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What are the major functions of the respiratory system?

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What are the major functions of the respiratory system?

  1. Supply body with oxygen for cellular respiration and dispose of carbon dioxide, a waste product of cellular respiration

    1. Respiratory and circulatory system are closely coupled

  2. Also functions in olfaction and speech

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What are the 4 processes involved in respiration?

Respiratory system

  1. Pulmonary ventilation (breathing): movement of air into and out of lungs

  2. External respiration: exchange of oxygen and carbon dioxide between lungs and blood

Circulatory system

  1. Transport of oxygen and carbon dioxide in blood

  2. Internal respiration: exchange of oxygen and carbon dioxide between systemic blood vessels and tissues

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Major organs

Upper respiratory

  • Nose and nasal cavity

  • Paranasal sinuses

  • Pharynx

Lower respiratory

  • Larynx

  • Trachea

  • Bronchi and branches

  • Lungs and alveoli

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Respiratory zone

  • Site of gas exchange

  • Composed of respiratory bronchioles, alveolar

  • Ducts & alveoli (microscopic)

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Conducting zone

  • Conduits that transport gas to and from gas exchange sites

  • Cleanses, warms, and humidifies air

  • Nose, nasal cavity, pharynx, larynx, trachea, bronchi, bronchioles, terminal bronchiole

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  • Only external portion of respiratory system

  • Divided into two regions:

    • External nose

    • Nasal cavity

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Functions of the nose

  1. Provides an airway for respiration

  2. Moistens and warms entering air

  3. Filters and cleans inspired air

  4. Serves as resonating chamber for speech

  5. Houses olfactory receptors

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Structural details of nasal cavity

Found within and posterior to external nose

  • Divided by midline nasal septum

    • Septum formed anteriorly by septal cartilage, and posteriorly by vomer bone and perpendicular plate or ethmoid bone

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Posterior nasal apertures (chognge)

Opening where nasal cavity turns into nasopharynx

  • Roof: formed by ethmoid and sphenoid bones

  • Floor: formed by hard palate (bone) and soft palate (muscle)

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Nasal Vestibule

Nasal cavity superior to nostrils

  • Lined with vibrissae (hairs) that filter coarse particles from inspired air

  • Rest of nasal cavity lined with mucous membranes

    • Olfactory mucosa: lines superior region of nasal cavity and contains olfactory epithelium

    • Respiratory mucosa: pseudostratified ciliated columnar epithelium that contains goblet cells and rests on lamina propria that contains many seromucous nasal glands

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The nose and paranasal sinuses

  1. Mucus and serous secretions also contain lysozymes and defensins (antibacterial peptides)

  2. Ciliated cells sweep contaminated mucus posteriorly towards throat

  3. Inspired air is warmed by plexuses of capillaries and veins in nasal cavity

  4. Nasal mucosa contains many sensory nerve endings that can cause sneezing to force particles out of cavity

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What are the 4 functions of the conchae?

  1. Increase mucosal area

  2. Enhance air turbulence

  3. During inhalation, conchae and nasal mucosa:

    1. Filter, heat, and moisten air

  4. During exhalation these structures:

    1. Reclaim heat and moisture

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Paranasal sinuses

  • Form ring around nasal cavities

  • Functions:

    • Lighten skull

    • Secrete mucus

    • Help to warm and moisten air

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  • Caused by viruses, bacteria, allergens

  • Inflammation of nasal mucosa

  • Nasal mucosa is continuous with mucosa of respiratory tract, so infections spread from nose to throat, to chest

  • Can also spread to tear ducts and paranasal sinuses, causing blockage of sinus passageways, resulting in sinusitis (inflamed sinuses)

    • Can lead to absorption of air resulting in sinus headache

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  • Funnel-shaped muscular tube that runs from base of skull to vertebra C6

    • Connects nasal cavity and mouth to larynx and esophagus

    • Composed of skeletal muscle

  • 3 regions:

    • Nasopharynx

    • Oropharynx

    • Larynopharynx

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  1. Air passageway posterior to nasal cavity

  2. Lining contains ciliated pseudostratified columnar epithelium

  3. Soft palate and uvula close nasopharynx during swallowing

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  1. Passageway for food and air from level of soft palate to epiglottis

  2. Lining consists of stratified squamous epithelium (protective)

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  1. Passageway for food and air

  2. Posterior to upright epiglottis

  3. Extends to larynx, where is is continuous with esophagus

  4. Lined with stratified squamous epithelium (protective)

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Larynx (voice box)

Extends from 3rd to 6th cervical vertebra and attaches to hyoid bone

  • Opens into laryngopharynx and is continuous with traches

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What are the three functions of the larynx?

  1. Provides patent (OPEN) airway

  2. Routes air and food into proper channels

  3. Voice production

    1. Houses vocal folds

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What does the framework of the larynx consist of?

9 hyaline cartilage (except for epiglottis), connected by membranes and ligaments

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Thyroid cartilage

Large, shield-shaped cartilage that resembles an upright open book; “spine” of book is the laryngeal prominence (Adam’s apple)

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Covers trachea during swallowing

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What are some factors of voice production?

  • Speech: intermittent release of expired air during opening and closing of glottis

  • Pitch is determined by length and tension of vocal cords

  • Loudness depends upon force of air

  • Chambers of pharynx and oral, nasal, and sinus cavities amplify and enhance sound quality

  • Sound is “shaped” into language by muscles of pharynx, tongue, soft palate, and lips

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Inflammation of the vocal folds that causes the vocal folds to swell, interfering with vibrations

  • Results in changes to vocal tone, causing hoarseness; in severe cases, speaking is limited to a whisper

  • Most often caused by viral infections but may also be due to overuse of the voice, very dry air, bacterial infections, tumours on the vocal folds, or inhalation of irritating chemicals

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  • “Windpipe” extends from larynx into mediastinum, where it divides into 2 main bronchi

  • About 4 inches long, 3/4 inch in diameter, and very flexible

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What are the three layers of the trachea?

  1. Mucosa: ciliated pseudostratified epithelium with goblet cells (protective secretions)

  2. Submucosa: connective tissue with seromucous glands supported by 16-20 C-shaped cartilage rings that prevent collapse of trachea

  3. Adventitia: outermost layer made of connective tissue

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Trachea divides to form right and left main (primary) bronchi

  • Right main bronchus wider, shorter, more vertical than left

  • Each main bronchus then branches into lobar (secondary) bronchi

    • 3 on right and 2 on left

    • Each lobar bronchus supplies one lobe

  • Each lobar bronchus branches into segmental (tertiary) bronchi

    • Segmental bronchi divide repeatedly

  • Branches become smaller and smaller

    • Bronchioles: less than 1 mm in diameter

    • Terminal bronchioles: smallest of all branches (less than 0.5 mm in diameter)

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Changes in structure in the bronchi

Support structures change

  • Cartilage rings become irregular plates

  • In bronchioles, elastic fibres replace cartilage altogether

Epithelium type changes

  • Pseudostratified columnar gives way to cuboidal

  • Cilia and goblet cells become more sparse

Smooth muscle increases

  • Allows bronchioles to provide substantial resistance to air passage (control over air flow)

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Respiratory zone begins where terminal bronchioles feed into respiratory bronchioles, which lead into alveolar ducts and finally into alveolar sacs (saccules)

  • Alveolar sacs contain clusters of alveoli

    • ~300 million alveoli make up most of lung volume

    • Sites of actual gas exchange

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Respiratory membrane

  • Blood air barrier that consists of alveolar and capillary walls along with their fused basement membranes

    • Very thin (~0.5um); allows gas exchange across membrane by simple diffusion

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What does the alveolar wall consist of?

  • Single layer of squamous epithelium (type I alveolar cells)

  • Scattered cuboidal type II alveolar cells secrete surfactant and antimicrobial proteins

  • Pores equalize pressure

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What are significant features of alveoli?

  1. Surrounded by fine elastic fibres and pulmonary capillaries

  2. Alveolar pores connect adjacent alveoli

    1. Equalize air pressure throughout lung

    2. Provide alternate routes in case of blockages

  3. Alveolar macrophages keep alveolar surfaces sterile

    1. 2 million dead macrophages/hour carried by cilia to throat and swallowed

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Occupy all of the thoracic cavity except for mediastinum

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site of vascular and bronchial attachment to mediastinum

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Superior tip, deep to clavicle

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Inferior surface that rests on diaphragm

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Found on mediastinal surface, it is the site for entry/exit of blood vessels, bronchi, lymphatic vessels, and nerves

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Left lung

Separated into superior and inferior lobes by oblique fissure

  • Smaller than right because of position of heart

  • Cardiac notch: concavity for heart to fit into

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Right lung

Separated into superior, middle, and inferior lobes

  • Superior and middle lobes separated by horizontal fissure

  • Middle and inferior lobes separated b oblique fissure

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Smallest subdivisions visible to naked eye; hexagonal segments served by bronchioles and their branches

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What are lungs composed of?

  • Mostly composed of alveoli; the rest consists of stroma, elastic connective tissue

    • Makes lungs very elastic and spongy

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How are the lobes divided?

Each lobe is further divided into bronchopulmonary segments

  • 10 on right and 8-10 on left

  • Separated by connective tissue septa

  • Each segment is served by its own artery, vein, and bronchus

    • If one segment is diseased, it can be individually removed

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Pulmonary circulation

  • Pulmonary arteries deliver systemic venous blood from heart to lungs for oxygenation

    • Branch profusely to feed into pulmonary capillary networks

  • Pulmonary veins carry oxygenated blood from respiratory zones back to heart

  • Low-pressure, high-volume system

  • Lung capillary endothelium contains many enzymes that act on different substances in blood

    • Ex: angiotensin-converting enzyme activates blood pressure hormone (ACE)

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Indirect regulation through the kidney

The renin-angiotensin-aldosterone mechanism

  • Decreased arterial blood pressure causes release of renin from kidneys (adrenal gland)

  • Renin enters blood and catalyzes conversion of angiotensinogen from liver to angiotensin I

  • Angiotensin-converting enzymes, especially from lungs, converts angiotensin I to angiotensin II

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Bronchial circulation

  • Bronchial arteries provide oxygenated blood to lung tissue

    • Arise from aorta and enter lungs at hilum

    • Part of systemic circulation, so are high pressure, low volume

    • Supply all lung tissue except alveoli

  • Bronchial veins anastomose with pulmonary veins

    • Pulmonary veins carry most venous blood back to heart

  • Conceptually similar to the coronary artery system

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Innervation of the lungs

  • Parasympathetic - constriction

  • Sympathetic - dilation

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Thin, double-layered serosal membrane that divides thoracic cavity into two pleural compartments and mediastinum

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Parietal pleura

Membrane on thoracic wall, superior face of diaphragm, around heart, and between lungs

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Visceral pleura

Membrane on external lung surface

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Pleural fluid

Fills slit-like pleural cavity between two pleurae

  • Provides lubrication and surface tension that assists in expansion and recoil of lungs

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Inflammation of pleurae that often results from pneumonia

  • Inflamed pleurae becomes rough, resulting in friction and stabbing pain with each breath

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Pleural effusion

  • Accumulation of fluid in pleural cavity

  • Plerua may produce excessive amounts of fluid, which may exert pressure on lungs, hindering breathing

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What are the two phases of pulmonary ventilation?

  1. Inspiration: gases flow into lungs

  2. Expiration: gases exit lungs

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Atmospheric pressure

  • Pressure exerted by air surrounding the body

  • 760 mm Hg at sea level = 1 atmosphere

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Respiratory pressures described relative to Patm

  • Negative respiratory pressure: < 1

  • Positive respiratory pressure: > 1

  • Zero respiratory pressure: = 1

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Intrapulmonary pressure (Ppul)

  • Pressure in alveoli

    • Also called intra-alveolar pressure

  • Fluctuates with breathing

  • Always eventually equalizes with Patm

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Intrapleural pressure (Pip)

  • Pressure in pleural cavity

  • Fluctuates with breathing

  • Always a negative pressure (<Patm and Ppul)

    • Usually always 4 mm Hg less than Ppul

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What are two inward forces that promote lung collapse?

  1. Lungs’ natural tendency to recoil

    1. Because of elasticity, lunges try to assume smallest size

  2. Surface tension of alveolar fluid

    1. Surface tension pulls on alveoli to reduce alveolar size

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What is one force that tends to enlarge the lungs?

Elasticity of chest wall pulls thorax outward

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How is negative Pip affected by the opposing forces of the lungs?

Negative Pip is affected by these opposing forces but is maintained by strong adhesive force between parietal and visceral pleurae

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Transpulmonary pressure

Pressure that keeps lung spaces open

  • Keeps lungs from collapsing

  • Greater transpulmonary pressure, the larger the lungs will be

Transpulmonary pressure = (Ppul - Pip)

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When will the lungs collapse?

Pip = Ppul

Pip = Patm

  • Negative Pip must be maintained to keep lungs inflated

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Lung collapse due to:

  1. Pneumothorax, air in pleural cavity

    1. Can occur from either wound in parietal pleura or rupture of visceral pleura

    2. Treated by removing air with chest tubes

    3. When pleurae heals, lung reinflates

  2. Plugged bronchioles, which cause collapse of alveoli

In most cases, atelectasis is a reversible process

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What does Pulmonary ventilation depend on?

Mechanical process that depends on volume changes in thoracic cavity

  • Volume changes lead to pressure changes

  • Pressure changes lead to flow of gases to equalize pressure

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Boyle’s law

Relationship between pressure and volume of a gas

  • Gases always fill the container they are in

    • If amount of gas is the same and container size is reduced, pressure will increase

  • Pressure (P) varies inversely with volume (V)

  • P1V1 = P2V2

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Intercostal muscles

Groups of muscles between the ribs

  • Help move the chest wall

  • Involved in mechanics of breathing

  • Will alternately expand or shrink the overall size of your chest cavity to drive pressure differences which are important for air inflow into the lungs

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Action of the diaphragm

When dome-shaped diaphragm contracts, it moves inferiorly and flattens out

  • Results in increase in thoracic volume

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Action of intercostal muscles

When external intercostals contract, rib cage is lifted up and out, much like when handle on a bucket is raised (outward as it moves upward)

  • Results in increase in thoracic volume

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What is the role of the diaphragm and intercostals in inspiration?

  1. As thoracic cavity volume increases, lungs are stretched as they are pulled out with thoracic cage

  2. Causes intrapulmonary pressure to drop by 1 mm Hg Ppul<Patm

  3. Because of difference between atmospheric and intrapulmonary pressure, air flows into lungs, down its pressure gradient, until Ppul=Patm

  4. During same period, Pip lowers to about 6 mm Hg less than Patm

These changes in pressure drive inspiration

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Quiet expiration is normally a passive process

  • Inspiratory muscles relax, thoracic cavity volume decreases, and lungs recoil

  • Volume decrease causes intrapulmonary pressure (Ppul) to increase by 1 mm Hg

  • Ppul>Patm so air flows out of lungs down its pressure gradient until Ppul=Patm

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Forced (deep) inspirations

Can occur during vigorous exercise or in people with pulmonary disease

  • Accessory muscles are also activated

    • Scalenes (neck), sternocleidomastoid (neck) and pectoralis minor (upper chest)

      • Erector spinae muscles of back also help to straighten thoracic curvature

    • Act to further increase thoracic cage size, creating a larger pressure gradient so more air is drawn in

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Forced expiration

An active process that uses oblique and transverse abdominal muscles, as well as internal intercostal muscles

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What are 3 physical factors that influence the ease of air passage and the amount of energy required for ventilation?

  1. Airway resistance

  2. Alveolar surface tension

  3. Lung compliance

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Airway resistance

  • Friction: major non-elastic source of resistance to gas flow; occurs in airways

  • F = P/R

  • Relationship between flow (F), pressure (P), and resistance (R):

    • ΔP - pressure gradient between atmosphere and alveoli (2 mm Hg or less during normal quiet breathing)

      • 2 mm Hg difference sufficient to move 500 mL of air

    • Gas flow changes inversely with resistance

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Why is resistance in respiratory tree usually insignificant?

  1. Diameters of airways in first part of conducting zone are huge

  2. Progressive branching of airways as they get smaller leads to an increase in total cross-sectional area

  • Any resistance usually occurs in medium-sized bronchi

  • Resistance disappears at terminal bronchioles, where diffusion is what drives gas movement

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What happens as airway resistance rises?

  • Breathing movements become more strenuous

  • Severe constriction or obstruction of bronchioles:

    • Can prevent life-sustaining ventialtion

    • Can occur during acute asthma attacks and stop ventilation

  • Epinephrine dilates bronchioles, reduces air resistance

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Alveolar surface tension

Surface tension: the attraction of liquid molecules to one another at a gas-liquid interface

  • Tends to draw liquid molecules closer together and reduce contact with dissimilar gas molecules

  • Water, which has very high surface tension, coats alveolar walls in a thin film

  • Tends to cause alveoli to shrink to smallest size

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Body’s detergent-like lipid and protein complex that helps reduce surface tension of alveolar fluid

  • Prevents alveolar collapse

  • Produced by type II alveolar cells

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Infant Respiratory Distress Syndrome (IRDS)

  • Insufficient quantity of surfactant in premature infants

  • Increased surface tension results in collapse of alveoli after each breath

  • Alveoli must be completely reinflated during each inspiration

  • Uses a tremendous amount of energy

  • Increased surface tension results in collapse of alveoli after each breath

  • 1% of newborns

  • Common in premature babies

    • Fetal lungs do produce adequate amounts of surfactant until last two months of development

  • Treatment includes surfactant treatment and ventilation

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Lung compliance

Measure of change in lung volume that occurs with given change in transpulmonary pressure

  • Measure of how much “stretch” the lung has

  • Normally high because of:

    • Distensibility of lung tissue

    • Surfactant, which decreases alveolar surface tension

  • The lower the lung compliance, the more energy is needed just to breathe

  • Higher lung compliance means it is easier to expand lungs

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How do you measure ventilation?

  • Several respiratory volumes can be used to assess respiratory status

  • Respiratory volumes can be combined to calculate respiratory capacities, which can give information on a person’s respiratory status

  • Respiratory volumes and capacities are usually abnormal in people with pulmonary disorders

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The act of measuring ventilation

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Original, cumbersome clinical tool used to measure patient’s respiratory volumes

  • Electronic measuring devices used today

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Tidal volume (TV)

Amount of air moved into and out of lung with each breath

  • Averages ~500 mL

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Inspiratory reserve volume (IRV)

Amount of air that can be inspired forcibly beyond the tidal volume (2100-3200 mL)

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Expiratory reserve volume (ERC)

Amount of air that can be forcibly expelled from lungs (1000-1200 mL)

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Residual volume (RV)

Amount of air that always remains in lungs

  • Needed to keep alveoli open

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Inspiratory capacity (IC)

Sum of TV + IRV

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Functional residual capacity (FRC)

Sum of RV + ERV

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Vital capacity (VC)

Sum of TV + IRV + ERV

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Total lung capacity (TLC)

Sum of all lung volumes


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What can a spirometry distinguish betwen?

  1. Obstructive pulmonary disease (defects in expelling air)

  2. Restrictive disease (defects in taking air in)

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Obstructive pulmonary disease

Increased airway resistance (ex: bronchitis, asthma)

  • TLC, FRC, RV may increase because of hyperinflation of lungs

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Restrictive disease

Reduced TLC due to disease (ex: tuberculosis) or exposure to environmental agents (ex: fibrosis)

  • VC, TLC, FRC, RC decline because lung expansion in compromised

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How can you measure the rate of gas movement?

Pulmonary functions tests

  • Forced vital capacity (FVC): amount of gas forcibly expelled after taking deep breath

  • Forced expiratory volume (FEV): amount of gas expelled during specific time interval of FVC

  • FEV1: amount of air expelled in 1st second

    • Healthy individuals can expel 80% of FVC in 1st second

    • Patients with obstructive disease exhale less than 80% in 1st second, whereas those with restrictive disease exhale 80% or more even with reduced FVC

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Anatomical dead space

Does not contribute to gas exchange

  • Consists of air that remains in passageways

    • ~150 ml out of 500 ml TV

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Alveolar dead space

Space occupied by nonfunctional alveoli

  • Can be due to collapse or obstruction

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Total dead space

Sum of anatomical and alveolar dead space

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How is dead space measured?

Through metabolic changes in CO2

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