Mechanics of Breathing

breathing

• bodily function that leads to ventilation of the lungs

• also known as external respiration

ventilation

• process of moving gases in (inspiration) and out (expiration) of lungs

mechanics of breathing

• describes structural and physiological bases of ventilation

diseases affecting ventilation

• obstructive conditions:

  • asthma

  • chronic obstructive pulmonary disease

  • lung cancer

• restrictive conditions:

  • intrinsic e.g. pulmonary fibrosis

  • extrinsic e.g. pneumothorax

Boyle’s law

• pV=constant

Ohm’s Law

• Q=ΔpR

  • Q= flow

  • p=pressure

  • R=resistance

relevance of pressure in breathing

• gas will flow through patent airways according to pressure gradient between atmosphere (barometric pressure) and alveoli

• inspiration: atmospheric pressure>alveolar pressure

• expiration: alveolar pressure> atmospheric pressure

generation of ΔP

• atmospheric pressure is constant

• ΔP generation is dependant on a cycle of pressure changes in the chest

respiratory muscles- inspiration

• quiet breathing:

  • diaphragm

  • external intercostals stabilise rib cage

• increasing effort:

  • diaphragm

  • external intercostals lift and expand rib cage

  • accessory muscles

  • neck muscles

  • shoulder girdle muscles

respiratory muscles- expiration

• quiet breathing:

  • elastic recoil of tissues

• increasing effort:

  • internal intercostals

  • abdominal wall muscles

thoracic cage expansion

• pleura are important in trasmitting thoracic cage expansion into lung volume expansion

• thoracic cage expansion exerts increasing negagtive pressure on intrapleural space

lungs at rest

• respiratory muscles at rest

• recoil of lung and chest wall are equal but opposite

• pressure along tracheobronchial tree is atmospheric

• no airflow

during inspiration

• inspiratory muscles contract and chest expands

• pleural and alveolar pressure becomes subatmospheric

• air flows into lungs

during expiration

• inspiratory muscles relax

• recoil of lung causes alveolar pressure to exceed pressure at airway opening

• air flows out of lung

disruption of expansion

• pneumothorax→ air leaks into thoracic cavity, so lung will collapse

• pleural effusion→ excess fluid build up in pleural space

spirometry

• most lung volumes can be measure via spirometry

• can be used to measure volume or flow

• lung capacity→ sum of two or more lung volumes- derived value

tidal volume

• volume of air moved in/out of lungs during normal breathing

• at rest→ 6-7ml/Kg

• during exercise→ 15ml/Kg

inspiratory reserve volume

• after normal expiration, take as deep a breath in as possible

• typical value (70kg male)→ 3000ml

expiratory reserve volume

• after normal inspiration, breathe out as deeply as possible

• typical value (70kg male)→ 1500ml

residual volume

• air remains in the lungs even after maximal expiration

• due to rigid nature of thorax and pleural attachments of lungs to chest wall→ prevents complete emptying of lungs

• cannot be measured by spirometry

• typical value (70kg male)→ 1000ml

lung capacities

• combinations of lung volumes

• total lung capacity→ TV+IRV+ERV+RV

• vital capacity→ TV+IRV+ERV

• functional residual capacity→ ERV+ RV

vital capacity

• after maximal inspiration make a maximal expiration

• typical value (70kg male)→ 5000ml

chest diseases affecting lung volumes/capacities

• restrictive lung diseases (pulmonary fibrosis):

  • reduced RV, FRC, VC, TLC

• obstructive lung disease e.g. asthma, COPD, emphysema

  • increased RV

  • TLC may be reduced (COPD) or increased (emphysema)

  • FRC increases in emphysema

diagram of lung volumes and capacities

what determines lung volumes

• balance between lung’s elastic recoil properties and properties of the muscles of the chest wall

• without external forces, elastic recoil of lung= lungs almost airless (10% TLC)

• without lung parenchyma, resting volume of chest wall increases (60% TLC)

FRC

• relaxation point of respiratory system when chest wall and lung recoil pressure are equal but opposite

alveolar and transpulmonary pressure

• alveolar pressure→ sum of pleural pressure and elastic recoil pressure

• transpulmonary pressure→ difference between alveolar pressure and pleural pressure