Respiratory System: Anatomy and Processes

• This section will cover the respiratory system
• Describes its functions, organ systems involved, anatomy and physiology

Respiratory System Functions

• Supplies the body with oxygen for cellular respiration
• Disposes of carbon dioxide, a waste product of cellular respiration
• Works with the circulatory system
• Functions include olfaction and speech

Respiration Processes

• Pulmonary ventilation (breathing)
  • Air movement into and out of the lungs
• External respiration
  • Oxygen and carbon dioxide exchange between the lungs and the blood
• Transport
  • Oxygen and carbon dioxide transport in the blood
• Internal respiration
  • Oxygen and carbon dioxide exchange between systemic blood vessels and tissues

Functional Anatomy

• Major organs of the respiratory system:
  • Upper respiratory organs
    • Nose and nasal cavity
    • Paranasal sinuses
    • Pharynx
  • Lower respiratory organs
    • Larynx
    • Trachea
    • Bronchi and branches
    • Lungs and alveoli

Lower Respiratory System

• Consists of the larynx, trachea, bronchi, and lungs
• Divided into two zones:
  • Conducting zone
    • Transports gas to and from gas exchange sites
    • No gas exchange occurs here
    • Includes all other respiratory structures
    • Cleanses, warms, and humidifies the air
  • Respiratory zone
    • Site of gas exchange
    • Consists of respiratory bronchioles, alveolar ducts, and alveoli

Larynx Anatomy and Function

• Extends from the 3rd to the 6th cervical vertebra
• Attaches to the hyoid bone
• Opens into the laryngopharynx
• Continuous with the trachea
• Functions:
  • Provides a patent airway (ability to breathe)
  • Routes air and food into proper channels
  • Voice production via vocal folds

Vocal Folds

• Vocal ligaments form the core of the true vocal cords
  • Contain elastic fibers, appearing white due to lack of blood vessels
  • Glottis: opening between vocal folds
  • Folds vibrate to produce sound as air rushes from the lungs
• Vestibular folds (false vocal cords)
  • Positioned superior to the vocal folds
  • Do not participate in sound production
  • Help to close the glottis during swallowing

Trachea

• A windpipe that extends from the larynx into the mediastinum
• Divides into two main bronchi
• 4 inches long and 3/4 inch in diameter
• Very flexible
• Wall composed of three layers:
  • Mucosa: ciliated pseudostratified epithelium with goblet cells
  • Submucosa: connective tissue with seromucous glands supported by 16–20 C-shaped cartilage rings (prevents collapse)
  • Adventitia: outermost layer made of connective tissue
• Trachealis
  • Consists of smooth muscle fibers connecting posterior parts of cartilage rings
  • It contracts to expel mucus during coughing
• Carina
  • Last tracheal cartilage that is expanded
  • Located at the point where the trachea branches into two main bronchi
  • Mucosa of carina is highly sensitive
    • Contact with a foreign object will trigger violent coughing

Clinical Imbalance - Cilia

• Smoking inhibits and destroys cilia
• Coughing is the only way to prevent mucus accumulation in the lungs without ciliary activity
• Morning "smoker's cough" will subside once ciliary function is restored after quitting smoking.
• Ciliary function usually recovers within a few weeks after a person stops smoking
• Tracheal obstruction is life-threatening
• Heimlich maneuver: a procedure where air in the victim's lungs is used to expel an obstructing object
  • If done incorrectly, it may lead to cracked ribs

Bronchi and Subdivisions

• Air passages undergo multiple orders of branching
• Branching is referred to as a bronchial tree
• Conducting zone structures give rise to respiratory zone structures

Conducting Zone Structures

• The trachea divides to form the right and left main (primary) bronchi
  • The right main bronchus is wider, shorter, and more vertical than the left
• Each main bronchus enters the hilum of one lung
• Each main bronchus then branches into lobar (secondary) bronchi
  • 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: the smallest of all branches
    • Less than 0.5 mm in diameter

Conducting Zone Changes

In the conducting zone from bronchi to bronchioles:

• Support structures change
  • Cartilage rings become irregular plates
  • Elastic fibers replace cartilage in bronchioles
• Epithelium type changes
  • Pseudostratified columnar changes to cuboidal
  • Cilia and goblet cells become more sparse
• Amount of smooth muscle increases
  • Allows bronchioles to substantially resist air passage

Respiratory Zone Structures

• The respiratory zone begins where terminal bronchioles feed into respiratory bronchioles
• leads 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

Respiratory Zone Structures of the Alveoli

• Blood air barrier is formed by alveolar and capillary walls with fused basement membranes
  • Very thin (~0.5 µm)
  • Allows gas exchange across the membrane via simple diffusion
• Alveolar walls consist of:
  • Single layer of squamous epithelium (type I alveolar cells)
  • Scattered cuboidal type II alveolar cells secrete surfactant and antimicrobial proteins
• Alveoli are:
  • Surrounded by fine elastic fibers and pulmonary capillaries
  • Alveolar pores which connect adjacent alveoli
    • Equalizes air pressure throughout the lung
    • Provides alternate routes in case of blockages
  • Alveolar macrophages keep alveolar surfaces sterile
    • 2 million dead macrophages/hour are carried by cilia to the throat and swallowed

Other Respiratory Structures

• Pleurae: thin, double-layered serosal membrane dividing the thoracic cavity into two pleural compartments and the mediastinum
  • Parietal pleura: membrane on the thoracic wall, superior face of the diaphragm, around the heart, and between the lungs
  • Visceral pleura: membrane on the external lung surface
• Pleural fluid fills the slit-like pleural cavity between the two pleurae
  • Provides lubrication and surface tension that assists in the expansion and recoil of the lungs

Mechanics of Breathing

• Pulmonary ventilation consists of two phases:
  • Inspiration: gases flow into the lungs
  • Expiration: gases exit the lungs
• Pulmonary ventilation
  • Boyle's law: relationship between pressure and volume of a gas
    • Gases always fill the container they are in
    • If the amount of gas is the same and the container size is reduced, the pressure will increase
    • Pressure (P) varies inversely with volume (V)
• Mathematically:
  • P1V1 = P2V2

Pressure Relationships

• Atmospheric pressure (Patm) is the pressure exerted by the air surrounding the body
  • 760 mm Hg at sea level = 1 atmosphere
• Respiratory pressures are described relative to Patm:
  • Negative respiratory pressure: less than Patm (e.g., 756 mm Hg)
  • Positive respiratory pressure: greater than Patm
  • Zero respiratory pressure: equal to Patm
• Intrapulmonary pressure (Ppul) is the pressure in the alveoli
  • Also called intra-alveolar pressure
  • It Fluctuates with breathing
  • Always eventually equalizes with Patm
• Intrapleural pressure (Pip) is the pressure in the pleural cavity
  • Fluctuates with breathing
  • Always a negative pressure (<Patm and <Ppul)
    • Usually always 4 mm Hg less than Ppul
  • Fluid level must be kept at a minimum
    • Excess fluid is pumped out by the lymphatic system
    • If fluid accumulates, positive Pip pressure develops, leading to lung collapse
• Two inward forces promote lung collapse:
  • Lungs' natural tendency to recoil
    • Lungs always try to assume the smallest size because of elasticity
  • Surface tension of alveolar fluid
    • Surface tension pulls on alveoli to try to reduce alveolar size
• Transpulmonary pressure is the pressure that keeps lung spaces open
  • Transpulmonary pressure = (Ppul - Pip)
  • Keeps lungs from collapsing
  • Greater transpulmonary pressure means that the lungs will be larger
  • Lungs will collapse if Pip = Ppul or Pip = Patm
    • Negative Pip must be maintained to keep lungs inflated

Clinical Imbalances - Lungs

• Atelectasis: lung collapse due to:
  • Plugged bronchioles (causes collapse of alveoli)
  • Pneumothorax: air in the pleural cavity
    • Can occur from a wound in the parietal pleura or rupture of the visceral pleura
    • Treated by removing air, usually with chest tubes
    • Lung will reinflate once the pleurae heal
• Pulmonary Ventilation Consists of inspiration and expiration
  • Mechanical process that depends on volume changes in the thoracic cavity
  • Volume changes lead to pressure changes, which lead to the flow of gases to equalize pressure
• Inspiration: active process involving inspiratory muscles (diaphragm and external intercostals)
  • Action of the diaphragm:
    • Dome-shaped diaphragm contracts and moves inferiorly, flattening out, which increases thoracic volume
  • Action of intercostal muscles:
    • External intercostals contract
    • Rib cage is lifted up and out, which increases thoracic volume
• As thoracic cavity volume increases, lungs are stretched as they are pulled out with the thoracic cage
  • Causes intrapulmonary pressure to drop by 1 mm Hg (Ppul < Patm)
• Forced (deep) inspirations can occur during vigorous exercise or in people with COPD
  • Accessory muscles are also activated:
    • Scalenes, sternocleidomastoid, and pectoralis minor
    • Erector spinae muscles of the back also help to straighten the thoracic curvature
    • They further increase the thoracic cage size, creating a larger pressure gradient so more air is drawn in

Expiration

• Quiet expiration is normally a passive process.
  • Inspiratory muscles relax
  • Thoracic cavity volume decreases
  • Lungs recoil
  • Volume decrease causes intrapulmonary pressure (Ppul) to increase by +1 mm Hg
  • Ppul > Patm
  • Air flows out of the lungs down its pressure gradient until Ppul = Patm
• Forced expiration is an active process It Uses oblique and transverse abdominal muscles, as well as internal intercostal muscles

Pulmonary Ventilation - Physical Factors

• Three physical factors influence the ease of air passage and the amount of energy required for ventilation:
  • Airway resistance
  • Alveolar surface tension
  • Lung compliance
• Airway resistance:
  • Friction is the major nonelastic source of resistance to gas flow and occurs in airways
  • The relationship between flow (F), pressure (P), and resistance (R):
  • ΔP – pressure gradient between the atmosphere and alveoli (2 mm Hg or less during normal quiet breathing)
    • 2 mm Hg difference is sufficient to move 500 ml of air
  • Gas flow changes inversely with resistance
• Resistance in the respiratory tree is usually insignificant
  • Diameters of airways in the first part of the conducting zone are large
  • Progressive branching of airways, as they get smaller, leads to an increase in total cross-sectional area
  • Resistance disappears at terminal bronchioles, where diffusion drives gas movement
• As airway resistance rises, breathing movements become more strenuous
• Severe constriction or obstruction of bronchioles
  • Can prevent life-sustaining ventilation
  • It can occur during acute asthma attacks
  • Epinephrine dilates bronchioles, reducing air resistance
• Alveolar surface tension
  • Surface tension: 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
    • Resists any force that tends to increase the surface area of the liquid
    • Water, which has very high surface tension, coats alveolar walls in a thin film
    • Tends to cause alveoli to shrink to their smallest size (collapse)
  • Surfactant: the body's detergent-like lipid and protein complex that helps reduce the surface tension of alveolar fluid
    • Prevents alveolar collapse
    • Produced by type II alveolar cells

Clinical Imbalance - Surfactant

• Insufficient quantity of surfactant in premature infants causes infant respiratory distress syndrome (IRDS)
  • Increased surface tension results in the collapse of alveoli after each breath
  • Alveoli must be completely reinflated during each inspiration
  • Uses a tremendous amount of energy
  • Common in premature babies
  • Fetal lungs do not produce adequate amounts of surfactant until the last two months of development
• Lung compliance: measure of the change in lung volume that occurs with a given change in transpulmonary pressure
  • Measures how much "stretch" the lung has
  • Normally high because of:
    • Distensibility of lung tissue
    • Surfactant, which decreases alveolar surface tension
  • Higher lung compliance means it is easier to expand lungs

Ventilation Assessment

• 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
• Spirometer: original clinical tool to measure a patient's respiratory volumes using electronic measuring devices
• Tidal volume (TV): amount of air moved into and out of the lung with each breath during quiet (normal) breathing
  • Averages ~500ml
• Inspiratory reserve volume (IRV): amount of air that can be inspired forcibly beyond the tidal volume (2100–3200 ml)
• Combinations of two or more respiratory volumes used to determine respiratory capacity
  • Inspiratory capacity (IC): sum of TV + IRV
  • Functional residual capacity (FRC): sum of RV + ERV
  • Vital capacity (VC): sum of TV + IRV + ERV
  • Total lung capacity (TLC): sum of all lung volumes (TV + IRV+ ERV + RV)
• Anatomical dead space: volume of the conducting respiratory passages (roughly 150 ml) that does not contribute to gas exchange
• Alveolar dead space: space occupied by nonfunctional alveoli
  • Can be due to collapse or obstruction
• Total dead space: sum of anatomical and alveolar dead space
• Minute ventilation: total amount of gas that flows into or out of the respiratory tract in 1 minute
  • Normal at rest = 6 L/min
  • Normal with exercise = up to 200 L/min
  • A rough estimate of respiratory efficiency
• Alveolar ventilation rate (AVR): flow of gases into and out of alveoli during a particular time
  • A better indicator of effective ventilation than minute ventilation because it accounts for dead space

Gas Exchange

• Gas exchange occurs between the lungs and blood, as well as between the blood and tissues
• External respiration: diffusion of gases between the blood and lungs
• Internal respiration: diffusion of gases between the blood and tissues
• Both processes are subject to the basic properties of gases and the composition of alveolar gas

Basic Gas Properties

• Dalton's law of partial pressures: the total pressure exerted by a mixture of gases is equal to the sum of the pressures exerted by each gas
• Partial pressure: pressure exerted by each gas in the mixture
  • Directly proportional to its percentage in the mixture
• Total atmospheric pressure equals 760 mm Hg
• Composition of gas
  • Nitrogen makes up ~78.6% of air
  • Oxygen makes up 20.9% of air/

Hemoglobin

• Oxygen is transported in the blood in two ways:
  • 1.5% is dissolved in plasma
  • 98.5% is loosely bound to each Fe of hemoglobin (Hb) in RBCs Each Hb molecule is composed of four polypeptide chains, each with an iron-containing heme group, so each Hb can transport four oxygen molecules
• Oxyhemoglobin (HbO2): hemoglobin-O2 combination
• Reduced hemoglobin (deoxyhemoglobin) (HHb): hemoglobin that has released O2
• The loading and unloading of O2 is facilitated by a change in the shape of Hb
  • As O2 binds, Hb changes shape, increasing its affinity for O2
  • As O2 is released, the Hb shape change causes a decrease in affinity for O2
• Fully saturated (100%): all four heme groups carry O2
• Partially saturated: when only one to three hemes carry O2
• The rate of loading and unloading of O2 is regulated to ensure adequate oxygen delivery to cells

Oxygen Delivery Influences

• Factors that influence oxygen delivery
  • P02
  • Other factors such as:
    • Temperature
    • Blood pH
    • PCO2
    • Concentration of BPG (2,3-biphosphoglycerate)
  • P02 heavily influences the binding and release of O2 with hemoglobin
    • The percent of Hb saturation can be plotted against P02 concentrations as an S-shaped curve called the oxygen-hemoglobin dissociation curve
  • In arterial blood:
    • P02 is 100 mm Hg and contains 20 ml of oxygen per 100 ml of blood (20 volume %)
    • Hb is 98% saturated
    • Further increases in P02 (as in deep breathing) produce minimal increases in O2 binding
  • In venous blood, P02 is 40 mm Hg and contains 15 volume % oxygen

Other Oxygen Factors

• Increases in temperature, H⁺, Pco2, and BPG modify the structure of hemoglobin
  • Results in a decrease in Hb's affinity for O2
  • Occurs in systemic capillaries at tissues
  • Enhances O2 unloading, causing a shift in the O2-hemoglobin dissociation curve to the right
• Decreases in these factors shift the curve to the left
  • Decreases oxygen unloading from the blood (at the lungs) As cells metabolize glucose, they use O2, causing: Increases in PCO2 and H⁺ in capillary blood Declining blood pH (acidosis) and increasing Pco2 cause the Hb-O2 bond to weaken
• Referred to as the Bohr effect
• O2 unloading occurs where it is needed most (at the tissue)
• Heat production in active tissue directly and indirectly decreases Hb affinity for O2, thus allowing increased O2 unloading to active tissues

CO2 Transport

• Carbon dioxide is transported in the blood in three forms:
  • Dissolved in plasma (7 to 10%) as PCO2
  • Chemically bound to hemoglobin (just over 20%)
    • CO2 is bound to the globin part of hemoglobin
    • Referred to as carbaminohemoglobin
  • As bicarbonate ions in plasma (about 70%).
    • Formation of bicarbonate involves CO2 combining with water to form carbonic acid (H2CO3), which quickly dissociates into bicarbonate and H+

Blood and CO2

• Primarily occurs in RBCs, where the enzyme carbonic anhydrase reversibly and rapidly catalyzes this reaction
• In systemic capillaries, after HCO3- is created, it quickly diffuses from RBCs into the plasma Outrush of HCO3- from RBCs is balanced as Cl- moves into RBCs from the plasma
• Referred to as the chloride shift In pulmonary capillaries, the processes occur in reverse
• HCO3 moves into RBCs while Cl- moves out of RBCs back into the plasma
• HCO3 binds with H⁺ to form H2CO3
• H2CO3 is split by carbonic anhydrase into CO2 and water
• CO2 diffuses into alveoli