anatomy and physiology of Respiratory System

8/3/2019 anatomy and physiology of Respiratory System

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T H E R E S P I R AT O R Y S Y S T E M A N D H O M EO S T A S I SThe respiratory system contributes to homeostasis by providing for the exchange of gasesoxygen andcarbon dioxidebetween the atmospheric air, blood, and tissue cells. It also helps adjust the pH of bodyfluids. Your bodys cells continually use oxygen (O2) for the metabolic reactions that release energy fromnutrient molecules and produce ATP. At the same time, these reactions release carbon dioxide (CO2).Because an excessive amount of CO2 produces acidity that can be toxic to cells, excess CO2 must beeliminated quickly and efficiently. The cardiovascular and respiratory systems cooperate to supply O2and eliminate CO2. The respiratory system provides for gas exchange intake of O2 and elimination ofCO2 and the cardiovascular system transports blood containing the gases between the lungs and bodycells. Failure of either system disrupts homeostasis by causing rapid death of cells from oxygenstarvation and buildup of waste products. In addition to functioning in gas exchange, the respiratorysystem also participates in regulating blood pH, contains receptors for the sense of smell, filters inspiredair, produces sounds, and rids the body of some water and heat in exhaled air. Like the digestive andurinary systems that will be covered in subsequent chapters, in the respiratory system there is anextensive area of contact between the external environment and capillary blood vessels. This area ofcontact allows the body to constantly renew and replenish the internal fluid environment that surroundsand nourishes every body cell.

The respiratory system consists of the nose, pharynx (throat), larynx (voice box), trachea(windpipe), bronchi, and lungs. Its parts can be classified according to either structure or function.

Structurally, the respiratory system consists of two parts:(1) The upper respiratory system includes the nose, pharynx, and associated structures.(2) The lowerrespiratory system includes the larynx, trachea, bronchi, and lungs.

Functionally, the respiratory system also consists of two parts:(1) The conducting zone consists of a series of interconnecting cavities and tubes both outside andwithin the lungs. These include the nose, pharynx, larynx, trachea, bronchi, bronchioles, and terminalbronchioles; their function is to filter, warm, and moisten air and conduct it into the lungs.(2) The respiratory zone consists of tissues within the lungs where gas exchangeoccurs . Theseinclude the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli; they are the main sites ofgas exchange between air and blood.


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PharynxThe pharynx (FAIR-inks), or throat, is a funnel-shaped tube about 13 cm (5 in.) long that starts at

the internal nares and extends to the level of the cricoid cartilage, the most inferior cartilage of the larynx(voice box). The pharynx lies just posterior to the nasal and oral cavities, superior to the larynx, and justanterior to the cervical vertebrae. Its wall is composed of skeletal muscles and is lined with a mucousmembrane. Contraction of the skeletal muscles assists in deglutition (swallowing). The pharynx functionsas a passageway for air and food, provides a resonating chamber for speech sounds, and houses thetonsils, which participate in immunological reactions against foreign invaders.

The pharynx can be divided into three anatomical regions: (1) Nasopharynx, (2)Oropharynx, and

(3) laryngopharynx. The muscles of the entire pharynx are arranged in two layers, an outer circular layerand an inner longitudinal layer. The superior portion of the pharynx, called the nasopharynx, liesposterior to the nasal cavity and extends to the soft palate. The soft palate, which forms the posteriorportion of the roof of the mouth, is an arch-shaped muscular partition between the nasopharynx andoropharynx that is lined by mucous membrane. There are five openings in its wall: two internal nares, twoopenings that lead into the auditory (pharyngotympanic) tubes (commonly known as the eustachiantubes), and the opening into the oropharynx. The posterior wall also contains the pharyngeal tonsil(adenoid). Through the internal nares, the nasopharynx receives air from the nasal cavity along withpackages of dustladenmucus. The nasopharynx is lined with pseudostratified ciliated columnarepithelium, and the cilia move the mucus down toward the most inferior part of the pharynx. Thenasopharynx also exchanges small amounts of air with the auditory tubes to equalize air pressurebetween the pharynx and the middle ear. The intermediate portion of the pharynx, the oropharynx, lies

posterior to the oral cavity and extends from the soft palate inferiorly to the level of the hyoid bone. It hasonly one opening into it, the fauces (FAW-sez _ throat), the opening from the mouth. This portion of thepharynx has both respiratory and digestive functions, serving as a common passageway for air, food,and drink. Because the oropharynx is subject to abrasion by food particles, it is lined with nonkeratinizedstratified squamous epithelium. Two pairs of tonsils, the palatine and lingual tonsils, are found in theoropharynx.The inferior portion of the pharynx, the laryngopharynx (larin_-go-FAIR-inks), orhypopharynx, beginsat the level of the hyoid bone. At its inferior end it opens into the esophagus (food tube) posteriorly andthe larynx (voice box) anteriorly. Like the oropharynx, the laryngopharynx is both a respiratory and adigestive pathway and is lined by nonkeratinized stratified squamous epithelium.


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LarynxThe larynx (LAIR-inks), or voice box, is a short passageway that connects the laryngopharynx

with the trachea. It lies in the midline of the neck anterior to the esophagus and the fourth through sixthcervical vertebrae (C4C6). The wall of the larynx is composed of nine pieces of cartilage. Three occursingly (thyroid cartilage, epiglottis, and cricoid cartilage), and three occur in pairs (arytenoid, cuneiform,and corniculate cartilages). Of the paired cartilages, the arytenoids cartilages are the most importantbecause they influence changes in position and tension of the vocal folds (true vocal cords for speech).The extrinsic muscles of the larynx connect the cartilages to other structures in the throat; the intrinsicmuscles connect the cartilages to one another.

The thyroid cartilage (Adams apple) consists of two fused plates of hyaline cartilage that form

the anterior wall of the larynx and give it a triangular shape. It is present in both males and females but isusually larger in males due to the influence of male sex hormones on its growth during puberty. Theligament that connects the thyroid cartilage to the hyoid bone is called the thyrohyoid membrane. Theepiglottis (epi- _ over; glottis _ tongue) is a large, leafshaped piece of elastic cartilage that is coveredwith epithelium. The stem of the epiglottis is the tapered inferior portion that is attached to the anteriorrim of the thyroid cartilage and hyoid bone. The broad superior leaf portion of the epiglottis isunattached and is free to move up and down like a trap door. During swallowing, the pharynx and larynxrise. Elevation of the pharynx widens it to receive food or drink; elevation of the larynx causes theepiglottis to move down and form a lid over the glottis, closing it off. The glottis consists of a pair of foldsof mucous membrane, the vocal folds (true vocal cords) in the larynx, and the space between them


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called the rima glottidis (RI-ma GLOT-ti-dis). The closing of the larynx in this way during swallowingroutes liquids and foods into the esophagus and keeps them out of the larynx and airways. When smallparticles of dust, smoke, food, or liquids pass into the larynx, a cough reflex occurs, usually expelling thematerial. The cricoid cartilage (KRI -koyd _ ringlike) is a ring of hyaline cartilage that forms the inferiorwall of the larynx. It is attached to the first ring of cartilage of the trachea by the cricotracheal ligament.The thyroid cartilage is connected to the cricoid cartilage by the cricothyroid ligament. The cricoidcartilage is the landmark for making an emergency airway called a tracheotomy.The paired arytenoid cartilages (ar_-i-TE -noyd _ ladlelike) are triangular pieces of mostly hyalinecartilage located at the posterior, superior border of the cricoid cartilage. They form synovial joints withthe cricoid cartilage and have a wide range of mobility. The paired corniculate cartilages (kor-NIK-u--la-t _ shaped like a small horn), horn-shaped pieces of elastic cartilage, are located at the apex of eacharytenoid cartilage. The paired cuneiform cartilages (KU--ne--i-form _ wedge-shaped), clubshapedelastic cartilages anterior to the corniculate cartilages, support the vocal folds and lateral aspects of theepiglottis. The lining of the larynx superior to the vocal folds is nonkeratinized stratified squamousepithelium. The lining of the larynx inferior to the vocal folds is pseudostratified ciliated columnarepithelium consisting of ciliated columnar cells, goblet cells, and basal cells. The mucus produced by thegoblet cells helps trap dust not removed in the upper passages. The cilia in the upper respiratory tractmove mucus and trapped particles down toward the pharynx; the cilia in the lower respiratory tract movethem up toward the pharynx.

TRACHEALying more or less in midline, in the lower part of neck and in the superior mediastinium, upper and iscontinous with larynx and lower end divides into right and left principle bronchi.Trachea is 10-15 cm in length and external diameter is 2 cm in male and 1.5 cm in female.Trachea has a fibroelastic wall supported by a cartilaginous skeleton formed by C shaped rings.is a cartilaginous and membranous tube, extending from the lower part of the larynx, on a level with thesixth cervical vertebra, to the upper border of the fifth thoracic vertebra, where it divides into the twobronchi, one for each lung.Upper end of trachea lies at lower border of cricoids cartilage opposite of 6 th cervical vertebrae, andlower end at thoracic 4th vertebrae.Relations

1. Anteriorlya. Manibarium sterniib. Sternothyroid nusclec. Thymusd. Left brachiocephalic and left common carotid artery.e. Some lymph nodes

2. Posteriorlya. Esophagusb. Vertebral column


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3. Right sidea. Right vagus and azygous vein

4. Left side

a. Left common carotid artery and left subclavian artery.

Blood supply1. Artery : inferior thyroid artery2. Vein : brachiocephalic vein3. Lymphatic drainage : pretracheal and paratracheal lymphnodes.

LUNGS


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y Lungs are the pair of respiratory organ situated in thoracic cavity. Right and left lung is separatedby mediastinium.

y Texture spongyy Colour in young brown, adults molted black due to deposition of carbon particles.y Weight right lung 600gm, left lung 550gms.y Shape Conical.

ApexBlunt and lies above level of anterior rib.Reaches 1-2 cm above medial 1/3rd of clavicle.Covered by cervical pleura and suprapleural membrane.Grooved by subclavian artery and vein.

BaseSemilunar and concaveRests on dome of diaphragm.Right sided dome is higher than left.

BORDERSy Anterior border

Thin, shorter than posterior border, shows a wide cardiac notch below the 4 th costal cartilage.y Posterior border

Thick and ill defined, it extends from C7-T10.y Inferior border

Seperates the base from costal and medial surface.y Costal surface

Large and convexy Medial surface

Divided in Posterior medial surface or mediastinial surface.


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DIFFERENCE BETWEEN RIGHT AND LEFT LUNG

RIGHT LUNG LEFT LUNG

2 fissure and 3 lobes 1 fissure and 2 lobes Anterior border is straight Ant. Border is interrupted by cardiac notchLarger and heavier in weight Smaller and lighter in weight

Shorter and broader Longer and narrower1 bronchial artery arises from 3rd posteriorintercostal artery of from left bronchial artery.

2 bronchial artery arises from descendingthoracic aorta

Veinous drainageThere are 2 bronchial veins on each side. The right bronchial vein drain into azygous vein. The leftbronchial vein drain either into left superior intercostal vein or into hemiazygous vein.

Lymphatic drainage1. Superficial vessels drain into peripheral lung tissue lying beneath pulmonary pleura these vessels

reach the hilum.2. Deep lymphatics drain the bronchial tree, pulmonary vessels and connective tissue septa. They

drain into bronchopulmonary nodes.

Nerve supply1. Parasympathetic nerve fibers are derived from vagus nerve

a. Motor nerve to bronchial muscles and on stimulation causes bronchospasm.b. Sensory nerves are responsible for stretch reflex of lungs and cough reflex.

2. Sympathetic nerve fibers are derived from 2nd 5th spinal segments.

Bronchial treey Trachea divides at the level of lower border of 4th thoracic vertebrae into primary principal bronchi

one for each lung.

y The right principal bronchus is 2.5 cm long, short, wide and more in line with trachea.y The let principal bronchus is 5 cm long, longer, narrower, more oblique.

o Each principal bronchus enters the lung through hilum and divides into secondary lobarbronchi, one for each lobe of lung

y 3 lobar bronchii on left side and 2 lobar bronchii on right side.o Each lobar bronchii divides into tertiary or segmental bronchi, 1 for each

bronchopulmonary segment.y There are 10 bronchopulmonary segment on each side of lung.

o The segmental bronchii divides repeatedly in very small branches called terminalbronchioles.

o Terminal bronchioles divides into respiratory bronchioles.o Each terminal bronchiole aerates a smallpar of lung called as pulmonary unit. It consists

of alveolar duct, atria, air saccules, pulmonary alveoli.


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Bronchopulmonary segmenty Pyramidal in shape with its apex directed towards the root of lung.y Each segment is surrounded by connective tissue which is continous on the surface with

pulmonary pleura. Thus bronchopulmonary segment are independent respiratory unit.

Relation to pulmonary arteryThe branches of pulmonary artery accompany the bronchii. The artery lies dorsolateral to the bronchus.Thus each segment has its own separate artery.

Relation to pulmonary veinThe pulmonary vein do not accompany the bronchii or pulmonary artery. They run in the intersegmentalplanes. Thus each segment has more than one vein and each vein drain more than one segment.


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FUNCTIONAL ANATOMY OF RESPIRATORY TRACT

Respiratory tract is the anatomical structure through which air moves in and out. It consists otnose, pharynx, larynx, trachea, bronchi and lungs.

Pleura

Each lung is enclosed by a bilayered serous membrane called pleura or pleural sac. The twolayers of pleura are the visceral and parietal layers. Visceral (inner) layer is attached firmly to the surtaceof the lungs. At hilum, it is continuous with paritetal (outer) layer, which is attached to the wall of the

thoracic cavity. The narrow space in between the two layers of pleura is called intrapleural space orpleural cavity. This space contains a thin film of serous fluid called pleural fluid. It is secreted by thevisceral layer of the pleura. It functions as the lubricant to prevent friction between two layers. It isinvolved in the creating negative pressure called intrapleural pressure within intrapleural space. In somepathological conditions, the pleural cavity expands with accumulation of air (pneumothorax), wa(hydrothorax), blood (hemothorax) or pus (Pyothorax)

TracheobronchialTree


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The trachea and bronchi are together called traches. bronchiat tree. It forms a part of air passage.The trachea biturcates into two main or primaj bronchi called right and left bronchi. Each primsi bronchusenters the lungs and divides into lobar secondary bronchi. The secondary bronchi divide is segmental ortertiary bronchi. In right lung, there are ts tertiary bronchi and, in left lung, there are eight tertiary bronchi.The tertiary bronchi divide several times w reduction in length and diameter into many generatior ofbronchiotes. When the diameter of bronchiols becomes 1 mm or less, it is called terminal bronchiTerminal bronchiole continues or divides into respirate bronchiole, which has a diameter of 0.5 mm.Generally, the respiratory tract is divided into two par upper respiratory tract and lower respiratory tract.

Upper Respiratory Tract

Upper respiratory tract includes all the structures frei nose up to vocal cords. Vocal cords are thefolds mucous membrane within larynx that vibrates to produi the voice.

Lower Respiratory Tract Lower respiratory tract includes trachea, bronchi and lungs.

1. DEFENSE MECHANISM

Lungs play important role in the immunological defense system of the body. The defense functions of thelungs are performed by their own defenses and by the presence of various types of cells in the mucousmembrane lining the alveoli of lungs. These cells are leukocytes, macrophages, mast cells, natural killercells and dendritic cells.

y Lungs Own DefensesThe epithelial cells lining the air passage secrete some innate immune factors called

defensins and cathelicidins. These substances are the antimicrobial peptides which play animportant role in lungs natural defenses.

y Defense throughLeukocytesThe leukocytes, particularly the neutrophils and lymphocytes present in the alveoli of

lungs provide defense mechanism against bacteria and virus. The neutrophils kill the bacteria byphagocytosis. Lymphocytes develop immunity against bacteria.

y Defense throughMacrophages

Macrophages engulf the dust particles and the pathogens, which enter the alveoli and thereby actas scavengers in lungs. Macrophages are also involved in the development of immunity byfunctioning as antigen presenting cells. When foreign organisms invade the body, themacrophages and other antigen presenting cells kill them. Later, the antigen from the organismsis digested into polypeptides. The polypeptide products are presented to T lymphocytes and Blymphocytes by the macrophages.Macrophages secrete interleukins, tumor necrosis factors (TNF) and chemokines . Interleukinsand TNF activate the general immune system of the body. Chemokines attract the white bloodcells towards the site of any inflammation.

y Defense throughMast CellMast cell is a large tissue cell resembling the basophil. The mast cell produces thehypersensitivity reactions like allergy and anaphylaxis (Chapter 17). It secretes heparin,

histamine, serotonin and hydrolytic enzymes.y Defense throughNatural Killer Cell

Natural killer (NK) cell is a large granular cell, considered as the third type of lymphocyte. UsuallyNK cell is present in lungs and other lymphoid organs. Its granules contain hydrolytic enzymes,which destroy the micro organisms. NK cell is said to be the first line of defense in specificimmunity particularly against viruses. It destroys the viruses and the viral infected or damagedcells, which may form the tumors. It also- destroys the malignant cells and prevents developmentof cancerous tumors. The NK cells secrete interferons and the tumor necrosis factors.


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y Defense through Dendritic CellsDendritic cells in the lungs play important role in immunity. Along with macrophages, these cellsfunction as antigen presenting cells.

2. MAINTENANCE OF WATER BALANCERespiratory tract plays a role in water loss mechanism. During expiration, water evaporatesthrough the expired air and some amount of body water is lost by this process.

3. REGULATION OF BODY TEMPERATUREDuring expiration, along with water, heat is also lost fr the body. Thus, respiratory tract plays arole in heat loss mechanism.

4. REGULATION OF ACIDBASE BALANCE

Lungs play a role in maintenance of acidbase balance of the body by regulating thecarbon dioxide content in blood. Carbon dioxide is produced during various metabolic reactionsin the tissues of the body. When it enters the blood, carbon dioxide combines with water formcarbonic acid. Since carbonic acid is unstable and splits into hydrogen and bicarbonate ions.

CO2+H2O H2CO3H*+HCOThe entire reaction is reversed in lungs when carbon dioxide is removed from blood intothe alveoli of lung.

H* + HCO H2C03 CO2 + H20As carbon dioxide is a volatile gas, it is practically blown out by ventilation.When metabolic activities are accelerated, m amount of carbon dioxide is produced in thetissues and the concentration of hydrogen ion is also increased leading to reduction in pH.The increased hydrogen concentration causes increased pulmonary ventilation i.e.hyperventilation by acting through various mechanisms like chemoreceptors in aorticand ca bodies and in medulla of the brain to hyperventilation, the excess of carbon dioxideremoved from the body fluids and the pH is brought bi to normal.

5. ANTICOAGULANT FUNCTIONMast cells in lungs secrete heparin, Heparin is anticoagulant and it prevents the intravascularclotting.

6. SECRETION OF ANGIOTENSIN CONVERTING ENZYME

Endothelial cells of the pulmonary capillaries secrete the angiotensin converting enzyme(ACE). It converts the angiotensin I into active angiotensin II which pIays important role inthe regulation of ECF volume and bIood pressure

7. SYNTHESIS OF HORMONALSUBSTANCESLung tissues are also known to synthesieze the hormonal substances, prostaglandins,acetylcholine and serotonin which have many physiological actions in body including regulation ofblood pressure.


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RESPIRATORYPROTECTIVE REFLEXESRespiratory protective reflexes are the reflexes that protect the lungs and air passage from foreignparticles. The respiratory process is modified by these reflexes order to eliminate the foreign particles orto prevent entry of these particles into the respiratory tract. The respiratory protective reflexes are:

1. COUGH REFLEX

Cough is a modified respiratory process characterized by forced expiration. It is the protectivereflex that occurs because of irritation of respiratory tract and some other areas such as externalauditory canal.

Causes

Cough is produced mainly by irritant agents. It is also produced by several disorders suchas cardiac disorders (congestive heart failure), pulmonary disorders chronic obstructivepulmonary disease COPD), and tumor in thorax which may exert pressure on larynx,trachea, bronchi, or lungs.

Mechanism

Cough begins with deep inspiration followed by forced expiration with closed glottis. Thisincreases the intrapleural pressure above 100 mm Hg. Then, glottis opens suddenly withexplosive outflow of air at a high velocity. The velocity of the airflow may reach 960km/hr. It causes expulsion of irritants out of the respiratory act.

Reflex Pathway

The receptors that initiate the cough are situated in several locations such as nose,

paranasal sinuses, larynx, pharynx, trachea, bronchi, pleura, diaphragm, pericardium,stomach, external auditory canal and tympanic membrane. Afferent nerve fibers pass viavagus, trigeminal, glossopharyngeal, and phrenic nerves. The center for cough reflex is inthe medulla oblongata. The efferent nerve fibers arising from the meduIlary center passthrough the vagus, phrenic, and spinal motor nerves. These nerve fibers activate theprimary and accessory respiratory muscles.

2. SNEEZING REFLEXSneezing is also a modified respiratory process characterized by forced expiration. It is aprotective reflex caused by irritation of nasal mucous membrane.

Causes

Irritation of the nasal mucous membrane occurs because of dust particles, debris, mechanicalobstruction of the airway, and excess fluid accumulation in the nasal passages.

MechanismSneezing starts with deep inspiration, followed by forceful expiratory effort with opened glottisresulting in expulsion of irritant agents out of respiratory tract.

Reflex PathwaySneezing is initiated by the irritation of nasal mucous membrane, the olfactory receptorsand trigeminal nerve endings present in the nasal mucosa. Afferent nerve fibers pass


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through the trigeminal and olfactory nerves. The sneezing center is in medulla oblongata.It is located diffusely in spinal nucleus of trigeminal nerve, nucleus sotitarius and thereticular formation of medulla. The efferent nerve fibers from the medullary center passvia tngeminal, facial, glossopharyngeal, vagus and intercostal nerves. These nerve fibersactivate the pharyngeal, tracheal and respiratory muscles.

MECHANISM OF RESPIRATION

Accessory lnspiratory MusclesSternocleidomastoid, scaleni, anterior serrati, elevators of scapulae and pectorals are the accessoryinspiratory muscles.

Inspiratory Muscles

Primary expiratory muscles are internal intercostal muscles, which are innervated by intercostalnerves.

Accessory expiratory muscles are the abdominal muscles.

MOVEMENTS OF THORACIC CAGE

Inspiration causes enlargement of thoracic cage. Thoracic cage enlarges because of increase inall diameters, viz. anteroposterior, transverse and vertical diameters, Increase in anteroposterior andtransverse diameters occurs due to the elevation of ribs. The vertical diameter of thoracic cage isincreased by the descent of diaphragm. In general, the change in the size of thoracic cavity occursbecause of the movements of four units of structures.

y Thoracic lid

y Upper costal series

y Lower costal series

y Diaphragm.


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1. Thoracic LidMovement of thoracic lid increases the anteroposterior diameter of thoracic cage. The thoracic lidis formed by manubrium sterni and the 1st pair of ribs. It is also called thoracic operculum. Due tothe contraction of scaleni muscles, the first ribs move upwards to a more horizontal position. Thisincreases the anteroposterior diameter of the thoracic cage.

2. Upper Costal SeriesMovement of upper costal series increases the anteroposterior and transverse diameter of thethoracic cage.

Pump handle movementThe upper costal series is constituted by 2nd to 6th pair of ribs. The contraction of externalintercostal muscles causes elevation of these ribs and upward and forward movement ofsternum. This movement is called pump handle movement, It increases anteroposterior diameterof the thoracic cage.

Bucket handle movementSimultaneously, the central portions of these ribs (arches of ribs) move upwards and outwards toa more horizontal position. This movement is called bucket handle movement and it increases thetransverse diameter of thoracic cage.

3. Lower Costal Series

Movement of lower costal series increases the transverse diameter of the thoracic cage.

Bucket handle movement


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It is formed by the 7 th to 10th pair of ribs. These ribs also show bucket handle movement byswinging outward and upward. This movement increase transverse diameter of the thoracic cage.The 11th and 12th pairs of ribs are the floating ribs, which are not involved in changing the size ofthoracic cage.

4. Diaphragm

Movement of diaphragm increases the vertical diameter of thoracic cage. Normally, beforeinspiration diaphragm is dome-shaped with convexity facing upwards. During inspiration, due tothe contraction, the muscle fibers are shortened. But, the central tendinous portion is drawndownwards so the diaphragm is flattened. Flattening of diaphragm increases the vertical diameterof the thoracic cage.

MOVEMENTS OFLUNGS

During inspiration, due to the enlargement of thoracic cage, the negative pressure is increased inthe thoracic cavity. It causes expansion of the lungs. During expiration the thoracic cavity decreases in

size to the preinspiral position. The pressure in the thoracic cage also comes back to the preinspiratorylevel. It compresses the Lung tissues so that, the air is expelled out of lungs.

Collapsing Tendency ofLungs

During expiration when air is expelled out, the lungs are expected to collapse. But it does nothappen. The lungs are under constant threat to collapse even under resting conditions because ofcertain factors.

Factors causing Collapsing Tendency ofLungs

Two factors are responsible for the collapsing tendency of lungs

1. Elastic property of lung tissues2. Surface tension

Elastic property of lung tissues: The elastic tissues Iungs show constant recoiling tendency and try tocollapse the lungs.

Surface tension: It is the tension exerted on the surface of the alveolar membrane by the fluid secretedfrom alveolar epithelium. fortunately, there are some factors which save the lungs from collapsing.

Factors preventing Collapsing Tendency ofLungs

In spite of the elastic property of the lungs and the surface tension in the alveoli of lungs, the collapsingtendency of lungs is prevented by two factors:

y The intrapleural pressure: It is the pressure in the pleural cavity which is always negative.Because of negativity, it keeps the lungs expanded and prevents the collapsing tendency oflungs produced by the elastic tissues.

y The surfactant. It is a substance secreted in alveolar epithelium. It reduces surface tensionprevents the collapsing tendency produced by surface tension.


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SURFACTANT

It is a surface acting material or agent that is responsible for lowering the surface tension of afluid. The surfactant that lines the epithelium of the alveoli in lungs is known as pulmonary surfactant andit decreases the surface tension on the alveolar membrane.

Source of secretion of pulmonary surfactant1. Type II alveolar epithelial cells in the lungs, which are called surfactant secreting alveolar cells orpneumocytes. The characteristic feature of these cells is the presence of microvilli on their alveolarsurface.

2. Clara cells, which are situated in the bronchioles. These cells are also called bronchiolar exocrinecells.

FUNCTIONS OFSURFACTANT

1. The surfactant reduces the surface tension in the alveoli of lungs and prevents thecollapsing tendency of lungs. Surfactant acts by the following mechanism:The phospholipid molecule in the surfactant has two portions. One portion of themolecules is hydrophilic. This portion dissolves in water and lines the alveoli. Theother portion is the hydrophobic portion which is directed towards the alveolar air.This surface of the phospholipid along with other portion spreads over the alveoliand reduces the surface tension. SPB and SP-C play active role in this process.

2. The surfactant is responsible for stabilization of the alveoli, which is necessary towithstand the collapsing tendency.

3. It plays an important role in the inflation of lungs after birth. In fetus, the secretionof surfactant begins after the third month. Until birth, the lungs are solid and notexpanded. Soon after birth, the first breath starts because of the stimulation ofrespiratory centers by hypoxia and hypercapnea. Although the respiratorymovements are attempted by the infant, the lungs tend to collapse repeatedly.

And, the presence of surfactant in the alveoli prevents the lungs from collapsing.

4. Another important function of surfactant is its role in defense within the lungsagainst infection and inflammation. The hydrophilic proteins SP-A and SP-Ddestroy the bacteria and viruses by means of opsonization. These two proteinsalso control the formation of inflammatory mediators.

RESPIRATORYPRESSURES

Two types of pressures are exerted in the thoracic cavity and the lungs during theprocess of respiration.

1. Intrapleural pressure or intrathoracic pressure

2. lntra-alveolar pressure or intrapulmonary pressure


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1. INTRAPLEURALPRESSURE / INTRATHORACIC PRESSURE

The intrapleural pressure is the pressure existing in pleural cavity, that is, in between thevisceral and parietal layers of pleura. It is exerted by the suction of fluid that lines thepleural cavity. It is also called intrathoracic pressure since it is exerted in the whole of

thoracic cavity.

NORMAL VALUES

Respiratory pressures are always expressed in relation to atmospheric pressure which is760 mm Hg. Under physiological conditions, the intrapleural pressure is always negative.

The normal values are:

y At the end of normal inspiration: -6 mm Hg (760 6 = 754 mm Hg)y At the end of normal expiration: -2 mm Hg (760 2 = 758 mm Hg)y At the end of forced inspiration: -30 mmHgy At the end of forced inspiration with closed glottis Mullers maneuver : -70 mm Hgy At the end of forced expiration with closed glottis Valsalva maneuver : + 50 mm Hg

2. INTRA-ALVEOLAR PRESSURE

Intra-alveolar pressure is the pressure existing in the alveoli of the lungs. It is also knownas intrapulmonary pressure.

Normal Values

Normally, intra-alveolar pressure is equal to the atmospheric pressure, which is 760mmHg. It becomes negative during inspiration and positive during expiration.

The normal values are:

y During normal inspiration: - 1 mm Hg (760 - 1 = 759 mm Hg)y During normal expiration: + 1 mm Hg (760 + 1 = 761 mm Hg)y At the end of inspiration and expiration: Equal to atmospheric pressure (760 mm Hg)y During forced inspiration with closed glottis (Mullees maneuver): 80 mm Hgy During forced expiration with closed glottis (Valsalva maneuver): + 100mm Hg

LUNG VOLUMES

1. TIDAL VOLUME (TV)


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Tidal volume is the volume of air breathed in and out of lungs in a single normalquiet respiration. Tidal volume signifies the normal depth of breathing.

Normal Value 500 mL (0.5 liter).

2. INSPIRATORY RESERVE VOLUME (IRV)

Inspiratory reserve volume is an additional volume of air that can be inspiredforcefully after the end of normal inspiration.

Normal Value 3300 mL (3.3 liters).

3. EXPIRATORY RESERVE VOLUME (ERV)

Expiratory reserve volume is the additional volume of air that can be expired outforcefully, after normal expiration.

Normal Value 1000 mL (1 liter).

4. RESIDUAL VOLUME (RV)

Residual volume is the volume of air remaining in the lungs even after forced expiration.Normally, lungs cannot be emptied completely even by forceful expiration. Some quantityof air always remains in u lungs even after the forced expiration. Residual volume issignificant because of two reasons:

y It helps to aerate the blood in between breathin and during expirationy It maintains the contour of the lungs.

Normal Value 1200 mL (1.2 liter)

LUNG CAPACITIES

Lung capacities are the combination of two or more lung volumes.

Lung capacities are of four types:

1. Inspiratory capacity

2. Vital capacity

3. Functional residual capacity

4. Total lung capacity.

A. INSPIRATORY CAPACITY (IC)


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Inspiratory capacity is the maximum volume of air tt. is inspired after normalexpiration (end expirator position). It includes tidal volume and inspiratory reservevolume.

IC = TV + IRV

= 500 + 3300 = 3800 mL

B. VITAL CAPACITY (VC)

It is the maximum volume of air that can be expelled, out forcefully after a deep(maximal) inspiration. capacity includes inspiratory reserve volume, tii volume andexpiratory reserve volume.

VC = IRV + TV + ERV

= 3300 + 500 + 1000 = 4800 ml.

Vital capacity is significant physiologically and determination is useful in clinicaldiagnosis.

C. FUNCTIONAL RESIDUAL CAPACITY (FRC)

It is the volume of air remaining in the lungs after normal expiration (after normal tidalexpiration). Functional dual capacity includes expiratory reserve volume and residualvolume.

FRC = ERV + RV

=1000+1200=2200mL

D. TOTALLUNG CAPACITY (TLC)

Total lung capacity is the volume of air present in the lungs after a deep (maximal)inspiration. It includes all the volumes.

TLC = IRV + TV + ERV + RV

= 3300 + 500 + 1000 + 1200 = 6000 ml

VENTILATION

1. PULMOMARY VENTILATION


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Pulmonary ventilation is a cyclic process, by which fresh air enters the lungs and anequal volume of air leaves the lungs. It is the volume of air moving in and out of lungs perminute in quiet breathing. It is also called respiratory minute volume (RMV).

y Normal value of pulmonary ventilation is 6000 ml (6 liters)/minute.y Pulmonary ventilation is the product of tidal volume (TV) and the rate of respiration

(RR). It is calculated by the formula:

Pulmonary Ventilation= Tidal volume x Respiratory rate

= 500 mL x 12/minute

= 6,000 mL/minute

2. ALVEOLAR VENTILATION

The alveolar ventilation is the amount of air utilized for gaseous exchange every minute.

The alveolar ventilation is different from pulmonary ventilation. In pulmonary ventilation,6ltr of air moves in and out of lungs in every minute. But the whole volume of air is notutilized for exchange of gases. The volume of air subjected for exchange of gases is thealveolar ventilation. The air trapped in the respiratory passage (dead space) does nottake part in gaseous exchange.

Normal value of alveolar ventilation is 4,200 mL (4.2 liters)/minute.

Alveolar ventilation = (Tidal volume - Dead space volume) x Respiratory rate

= (500150) x 12

= 4,200 mL (4.2 liters )/minute.

DEAD SPACE

Dead space is defined as the part of the respiratory tract, where gaseous exchange doesnot take place. The air present in the dead space is called dead space air. The parts of

respiratory tract, which form the dead space, are nose, pharynx, trachea, bronchi and branches ofbronchi up to terminal bronchioles. These structures serve only as the passage for air movement.Gaseous exchange does not take place in these structures.

TYPES OF DEAD SPACEDead space is of two types:I. Anatomical dead spaceII. Physiological dead space.

1. Anatomical Dead Space


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It is the volume of respiratory tract from nose up to terminal bronchiole.

2. Physiological Dead Space

Physiological dead space includes the anatomical dead space plus two additionalvolumes. These two additional volumes are generally considered as wasted Ventilation Theadditional volumes included in physiological dead space are:

a. The air in the alveoli, which are nonfunctioning. In some of the respiratory diseases,alveoli do not function because of dysfunction or destruction of alveolar membrane.

b. The air in the alveoli, which do not receive adequate blood flow. Gaseous exchange doesnot take place during inadequate blood supply.

Normal value of dead space is 150 ml

REGULATION OF RESPIRATION

Respiration is a reflex process. But it can be controlled voluntarily. Voluntary arrest of respiration(voluntary apnea) is possible but only for a short period of about 40 seconds. However, bypractice, breathing can be withheld for a long period. At the end of that period, the person isforced to breathe.

Respiration is subjected to variation even under normal physiological conditions. Emotion andexercise increase the rate and force of respiration. Rest and sleep decrease the rate and forceof respiration. But the altered pattern of respiration is brought back to normal within a short timeby some regulatory mechanisms in the body.

The pattern of respiration is regulated by two mechanisms:

A. Nervous or neural mechanism

B. Chemical mechanism.

NERVOUSMECHANISM


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Nervous mechanism regulates respiration by reflex process. This mechanism includesrespiratory centers, afferent nerves and efferent nerves.

RESPIRATORY CENTERS

Respiratory centers are group of neurons, which control the rate, rhythm and force ofrespiration. These centers are bilaterally situated in reticular formation of the brainstem.

Depending upon the situation in the brainstem, the respiratory centers are classified into twogroups:

1. Medullary centers which are made up ofa. Dorsal respiratory group of neuronsb. Ventral respiratory group of neurons

2. Pontine centers which area. Pneumotaxic centerb. Apneustic center.

MEDULLARY CENTERS

1. Dorsal Respiratory Group ofNeurons

Situation

Dorsal respiratory group of neurons are diffusely situated in nucleus of tractussolitarius which is present in the upper part of the medulla oblongata. Formerly theseneurons were collectively called inspiratory center. All the neurons of dorsal respiratorygroup are inspiratory neurons which generate inspiratory ramp by the virtue of their

autorhythmic property.

Function

Dorsal group of neurons are responsible for basic rhythm of respiration

2. Ventral Respiratory Group ofNeurons

Situation

Ventral respiratory group of neurons are present in nucleus ambiguous and

nucleus retro-ambiguous. These two nuclei are situated in the medulla oblongata anteriorand lateral to the nucleus of tractus solitarius. Earlier the ventral group neurons werecollectively called expiratory center.

Ventral group has both inspiratory and expiratory neurons. The inspiratory neuronsare found in the central area of the group. The expiratory neurons are in the caudal androstral areas of the group.

Function


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Normally, the ventral group neurons are inactive during quiet breathing andbecome active during forced breathing. During forced breathing, these neurons stimulateboth inspiratory muscles and expiratory muscles.

Experimental evidence

Electrical stimulation of the inspiratory neurons in ventral group causes contraction

of inspiratory muscles and prolonged inspiration. Stimulation of expiratory neuronscauses contraction of expiratory muscles and prolonged expiration.

PONTINE CENTERS

1. Pneumotaxic Center

Situation :- The pneumotaxiC center is situated in dorsolateral part of reticular formationin upper pons. Ills formed by the neurons of medial parabrachial and subparabrachiat

nuclei. The subparabraChial nucleus is also called ventral parabrachial or KUuliker-Fusenucleus.

Function

The primary function of pneumotaxic center is to control the medullary respiratorycenters, particularly the dorsal group neurons. It acts through apneustic center. Thepneumotaxic center inhibits the apneustic center so that the dorsal group neurons areinhibited. Because of this inspiration stops and expiration starts. Thus, the pneumotaxiccenter influences the switching between inspiration and expiration.

The pneumotaxic center increases the respiratory rate by reducing the duration ofinspiration.


Stimulation of pneumotaxic center does not produce any typical effect, exceptslight prolongation of expiration by inhibiting the dorsal respiratory group of neuronsthrough apneustic center. Destruction or inactivation of pneumotaxic center results inapneusis. Apneusis is an abnormal pattern of respiration or breathing irregularitycharacteTized by prolonged inspiration followed by short, inefficient expiration.

2. APNEUSTIC CENTER

Situation :- The apneustic center is situated in reticular formation of lower pons.

Function This center increases the depth of inspiration by acting directly on the dorsalgroup neurons.



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The stimulation of apneustic center causes apneusis.

CONNECTIONS OF RESPIRATORY CENTERS

Efferent Pathway

The nerve fibers from the respiratory centers leave brainstem and descend in anteriorpart of lateral columns of spinal cord. These nerve fibers terminate on the motor neurons in theanterior horn cells of cervical and thoracic segments of spinal cord. From the motor neurons ofspinal cord two sets of nerve fibers arise:

y Phrenic nerve fibers (C3 C5) which supply the diaphragmy The intercostal nerve fibers (T1 T11) which supply the external intercostal

muscles.

Vagus nerve also contains some efferent fibers from the respiratory centers.

Afferent Pathway

Impulses from peripheral chemoreceptors and baroreceptors are carried to the respiratorycenters by the branches of glossopharyngeal and vagus nerves. Vagal nerve fibers also carryimpulses from the stretch receptors of lungs to the respiratory centers. Thus, the respiratorycenters receive afferent impulses from different parts of the body and, modulate the movementsof thoracic cage and lungs accordingly through efferent nerve fibers.

ROLE OFMEDULLARY CENTERS

1. Rhythmic discharge of inspiratory impulses

Dorsal respiratory group neurons are responsible for the normal rhythm of

respiration. These neurons maintain the normal rhythm of respiration by rhythmicdischarge of impulses (action potentials). These impulses are transmitted to therespiratory muscles by the fibers of phrenic and intercostals nerves.

2. lnspiratory ramp

Inspiratory ramp is the pattern of discharge from dorsal respiratory group neuronscharacterized by steady increase in amplitude of the action potential. The firing of theseneurons is not like a sudden outburst and it discharge is also not uniform. To start with,


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the amplitude of the action potential is low, It is due to the activation of only few neurons.Later, more and more neurons are activated leading to gradual increase in the amplitudeof the action potential in a ramp fashion. The impulses of this type of firing from dorsalgroup neurons are called inspiratory ramp signals.

The impulses from dorsal group neurons are produced continuously but only for aperiod of 2 sec during which inspiration occurs. After 2 seconds, ramp signals stop

abruptly and do not appear for another 3 seconds. The switching off ramp signal causesexpiration. At the end of 3 seconds, the inspiratory ramp signals reappear in the samepattern and the cycle is repeated.

Normally, during inspiration, the dorsal respiratory group neurons inhibit expiratoryneurons of ventral group and during expiration; the expiratory neurons inhibit the dorsalgroup neurons. Thus, the medullary respiratory centers control each other.

Significance of inspiratory ramp signals

The significance of inspiratory ramp signals is that there is a slow and steadyinspiration so that, the filling of lungs with air is also steady.

ROLE OFPONTINE CENTERS

Pontine respiratory centers regulate the medullar centers. The apneustic centeraccelerates the activity of dorsal group neurons and the stimulation of this center causesprolonged inspiration. The pneumotaxic center inhibits the apneustic center and restrictsthe duration of inspiration.

HERING-BREUER REFLEX

Hering-Breuer reflex is a protective reflex that restricts the inspiration and prevents overstretching of lung tissues. It is initiated by the stimulation of stretch receptors of air passage.

Stretch receptors are the receptors which give response to stretch of the tissues. Thesereceptors are situated on the wall of the bronchi and bronchioles.

During inspiration, the lungs expand. This causes retching of lungs and the air passage. So the

stretch receptors are stimulated. The impulses from stretch receptors are transmitted by vagalafferent fibers to the respiratory centers. The impulses actually inhibit the dorsal group neuronsand so inspiration stops and expiration starts. Thus, the overstretching lung tissues areprevented.

However, Hering-Breuer reflex does not operate during quiet breathing. It operates, only whenthe tidal volume increases beyond 1000 mL.


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This reflex is also called Hering-Breuer inflation reflex since it restricts the inspiration and limitsthe over stretching of lung tissues. The reverse of this reflex is called Hering-Breuer deflationreflex and it takes place during expiration. During expiration as the stretching of lungs isabolished, the deflation of lungs occurs.

IMPULSESFROM BARRORECEPTOR

The baroreceptors are the receptors which give response to change in blood pressure. Thesereceptors are also called pressoreceptors.

Function

The baroreceptors in carotid sinus and arch of aorta give response to increase in bloodpressure. Whenever arterial blood pressure increases, baroreceptors are activated and sendinhibitory impulses to medulla oblongata. This causes decrease in blood pressure and inhibitionof respiration. However, in physiological conditions, the role of baroreceptors in regulation ofrespiration is insignificant.


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CHEMICALMECHANISM OF RESPIRATION

The chemical mechanism of regulation of respiration is operated through the

chemoreceptors. The chemoreceptors are the sensory nerve endings, which give response tochemical changes in blood. The chemoreceptors are stimulated by the changes in the chemicalconstituents of the blood such as:

1. Hypoxia (decreased P02)

2. Hypercapnea (increased PCO2)

3. Increased hydrogen ion concentration.

Types of chemoreceptors

Chemoreceptors are classified into two groups:

1. Central chemoreceptors

2. Peripheral chemoreceptors.

1. CENTRAL CHEMORECEPTORS

The chemoreceptors present in the brain are called the central chemoreceptors.

Situation :- Central chemoreceptors are situated in the deeper partof medullaoblongata, close to the dorsal respiratory group of neurons. This area is known aschemosensitive area and the neurons are called chemoreceptors. The chemoreceptorsare in close contact with blood and cerebrospinal fluid.

Mechanism of Action

The central chemoreceptors are connected withrespiratory centers particularly the dorsalrespiratory group of neurons through synapses. These chemoreceptors act slowly but

effectively. The central chemoreceptors are responsible for 70-80% of increasedventilation through chemical regulatory mechanism.

The main stimulant for the central chemoreceptors is the increased hydrogen ionconcentration. However, if hydrogen ion concentration increases in the blood, it cannotstimulate the central chemoreceptors because, the hydrogen ions from blood cannotcross the blood- brain barrier and blood cerebrospinal fluid barrier


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On the other hand, if carbon dioxide increases in the blood, it can easily cross the blood-brain barrier and blood cerebrospinal fluid barrier and enter the interstitial fluid of brain orthe cerebrospinal fluid. There, the carbon dioxide combines with water to form carbonicacid. Since carbonic acid is unstable, it immediately dissociates into hydrogen ion andbicarbonate ion.

CO2 + H20 H2C03 H+ + HCO3

The hydrogen ions stimulate the central chemoreceptors. From chemoreceptors, thestimulatory impulse is sent to dorsal respiratory group of neurons causing, increasedventilation (increased rate and force breathing). Because of this, the excess carbondioxide is washed out and the respiration is brought back to normal. Lack of oxygen doesnot have significant effect on the central chemoreceptors except that it generallydepresses the overall function of brain.

2. PERIPHERAL CHEMORECEPTORS

Chemoreceptors present in the carotid and aortic region are called peripheralchemoreceptors.

Mechanism of action

Reduction in partial pressure of oxygen is the most potent stimulant for the peripheralchemoreceptors. Whenever, the partial pressure of oxygen decreases, the


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chemoreceptors are stimulated and send impulses through aortic and Herings nerves.These impulses reach the respiratory centers, particularly the dorsal group of neuronsand stimulate them. Dorsal group of neurons send stimulatory impulses to respiratorymuscles resulting in increased ventilation. This provides enough oxygen and rectifies thelack of oxygen.

The peripheral chemoreceptors are mildly sensitive to the increased partial pressure of

carbon dioxide and increased hydrogen ion concentration.

Documents

anatomy and physiology of Respiratory System