Respiratory System

Dr. B P Kumar

Introduction

  • The respiratory system is functionally divided into two portions: the conducting portion and the respiratory portion.

  • The conducting portion includes the upper respiratory tract, which consists of the nasal cavity, pharynx, and larynx, and parts of the lower respiratory tract, namely the trachea, bronchi, and bronchioles.

  • The main function of the conducting portion is to provide a passage for air and to condition the inspired air.

  • The respiratory portion includes the terminal parts of the lower respiratory tract such as respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

  • The respiratory portion is involved in the exchange of oxygen and carbon dioxide between the blood and inspired air.

General structure

In general, the respiratory tract is made up of four coats.

  • Mucosa

    • It includes the epithelial lining and the underlying lamina propria.

    • The epithelium is usually pseudostratified ciliated columnar epithelium with goblet cells.

  • Submucosa

    • It is a layer of loose connective tissue containing mixed glands.

  • Cartilage layer

    • This layer is mostly formed by hyaline cartilage along with smooth muscle.

  • Adventitia

    • It is a layer of fibroelastic connective tissue that merges with the surrounding tissue.

 

Nasal cavity

General features

  • The nasal cavity is the beginning of the respiratory system.

  • It extends from the nostrils in front to the posterior nasal apertures behind.

  • Its framework is formed by bones and cartilages covered by mucous membrane.

  • It is divided into right and left halves by the nasal septum.

Structure

Histologically, based on the lining epithelium, the nasal cavity is divided into three regions.

1. Vestibule

  • It is the anterior dilated part of the nasal cavity and is lined by skin.

  • It contains thick short hairs called vibrissae, which filter large particles from the inspired air.

2. Olfactory region

  • This region is confined to a smaller area and is present in the roof and adjoining parts of the nasal septum and lateral wall of the nasal cavity.

  • It is covered by a thick yellow olfactory mucosa which functions as the organ of smell.

  • The olfactory mucosa is composed of a thick olfactory epithelium and the underlying connective tissue called lamina propria.

  • The lamina propria contains serous glands (bowman’s glands), bundles of olfactory nerves (fila olfactoria), blood vessels, and lymphatics.

  • The secretion of bowman’s glands moistens the olfactory epithelium and acts as a solvent for odoriferous substances, thereby stimulating the olfactory cells.

  • The olfactory epithelium consists of the following three types of cells.

(a) Olfactory cells

  • These are modified bipolar nerve cells.

  • Each cell has a cell body, a dendrite, and an axon.

  • The cell bodies contain round nuclei and are found at different levels, forming a broad zone of round nuclei.

  • The dendrites extend to the surface and end in knob-like olfactory vesicles from which nonmotile olfactory hairs arise.

  • The axons pass into the lamina propria and form bundles of olfactory nerves (fila olfactoria).

(b) Sustentacular cells

  • These are tall columnar supporting cells.

  • They bear microvilli on their free surface.

  • Each cell has a broad superficial part with an oval nucleus and a slender deep part attached to the basement membrane.

  • Their nuclei lie near the free surface at the same level, forming a narrow zone of oval nuclei.

  • The presence of yellow lipofuscin pigments suggests a phagocytic function.

(c) Basal cells

  • These cells are conical in shape.

  • They do not reach the surface.

  • They give rise to other cell types.

3. Respiratory region

  • It occupies the remaining area of the nasal cavity.

  • It is covered by respiratory mucosa, which is pink in color and thinner than the olfactory mucosa.

  • The respiratory mucosa is firmly adherent to the underlying periosteum or perichondrium.

  • It consists of respiratory epithelium and underlying lamina propria.

  • The respiratory epithelium is pseudostratified ciliated columnar epithelium with goblet cells.

  • The lamina propria contains mixed glands, lymphatic nodules, and rich vascular plexuses.

  • The respiratory epithelium includes the following cell types.

(a) Ciliated cells

  • These are columnar cells with cilia on their free surface.

  • They are the most abundant cell type.

  • The cilia beat towards the pharynx.

(b) Goblet cells

  • These are flask-shaped mucus-secreting cells.

(c) Brush cells

  • These are columnar cells with microvilli on their free surface.

  • They may have a sensory function.

(d) Basal cells

  • These are small pyramidal cells.

  • They do not reach the surface.

  • They give rise to other cell types.

(e) Granule cells

  • These are small round cells with numerous cytoplasmic granules.

  • They form part of the apud cell series.

 

Pharynx

General features

  • Pharynx is a fibromuscular tube extending from the base of the skull to the level of the sixth cervical vertebra, where it becomes continuous with the oesophagus.

  • It lies behind the nasal cavity forming the nasopharynx, behind the oral cavity forming the oropharynx, and behind the larynx forming the laryngopharynx.

Structure

  • Pharynx is composed of four coats.

1. Mucosa

  • It consists of epithelium and lamina propria.

  • The epithelium is pseudostratified ciliated columnar type in the nasopharynx.

  • The epithelium is stratified squamous type in the oropharynx and laryngopharynx.

  • Aggregation of lymphatic nodules in the lamina propria of the posterior wall and around the opening of the auditory tube in the nasopharynx forms the pharyngeal tonsil and tubal tonsils respectively.

  • The palatine tonsil in the lateral wall of the oropharynx and the lingual tonsil in the pharyngeal part of the tongue are described under the lymphatic system.

2. Submucosa

  • It is formed by loose areolar connective tissue called the pharyngobasilar fascia.

3. Muscle coat

  • This layer is composed of skeletal muscle.

  • The muscle fibers are arranged into an inner longitudinal layer and an outer circular layer.

  • The circular layer is formed by the constrictor muscles of the pharynx.

4. Adventitia

  • It is formed by fibroelastic connective tissue (buccopharyngeal fascia).

 

Larynx

General features

  • Larynx is a specialised organ responsible for the production of voice.

  • It houses the vocal cords.

  • Above, it opens into the laryngopharynx, and below it is continuous with the trachea.

  • It has a cartilaginous framework made up of nine cartilages, consisting of three paired and three unpaired cartilages.

  • These cartilages are connected to each other by membranes and ligaments and are moved by skeletal muscles.

  • The cartilages are either hyaline or elastic in nature.

  • The hyaline cartilages include:

    • Thyroid cartilage (unpaired)

    • Cricoid cartilage (unpaired)

    • Arytenoid cartilages (paired)

  • The elastic cartilages include:

    • Epiglottis (unpaired)

    • Cuneiform cartilages (paired)

    • Corniculate cartilages (paired)

Structure

  • Most of the larynx is lined by pseudostratified ciliated columnar epithelium with goblet cells.

  • In areas subjected to friction or contact with food, it is lined by stratified squamous epithelium.

  • Examples of such areas include the anterior surface and upper part of the posterior surface of the epiglottis and the vocal folds.

  • The lamina propria may contain seromucous glands and lymphatic tissue.

 

Trachea

General features

  • Trachea is a flexible fibroelastic cartilaginous tube that allows expansion in width and extension in length during inspiration.

  • It measures about 10 cm in length and 2 cm in width.

  • It extends from the lower border of the cricoid cartilage at the level of c6 to its bifurcation at the level of t4 into right and left bronchi.

  • It is also called the windpipe and serves as the airway through which respiratory air is transported.

  • It is supported by 16–20 c-shaped rings of hyaline cartilage that keep the lumen patent.

  • The free posterior ends of the c-shaped cartilages are connected by a band of smooth muscle called trachealis and a fibroelastic ligament.

  • The ligament prevents overdistension of the lumen, while contraction of the muscle reduces the diameter during coughing, increasing the velocity of expired air for cleaning the air passages.

Structure

The trachea is composed of four coats, similar to the general plan of the respiratory tract.

1. Mucosa

  • It is composed of epithelium and lamina propria.

  • The epithelium is thick and consists of pseudostratified ciliated columnar epithelium with goblet cells.

  • The lamina propria is made of fibroelastic vascular connective tissue with longitudinally oriented elastic fibers.

  • Lymphocytes and mast cells are abundant in the lamina propria.

2. Submucosa

  • It is formed by loose connective tissue located deep to the lamina propria.

  • It contains mixed glands.

  • There is no clear demarcation between the lamina propria and the submucosa.

3. Cartilage and smooth muscle layer

  • This layer is formed by c-shaped rings of hyaline cartilage.

  • The posterior free ends of the cartilage rings are bridged by smooth muscle and a fibroelastic ligament.

4. Adventitia

  • It is composed of fibroelastic connective tissue containing neurovascular structures.

 

Lungs

General features

  • Lungs are the principal organs of respiration and are situated in the thoracic cavity on either side of the mediastinum.

  • Each lung is conical in shape and is covered by visceral pleura.

  • Lungs contain the terminal parts of the bronchial tree, including intrapulmonary bronchi, bronchioles, respiratory bronchioles, and lung parenchyma consisting of alveolar ducts and alveoli, along with blood vessels.

Structure

  • The lining epithelium of the bronchial tree gradually decreases in thickness towards the distal parts.

  • Glands and goblet cells also decrease gradually and disappear distally.

  • Cartilaginous support decreases, while elastic fibers increase towards the periphery.

1. Intrapulmonary bronchus (secondary and tertiary bronchi)

  • Each principal bronchus divides into secondary (lobar) bronchi, which further divide into tertiary (segmental) bronchi.

  • The following layers are seen from inside to outside.

(a) Mucosa

  • It consists of epithelium and lamina propria.

  • The epithelium is pseudostratified ciliated columnar with few goblet cells.

  • The lamina propria is rich in longitudinally arranged elastic fibers.

  • The mucosa is thrown into folds due to contraction of smooth muscle.

(b) Smooth muscle layer

  • It consists of spirally arranged criss-cross bundles.

  • In sections, it appears discontinuous.

(c) Submucosa

  • It contains few seromucous glands.

(d) Cartilage layer and adventitia

  • Hyaline cartilage is present as isolated plates instead of c-shaped rings.

 

2. Bronchiole

  • Bronchioles are formed by repeated division of tertiary bronchi.

  • Each bronchiole enters a pulmonary lobule and divides into 5–7 terminal bronchioles.

  • Terminal bronchioles have a diameter of less than 1 mm.

  • Bronchioles show the following features:

    • Simple columnar or cuboidal ciliated epithelium.

    • Goblet cells are absent and replaced by clara cells.

    • Clara cells secrete a protective glycoprotein.

    • Thick smooth muscle layer under autonomic control.

    • Many elastic fibers.

    • Absence of glands and cartilage.

 

3. Respiratory bronchiole

  • Respiratory bronchioles arise from terminal bronchioles.

  • They form the transitional zone between conducting and respiratory portions.

  • Their walls are interrupted by alveolar outpouchings.

  • The lining epithelium between alveoli is ciliated cuboidal with clara cells.

  • They divide distally into alveolar ducts.

4. Alveolar duct

  • Alveolar ducts arise from respiratory bronchioles.

  • They are lined by squamous epithelial cells supported by smooth muscle and fibroelastic tissue.

  • Numerous alveoli open into the ducts, giving a knob-like appearance in sections.

  • Alveolar ducts open into atria, which communicate with alveolar sacs and alveoli.

 

5. Alveoli

  • Alveoli form the lung parenchyma.

  • They are sac-like terminal expansions of the bronchial tree.

  • They are the site of gaseous exchange between air and blood.

  • Alveoli are compactly packed and appear honeycomb-like in sections.

  • Adjacent alveoli are separated by interalveolar septa.

  • The interalveolar septum contains capillaries, reticular and elastic fibers, and occasional fibroblasts, mast cells, and macrophages.

  • Alveolar pores of kohn may be present and help equalize air pressure.

  • Elastic fibers allow expansion during inspiration and passive recoil during expiration.

  • Reticular fibers prevent overdistension of alveoli.

 

Cells of alveolar lining epithelium

(a) Type i pneumocytes

  • Extremely thin squamous cells.

  • Cover about 97% of alveolar surface.

  • Contain pinocytic vesicles.

  • Form part of the blood–air barrier.

(b) Type ii pneumocytes

  • Cuboidal cells present in small groups.

  • Cover about 3% of alveolar surface.

  • Contain lamellated bodies.

  • Secrete pulmonary surfactant.

  • Surfactant reduces surface tension, prevents alveolar collapse, and has bactericidal properties.

 

Alveolar macrophages

  • Alveolar macrophages are derived from monocytes.

  • They form part of the mononuclear phagocytic system.

  • They phagocytose inhaled dust and carbon particles.

  • They may migrate to the alveolar surface and are expelled through sputum, which has diagnostic importance.

 

MCQs

1. The respiratory system is functionally divided into:

A. Upper and lower portions
B. Conducting and respiratory portions
C. Central and peripheral portions
D. Internal and external portions

Answer: B

2. The functional unit of the respiratory system is:

A. Trachea
B. Bronchus
C. Bronchiole
D. Alveolus

Answer: D

3. The conducting portion of the respiratory tract includes:

A. Alveoli
B. Alveolar ducts
C. Bronchioles
D. Trachea

Answer: D

4. The nasal cavity is divided into right and left halves by:

A. Concha
B. Turbinate
C. Nasal septum
D. Vomer

Answer: C

5. The vestibule of the nasal cavity is lined by:

A. Pseudostratified epithelium
B. Stratified squamous epithelium
C. Skin
D. Simple cuboidal epithelium

Answer: C

6. Vibrissae are present in:

A. Olfactory region
B. Respiratory region
C. Vestibule
D. Nasopharynx

Answer: C

7. Olfactory mucosa is yellow in color because of:

A. Mucus secretion
B. Lipid content
C. Lipofuscin pigment
D. Keratin

Answer: C

8. Bowman’s glands are present in:

A. Respiratory mucosa
B. Olfactory mucosa
C. Trachea
D. Bronchi

Answer: B

9. Bowman’s glands secrete:

A. Mucus
B. Serous fluid
C. Sweat
D. Surfactant

Answer: B

10. Olfactory cells are:

A. Unipolar neurons
B. Multipolar neurons
C. Bipolar neurons
D. Pseudounipolar neurons

Answer: C

11. Sustentacular cells mainly provide:

A. Sensation
B. Support
C. Secretion
D. Movement

Answer: B

12. Basal cells of olfactory epithelium are responsible for:

A. Secretion
B. Support
C. Regeneration
D. Sensation

Answer: C

13. Respiratory epithelium is:

A. Stratified squamous epithelium
B. Simple cuboidal epithelium
C. Pseudostratified ciliated columnar epithelium
D. Simple squamous epithelium

Answer: C

14. The main function of respiratory mucosa is:

A. Voice production
B. Smell
C. Conditioning of air
D. Immunity

Answer: C

15. Pharynx extends from:

A. C1 to C4
B. C2 to C7
C. Base of skull to C6
D. C3 to T1

Answer: C

16. Nasopharynx is lined by:

A. Stratified squamous epithelium
B. Simple cuboidal epithelium
C. Pseudostratified ciliated columnar epithelium
D. Simple squamous epithelium

Answer: C

17. Palatine tonsil is located in:

A. Nasopharynx
B. Oropharynx
C. Laryngopharynx
D. Trachea

Answer: B

18. Muscle coat of pharynx is composed of:

A. Smooth muscle only
B. Cardiac muscle
C. Skeletal muscle
D. Elastic fibers

Answer: C

19. The organ of voice production is:

A. Pharynx
B. Trachea
C. Larynx
D. Bronchus

Answer: C

20. Total number of laryngeal cartilages is:

A. 7
B. 8
C. 9
D. 10

Answer: C

21. Which is an elastic cartilage of larynx?

A. Thyroid
B. Cricoid
C. Arytenoid
D. Epiglottis

Answer: D

22. Vocal folds are lined by:

A. Pseudostratified epithelium
B. Stratified squamous epithelium
C. Simple squamous epithelium
D. Cuboidal epithelium

Answer: B

23. Trachea extends from:

A. C4 to T6
B. C6 to T4
C. C3 to T5
D. C5 to T3

Answer: B

24. Tracheal cartilage is:

A. Complete ring
B. Flat plate
C. C-shaped
D. Elastic plate

Answer: C

25. Trachealis muscle helps in:

A. Inspiration
B. Voice production
C. Swallowing
D. Coughing

Answer: D

26. Tracheal epithelium is:

A. Simple squamous
B. Stratified squamous
C. Pseudostratified ciliated columnar
D. Cuboidal

Answer: C

27. Intrapulmonary bronchi contain:

A. C-shaped cartilage
B. Elastic cartilage
C. Cartilage plates
D. No cartilage

Answer: C

28. Goblet cells gradually disappear in:

A. Trachea
B. Bronchi
C. Bronchioles
D. Pharynx

Answer: C

29. Bronchioles are characterized by absence of:

A. Smooth muscle
B. Elastic fibers
C. Cartilage
D. Epithelium

Answer: C

30. Clara cells are found in:

A. Trachea
B. Bronchi
C. Bronchioles
D. Alveoli

Answer: C

31. Clara cells secrete:

A. Mucus
B. Surfactant
C. Glycoprotein
D. Enzymes

Answer: C

32. Bronchoconstriction is caused by stimulation of:

A. Sympathetic nerves
B. Vagus nerve
C. Phrenic nerve
D. Intercostal nerve

Answer: B

33. Respiratory bronchioles represent:

A. Conducting zone
B. Respiratory zone
C. Transitional zone
D. Excretory zone

Answer: C

34. Alveolar ducts are lined by:

A. Cuboidal epithelium
B. Columnar epithelium
C. Squamous epithelium
D. Stratified epithelium

Answer: C

35. Lung parenchyma mainly consists of:

A. Bronchi
B. Bronchioles
C. Alveoli
D. Trachea

Answer: C

36. Alveoli give the lung its:

A. Hard texture
B. Solid structure
C. Spongy appearance
D. Fibrous nature

Answer: C

37. Interalveolar septum contains:

A. Cartilage
B. Smooth muscle
C. Capillaries
D. Serous glands

Answer: C

38. Pores of kohn help in:

A. Surfactant secretion
B. Air passage between alveoli
C. Blood circulation
D. Mucus secretion

Answer: B

39. Elastic fibers in alveoli help in:

A. Gas diffusion
B. Expansion only
C. Passive recoil
D. Surfactant formation

Answer: C

40. Type i pneumocytes cover about:

A. 50% alveolar surface
B. 70% alveolar surface
C. 90% alveolar surface
D. 97% alveolar surface

Answer: D

41. Type i pneumocytes are:

A. Cuboidal
B. Columnar
C. Squamous
D. Stratified

Answer: C

42. Type ii pneumocytes secrete:

A. Mucus
B. Glycoprotein
C. Pulmonary surfactant
D. Enzymes

Answer: C

43. Lamellated bodies are found in:

A. Type i pneumocytes
B. Macrophages
C. Type ii pneumocytes
D. Clara cells

Answer: C

44. Pulmonary surfactant function is to:

A. Increase surface tension
B. Decrease surface tension
C. Increase diffusion distance
D. Cause collapse of alveoli

Answer: B

45. Alveolar macrophages are derived from:

A. Neutrophils
B. Lymphocytes
C. Monocytes
D. Eosinophils

Answer: C

46. Alveolar macrophages are also called:

A. Foam cells
B. Dust cells
C. Kupffer cells
D. Plasma cells

Answer: B

47. Blood–air barrier is formed mainly by:

A. Type ii pneumocytes
B. Alveolar macrophages
C. Type i pneumocytes and capillary endothelium
D. Clara cells

Answer: C

48. Reticular fibers in alveoli:

A. Allow gas exchange
B. Secrete surfactant
C. Prevent overdistension
D. Cause elasticity

Answer: C

49. The respiratory epithelium cilia beat towards:

A. Alveoli
B. Lungs
C. Pharynx
D. Pleura

Answer: C

50. The main site of gaseous exchange is:

A. Bronchiole
B. Respiratory bronchiole
C. Alveolar duct
D. Alveolus

Answer: D