dead space is approximately 150 mL, and respiratory rate is 12/min:

16. Pulmonary Circulation and Perfusion

Pulmonary circulation carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium.

Pulmonary capillary blood flows around alveoli and provides the vascular surface required for gas exchange.

Perfusion

Perfusion refers to blood flow through pulmonary capillaries. Effective gas exchange requires appropriate matching between ventilation of alveoli and perfusion of pulmonary capillaries.

17. Gas Exchange in Detail

Gas exchange is the movement of oxygen and carbon dioxide across the respiratory membrane according to their partial-pressure gradients.

External Respiration

External respiration occurs between alveolar air and pulmonary capillary blood.

Oxygen moves from alveolar air into blood because the partial pressure of oxygen is higher in alveolar air than in incoming venous blood.

Carbon dioxide moves from pulmonary capillary blood into alveolar air because the partial pressure of carbon dioxide is higher in venous blood than in alveolar air.

Internal Respiration

Internal respiration occurs between systemic capillary blood and tissue cells. Oxygen moves from blood to tissues, while carbon dioxide moves from tissues into blood.

Factors Affecting Gas Exchange

1. Partial-pressure gradient.
2. Thickness of respiratory membrane.
3. Surface area available for diffusion.
4. Diffusion coefficient of the gas.
5. Ventilation-perfusion matching.
6. Solubility of the gas.

Fick's Law of Diffusion

The rate of diffusion across a biological membrane increases with surface area and partial-pressure gradient and decreases with membrane thickness.

18. Partial Pressure of O₂ and CO₂

Partial pressure is the pressure contributed by an individual gas within a mixture of gases. Respiratory gas exchange occurs because gases move down their partial-pressure gradients.

Alveolar and Blood Values

Why Is Alveolar PO₂ Lower Than Atmospheric PO₂?

Atmospheric air is not identical to alveolar air. Inspired air becomes humidified in the respiratory tract, and oxygen is continuously removed from alveolar air into pulmonary blood while carbon dioxide is continuously added from blood.

Why Does CO₂ Diffuse Despite a Smaller Gradient?

Carbon dioxide is considerably more soluble in biological membranes than oxygen. Therefore, it diffuses efficiently even with a smaller partial-pressure gradient.

19. Transport of Oxygen in Blood

Oxygen is transported from lungs to tissues mainly in two forms: dissolved in plasma and reversibly bound to haemoglobin in red blood cells.

1. Dissolved Oxygen

A small proportion of oxygen is physically dissolved in plasma. Although quantitatively small, dissolved oxygen determines the measured partial pressure of oxygen in blood.

2. Haemoglobin-Bound Oxygen

Most oxygen is transported bound reversibly to the iron-containing haem groups of haemoglobin.

Each haemoglobin molecule can bind up to four oxygen molecules.

Oxygen Loading in Lungs

In pulmonary capillaries, the relatively high PO₂ promotes binding of oxygen to haemoglobin.

Oxygen Unloading in Tissues

In systemic tissues, the lower PO₂ and local chemical conditions promote oxygen release from haemoglobin.

Factors Promoting Oxygen Unloading

• Decreased PO₂.
• Increased PCO₂.
• Increased H⁺ concentration / decreased pH.
• Increased temperature.
• Increased 2,3-BPG in red cells.

20. Transport of Carbon Dioxide in Blood

Carbon dioxide produced by tissues is transported to the lungs in three major forms.

Bicarbonate Formation

In red blood cells, carbon dioxide combines with water to form carbonic acid. This reaction is rapidly catalysed by the enzyme carbonic anhydrase.

The bicarbonate ion leaves the red blood cell in exchange for chloride ions. This is called the chloride shift.

At the Lungs

The reactions reverse. Bicarbonate enters red blood cells, combines with hydrogen ions and forms carbonic acid. Carbonic acid is converted into carbon dioxide and water. CO₂ then diffuses into alveoli and is exhaled.

21. Oxygen-Haemoglobin Dissociation Curve

The oxygen-haemoglobin dissociation curve shows the relationship between the partial pressure of oxygen and the percentage saturation of haemoglobin with oxygen.

Shape

The curve is sigmoid because oxygen binding to haemoglobin is cooperative. Binding of one oxygen molecule increases the affinity of haemoglobin for subsequent oxygen molecules.

Importance

The plateau at high PO₂ helps maintain substantial haemoglobin saturation in the lungs even when alveolar PO₂ changes moderately. The steep portion at lower PO₂ facilitates oxygen unloading in tissues.

Right Shift

A rightward shift indicates decreased haemoglobin affinity for oxygen and therefore enhanced oxygen unloading.

• Increased CO₂.
• Increased H⁺.
• Decreased pH.
• Increased temperature.
• Increased 2,3-BPG.

Bohr Effect

The Bohr effect refers to the reduction in haemoglobin affinity for oxygen caused by increased CO₂ and H⁺ concentration. This promotes oxygen unloading in metabolically active tissues.

22. Haldane Effect

The Haldane effect describes the influence of oxygenation of haemoglobin on carbon dioxide transport.

Deoxygenated haemoglobin has a greater capacity to bind carbon dioxide and hydrogen ions than oxygenated haemoglobin. Therefore, in systemic tissues, deoxygenation of haemoglobin facilitates uptake and transport of CO₂.

In the lungs, oxygen binding to haemoglobin reduces its affinity for CO₂ and H⁺, facilitating CO₂ unloading and exhalation.

23. Ventilation-Perfusion Relationship

The ventilation-perfusion ratio describes the relationship between alveolar ventilation and pulmonary blood flow.

For the whole lung at rest, the average V/Q ratio is approximately 0.8.

Low V/Q

A low V/Q ratio means that ventilation is inadequate relative to perfusion. Blood leaving such regions may remain relatively underoxygenated.

High V/Q

A high V/Q ratio means that ventilation is relatively high compared with perfusion. A physiologically extreme example is dead space, where ventilation occurs with little or no effective perfusion.

Importance

Efficient gas exchange requires appropriate matching of ventilation and perfusion. V/Q mismatch is an important mechanism contributing to impaired oxygenation in several pulmonary disorders.

24. Regulation of Respiration

Breathing is regulated by respiratory centres in the brainstem, together with sensory input from chemoreceptors and pulmonary receptors.

Medulla Oblongata

The medulla contains major neuronal networks involved in generating the basic respiratory rhythm and coordinating inspiration and expiration.

Pons

Pontine respiratory centres modify and coordinate the respiratory pattern and contribute to smooth transitions between inspiration and expiration.

Chemoreceptors

Central Chemoreceptors

Central chemoreceptors respond mainly to changes related to CO₂ and hydrogen ion concentration in the brain extracellular fluid. An increase in arterial CO₂ generally increases respiratory drive.

Peripheral Chemoreceptors

Peripheral chemoreceptors are located mainly in the carotid and aortic bodies. They respond to decreased arterial PO₂ and also respond to changes in CO₂ and pH.

Basic Feedback Mechanism

25. Complete 10-Mark Long Questions

26. Separate 5-Mark Preparation

1. Functions of Respiratory System

• Oxygen uptake.
• Carbon dioxide removal.
• Maintenance of acid-base balance.
• Voice production.
• Air filtration, warming and humidification.
• Defense against inhaled particles.

2. Conducting and Respiratory Zones

The conducting zone extends from nose to terminal bronchioles and conducts, filters, warms and humidifies air. The respiratory zone consists of respiratory bronchioles, alveolar ducts and alveoli and is responsible for gas exchange.

3. Histology of Trachea

• Pseudostratified ciliated columnar epithelium.
• Goblet cells.
• Submucosal seromucous glands.
• C-shaped hyaline cartilage.
• Trachealis muscle posteriorly.

4. Histology of Alveoli

• Type I pneumocytes for gas exchange.
• Type II pneumocytes produce surfactant.
• Alveolar macrophages provide defense.
• Thin respiratory membrane.
• Dense pulmonary capillary network.

5. Mechanism of Inspiration

1. Diaphragm contracts.
2. External intercostals contract.
3. Thoracic volume increases.
4. Alveolar pressure decreases.
5. Air enters the lungs.

6. Mechanism of Expiration

1. Inspiratory muscles relax.
2. Thoracic volume decreases.
3. Elastic recoil of lungs occurs.
4. Alveolar pressure increases.
5. Air leaves lungs.

7. Gas Exchange

Oxygen diffuses from alveoli to blood because alveolar PO₂ is higher than venous blood PO₂. Carbon dioxide diffuses from blood to alveoli because venous PCO₂ is higher than alveolar PCO₂.

8. Partial Pressure

Partial pressure is the pressure exerted by an individual gas in a mixture. At sea level, atmospheric PO₂ is approximately 159 mmHg, while alveolar PO₂ is approximately 100 mmHg and alveolar PCO₂ is approximately 40 mmHg.

9. Oxygen Transport

Oxygen is transported mainly bound to haemoglobin and a small amount dissolved in plasma. Haemoglobin binds oxygen reversibly to form oxyhaemoglobin.

10. Carbon Dioxide Transport

• Dissolved in plasma.
• As carbamino compounds.
• Mostly as bicarbonate.

11. Bohr Effect

Increased CO₂ and H⁺ decrease haemoglobin affinity for oxygen and promote oxygen unloading in tissues.

12. Haldane Effect

Deoxygenated haemoglobin carries more CO₂ and H⁺, while oxygenation of haemoglobin in lungs promotes CO₂ unloading.

13. Surfactant

Pulmonary surfactant is produced by type II pneumocytes. It lowers alveolar surface tension, increases lung compliance and reduces the tendency of alveoli to collapse.

14. V/Q Ratio

V/Q represents the relationship between alveolar ventilation and pulmonary perfusion. The average whole-lung value is approximately 0.8 at rest.

15. Regulation of Respiration

Respiration is regulated by brainstem respiratory centres and chemoreceptors. Increased CO₂ generally stimulates ventilation, while peripheral chemoreceptors also respond strongly to low arterial PO₂.

27. Separate 2-Mark Preparation

28. Quick Revision

Respiratory Tract Sequence

Gas Exchange

Important Partial Pressures

Oxygen Transport

Carbon Dioxide Transport

Breathing Mechanism

Important Exam Points

Diagrams You Should Practice for 10-Mark Questions

Selected Clinical Correlations

Asthma

Asthma involves variable airway narrowing and increased airway resistance. Bronchial smooth-muscle constriction, inflammation and mucus production can impair airflow.

Pneumonia

Infection and inflammatory material within alveoli can interfere with effective ventilation and gas exchange.

Pulmonary Edema

Accumulation of fluid within the pulmonary interstitium or alveolar spaces can increase diffusion distance and impair oxygen transfer.

Emphysema

Destruction of alveolar walls reduces the effective surface area available for gas exchange and alters elastic recoil.

Pulmonary Embolism

Obstruction of pulmonary blood flow creates regions that may be ventilated but poorly perfused, producing a high V/Q state.

How to Write a 10-Mark Respiratory LQ

For physiology-heavy questions such as gas exchange, oxygen transport and breathing mechanism, emphasize the sequence of events and use pressure values and equations where appropriate. For anatomy-heavy questions, emphasize labelled structure, histology and functional correlation.

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