Module 4.1 Respiratory system

Functions of the respiratory system

• gas exchange

• pulmonary ventilation

• acid-base regulation

• phonation

• speech production

• swallowing coordination

• airway protection

anatomy of the respiratory system

• upper respiratory tract: nasal cavity, pharynx, oral cavity

• larynx: thyroid cartilage, cricoid cartilage, epiglottis, arytenoids

• vocal folds: true vocal folds, vestibular folds

• lower respiratory tract: trachea, bronchial tree, respiratory zone

• lungs: 4 right lobes, 2 left lobes

• pleurae: visceral pleura, parietal pleura, pleural cavity

Boyle’s law

At constant temperature, P x V = constant

• when V increase, pressure decrease → air flows in

• when V decrease, P increase → air flows out

types of key pressures

• atmospheric pressure (Patm)

• Intra-alveolar pressure (Palv)

• Intrapleural pressure (PiP)

• Transpulmonary pressure

Inhalation

• active - requires muscular effort

• inspiratory muscles contract

• thoracic volume increases in 3D

• intrapleural pressure drops

• transpulmonary pressure gradient increases → lungs expand passively with chest wall

• alveolar wall increases → palv drops 1-2mmHg below Patm

• pressure gradient drives air into lungs

• airflows until Palv = Patm

Exhalation (quiet exhalation)

• passive - no muscular energy needed

• inspiratory muscles relax → elastic recoil of lungs and chest wall

• thoracic volume decreases

• intrapleural pressure returns

• alveolar volume decreases → palv rises above patm

• pressure gradient drives air out

• airflows until Palv = Patm

forced inhalation

• recruits accessory muscles

• scalenes, sternocleidomastoid, pectoralis minor

• elevate upper ribs and sternum to further expan thorax

forced exhalation

• internal intercostals depress rib cage

• abdominal muscles compress viscera, push diaphragm up

• speech is exhalation

alveoli

• functional units of gas exchange

cell types of alveoli

• type I pneumocytes - thin, flat cells, sites of gas exchange

• type II pneumocytes - cuboidal, secrete surfactant, regenerate type I cells after

• alveolar macrophages - phagocytose particles and pathogens

the respiratory membrane of alveoli

• only 0.5-1 micrometer thick

• gases diffuse across this membrane by simple diffusion down partial pressure gradients

• pulmonary capillaries envelop alveoli

surfactant

a phospholipid-protein mixture secreted by Type II pneumocytes

• reduces surface tension at air-water interface within alveoli

• without it, surface tension would collapse small alveoli

• allows alveoli of different sizes to coexist without smaller ones emptying into larger

• developed at the end of pregnancy

4 primary non-overlapping lung volumes

• tidal volume: air moved per breath during quiet breathing

• inspiratory reserve volume: extra air inhaled forcefully

• expiratory reserve volume: extra air exhaled forcefully

• residual volume - air remaining after maximal exhalation

factors reducing lung volumes

• obesity, pregnancy - reduce ERV and FRC

• restrictive lung diseases (fibrosis)

• ageing - loss of elastic recoil

• neuromuscular weakness - reduces inspiratory and expiratory force

inspiratory capacity

tidal volume + inspiratory reserve volume

• maximum air inhaled from resting end-expiration

functional residual capacity

expiratory reserve volume + residual volume

• air remaining at end of quiet exhalation

vital capacity

TV + IRV + ERV

• maximum air moved in one breath

total lung capacity

Vital capacity + Residual volume

• max air the lung can hold

Dead space

• inhaled air that does not participate in gas exchanges

anatomical dead space

air trapped in conducting airways (nose, trachea, bronchi, bronchioles) which don’t contain alveoli

alveolar dead space

ventilated alveoli that are not perfused (no blood flow) e.g. due to pulmonary embolism

physiological dead space

the total sum of both anatomical and alveolar dead space

physiological factors affecting lung volumes and capacities

• body size

• age

• sex

• posture

• physical fitness

how does body size affect lung volume and capacities

taller individuals have larger lungs

how does age affect lung volumes and capacities

• volumes pear around 25 years, decline 30mL/year after 30

how does sex affect lung volumes and capacities

• males typically have 20-25% higher volumes than females

how does physical fitness affect lung volumes and capacities

• trained athletes have higher volumes and capacities and endurance

how does posture affect lung volume/capacity?

• upright increases FRC

• supine decreases FRC (diaphragm pushed up)

pathological factors affecting lung volumes and capacities

• obstructive disease (COPD, asthma)

• restrictive disease (fibrosis, obesity)

• neuromuscular disease (MND, SCI, Parkinson’s)

• Pleural effusion or pneumothorax

Minute ventilation (VE)

Tidal volume x respiratory rate

normal is 6-9L/min

alveolar ventilation (VA)

VA = (TV-Dead Space) x RR

what are rate and depth of ventilation controlled by

• chemoreceptors detecting arterial PaCO2 and pH

• exercise → increase metabolic CO2 → increased rate and depth (up to 60-100 L/min)

• voluntary control via cortex - critical for speech, singing, breath-holding

factors affecting airway resistance

• airway resistance opposes airflow

• poiseuille’s law: resistance is poroportional to 1/r^4

• halving radius, increases resistance by 16x

• bronchospasm (asthma)

• mucus hypersecretion

• mucosal oedema

• dynamic airway collapse (COPD)

compliance in ventilation

• compliance = change of volume/change of pressure

• how easily the lung expands per unit pressure change

• high compliance = easy to expand

elasticity in ventilation

• elasticity - the tendency of lung tissue to return to its original shape after stretching

• provided by elastin fibres and surface tension

• lung recoil drives passive exhalation and speech production

clinical alterations to compliance and elasticity

• increase compliance (decrease recoil) - emphysema: elastin destruction; lungs inflate easily but fail to recoil; air trapping

• decrease compliance (increase stiffness) - pulmonary fibrosis, ARDS, pneumonia: stiff lungs require greater inspiratory effort

pressure of pulmonary circulation

25/8 mmHg - much lower than systemic (120/80 mmHg)

hypoxic vasoconstriction

low oxygen in alveolars → vasoconstrictions → redirects blood to better ventilated alveoli

• V/Q (Ventilation-perfusion) matching: ratio optimised for efficient gas exchange

• pulmonary hypertension: elevated pressure → right heart strain → oedema

the pulmonary-systemic circuit: connecting lung and body

left atrium → mitral valve → left ventricle → aorta → systemic arteries → tissue delivery → venous return via vena cava → right heart → lungs

how is oxygen transported in blood

2 forms

• dissolved in plasma

• bound to haemoglobin

oxyhaemoglobin dissociation curve

• sigmoidal shape due to cooperative binding - binding of O2 increases likelihood for the next

Factors shifting curve right (more O2 released to tissues)

• increase temperature

• increase CO2

• decrease pH

• increase 2, 3-BPG (Bohr effect)

Factors shifting curve left (less O2 released)

• decrease temperature

• decrease CO2

• increase pH

How is CO2 transported in blood

• dissolved in plasma

• as bicarbonate ions

• as carbaminohaemoglobin

Haldane effect

• deoxygenated Hb binds CO2 more readily than oxygenated Hb

• in tissues: Hb gives up O2 → takes up more CO2

• in lungs: Hb gains O2 → releases CO2

gas exchange: external respiration (lungs)

• gas exchange across the respiratory membrane is driven by diffusion

• gradients drive diffusion

• partial pressures

  • O2 diffuses from alveolus into blood → blood leaves as PO2

  • CO2 diffuses from blood into alveolus → exhaled

what are factors that impair external respiration

• thickened membrane → decreases diffusion rate

• V/Q mismatch → ventiled but unperfused alveoli or perfused but unventilated

• decreased surface area

Gas exchange: internal respiration (tissues)

in systemic capillaries, gradients are reversed

diffusion

• O2 diffuses from blood

• CO2 diffuses from cells into blood

• Blood leaves as PO2, PCO2

the respiratory centre (neural control)

• located in the brainstem (medulla oblongata + pons)

  • medullary centres: dorsal respiratory groups, ventral respiratory group

• pontine centres

  • pneumotaxic centre

  • apneustic centre

• voluntary override

  • cerebral cortex

  • limbic system

chemoreceptors

• central chemoreceptors (medulla)

• peripheral chemoreceptors

central chemoreceptors (medulla)

• respond to increased CO2

• CO2 drives the breathing

peripheral chemoreceptors

• carotid bodies (CNIX)

• aortic bodies (CNX)

major effector pathways

• phrenic nerves

• intercostal nerves

• vagus nerve

• autonomic system/sympathetic chain: bronchial smooth muscle

key reflexes modulating breathing

• hering-breurer reflex - reflex to prevent over-inflation

• irritant receptors - glottis closed, develop high pressure underneath, glottis opens to cough

• J-receptors: activated by pulmonary oedema → rapid shallow breathing

• proprioceptors in muscles/joints → increase ventilation at start of exercise

higher centre influences on respiration

• hypothalamus - raises ventilation in response to pain and temperature

• limbic system - emotional breathing

• cortex - voluntary breathing