Respiratory System: Structure and Function

Respiratory System Overview

• The respiratory system divides into two major portions: conducting and respiratory

Conducting Portion

• Includes the nasal cavity, nasopharynx, larynx, trachea, bronchi, and bronchiole, and terminal bronchiole
• Specialized to carry air to and from gas exchange sites
• Kept potent by hard or firm connective tissue in walls to prevent collapse during respiration
• Functions include conditioning, competeting, clearing the air and regulating humidity

Respiratory Portion

• Includes the respiratory bronchiole, alveolar duct, alveolar sac, and alveoli
• Specialized for O2 and CO2 exchange between blood capillaries and air

Functions of the Respiratory System

• Conducting and gaseous exchange
• Phonation and olfaction
• Body temperature regulation
• Excretion
• Acid-base balance
• Secretion
• Blood pressure regulation
• Filtration/Cleaning of air via ciliated epithelial cells that sweep dust particles trapped in mucous

Nasal Cavity

• Consists of three regions: vestibular, respiratory, and olfactory

Vestibular Region

• Anterior portion of the nasal cavity
• Anterior cartilaginous part lined with keratinized stratified squamous epithelium (skin)
• Contains short, stiff hairs and numerous sebaceous, and sweet glands in the lamina propria
• Hairy skin lines the vestibular region in horses
• Posterior part of the vestibular region is lined with non-keratinized stratified squamous epithelium
• Submucosa has dense connective tissue and blood vessels

Respiratory Portion

• Posterior part of the nasal cavity
• Covered with pseudostratified columnar ciliated epithelium with goblet cells in between
• Lamina propria is highly vascularized with seromucoid glands, plasma cells, lymphoid elements

Olfactory Region

• Located in the roof of the nasal cavity and encompasses the superior aspect of nasal septum and superior concha
• Responsible for odor perception
• Olfactory epithelium contains olfactory, sustentacular, and basal cells
• Lamina propria of connective tissue underlies the olfactory epithelium, with fibroelastic connective tissue rich in vascular plexus cavernous tissue
• Bowman's glands are tubulo-alveolar serous glands that secrete fluid on the olfactory mucosa acting as a solvent for odorous substances

Olfactory Cells

• Bipolar nerve cells with an apical dendrite, basally directed
• Spheroidal or oval cell body (soma) with a rounded nucleus
• Apical dendrite forms a bulb (olfactory vesicle) projecting above sustentacular cells, containing SER, mitochondria, microtubules, and a basal body
• Vesicle projects 6-12 cilia into the surface and these are considered receptors
• Basal process (axon) goes into connective tissue as an unmyelinated fiber of the olfactory nerve, penetrating the basal lamina and joining similar axons to form bundles of nerve fibers
• Nerve fibers penetrate the cribriform plate of ethmoidal bone to synapse

Sustentacular Cells

• Tall columnar cells, wide near the surface and narrow near the basement membrane
• Basal end is bifurcated with lateral indentations
• Surface covered with microvilli
• Oval nucleus situated in the apical 1/3 of the cell
• Function is to support olfactory cells

Basal Cells

• Small, short basophilic cells
• Star-shaped or pyramidal cells don't reach the epithelial surface
• Rounded, darkly stained nuclei lie between olfactory receptor cells and supporting cells
• Act as stem cells that replace sustentacular cells

Nasopharynx

• Communicates the nasal cavity with the larynx
• Nasal side lined by respiratory epithelium
• Oral surface lined by stratified squamous epithelium
• Lamina propria of the posterior aspect contains mucous glands, blood vessels, and pharyngeal tonsils

Larynx

• Irregular tube connecting the pharynx with the trachea
• Laryngeal wall has mucosa, cartilage and striated muscles in connective tissue sheath
• Pairs of mucosal folds project inward from its wall
• The upper pair is called the vestibular fold and the lower pair is the vocal folds
• Vocal folds vibrate to produce sound and protect the larynx from the entrance of foreign particles
• Anterior surface of epiglottis and vocal cords are covered by non-keratinized stratified squamous epithelium
• The rest of the larynx is lined by respiratory epithelium
• Lamina propria formed of connective tissue containing mucous secreting glands
• Larynx is held open by laryngeal cartilage
• Thyroid and cricoid cartilages are hyaline, while the epiglottis, corniculate, and cuneiform are elastic
• Extrinsic muscles of the larynx attach to the hyoid bone and help with deglutition
• Intrinsic muscles join thyroid and cricoid cartilages altering vocal cord tension for phonation

Trachea

• Tube connected to the larynx that bifurcates into 2 main bronchi
• Wide, flexible air tube from larynx to thorax
• Reinforced by 20 C-shaped incomplete rings of hyaline cartilage located anteriorly and laterally
• Posterior wall contains a thick band of transverse interlacing bundles of smooth muscles
• Concerned with conduction and conditioning of air

Trachea Structure

• Wall formed of 4 layers: mucosa, submucosa, cartilaginous coat, and fibrosa

I - Mucosa

• Consists of respiratory epithelium (pseudostratified columnar ciliated with goblet cells), lamina propria, and elastic membrane

II - Submucosa

• Formed of areolar connective tissue with blood vessels, nerves, lymphoid follicles, and seromucous tracheal glands
• Gland ducts open into the surface

III - Cartilaginous Coat

• Made of C-shaped hyaline cartilage deficient posteriorly
• Tracheal muscles attach: outside cartilage ends in dogs/cats, inside in ruminants/horses/pigs, between cartilage ends in humans

IV - Fibrous Layer

• Thin, fibrous connective tissue surrounds the trachea

Bronchial Tree

• Trachea divides into two main bronchi (extra pulmonary), which enter the hila of the lungs
• Inside lungs they become lobar secondary bronchi, segmental tertiary bronchi, subsegmental bronchi, terminal bronchioles, then respiratory bronchioles
• Respiratory bronchioles lead to alveolar ducts, alveolar sacs, and alveoli

Extra Pulmonary Bronchus

• Shares similar structure to trachea, except narrower in diameter

Bronchioles

• Small tubes with a diameter of less than 1mm
• Exist as 1ry, 2ry, 3ry or terminal bronchioles
• Wall consists of mucosa, submucosa, and connective tissue fibrosa

Bronchioles Mucosa

• Folded (decreases distally)
• Large bronchioles are lined by ciliated simple columnar epithelium with occasional goblet cells
• Smaller bronchioles transition to ciliated simple cuboidal epithelium alternating with Clara cells and no goblet cells
• Terminal bronchioles are the smallest with no goblet cells and an increase in Clara cells

Clara Cells

• Tall columnar cells with rounded apices that may project above ciliated cells
• Possess small nucleus, well-developed SER, mitochondria, RNA, Golgi body, and secretory granules
• Secrete a fluid layer of protein that is continuous with the surfactant
• Act as regenerator cells to replace died cells
• Lamina propria is rich in elastic fibers
• Arrangement includes spirally-arranged smooth muscle fibers ensheathed in elastic fibers
• Act as a sphincter to regulate air entering the respiratory portion
• Outer connective tissue fibrosa is a thin layer containing blood vessels and nerve fibers

Terminal Bronchiole

• End of conducting portion and is the smallest of the conducting parts
• Thin wall formed of folded mucosa
• Mucosa lined by simple columnar epithelium in 1ry and 2ry or simple cuboidal epithelium with Clara cells and goblet cells
• Supported by connective tissue (lamina propria) containing blood vessels, nerves, lymphocytes, and elastic fibers
• Consist of circular and oblique smooth muscles that squeeze terminal bronchioles to enter the air
• Perichondral connective tissue has collagenous and elastic fibers, no gland and no cartilages

Respiratory Portion

• Consists of respiratory bronchiole, alveolar duct, alveolar sac, and alveoli

Respiratory Bronchioles

• Result from bifurcation of terminal bronchioles
• Wall interrupted by thin-walled, pouch-like structures called alveoli
• Some lining cells produce surfactant (lipoprotein that reduces surface tension)
• Scattered cells are ciliated, while others have mitochondria

Alveolar Ducts

• Each alveolar duct ends as a blind out-pushing of two or more small clusters of alveoli called alveolar sacs
• Lined with simple cuboidal or simple squamous, non-ciliated epithelium
• The lamina propria is formed of CT containing matrix of elastic and collagenous fibers and network of smooth muscles around the opening of alveolar sacs and alveoli
• Smooth muscle disappears at the distal end of alveolar ducts

Alveolar Sacs

• Pockets of pulmonary alveoli which open into common central spaces leading to alveoli and exist in the lungs

Alveoli

• The structural and functional units of respiratory gas exchange
• Small polyhedral air spaces with one wall lacking to allow air diffusion
• In contact with blood capillaries
• Air and blood are separated by alveolar and capillary walls allowing gas diffusion
• Alveolar pores (Lamber's sinus) are small opening facilitating equal pressure and collateral respiration
• Lined by alveolar epithelium and separated by interalveolar septum

Interalveolar Septum

• The partition between adjacent alveoli
• Made from connective tissue and leucocytes

Alveolar Epithelium

• Formed of two types of cells: Pneumocyte type I, and Pneumocyte type II

Pneumocyte Type I (Squamous Cells)

• Flattened simple squamous cells with flat, dark nuclei
• Line most of the alveolar wall
• Differentiated cells that cannot divide
• Comprise 97% of alveolar wall surface area
• Connected by tight junctions and desmosomes to prevent tissue fluid leakage
• Rest on clear basal lamina and fuse with the basal lamina of adjacent endothelial cells of blood capillaries
• Specialized to provide minimum thickness for gas diffusion (25 nm width)

Type II Pneumocyte (Cuboidal Cells)

• Cuboidal cells with central rounded vesicular nuclei
• Are less numerous: Make up 3% of alveolar wall
• Attach to pneumocyte type I by tight junction and desmosomes
• Exhibit vacuolated cytoplasm containing characteristic large electron dense multilamellar granules with phospholipids glycosaminoglycans and protein
• Have the capacity to divide and act as progenitor cells for type I and type II pneumocytes
• Secrete surfactant (phospholipid layer) released by exocytosis spreading as a film that coats the alveolar wall to reduce alveolar surface tension which prevents alveolar collapse especially in new born infants

Blood Air Barrier

• The wall separating alveolar air from circulating blood in lung capillaries
• Facilitates gas exchange between alveoli lumen and blood capillaries
• Composed of squamous and cuboidal alveolar cells' cytoplasm, the basement membrane of alveolar cells I and II, endothelium basement membrane, endothelium cell cytoplasm, and surfactant layer on alveolar wall
• Prevents tissue fluid leakage into alveoli

Alveolar Capillaries

• Very thin and lined by continuous endothelium
• Share in the formation of the blood air barrier

Alveolar Macrophages

• Dust cells, mononuclear phagocytes in the interalveolar septum and alveoli
• Originate from blood monocytes migrating through blood capillary walls to the alveolar space or from histocytes outside the alveoli

Heart Failure Cells

• Located in the alveolar capillaries
• Originate from monocytes
• Engulf ruminants of blood cells and phagocytose iron pigment from the ruptured erythrocytes in certain heart diseases because heart failure may cause failure to distribute all blood coming from the lung

Lungs

• The capsule (pleura): thin layer of connective tissue that covers the lung composed of 2 outer (parietal) and inner (visceral) layers
• Lung is composed of framework tissue: the capsule (pleura) and the interalveolar septum
• Contains fibroelastic membrane rich in elastic and collagenic fibers and macrophages, blood, lymph and nerves
• Visceral pleura is composed of simple squamous epithelium (mesothelium) resting on a layer of elastic fibers
• Subserosa has collagenous fibers
• Capsule is thick in cattle and camels, but thin in carnivores

Interalveolar Septum

• Consists of smooth muscle fibers that divide the lung into several lobules (clear in cattle, 1-2cm)
• Contains bronchi, blood vessels, nerves, lymph nodes, and lymph vessels
• Septa are thick in cattle and camels but thin in carnivores