The Respiratory System

Functions: • Provides a way to exchange O2 and CO2 between the atmosphere and the blood – oxygen is used by the cells of the body solely for the process of aerobic respiration – carbon dioxide is a waste product of aerobic respiration and must be removed from the body • Regulation of body pH • Protection from inhaled pathogens and irritating substances • Vocalization

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The Respiratory System • Together, the respiratory system and the circulatory system deliver O2 to cells and remove CO2 from the body through 3 processes 1. Pulmonary ventilation (breathing) • movement of air into and out of the lungs • Inspiration/inhalation and expiration/expiration 2. Gas Exchange • O2 and CO2 are exchanged between the air in the lungs and the blood • O2 and CO2 are exchanged between the blood and the cells 3. Transport • movement of O2 and CO2 between the lungs and cells 2

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Organization of the Respiratory System • Anatomically, the respiratory system includes the: – upper respiratory tract (mouth, nasal cavity, pharynx and larynx) – lower respiratory tract (the trachea, 2 primary bronchi, the branches of the primary bronchi and the lungs) • Functionally, the respiratory system includes the: – the conducting zone (semi-rigid airways) lead from the external environment of the body to the exchange surface of the lungs – the exchange surface (respiratory zone) consists of the alveoli which are a series of interconnected sacs (surrounded by pulmonary capillaries) which expand and collapse during ventilation and allows oxygen and carbon dioxide to be exchanged between the air in the lungs and the blood 4

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Cross Section Through the Thoracic Cavity Anterior Breast Sternum Ribs

Pericardial cavity Heart Left lung

Right lung

Visceral pleura

Aorta

Pleural cavity

Vertebra

Parietal pleura

Spinal cord

Posterior

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The Thorax and Respiratory Muscles • The bones of the spine and ribs and their associated skeletal muscles form the thoracic cage • Contraction and relaxation of these muscles alter the dimensions of the thoracic cage which promotes ventilation – 2 sets of intercostal muscles connect the 12 pairs of ribs – additional muscles (sternocleidomastoid and scalenes) connect the head and neck to the sternum and the first 2 ribs – a dome-shaped sheet of skeletal muscle called the diaphragm forms the floor – the abdominal muscles also participate in ventilation

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The Pleural Membranes and Fluid • Within the thorax are 2 double layered pleural sacs surrounding each of the 2 lungs • Parietal pleura – lines the interior of the thoracic wall and the superior face of the diaphragm • Visceral pleura – covers the external surface of the lungs (alveoli) • A narrow intrapleural space between the pleura is filled with 25 mL of pleural fluid which holds the 2 layers together by the cohesive property of water – serves to lubricate the area between the thorax and the outer lung surface – holds the lungs tight against the thoracic wall • prevents lungs from completely emptying even after a forceful expiration 9

The Lungs and Bronchial Tree • Lungs are crowded by adjacent organs; they neither fill the entire ribcage, nor are they symmetrical – Right lung • Shorter than left because the liver rises higher on the right • Has three lobes—superior, middle, and inferior—separated by horizontal and oblique fissure – Left lung • Taller and narrower because the heart tilts toward the left and occupies more space on this side of mediastinum • Has indentation—cardiac impression • Has two lobes—superior and inferior separated by a single oblique fissure 10

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Passageway Through the Respiratory Tract 1.

2.

Air enters the upper respiratory tract through either the mouth or nose and passes through the pharynx – warms and humidifies (adds H2O) inspired air – hair in the nose filters inspired air of any dust

Air then passes through the larynx or “voice box” – contains the vocal cords (bands of connective tissue) which tighten and vibrate to produce sound

Pharynx: Nasopharynx Oropharynx Laryngopharynx

• Nasopharynx passes only air and is lined by pseudostratified columnar epithelium

• Oropharynx and laryngopharynx pass air, food, and drink and are lined by stratified squamous 12 epithelium

3. Air continues into the lower respiratory tract through the trachea which is a semi-flexible tube held open by C-shaped rings of cartilage 4. The distal end of the trachea splits into 2 primary bronchi which lead to the 2 lungs branch repeatedly into progressively smaller bronchi • the walls of the bronchi are supported by cartilage

The Trachea

• Trachea (windpipe)—a rigid tube about 12 cm (4.5 in.) long and 2.5 cm (1 in.) in diameter – Found anterior to esophagus – Supported by 16 to 20 C-shaped rings of hyaline cartilage – Reinforces the trachea and keeps it from collapsing when you inhale – Opening in rings faces posteriorly toward esophagus – Trachealis muscle spans opening in rings • Gap in C allows room for the esophagus to expand as swallowed food passes by • Contracts or relaxes to adjust airflow

Trachea (Cross Section)

14 4 µm

The Trachea • Inner lining of trachea is a ciliated pseudostratified columnar epithelium – Composed mainly of mucus-secreting cells, ciliated cells, and stem cells – Mucociliary escalator: mechanism for debris removal • Mucus traps inhaled particles • Upward beating cilia drives mucus toward pharynx where it is swallowed • Middle tracheal layer—connective tissue beneath the tracheal epithelium – tracheal cartilage

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Middle and Distal Respiratory Tract • Bronchi send air into the bronchioles – these airways are supported by smooth muscle only • contraction causes bronchoconstriction which decreases the airway diameter and makes ventilation more difficult – increases airway resistance to decrease flow • relaxation causes bronchodilation increases the airway diameter which makes ventilation easier – decreases airway resistance to increase flow • branches into respiratory bronchioles which begins the Respiratory Zone. • Bronchioles move air into the blind sacs called alveoli where gas exchange occurs (respiratory zone) – approximately 150 – 300 million per lung 17

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Anatomy of Alveoli • Composed of very thin (simple) epithelial tissue consisting of 2 predominant alveolar cell types – Type I (squamous) alveolar cells • allows for very rapid exchange of O2 and CO2 – Type II or great (cuboidal) alveolar cells • secrete surfactant into the alveolar lumen • Exterior surface is surrounded by large numbers of blood capillaries for gas exchange and large numbers of elastic fibers to aid in lung recoil during exhalation • White blood cells (macrophages) within the lumen of the alveoli protect against inhaled pathogens • Alveoli represents an enormous surface area for gas exchange (2800 square feet or half of a football field) 19

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(a) Normal

Fluid and blood cells in alveoli Alveolar walls thickened by edema (b) Pneumonia

Confluent alveoli

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Properties of Alveoli • Compliant – ability to be easily stretched or deformed – allows lungs to fill up with air during inspiration – attributed by the very thin Type I alveolar cells

• Elastic – ability to resist being stretched or deformed – allows lungs recoil (deflate) during expiration – attributed by: • interior (luminal) surface covered with a thin film of water which creates surface tension at the air-fluid interface (surface) of the alveoli • the elastic fibers surrounding the alveoli 22

Alveolar Surface Tension and Elasticity • During inhalation the alveoli expand and adjacent water molecules on the luminal surface are pulled apart from one another causing the H-bonds between them to be stretched (like a spring) creating tension • During exhalation the tension within the H-bonds is released which returns the water molecules to their original spacing pulling the alveoli inward allowing them to recoil

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Surfactant • Type II alveolar cells secrete surfactant (“surface active agent”) which is a fluid consisting of amphiphilic molecules into the lumen of the alveoli • These molecules disrupt the cohesive forces between water molecules by inserting themselves between some of the water molecules preventing H-bonds from forming and thus decreases the surface tension of the water on the luminal surface • Reducing surface tension simultaneously increases compliance and reduces elasticity of the alveoli which greatly decreases the amount of effort needed to inflate the lungs while retaining the ability to deflate the lungs • Without surfactant, the muscles of respiration cannot contract with enough force to overcome the alveolar surface tension resulting in the inability to breathe 24

Pulmonary Ventilation • The movement of air into and out of the airways – occurs as a result of increasing and decreasing the dimensions of the thoracic cavity through the contraction and relaxation of the skeletal muscles of respiration • Since the alveoli are “stuck” to the interior surface of the thorax via the pleura, dimensional changes in the thoracic cavity result in the same dimensional changes in the alveoli

• Dimensional changes in the alveoli create air pressure changes in the alveoli as expressed by Boyle’s Law

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Boyle’s Law • The mathematical inverse relationship that describes what happens to the pressure of a gas or fluid in a container following a change in the volume (dimensions) of the container – If the volume of a container increases, then pressure within the container must decrease – If volume of a container decreases, then pressure within the container must increase V1 x P1 = V2 x P2 V = volume of a container P = pressure within the container • force of collisions between molecules within the container and the wall of the container • determined by the “concentration” of molecules within the 26 container

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Pulmonary Ventilation • Changes in the pressure in alveolar air (alv) create air pressure gradients between the air in the alveoli and the atmospheric air that surrounds our bodies (atm) which drive air flow into and out of the lungs

• Air always flows from an area of higher pressure to an area of lower pressure – When alv < atm inspiration occurs • air flows into the lungs – When alv > atm expiration occurs • air flows out of the lungs – When alv = atm no air flow occurs • at transition between inspiration and expiration 28

Inspiration • Before inspiration, the alv (0 mm Hg) = atm (0 mm Hg) (no air movement) • Expansion of the thoracic cavity (by the contraction of the diaphragm, the external intercostals, the scalenes and the sternocleidomastoid) pulls the alveoli open which increases their volume and decreases their pressure (-1 mm Hg) – the alveolar pressure decreases below atmospheric pressure, creating a pressure gradient resulting in inspiration • As the alveoli fill with air (more molecules), the alv pressure increases until it equals atm pressure • Inspiration ends when alv (0 mm Hg) = atm (0 mm Hg)

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Expiration • Expiration is a passive process that does not require muscle contraction to occur • Before expiration, (at end of previous inspiration), the alv (0 mm Hg) = atm (0 mm Hg) (no air movement) • Expiration begins as action potentials along the nerves that innervate the muscles of inspiration cease allowing these muscles to relax returning the diaphragm and ribcage to their relaxed positions – allows the alveoli to collapse which decreases their volume and increases their pressure (1 mm Hg) • the alveolar pressure increases above atmospheric pressure, creating a pressure gradient resulting in quiet (passive) expiration • As the alveoli empty with air, the alv pressure decreases until it equals atm pressure • Expiration ends when alv (0 mm Hg) = atm (0 mm Hg)

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Control of Ventilation • Ventilation occurs automatically whereby the contraction of the skeletal muscles of respiration are controlled by a spontaneously firing network of neurons in the brainstem but can be controlled voluntarily up to an extent

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Voluntary Control of Breathing • Originates in the motor cortex of frontal lobe of the cerebrum – Sends impulses down corticospinal tracts to respiratory neurons in spinal cord, bypassing brainstem • Limits to voluntary control – Breaking point: when CO2 levels rise to a point when automatic controls override one’s will

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Respiratory Centers of the Medulla • The dorsal respiratory group (DRG) is the pacesetter for ventilation where in a person at rest initiates bursts of action potentials every 5 seconds setting a quiet ventilation rate of 12 breaths/minute

– action potentials travel down the phrenic nerve stimulating the diaphragm and the intercostal nerves stimulating the external intercostals – periods of time between these bursts action potentials allow for expiration as the muscles relax

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Receptors of Respiration • Various chemoreceptors (monitoring changes in H+, CO2 or O2) initiate reflexes which alter the firing of action potentials by the DRG promoting different ventilation patterns • An increase in either CO2 (hypercapnia) or H+ will stimulate the DRG and result in an increase in respiration rate and depth (hyperventilation) • A decrease in either CO2 or H+ will inhibit the DRG and result in a decrease in respiration rate and depth (hypoventilation) • Only a substantial decrease in systemic arterial O2 ( 7.4) can denature proteins and depress the CNS • Chemoreceptors will inhibit the DRG to decrease the ventilation rate and depth (hypoventilation) – removes CO2 from the body more slowly resulting in an increase in CO2 levels – causes the reaction to proceed to the right increasing the amount of H+ • decreasing the pH of the body back to 7.4 61

Transport of CO2 as HCO3– • The high [HCO3-] in the RBC promotes the diffusion of HCO3- out of the RBC into blood plasma – HCO3- is more soluble than CO2 therefore more can be carried – the volume of plasma is greater than the collective volume of the cytosol of the RBCs and thus has a greater capacity to carry HCO3(CO2) – Cl- diffuses from the plasma into the RBC to electrically counterbalance the diffusion of HCO3- out of the RBC (chloride shift) • HCO3- circulates back to the lungs in the plasma

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Conversion of HCO3– to CO2 H+ + HCO3- → H2CO3 → CO2 + H2O • As the blood flows through the pulmonary capillaries, CO2 diffuses out of the plasma and RBCs and enters the alveoli, which decreases the amount of CO2 in the RBC • HCO3- diffuses from the plasma into the RBCs which increases the amount of HCO3- in the RBC – Cl- diffuses out of the RBC (reverse chloride shift) • In the RBC, H+ and HCO3- combines to form H2CO3 – H2CO3 is then converted by carbonic anhydrase to CO2 and H2O • CO2 diffuses out of the RBC and into the alveoli and removed from the body on the next expiration

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