The Respiratory System

Written By Anjani Mishra

General Features of R.S.

Respiration has been defined as the osmotic and chemical process or processes by which a plant or an animal absorbs oxygen and gives off the products(specially carbon dioxide) formed by the oxidative activities of the tissues.

The respiratory system consists of a conducting portion, a respiratory portion, and a pumping mechanism by which air is alternately drawn in(inspiration) and expelled out(expiration) from the system.

The conducting portion is the part through which the air passes to reach the respiratory portion, where the actual exchange of oxygen for the products of oxidation takes place. It comprises,

Conducting portion

1. Nose (external nose, nasal cavity, and posterior nares),
2. The part of pharynx,
3. Larynx,
4. Trachea, and within the
5. Lungs, the bronchi and bronchioles as far as the respiratory bronchioles.

Respiratory portion

6. The respiratory bronchioles,
7. The alveolar ducts,
8. The alveolar sacs, and
9. The pulmonary alveoli.

The essential components of the pumping apparatus are;

1. The two pleural sacs which envelope the lungs and form vacuum chamber around them,

2. The skeleton of the thorax, sometimes referred to as the thoracic cage, and its associated muscles, and

3. The diaphragm.

Fig: Diverticulum nasi in horse

Fig: Parts of nasal cavity

NASAL CAVITY

The nasal cavities(left & right) which are separated by osteo-cartilagenous partition extends from nostrils(anterior nares) to choanae(posterior nares). These are formed by bones and cartilages and are lined by mucous membrane.

The nasal cavity is subdivided into three regions, namely;

1. Vestibular region

2. Respiratory region

3. Olfactory region

1. 1. Vestibular region:- it consists of;

A. Mucosa: the mucosa of the vestibule is moderately pigmented.

i) Lamina epithelia- lined with stratified squamous epithelium as epidermis.

ii) Lamina propria- composed of compact collagenous tissue that contains many serous gland.

B. Sub-mucosa: similarly composed of firm substance as L. propria in which there is abundance of large blood vessels and nerves. In horse the external skin, bearing hair and sebaceous and tubular glands is continued into the vestibule and the diverticulum nasi.

Fig: Nasal Cavity, Vestibule, Dog

4. Cartilage, 5. Cavernous vein, 13. Serous gland, 14. Stratified squamous epithelium

2. Respiratory region:- it consists of;

A. Mucosa: the mucosa consists of,

i) Lamina epithelia- lined with stratified columnar and finally into pseudostratified epithelium(ciliated) with goblet cells.

ii) Lamina propria- composed of compact collagenous tissue that contains many serous gland.

B. Sub-mucosa: composed of many elastic fibers, rich in leukocytes with lymph nodules in certain places. It also contains tubulo-alveolar glands mainly serous, although some mucous and mixed glands are found.

Fig: Respiratory epithelium, Nasal Concha, Dog

2. Bone, 5. Cavernous vein, 6. Goblet cell with pseudostratified epithelium(ciliated)

3. Olfactory region:- it consists of;

A. Mucosa: the olfactory mucosa differs from that of the respiratory region mainly in that it is thicker and has a different colour caused by pigments granules in the epithelium.

In horse and ox- yellowish

Sheep- yellow

Goat- dark

Pig- brown

Dog- gray

i) Lamina epithelia- It consists of three types of cells, namely; supporting or sustentacular cells, olfactory cells, and the basal cells.

Supporting cells- are elongated columnar cells with oval nuclei. The cytoplasm consists of fine granules and often pigmented.

Olfactory cells- are spheroid or ovoid shape nerve cells with large spheroid nuclei. It consists of peripheral and a central process.

Basal cells- are pseudostratified columnar cells with large and round to oval shape nuclei. The cytoplasm may appear light pink or slightly bluish.

ii) Lamina propria- is composed of collagen fiber and contains the tubular olfactory glands that contain yellow pigment granules and elaborate a mucoserous secretion.

B. Sub-mucosa: resembles that of the other sections of the nasal cavity.

Fig: Olfactory Epithelium, Nasal Cavity, Dog

1. Basal cells, 3. Bowman’s gland, 5. Cavernous vein, 9. Olfactory cells, nuclei, 11. Sensory hairs, 16. Supporting cells, nuclei

PHARYNX

The pharynx is a funnel shaped musculo-membranous sac/space between the digestive and respiratory system. The pharynx is sub-divided into nasopharynx and oropharynx.

Structure

The wall of pharynx is made of ;

1. Mucosa

2. The thin internal pharyngeal fascia

3. Striated muscles

4. The outer pharyngeal fascia

5. The loose adventitia

Fig: Oropharynx, dog

3. Duct, 8. Mixed gland, 12. Skeletal muscle, 13. Stratified squamous epithelium

Fig: Oropharynx, dog

2. Elastic fibers, 3. Lamina propria, 4. Mixed glands, 11. Stratified squamous epithelium

1. Mucosa

a. Lamina epithelia -

Oropharynx- It is lined with stratified squamous epithelium

Nasopharynx- It is lined with pseudo-stratified ciliated epithelium

b. Lamina propria- It is composed of fibroelastic tissue, it has a papillary body and an abundance of lymphatic tissue (in the lymph nodules and pharyngeal tonsils). In oropharynx tubular mucous gland are also found where as in nasopharynx both serous and mucous glands are found where serous is dominant.

2. The thin internal pharyngeal fascia

It forms the deep boundry of the mucosa. It contains elastic fiber nets, which are continued peripherally into the intermuscular connective tissue (C.T.)

3. Striated muscles

Consists of partly of inner longitudinal and mostly of outer circular fibers.

4. The outer pharyngeal fascia

A tough fibrous membrane composed of elastic fiber nets.

5. The loose adventitia

It consists of loose connective tissue.

Fig: Nasopharynx, dog

3. Duct, 6. Mixed gland, 7. Pseudostratified epithelium, 8. Skeletal muscle

LARYNX

The larynx is a rigid, short tube/passage for air between the pharynx and trachea. It is a complex valvular apparatus, which regulates the volume of air in respiration, prevents aspiration of foreign bodies, and is the chief organ of voice.

Structure

The laryngeal wall consists of;

1. Mucosa

Lamina epithelia

Lamina propria

2. Sub mucosa

3. Middle layer

4. Muscular layer

Fig: Epiglottis, dog

1. Adipose tissue, 4. Elastic cartilage, 5. Lamina propria, 6. Mixed gland, 9. Stratified squamous epithelium(non-keratinized), 10. Sub-mucosa

1. Mucosa

a. Lamina epithelia -

It is lined by pseudostratified ciliated epithelium except epiglottis, arytenoid cartilages and the aryepiglottic folds which are lined by stratified squamous epithelium (non-keratinized). Taste buds are found in the laryngeal portion of the epiglottic folds in ruminants, swine and carnivores.

b. Lamina propria - Composed of connective tissue containing numerous elastic fibers. It contains serous, mucuos and mixed glands and much lymphatic tissue, including lymph nodules.

2. Sub mucosa

It is a thin layer except at the base of epiglottis

3. Middle layer

Middle layer is mainly composed of cartilages, ligaments and a connecting fibrous membrane. The cartilages which forms the laryngeal skeleton are thyroid, cricoid, arytenoid, and epiglottis. All are composed of hyaline cartilage except epiglottis with its cuneiform processes and vocal and corniculate processes of arytenoid cartilages which are composed of elastic cartilage.

4. Muscular layer

It is mainly composed of striated fiber.

Fig: Arytenoid cartilage, vocal fold, l.s., cat

2. Arytenoid cartilage, 14. Stratified squamous epithelium, 18. Vocal ligament

TRACHEA

The trachea or windpipe is a flexible tube composed of fibro elastic C.T. and cartilages, which extends from the larynx caudally at the level of 2^nd cervical vertebra to the level of 5^th thoracic vertebra.

Fig: Trachea, x.s., sheep

1. Adipose tissue, 2. Elastic fibers, 3. Fibroelastic membrane, 5. Hyaline cartilage,
9. Pseudostratified epithelium, 11. Trachealis muscle

Structure

The wall of trachea consists of following layers-

1. Mucosa:

i) L. epithelia- It is lined with pseudostratified epithelium(ciliated) on a basement membrane with many goblet cells.

ii) L. Propria- It consists of fine fibers(elastic), mixed tubular glands(sero-mucous), lymphocyte oftenly, with blood vessels. [lymphnodules-specially in sheep and cat]

iii) L.m. mucosa- Absent. The lamina propria & sub-mucosa is separated by a firbro-elastic membrane consists of collagenous and longitudinal elastic fibers and takes the place of muscularis mucosae.

2. Sub-mucosa:

The sub-mucosa is rich in elastic fiber and fat and blends into the perichondrium in the region of cartilaginous rings. It also contains mixed tubular glands, blood vessels and nerves.

3. Muscularis: (present only dorsally)

It consists of dorsal strip of transverse smooth muscle fiber.
Horse, ruminant, pig - lies inside the end of rings between them and the mucosa
Dog, Cat - lies outside the rings

4. Adventitia:

It is the outermost layer, that surrounds the entire organ composed of collagenous and elastic fiber are loose and rich in adipose tissue, vessels and nerves.

LUNGS

Structure

The structure of lung is a lace work of alveoli separated by thin walled inter-alveolar septa. This is traversed by a system of intra-pulmonary bronchi, bronchiole and alveolar duct, which open into alveolar sacs and alveoli.

The lungs are sub-divided into two portions;

1. Conducting portion

a. Extra-pulmonary bronchus

b. Intra-pulmonary bronchus

i) Primary, ii) Secondary, & iii) Tertiary

c. Intra-pulmonary bronchiole

i) Primary, ii) Terminal

2. Respiratory portion

a. Respiratory bronchiole

b. Alveolar duct

c. Alveolar sac

d. Alveoli

b. Intra-pulmonary bronchus- They are lined by respiratory epithelium. Their wall contain irregular plates of hyaline cartilage. Smooth muscle bundles separate the lamina propria from sub-mucosa, which contain sero-mucous gland.

Fig: Primary bronchus, x.s., dog

5. Hyaline cartilage, 6. Lamina propria, 7. Mixed gland, 8. Muscularis, 9. Pseudostratified epithelium

Fig: Bronchus (Secondary), x.s., cow

1. Alveolus, 6. Hyaline cartilage, 7. Lamina propria, 9. Mixed gland, 10. Muscularis,

11. Pseudostratified epithelium

Fig: Small bronchus (Tertiary) and bronchioles, x.s., cat

1. Alveolus, 2. Bronchiole, 6. Hyaline artilage, 11. Pseudostratified epithelium, 12. Serous gland

c. Intra-pulmonary bronchiole

i) Primary bronchiole

It is lined with ciliated columnar to simple cuboidal with clara cells & goblet cell occasionally
Lamina propria is composed largely of smooth muscle and elastic fibers
The sub-mucosa has serous glands
The wall have no cartilage

Fig: Large bronchiole (Priamary), x.s., cat

1. Alveolus, 2. Bronchiole, 3. Columnar epithelium(ciliated), 8. Lymphatic nodule,

10. Muscularis, 12. Serous gland

ii) Terminal bronchiole

It is lined with simple cuboidal with many clara cells (no goblet cell)
Lamina propria is composed of fibro-elastic C.T.
1-2 layer of smooth muscle surround the lamina propria

Fig: Bronchioles (terminal bronchiole), pig

3. Alveolus, 4. Bronchiole, 6. Elastic fibers, 8.Muscularis

2. Respiratory portion

a. Respiratory bronchiole

Mucosa-similar to T. bronchiole, except that their walls are interrupted by numerous saclike alveoli
It is lined with ciliated cuboidal and clara cells with squamous alveolar cells
Smooth muscle and elastic C.T. present below epithelium

Fig: Respiratory bronchiole, l.s., sheep

3. Alveolus, 5. Cuboidal epithelium, 9. Respiratory bronchiole, 10. Smooth muscle

b. Alveolar duct

Arises from R. bronchiole and are thin walled, studded with alveolar sacs and alveoli.
It is lined with squamous alveolar cell.
A network of smooth muscle cells, collagen fiber present in the lamina propria surrounding the rim of alveoli.

Fig: Alveolar duct and alveolar sacs, sheep

1. Alveolar duct, 2. Alveolar sac, 3. Alveolus, 10. Smooth muscle

c. Alveolar sac

It is a simple sac that give rise to a number of alveoli.
It is lined with squamous alveolar cells.
Collagen and elastic fibers are present beneath the epithelium supporting the wall.
No smooth muscles are found here.

d. Alveoli

Alveoli are sac like evaginations (200mm) of respiratory bronchioles, alveolar ducts, and alveolar sacs.
They are responsible for the spongy structure of the lungs.
They are separated from each other by inter-alveolar septae, which may contain one or more alveolar pores.
An inter-alveolar septum is a wall or partition, between two adjacent alveoli. It consists of two thin squamous epithelial layer where capillaries, elastic and reticular fibers, C.T. matrix and cells are present.

Fig: Alveoli, cat

1. Alveolar septum, 2. Alveolus, 6. Erythrocyte in capillary, 7. Macrophage,

10. Type-ll alveolar cell

Alveolar cells: It is of 2 types;

Alveoli are lined by two types of highly attenuated simple squamous epithelial cells; Type-I & Type-II pneumocytes.

Type-I Pneumocyte (Squamous alveolar cells)

Covers 97% of the alveolar surface
Extremely thin (25nm)
Provide a barrier of minimal thickness that is permeable to gases

Type-II Pneumocyte

· Cover 3% of the alveolar surface

· Rounded cells with microvilli at their free surface

· Synthesize surfactant, that lower alveolar surface tension

Alveolar macrophage (dust cell)

Lies in the inter-alveolar septum and often seen on the alveolar surface
Remove dust, bacteria, carbon particles

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The Respiratory System | Histology | Anjani Mishra