Human beings survive Three (3) weeks without food, Three (3) days without water, and only Three (3) minutes without oxygen. Trillions of human body cells would require a supply of uninterrupted stores of oxygen in order to perform functions. The major functions of the respiratory system are to supply oxygen to the body and dispose of carbon dioxide from the body.
4 Processes of Respiration
- Pulmonary ventilation
- moving air into (inflow) and out (outflow) between the atmosphere and the lung alveoli
- Respiratory gas exchange –diffusion of oxygen and diffusion of carbon dioxide between the alveoli and the blood
- External respiration –gas exchange between the lungs and the blood
- Internal respiration –gas exchange between systemic blood vessels and tissues
The respiratory tract can be divided anatomically and functionally. Anatomically, it can be divided into the upper or lower respiratory tract with the Larynx as the point of distinction. Functionally, the respiratory tract can be divided into the conducting zone and the respiratory zone.
- The upper respiratory system
- Nose à nasal cavity à oropharynx à nasopharynx àLarynx
- The lower respiratory system
- Trachea à bronchi and bronchioles à alveoli
- The upper respiratory system
- Conducting zone
- nose, pharynx, larynx, trachea, bronchi, bronchioles, terminal bronchioles
- provide fairly rigid conduits for air to reach the gas exchange sites
- cleanse, humidify, and warm incoming air
- Respiratory zone
- respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli actual site of gas exchange
- Conducting zone
Nose and Paranasal Sinuses
- The nose is the only part of the respiratory system that is visible externally
- Functions of the nose:
- Provides airway for respiration
- Warms, moistens, and filter the air that enters
- Resonating chamber for speech
- Houses the smell or olfactory receptors
- Divided into External nose and Internal nasal cavity
- Consists of Apex, External nares (Nostrils), Dorsum nasi, and Alae Nasi
- Skeleton of the external nose
- Bony framework
- nasal bones
- frontal processes of maxilla
- nasal part of frontal bone
- Cartilaginous framework
- septal & lateral cartilages
- major/minor alar nasal cartilages
- Bony framework
Internal Nasal Cavity
- Lies in and posterior to the external nose
- Divided into Right & Left halves by nasal septum formed by
- Anteriorly: septal cartilage
- Posteriorly: vomer bone and perpendicular plate of the ethmoid bone
- Continuous posteriorly with the nasal portion of the pharynx through the posterior nasal apertures, also called the choanae
- Roof is formed by the sphenoid, cribriform plate of ethmoid, frontal & nasal bones, & the nasal cartilages
- Floor is formed by the palatine process of maxilla & horizontal plate of the hard palate, which separates the nasal cavity from the oral cavity below
- The respiratory mucosa lines most of the nasal cavity. The respiratory mucosa is a pseudostratified ciliated columnar epithelium, containing scattered goblet cells, that rests on a lamina propria richly supplied with seromucous nasal glands.
- Secretions of nasal mucosa contain bactericides, lysozymes, lactoferrin, & Immunoglobulin A
3 Regions of the Nasal Cavity
- Nasal vestibule
- lumen lined with skin
- Inferior region bearing sebaceous & sweat glands, coarse hairs (vibrissae)
- Vibrissae or hair filter coarse particles such as dust and pollen from inspired air
- Respiratory region
- encompass the lateral & medial walls, the nasal roof, floor & conchae
- Nasal Conchae:
- Increases surface area of the nasal cavity
- Inspired dust particles are removed from the air by mucous membrane
- Meatus: Located inferolateral to each concha
- Superior meatus
- receives opening of post ethmoidal sinus
- Middle meatus
- receives opening of maxillary sinus via the hiatus semilunares
- Inferior meatus
- contains the opening of the nasolacrimal duct
- Superior meatus
- Chemosensory / olfactory area
- contains sensory terminals of the olfactory nerve
- Anterior & posterior ethmoidal branches of the ophthalmic artery
- ethmoidal & frontal sinuses
- Sphenopalatine branch of the maxillary artery
- mucosa of conchae, meatuses and septum
- Septal branch of sup labial ramus of facial artery
- vestibular septum
- Infraorbital, superior, anterior, and post alveolar branches of the maxillary artery
- mucosa of maxillary sinus
- Pharyngeal branch of the maxillary artery
- sphenoidal sinus
- sphenopalatine vein
- facial vein
- ophthalmic vein
- submandibular nodes
- parotid nodes
- upper deep cervical nodes
- retropharyngeal nodes
- Nerves of ordinary sensation
- Mediate sensations of pain, touch, and temperature
- Trigeminal fibers sensitive to noxious chemicals
- Derived from maxillary nerve & nasociliary branch of ophthalmic nerve
- Olfactory nerves
- from sensory cells of area olfactoria, passing into the cranial cavity via foramina of the cribriform plate
- Autonomic nerves
- Sympathetic postganglionic vasomotor fibers to the nasal blood vessels
- Postganglionic parasympathetic fibers provide secretomotor supply to nasal glands
- Rudimentary or absent at birth
- lighten the skull, and they may help filter, warm and moisten the air.
- The mucus they produce ultimately flows into the nasal cavity, and the suctioning effect created by nose blowing helps drain the sinuses.
- Lined with mucoperiosteum & filled with air
- Act as resonators to the voice
- When apertures of sinuses are blocked, or become filled with fluid, the quality of voice is markedly changed
***Clinical Importance: Cold viruses, streptococcal bacteria, and various allergens can cause rhinitis. The inflamed nasal mucosa is accompanied by excessive mucus production, nasal congestion, and postnasal drip. Nasal cavity infections often spread into the tear ducts and paranasal sinuses causing sinusitis.
- Frontal Sinus
- Contained within frontal bone posterior to superciliary arches
- Roughly triangular in shape
- Extends upward above medial end of eyebrow and backward into medial part of the roof of the orbit
- Aperture opens into middle meatus via the ethmoidal infundibulum or thro’ the frontonasal duct
- Well-developed between 7th and 8th years, reach full size after puberty
- More prominent in males
***Clinical importance: Frontal Sinusitis may erode the anterior cranial fossa that could possibly lead to brain abscess or meningitis
- Ethmoidal Sinus
- Lie between the upper part of Nasal Cavity and the orbit
- Separated by the paper-thin orbital plate of ethmoid (poor barrier to infection)
- Small thin-walled cavities, formed by frontal, maxillary, lacrimal, sphenoid & palatine bones
***Clinical importance: Ethmoidal sinusitis may erode the medial wall of the orbit which is a poor barrier to infection. This may cause orbital cellulitis that may spread to the cranial cavity.
- Sphenoidal Sinus
- Situated post to the upper part of Nasal Cavity, within the sphenoid bone
- Each sinus opens into the sphenoethmoidal recess above superior concha
- The pituitary gland lies above this sinus and can be reached by a transsphenoidal approach
- Maxillary Sinus
- Largest air sinus of the nose
- Pyramidal in shape
- Roots of 1st & 2nd premolars, 3rd molar, and root of canine project up into the sinus
***Clinical Importance: Occasionally, thin layer of compact bone enclosing roots of teeth is absent. If it is absent, a tooth extraction may result in fistula and an infected tooth may produce sinusitis
- The pharynx or mostly known as the throat connects the nasal cavity and mouth superiorly to the larynx and esophagus inferiorly. It is divided into 3 regions namely the nasopharynx, oropharynx, and laryngopharynx.
- lies posterior to the nasal cavity and extends to the soft palate.
- During swallowing, the soft palate and its pendulous uvula move superiorly, an action that closes off the nasopharynx and prevents food from entering the nasal cavity
- receives air from the nasal cavity along with packages of dust-laden mucus.
- serves only as an air passageway
- lined with pseudostratified ciliated columnar epithelium
- the cilia move the mucus down towards the inferior portion of the pharynx.
- also exchanges small amounts of air with the auditory tubes to equalize air pressure b/w pharynx and middle ear.
- has both respiratory and digestive functions, serving as a common passageway for air, food and drink.
- subjected to abrasion by food particles
- lined with nonkeratinized stratified squamous epithelium.
- has 2 pairs of tonsils, the palatine and lingual tonsils.
- opens into the esophagus posteriorly and the larynx (voice box) anteriorly.
- serves as a passageway for food and air and is lined with a stratified squamous epithelium
- has both respiratory and a digestive pathway
- lined by nonkeratinized stratified squamous epithelium.
- ‘Voice box’
- extends 5 cm (2 inches) from the level of the third to the sixth cervical vertebra
- A specialized organ that provides a protective sphincter at the inlet of air passages; provides patent airway
- Act as a switching mechanism for air and food passages into their proper channels
- Responsible for voice production- it houses the vocal cords
- Above, it opens into the laryngopharynx
- Below, is continuous with the trachea
- Except for the epiglottis, all laryngeal cartilages are hyaline cartilages.
- Thyroid cartilage
- Largest of the laryngeal cartilages
- 2 laminae of hyaline cartilage meet in the midline forming the prominent V angle of the Adam’s apple
- Laryngeal prominence is well marked in men, scarcely visible in women because male sex hormones stimulate its growth during puberty
- Cricoid cartilage
- Inferior to the thyroid cartilage
- Formed from a complete ring of hyaline cartilage
- The skeletal foundation of the larynx
- Forms a complete ring around the airway
- Articulated by synovial joints to the thyroid cartilage and the 2 arytenoids
- Arytenoid cartilage
- Pyramidal in shape
- Situated at the back of the larynx, on the lateral part of the upper border of the cricoid’s lamina
- Important because they anchor the vocal folds
- Corniculate cartilage
- 2 small nodules that articulate w/ apices of arytenoid cartilages
- Gives attachment to the aryepiglottic folds
- Cuneiform cartilage
- 2 small rod-shaped pieces of cartilages
- Serves as support for the aryepiglottic folds
- Leaf-shaped elastic cartilage, situated behind the root of the tongue
- Connected in front to the body of the hyoid bone, & by its stalk to the back of thyroid cartilage
- elastic cartilage and is almost entirely covered by a taste bud– containing mucosa
***Clinical Importance: Epiglottitis, inflammation of the epiglottis, may cause life-threatening airway obstruction especially in the pediatric population.
- Vocal folds
- Lying under the laryngeal mucosa on each side are the vocal ligament composed largely of elastic fibers, form the core of mucosal folds called the vocal folds, or true vocal cords, which appear pearly white because they lack blood vessels
- Superior to the vocal folds is a similar pair of mucosal folds called the vestibular folds, or false vocal cords. Provide no direct part in sound production but help to close the glottis during swallowing.
**Clinical importance: Laryngitis or the inflammation of the vocal folds causes swelling of the vocal cords, interfering with the vibrations. These interferences cause the hoarseness of voice. Causes would include overuse of the voice, very dry air, bacterial infections, tumors or irritating chemical agents.
- Commonly known as ‘Windpipe’
- Begins at the inferior border of the cricoid cartilage (C6) as a continuation of the larynx and
- bifurcates into the right and left main stem bronchi at the level of the sternal angle
- 10–12 cm (about 4 inches) long and 2 cm (3/4 inch) in diameter
- Flexible and mobile
- Tracheal wall has many layers: mucosa, submucosa, and adventitia—plus a layer of hyaline cartilage
- goblet cell–containing pseudostratified epithelium
- cilia continually propel debris-laden mucus toward the pharynx
- contains seromucous glands that help produce the mucus in the trachea
- supported by 16 to 20 C-shaped rings of hyaline cartilage
- The outermost layer of connective tissue
- Tracheal Cartilage
- 16-20 in number
- forms an incomplete ring surrounding anterior two-thirds of tracheal circumference
- connected by smooth muscle fibers of the trachealis and by soft connective tissue
- Carina– downward and backward projection of the last tracheal cartilage
- marking the point where the trachea branches into the two main bronchi
- The bronchi branch from the trachea into each lung and create the network of intricate passages that supply the lungs with air.
- Right principal bronchus
- Wider, shorter, & more vertical than the Left
- About 2.5 cm long
- Enters Right lung at the hilus where it divides into secondary branches
- Greater width & more vertical course explain why Foreign Body enters it more often than the Left
- Left Principal bronchus
- Narrower, & less vertical than the R
- About 5 cm long
- Each main bronchus subdivides into lobar (secondary) bronchi—three on the right and two on the left
- The lobar bronchi branch into segmental (tertiary) bronchi, which divide repeatedly into smaller bronchi
- Once the passage is smaller than 1 mm in diameter, these are called Tiniest of which are less than 0.5 mm in diameter
- thin-walled air sacs
- The respiratory bronchioles lead into winding alveolar ducts that would lead to alveoli sacs or saccules which are terminal clusters of alveoli
- Walls of alveoli involve a single layer of squamous epithelial cells, called Type I alveolar cells. It is integral to maintain the barrier function of the alveoli at the same time being involved in the gas exchange.
- Alveoli are densely covered with pulmonary capillaries to form the respiratory membrane
- 5-μm-thick blood air barrier that has blood flowing past on one side and gas on the other
- Gas exchanges occur readily by simple diffusion
- Type II alveolar cells secrete surfactant that coats alveolar surfaces
- Apex, the superior tip of the lung is situated deep to the clavicle
- Base of the lung rests on the diaphragm
- Hilum is an indentation on the mediastinal surface of the lung where systemic and pulmonary blood vessels, nerves, and bronchi enter and leave the lungs
- Left lung is smaller than the right because of the cardiac notch noted to accommodate the heart.
- Right lung
- 3 lobes: Superior lobe, Middle lobe, Inferior lobe
- Divided by the oblique and horizontal fissures
- Has grooves for various structures (e.g., Superior Vena Cava, arch of azygos vein, esophagus).
- Left lung
- 2 lobes: Superior lobe, Inferior lobe
- Divided by the oblique fissure
- Has presence of the lingula, a tongue-shaped portion of the superior lobe that is equivalent to the middle lobe of the right lung.
- Contains a cardiac notch with cardiac impression and groove for various structures (e.g., aortic arch, descending aorta, left subclavian artery).
- Pyramid shaped Bronchopulmonary segments are present in each lobe
- Right lung has 10 bronchopulmonary segments
- Left lung is variable and consists of 8 to 10 segments
- Each segment has its own artery and vein
- Important to note clinically because pulmonary disease is often confined to one or a few segments which allows diseased segment to be removed surgically without impairing blood supply and damaging the neighboring segments
Blood and Nerve Supply
- Lung is perfused in two circulations, the pulmonary and the bronchial, which differ in size, origin, and function
- Pulmonary Circulation of the Lungs
- Pulmonary arteries carry systemic venous blood to be oxygenated in the lungs
- Pulmonary veins carry freshly oxygenated blood from the lungs to the heart for systemic circulation
- Bronchial Circulation of the lungs
- Unlike pulmonary circulation, bronchial arteries carry oxygenated systemic blood to the lung tissue.
- Tiny bronchial veins drain some systemic venous blood from the lungs, but there are multiple anastomoses between the two circulations, and most venous blood returns to the heart via the pulmonary veins.
- Innervation of the lungs
- Parasympathetic and sympathetic motor fibers, and visceral sensory fibers
- Sympathetic fibers would cause bronchodilation
- Parasympathetic fibers would cause bronchoconstriction
- Phrenic nerve innervates the fibrous pericardium, the mediastinal and diaphragmatic pleurae, and the diaphragm for motor and its central tendon for sensory functions.
- Parietal pleura is a serous membrane lining that covers the thoracic wall, mediastinum, and superior face of the diaphragm
- Named based on location, for example, the Costovertebral pleura covers the internal thoracic wall and vertebral bodies; the diaphragmatic pleura covers the thoracic surface of the diaphragm, the mediastinum covers the lateral boundary of the mediastinum; the cervical pleura or cupula is a dome of pleura that projects into the neck above the neck of the first rib to cover the apex of the lung
- Visceral pleura or pulmonary pleura is a thin serous membrane to cover each external lung surface. It adheres to the surface of the lungs and dips into all of the fissures. Insensitive to pain but is sensitive to stretch and contains vasomotor fibers and sensory endings of vagal origin.
- Pleurae produce pleural fluid, which fills the pleural cavity in between that makes both lungs glide easily over the thoracic wall during breathing.
- Costodiaphragmatic recesses – pleural recesses formed by the costal and diaphragmatic reflection
- Costomediastinal recesses– costal and mediastinal pleurae meet anteriorly behind the sternum
**Clinical importance: Pneumothorax is an increase in the air within the pleural cavity, which then leads to an increase in intrapleural pressure and loss of negative pressure, which leads to lung collapse. Symptoms are chest pain and dyspnea. This can be treated by draining pleural air by needle aspiration or chest tube thoracostomy.
- Pleurae also compartmentalize the thoracic cavity into three—the central mediastinum containing the heart and the two lateral pleural compartments each containing a lung. This limits the spread of infection and prevents the lung and the heart to interfere with each other.
- Respiratory pressures are always described relative to atmospheric pressure
- At sea level, atmospheric pressure is 760 mm Hg
- atmospheric pressure = 760 mm Hg = 1 atm
- consists of inspiration and expiration
- the inspiratory muscles: diaphragm and external intercostal muscles are activated
- involves the process of contraction of the diaphragm contraction thereby increasing the vertical diameter of the thorax and the enlargement of the pleural cavities and lungs that would reduce the intrapulmonary pressure creating a negative pressure.
- Largely a passive process that depends more on lung elasticity than on muscle contraction. When the inspiratory muscles relax and resume their resting length, the rib cage descends due to gravity and the lungs recoil
- Tidal, inspiratory reserve, expiratory reserve, and residual
- Tidal Volume (TV)
- Amount of air moves into and out of the lungs with each normal quiet breathing
- 500 mL
- Inspiratory reserve volume (IRV)
- amount of air that can be inspired forcibly beyond the tidal volume
- 2100 to 3200 mL
- Expiratory reserve volume (ERV)
- amount of air that can be expelled from the lungs after a normal tidal volume expiration
- 1000 to 1200 ml
- Residual volume (RV)
- helps to keep the alveoli open and prevent lung collapse even after the most strenuous expiration
- 1200 mL
- Inspiratory, functional residual, vital, and total lung capacities
- consist of two or more lung volumes
- Inspiratory capacity (IC)
- total amount of air that can be inspired after a normal tidal volume expiration
- it is the sum of TV and IRV
- Functional residual capacity (FRC)
- amount of air remaining in the lungs after a normal tidal volume expiration
- it is the combined RV and ERV
- Vital capacity (VC)
- the total amount of exchangeable air
- It is the sum of TV, IRV, and ERV
- Total lung capacity (TLC
- is the sum of ALL lung volumes
- Anatomical dead space
- volume of these conducting zone conduits, does not contribute to gas exchange in the alveoli
- 150 mL
- Alveolar dead space
- some alveoli cease to act in gas exchange in cases of alveolar collapse or mucosal obstruction
- Total dead space
- Sum of non-useful volumes
Pulmonary Function Tests
Spirometry is most useful for evaluating losses in respiratory function
- Cannot diagnose specific diseases but can distinguish between restrictive and obstructive diseases
|Obstructive diseases||Restrictive diseases|
Increased airway resistance in cases such as Chronic Bronchitis
|Limited lung expansion
Reduced Total lung capacity caused by diseases such as Tuberculosis or exposure to environmental agents
|Increased TLC, FRC, and RV||Decreased VC, TLC, FRC, and RV|
Forced vital capacity (FVC)
- amount of gas expelled when a subject takes a deep breath and then forcefully exhales maximally and as rapidly as possible
- Healthy lungs can expel 80% of the FVC within 1 second
Forced expiratory volume (FEV)
- amount of air expelled during specific time intervals of the FVC test
- Oxygen is poorly soluble in water
- only 1.5% of the oxygen transported is carried in the dissolved form
- 5% of the oxygen is carried from lungs to tissues in combination with hemoglobin
- Hemoglobin has four polypeptide chains. Each chain is bound to an iron-containing heme group
- Each hemoglobin can combine with four oxygen molecules
- Hemoglobin-oxygen combination is called oxyhemoglobin
- Inadequate oxygen delivery to body tissues
- Anemic Hypoxia- poor oxygen delivery caused by few or abnormal red blood cells
- Ischemic Hypoxia- blocked blood circulation
- Histotoxic Hypoxia- body cells unable to use oxygen caused by metabolic poisons such as cyanide
- Hypoxemic Hypoxia- reduced arterial partial pressure of oxygen
- Carbon monoxide poisoning- hemoglobin’s affinity for carbon monoxide is more than 200 times greater than its affinity for oxygen
**Hyperbaric therapy or 100% oxygen are given until the carbon monoxide has been cleared from the body
Our respiration is primarily maintained and controlled by a network of automatic neurons located bilaterally from the medulla oblongata and pons of the brainstem. They generate respiratory rhythm and would receive regulatory inputs. It is divided into three major groups of neurons. We’ll discuss them here.
- Medullary Respiratory Centers
- (1) Ventral Respiratory Group (VRG)
- Groups of neurons that fire during inspiration and expiration
- During inspiration, neurons fire impulses along phrenic and intercostal nerves to excite the inspiratory muscles, the diaphragm and external intercostal musclesà thorax expands and air rushes into the lungs
- During expiration, expiratory neurons fire, the inspiratory muscles relax and the lungs recoil
- This cycle repeats and produces 12-16 breaths per minute respiratory rate which is normal
- (2) Dorsal Respiratory Group (DRG)
- Inspiratory center with same tasks performed by the Ventral Respiratory Group
- Integrates input from chemoreceptors and peripheral stretch
- (3) Pontine Respiratory Centers
- Influences medullary neurons
- ‘pneumotaxic center’ located dorsally in the superior portion of the pons
- Lesions in this area can cause ‘apneustic breathing’ with very prolonged inspiration
- Transmit impulses to the VRG of the medulla that modifies the breathing rhythms during sleep, exercise, and vocalization.
- (1) Ventral Respiratory Group (VRG)
In conclusion, our respiratory system is very vital in our survival and body homeostasis. Other body systems will not work if our respiratory system is failing. From a simple muscle activity to the complex neuronal activity, we would need our respiratory system to provide oxygen and to dispose carbon dioxide. The processes are intricate and complicated but amazing how each part functions systematically and synchronously to keep us functional.
- Oxygen Supply and CO2 disposal are the major functions of the respiratory system
- There are 4 processes of respiration namely, Pulmonary ventilation, Respiratory gas exchange, and External and Internal respiration.
- The respiratory system is divided into anatomical or functional. Anatomically, it is divided into the upper and lower respiratory tract. Functional division is separated into conducting and respiratory zone.
- The nose is the only part of the respiratory system that is visible externally. It is divided into the External nose and the Internal nasal cavity.
- Paranasal sinuses lighten the skull, and they may help filter, warm and moisten the air. There are four sinuses namely the frontal sinus, maxillary, ethmoid, and sphenoid sinuses.
- The Pharynx has three divided regions namely the nasopharynx, oropharynx, and laryngopharynx.
- The larynx is known as the voice box. It divides the respiratory system anatomically. There are many cartilages in the larynx namely thyroid, cricoid, arytenoid, corniculate, cuneiform, and epiglottis.
- Trachea or windpipe has 16 to 20 cartilages. The carina, the last cartilage, is the point of bifurcation.
- Bronchi, the right and the left, are distinct. The right is wider, shorter, and more vertical than the left making it more prone for a foreign body to lodge.
- Alveoli has two alveolar cells. Type I is for the wall of the alveoli that involves gas exchange while type II secretes surfactant.
- The right lung has three lobes and two fissures while the left lung has two lobes and is divided by one fissure.
- The lung has two pleurae, the parietal and the visceral. The parietal covers the thoracic wall while the visceral covers each lung.
- Oxygen is attached to hemoglobin to be carried from the lungs to the tissues.
- Respiratory centers are found in the medulla oblongata and the pons. The Dorsal respiratory group mainly causes inspiration. The Ventral group causes expiration. The Pneumotaxic center controls the rate and depth of breathing.
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