functional anatomy & lung mechanics

These flashcards cover the fundamental anatomy, respiratory muscles, lung volumes, capacities, and ventilation equations as discussed in the Respiratory Physiology lecture.

respiratory system

system responsible for the exchange of gases in the body that consist of the

• gas exchanging organ - alveoli in the lungs & pulmonary capillaries

• ventilatory pump - chest wall, diaphragm, areas in the brain that control respiration

  • chest wall - rib cage + muscles of respiration

respiration

1. pulmonary ventilation - inflow & outflow of air between the atmosphere & lung alveoli

2. diffusion of O₂ & CO₂ between alveoli & blood in pulmonary capillaries

3. transport of gases - accomplished by circulation

4. regulation of ventilation

respiratory muscles

muscles involved in the process of breathing which expand and contract the thoracic cavity to facilitate airflow in and out of the lungs

• primary (inspiration)

  • diaphragm

  • external intercostal

• accessory muscles (inspiration)

  • sternocleidomastoid, scalenes (anterior, middle, posterior), pectoralis major & minor, serratus anterior, serratus posterior superior

• muscles of expiration (active/forced)

  • abdominal muscles (rectus abdominis, external oblique, internal oblique, & transversus abdominis)

  • internal intercostals (excluding the interchondral portion), innermost intercostals, subcostals

  • serratus posterior inferior, quadratus lumborum

normal inspiratory muscles

muscles primarily responsible for increasing thoracic volume during inhalation

• diaphragm - flatten to push abdominal contents downwards

• external intercostals - lift ribs upwards & outward

accessory inspiratory muscles

muscles that assist the diaphragm & external intercostals during deep or labored breathing (ex. exercise or respiratory distress)

• sternocleidomastoid

• scalenes (anterior, middle, posterior)

quiet expiration

a passive process driven by the relaxation of inspiratory muscles and the elastic recoil of the lungs and chest wall

active expiratory muscles

muscles recruited during forced expiration (exercise, coughing, respiratory distress) to increase intrathoracic pressure and expel air

• internal intercostals → pull ribs downward + inward

• abdominal muscles (rectus, obliques, transversus) → compress abdominal cavity → push diaphragm upward

pump handle movement

movement where ribs move upwards and forwards increasing the anteroposterior diameter of the rib cage during inhalation

• primarily involves the upper ribs (2–6)

bucket handle movement

movement where ribs move upwards and outwards (laterally) increasing the transverse diameter of the rib cage

• primarily involves the lower ribs (7–10)

lungs - gross anatomy

structural organization of the lungs, including lobes, airway branching, and relationship to the diaphragm

• right lung: 3 lobes (superior, middle, inferior)

• left lung: 2 lobes (superior, inferior) → cardiac notch for the heart

• trachea divides into right and left main bronchi → further branching into lobar and segmental bronchi

• lungs rest on the diaphragm → primary muscle of inspiration

pleura

a thin, double-walled serosa surrounding the lungs

• composed of an outer layer lining the thoracic wall (parietal) & an inner layer covering the lung surface (visceral)

• the space between the layers = pleural cavity

• produces a small amount of lubricating fluid (≈ 10–15 mL) to reduce friction during breathing

pleural effusion

accumulation of excess fluid within the pleural cavity

• restricts lung expansion and reduces ventilation

• appears as opacity on imaging

phrenic nerve

motor and sensory nerve arising from C3–C5 that innervates the diaphragm and conveys sensation from the central diaphragmatic pleura

• provides motor innervation → diaphragm

• carries sensory fibers from the central diaphragmatic pleura and pericardium

• irritation of these structures (e.g., splenic injury, subdiaphragmatic inflammation) may be perceived as referred pain at the shoulder tip due to shared C3–C5 dermatomes

pneumothorax

collapse of the lung caused by air entering the pleural cavity

• air leaks from the lung or chest wall into the space between lung and thoracic wall

• accumulated air prevents lung expansion → partial or complete collapse

• commonly due to chest trauma or underlying lung disease (COPD, asthma, cystic fibrosis, tuberculosis)

lungs - arterial blood supply

• pulmonary deliver deoxygenated blood from the right ventricle → lungs for gas exchange

• bronchial provide oxygenated blood to the conducting airways and supporting lung structures

lungs - venous blood supply

• pulmonary return oxygenated blood → left atrium

• bronchial drain deoxygenated blood from part of the airway circulation (with some drainage entering pulmonary veins)

lungs - parasympathetic innervation

cholinergic input that increases airway smooth muscle tone and reduces airflow

• stimulation of M₃ muscarinic receptors →bronchoconstriction

• increases airway resistance and limits airflow

• mediated by vagal efferents to bronchial smooth muscle

lungs - sympathetic innervation

adrenergic input that relaxes airway smooth muscle and increases airflow

• activation of bronchial smooth muscleβ₂ receptors → bronchodilation

• enhances airflow during stress, exercise, or increased metabolic demand

• β₂ agonists (epinephrine, isoproterenol, albuterol) mimic this effect and are used to treat bronchial asthma

respiratory system organization

upper airways → nose, mouth, pharynx, larynx

conducting zone → extends from the top of the trachea to the beginning of respiratory bronchioles

respiratory zone → extends from the respiratory bronchioles down

upper airways

structures that condition incoming air before it reaches the lower respiratory tract

• includes the nose, mouth, pharynx, and larynx

• responsible for filtering, warming, and humidifying inspired air

• also provides pathways for speech and swallowing

conducting zone

portion of the respiratory tract that moves air but does not participate in gas exchange

• extends from the top of the trachea → beginning of the respiratory bronchioles

  • includes trachea, bronchi, and bronchioles

• functions in air transport, humidification, filtration, and regulation of airflow

respiratory zone

region where gas exchange occurs within the distal airways

• extends from the respiratory bronchioles → alveolar sacs

  • includes respiratory bronchioles, alveolar ducts, and alveoli

• site of O₂–CO₂ exchange between air and blood

Weibel’s 23-generation Model

an anatomical framework describing airway branching from trachea to alveolar sacs

• 23 generations of branching airways

  • generation 0: trachea

  • generations 1–16: conducting zone (no gas exchange)

  • generations 17–23: respiratory zone (gas exchange)

bronchi

cartilaginous airways that conduct air into progressively smaller branches

• contain cartilage plates that maintain airway patency

• lined with ciliated epithelium and mucus‑producing cells

• part of the conducting zone (no gas exchange)

• branch repeatedly before transitioning into bronchioles

bronchioles

small, non‑cartilaginous airways that regulate airflow resistance

• no cartilage; walls supported by smooth muscle

  • lined with ciliated epithelium proximally

• diameter <1 mm

• include terminal (conducting zone) & respiratory (start of respiratory zone)

ciliated mucosal epithelium

airway lining specialized for particle clearance and mucociliary transport

• extends from the trachea → proximal respiratory bronchioles

• cilia beat upward to remove debris and pathogens

• works with mucus to protect lower airways

• gradually disappears in distal respiratory bronchioles and alveoli

generation 0

the first division of the airway tree responsible for conducting inspired air

• corresponds to the trachea

• supported by cartilage and lined with ciliated epithelium

• marks the start of the conducting zone

generations 1–16 (conducting zone)

airway generations that conduct air but do not participate in gas exchange

• includes bronchi and bronchioles

  • cartilage present in bronchi; absent in bronchioles

• diameter progressively decreases

• ciliated epithelium present throughout

• functions in air transport, humidification, filtration

generations 17–23 (respiratory zone)

distal airway generations where gas exchange occurs

• includes respiratory bronchioles, alveolar ducts, alveolar sacs

  • no cartilage

• thin walls for diffusion

• site of O₂–CO₂ exchange with pulmonary capillaries

central airways

large proximal airways that provide rigid, high‑velocity airflow

• include the trachea and larger bronchi

• supported by cartilage → semi‑rigid

• airflow = fast & turbulent

  • contribute the highest resistance due to large diameter but intense turbulence

peripheral airways

small, compliant distal airways that regulate airflow distribution

• include the bronchioles

  • no cartilage; walls contain smooth muscle

• arranged in parallel → reducing total resistance

  • airflow = slower and more laminar

airway resistance – peak location

region of the respiratory tract where total airflow resistance is greatest

• medium‑sized bronchi generate the highest resistance

  • branching at this level creates turbulent airflow → increasing resistance

• resistance decreases in smaller bronchioles because many parallel pathways lower total resistance

• trachea = high turbulence but less total resistance than medium bronchi

type I pneumocytes

large, thin epithelial cells forming the primary gas‑exchange surface of the alveoli

• cover ~95% of alveolar surface area

• extremely flat → minimize diffusion distance

• form part of the air–blood barrier with capillary endothelium

• essential for efficient oxygen and carbon dioxide exchange

type II pneumocytes

cuboidal alveolar cells responsible for surfactant production and epithelial repair

• synthesize and secrete surfactant → reduces alveolar surface tension

• act as progenitor cells → differentiate into type I pneumocytes after injury

  • proliferate during damage to restore alveolar lining

• occupy smaller surface area but are more numerous than type I cells

respiratory membrane

the thin diffusion barrier where gas exchange occurs between alveolar air and pulmonary capillary blood

• composed of alveolar epithelium, fused basement membranes, and capillary endothelium

• extremely thin (~0.6 μm) → minimize diffusion distance

• large total surface area (~70 m²) → maximize gas exchange

• site where O₂ enters blood and CO₂ leaves blood by simple diffusion driven by partial pressure gradients

alveolar surface area

structural arrangement that maximizes diffusion capacity in the lungs

• created by millions of small alveoli tightly packed within the thoracic cavity

• pulmonary capillaries coil around alveoli to expand total surface area

• large surface area (~70 m²) enables efficient gas exchange

respiratory tract particle filtration

size‑dependent mechanisms that trap inhaled particles & protect the lower respiratory tract from harmful debris

• > 10 µm: trapped in upper airways (e.g., nasal hair)

• 2–10 µm: reach trachea/bronchi → reflex bronchoconstriction + coughing & mucociliary escalator

• < 2 µm: reach alveoli → cleared by PAMs

mucociliary escalator

a defense mechanism involving coordinated ciliary system that clears trapped particles from conducting airways

• cilia beat upward toward the pharynx

• removes particles 2–10 µm trapped in trachea / bronchi

• works with mucus to protect lower airways

• impaired in smoking, chronic bronchitis, and cystic fibrosis

pulmonary alveolar macrophages (PAMs)

immune cells that clear small inhaled particles and pathogens in the alveoli

• remove particles < 2 µm

• phagocytose microbes and debris

• activated in infection, inflammation, and dust exposure

secretory IgA (respiratory tract)

immunoglobulin that protects mucosal surfaces from respiratory infections

• produced by airway mucosa

• neutralizes pathogens before they invade tissue

• key component of mucosal immunity

angiotensin converting enzyme (ACE)

enzyme in produced by pulmonary capillary endothelial cells that activates angiotensin II

• converts angiotensin I → angiotensin II

• helps regulate blood pressure and vascular tone

prostaglandins (PGE₂) – pulmonary epithelium

locally acting lipid mediators that modulate airway physiology

• protect epithelial surfaces

• regulate inflammation

• influence airway tone

histamine (respiratory tract)

inflammatory mediator involved in allergic and immune responses

• released by mast cells

• contributes to airway inflammation

• plays a role in allergic reactions and bronchial hyperreactivity

spirometer

a device that measures the volume of air inspired and expired to track changes in lung volume

• floating drum rises and falls with inspiration and expiration

• measures TV, IRV, ERV, and capacities derived from them

• cannot measure RV, FRC, or TLC because they contain residual volume

tidal volume (V_T)

the volume of air that moves into the lung with each quiet inspiration

• typically ~ 500 ml (~ 6–8 mL/kg)

inspiratory reserve volume (IRV)

the volume of air that can still be breathed in after a normal inspiration

• forced inhalation → after V_T

• ~ 3,100 mL for ♂ & 1,900–2,500 mL for ♀

expiratory reserve volume (ERV)

the volume of air that can still be breathed out after a normal inspiration

• forced exhalation → after V_T

• ~ 1,100–1,200 mL for ♂ & 700–800 mL for ♀

residual volume (RV)

the volume of air remaining in the lung after maximal expiration

• cannot be expelled & measured by direct spirometry

• prevents alveolar and airway collapse (atelectasis)

  • reduces the pressure needed for reinflation

  • minimizes energy expenditure during breathing

inspiratory capacity (IC)

the air that can be breathed in after a normal exhalation

• calculated as IRV + V_T

functional residual capacity (FRC)

the volume of gas in the lungs after a normal expiration; the equilibrium (resting) point where outward chest wall recoil balances inward lung recoil

• calculated as RV + ERV

• acts as a buffer for gas exchange between breaths

• measured without muscle contraction → usually unaffected by skeletal muscle weakness

• lowest pulmonary vascular resistance

• increases with height & age → loss of elastic tissue

vital capacity (VC)

the maximum volume of gas that can be expired after a maximal inspiration

• calculated as IC + ERV

  • IRV + V_T + ERV

forced vital capacity (FVC)

the total amount of air that can be forcibly exhaled after a maximal inhalation

total lung capacity (TLC)

the total volume of gas present in the lungs after a maximal inspiration

• calculated as VC + RV

  • IRV + V_T + ERV + RV

helium dilution technique

an indirect measurement method for lung volumes based on the law of conservation of mass

• He = poorly soluble & stays in airways

• C₁ × V₁ = C₂ × (V₁ + V₂)

  • before equilibration: known C₁ and V₁

  • after equilibration: C₂ reflects mixing with lung volume V₂

• valve opened at end of quiet expiration → measures FRC

• valve opened after maximal expiration → measures RV

dead space (DS)

regions of the respiratory system where ventilation does not participate in gas exchange

• anatomic dead space: conducting airways

• alveolar dead space: unperfused alveoli

• physiologic dead space: total dead space

• measured using capnography (anatomic) and Bohr equation (physiologic)

anatomic dead space

the volume of the conducting airways (ending at terminal bronchioles) where no gas exchange occurs

• extends from nose/mouth → terminal bronchioles

• proportional to body weight in pounds (≈ weight in mL)

• contains air that is ventilated but not perfused

• contributes to wasted ventilation

physiological dead space

the total dead space in the lungs

• sum of anatomic + alveolar dead space

• equals anatomic dead space in healthy individuals

• increases when alveoli lose perfusion

alveolar dead space

alveoli that are ventilated but not perfused

• contain air but lack capillary blood flow

• increases in conditions like pulmonary embolism

• normally minimal / absent in healthy lungs

volumetric capnography

method using exhaled CO₂ to estimate anatomic dead space

• phase I: no CO₂ → pure anatomic dead space air

• phase II: rising CO₂ → mix of dead space + alveolar air

  • anatomic dead space = volume of exhaled gas at midpoint of phase II transition

• phase III: plateau → alveolar air

Bohr’s equation

formula calculating physiologic dead space from arterial CO₂ & mixed expired CO₂

• $\bm{V_{D}=V_{T}\times\frac{P_{A}CO_2-P\overline{_{E}}CO_2}{P_{A}CO_2}}$

  • V_T = tidal volume (mL)

  • P_ACO₂ = P_A of arterial blood CO₂ (mmHg)

  • P_ECO₂ = P_A of mixed expired air CO₂ (mmHg)

• larger difference = more wasted ventilation

• detects ventilation–perfusion mismatch

$$\bm{V_{T}\times\frac{P_{A}CO_2-P\overline{_{E}}CO_2}{P_{A}CO_2}}$$

Bohr’s equation - physiologic dead space (V_D)

ventilation rate

frequency of breathing cycles per minute

• number of breaths taken per minute

  • normal resting value ≈ 12–20 breaths/min

• used to calculate minute ventilation and alveolar ventilation

total (minute) ventilation

total volume of air moving in or out of the respiratory system per minute

• tidal volume (V_T) × respiratory rate (RR)

  • V_T = mL & RR = breaths/min

  • example: 500 mL × 12 breaths/min = 6 L/min

• includes dead space + alveolar ventilation

tidal volume (V_T) × respiratory rate (RR)

total (minute) ventilation

alveolar ventilation (V_A)

volume of fresh air delivered to the respiratory zone (alveoli) per minute

• (V_T − V_D) × (RR)

  • V_T = tidal volume (mL)

  • V_D = dead space volume (mL)

  • RR = respiratory rate (breaths/min)

  • example: (500 − 150) × 12 = 4.2 L/min

• excludes anatomic dead space

• determines CO₂ elimination

(V_T − V_D) × (RR)

alveolar ventilation

alveolar ventilation (V_A) - P_CO2

relationship between alveolar ventilation & partial pressure of CO₂ in alveolar air

• $\bm{\frac{V_{CO_2}\times K}{P_{A}CO_2}}\Rightarrow\mathbf{P_{A}CO_2}=\bm{\frac{V_{CO_2}\times K}{V_{A}}}$

  • V_CO2 = rate of CO₂ production (mL/min)

  • P_ACO₂ = P_A of arterial blood CO₂ (mmHg)

  • K = constant

• describes inverse relationship: ↑ ventilation → ↓ P_ACO₂

$$\frac{V_{CO_2}\times K}{P_{A}CO_2}$$

alveolar ventilation - constant CO₂

alveolar ventilation – constant CO₂

relationship between alveolar ventilation and arterial CO₂ when metabolic CO₂ production is unchanged

• varies inversely

  • ↑ ventilation → ↓ P_ACO₂

  • ↓ ventilation → ↑ P_ACO₂

• described mathematically by: $\bm{V_{A}=\frac{V_{CO_2}\times K}{P_{A}CO_2}}$

• basis of hyperventilation (low P_ACO₂) and hypoventilation (high P_ACO₂)

alveolar ventilation – exercise

ventilatory adjustments required to maintain normal P_ACO₂ during increased CO₂ production

• V_CO2 increases (often doubles or more) → to keep PaCO₂ ≈ 40 mm Hg → ventilation increase proportionally

  • ↑ CO₂ production → ↑ V_A

• prevents exercise‑induced hypercapnia

• same governing equation: $\mathbf{P_{A}CO_2}=\bm{\frac{V_{CO_2}\times K}{V_{A}}}$

$$\mathbf{P_{A}CO_2}=\bm{\frac{V_{CO_2}\times K}{V_{A}}}$$

alveolar ventilation – exercise

point A

total lung capacity (TLC) - VC + RV

point B

functional residual capacity (FRC) - RV + ERV

point C

inspiratory reserve volume (IRV)

point D

expiratory reserve volume (ERV)

point E

forced vital capacity (FVC)

point F

residual volume (RV)

point G

inspiratory capacity (IC) - IRV + VT

point H

tidal volume (VT)

point I

vital capacity (VC) - IRV +VT +ERV