BIOMECHANICS OF RESPIRATION

20-Mark Answer

REFERENCES (as noted in PDF)

• JSF by Cynthia C. Norkin, 4th ed. / Human Anatomy by BD Chaurasia, 5th ed.
• Therapeutic Exercise by Carolyn Kisner, 6th ed. / K. Santhigiri
• Balloch of Respiratory Physiology

1. RIB CAGE - GENERAL STRUCTURE

• Manubrium (top)
• Body
• Xiphoid process
• Sternal angle / Sternal notch

2. ARTICULATIONS OF THE RIB CAGE

A. Manubriosternal Joint (7)

• Manubrium and the body of the sternum articulate at the level of the 2nd rib
• This joint is known as the STERNAL ANGLE OF LOUIS
• The xiphoid process joins the inferior aspect of the sternal body at the level of the 5th costal cartilage

B. Costoverterbal Joint (CV Joint) (7)

• Synovial joint
• Formed by:
  • Head of ribs
  • Two adjacent vertebral bodies
  • Interposed vertebral disc
• CV joint is formed by the articulation of the costal tubercle on the transverse process of the corresponding vertebra
• There are 10 pairs of CV joints (vertebrae T1 to T10 - ribs the same number)
• 3 Major Ligaments support the CV joint:
  1. Superior CT ligament
  2. Lateral CT ligament
  3. Costo-transverse ligament
• Ribs 2-9 have typical CV joints, each having 2 articular facets called "DEMIFACETS"
• Ribs 1 & 10 have atypical CV joints - articulate with only one vertebra (more mobile than typical CV joints)

C. Costochondral Joint (CC Joint) (7)

• Formed by articulation of the 1st through 10th ribs anterolaterally with costal cartilage
• No synovial - have no apparent support (CS joint)
• Interchondral joint (formed by the articulation of the costal cartilage of ribs 6 to 7 anteriorly with the sternum)
• Supported by 2 structures: Capsule and Interchondral ligaments
• 9th through 10th CC - each articulate with the cartilage immediately above them

D. Costosternal Joint (CS Joint) (54)

• (See costochondral section above - CS joints have no apparent synovial support)

E. Intercostal Joint (54)

F. Costotransverse Joint (57)

3. MUSCLES OF INSPIRATION

Primary Muscle: DIAPHRAGM

• Large dome-shaped sheet of muscle
• Separating the thoracic and abdominal cavities

• Contraction of diaphragm pushes the abdominal contents downward and outward (gutward)
• This increases the vertical dimension of the thoracic cavity
• Elevates the margin of lower ribs
• Increases the transverse diameter of the thorax
• Inspiration is also assisted by the external intercostal muscles connecting higher ribs to lower ribs
• When it contracts, it causes expansion of the ribcage in all directions

Accessory Muscles of Inspiration:

• SCM (Sternocleidomastoid)
• Scalene muscles
• Serratus anterior
• Pectoralis minor (or pectoralis major)
• Elevators of scapula

4. MUSCLES OF EXPIRATION

Mechanism of Expiration:

• The abdominal wall muscles collapse the ribcage
• Broad, flat muscles attached to the rib cage and pelvis
• Intercostal muscles (internal intercostals):
  • Pull from higher rib down to lower rib
  • Causes the collapse of the rib cage

5. MOVEMENTS OF THE RIB CAGE

A. PUMP HANDLE MOVEMENT (Upper Costal Series)

• Formed by the 1st pair of ribs
• Contraction of scaleni muscles moves the 1st rib in a horizontal plane
• This draws the manubrium sternum upward and forward
• Increasing Antero-Posterior (AP) diameter of the chest

B. BUCKET HANDLE MOVEMENT (Lower Costal Series)

• Formed by the 7th to 10th pairs of ribs
• Also shows bucket handle movement by swinging outward and upward, thereby changing the diameter of the thoracic cage
• 11th and 12th ribs do not take part in changing the diameter of the thoracic cage

• Contraction of external intercostal muscles elevates the ribs
• Sternum moves upward and forward (Pump Handle)
• Ribs move outward and upward simultaneously (Bucket Handle)
• Increases transverse diameter

6. DIAPHRAGM - DETAILED MECHANICS

• During inspiration: due to contraction, the fibres shorten, but the central tendinous portion is drawn downward and the diaphragm Plateau flattens
• This increases the vertical diameter of the rib cage
• 11th and 12th ribs do not take part in changing diameter of the thoracic cage

7. MOVEMENT OF LUNGS

During Inspiration:

• Enlargement of thoracic cavity (TC) causes expansion of the lungs
• -ve (negative) pressure increases in TC

During Expiration:

• Pressure comes back to pre-inspiratory level with compressed lungs
• Tissues return to pre-inspiratory state

8. COLLAPSING TENDENCY OF LUNGS

1. Elastic property of lung tissue - causes collapse
2. Surface tension - acting over the surface of the alveoli

• Intrapleural pressure - "it is always -ve" (negative)
• This keeps lungs expanded and prevents collapse

• Substance secreted at alveolar-capillary surface
• Reduces surface tension
• Surface tension = Surface tension of the lung × 2 (surface tension)
• Pressure needed to keep a spherical shape = Laplace's Law
• Surfactant = mixture of phospholipids → reduces surface tension → less alveolar collapse
• "Surfactant product" starts at 6-7th month of gestation - secreted by Type II alveolar cells
• Approx 100 nm in adults

9. RESPIRATORY PRESSURES

A. Intrapleural Pressure

• Existing in the pleural cavity, between visceral and parietal pleura
• Also called: Intra-thoracic Pressure
• Expressed in adults as atmospheric pressure
• Equals -6 mmHg

B. Intra-Alveolar Pressure (= Alveolar Pressure / Intra-pulmonary Pressure)

• Existing in the alveoli
• Intra-alveolar pressure (IAP) = atmospheric pressure
• Becomes -ve (negative) during inspiration, then 0 (atmospheric) at end-inspiration
• Alveolar pressure is 0 prior to the onset of inspiration

C. Pleural Pressure

• -ve (negative) → subatmospheric
• Reflects the balance of opposing recoil forces of lung and chest wall
• Contraction of TM lowers pleural and alveolar pressure

10. COMPLIANCE OF THE RESPIRATORY SYSTEM

• ΔVolume / ΔPressure (Change in volume / Change in pressure)

• During diaphragmatic inspiration, the abdomen becomes the fulcrum for rib expansion of the rib cage
• Compliance of the abdomen is a factor in the inspiratory movement of the thorax

Lung Compliance:

• Lungs are more compliant during expiration (shown in the Pressure-Volume loop in the notes)
• Compliance diaphragm = Healthy person ~200 ml/cmH₂O (no range)

Measurement of Compliance:

• Subject inspires a known amount of air at regular steps
• At each step, air is inspired; at end of each step - intrapleural pressure is measured by an oesophageal balloon
• The air will return to the original pre-inspiratory level
• The values of Volume vs Pressure are plotted to obtain the Compliance Curve

11. MECHANICS OF INSPIRATION AND EXPIRATION

Mechanism of Inspiration:

• Inspiration continues as long as pressure in the mouth exceeds lung recoil force that exceeds pressure in the alveoli
• At the end of inspiration → pleural pressure matches the intra-alveolar pressure
• This is exceeded by the intra-alveolar pressure (IAP), which becomes higher than the atmospheric pressure, and air flow ceases

Mechanism of Expiration:

• During expiration: Alveolar pressure > atmospheric pressure
• Direction of air flow reverses
• Respiratory system recoils passively

SUMMARY TABLE

Note on references in the PDF: JSF (Joint Structure and Function) by Cynthia C. Norkin 4th ed., BD Chaurasia Human Anatomy 5th ed., Therapeutic Exercise by Carolyn Kisner 6th ed., and Balloch of Respiratory Physiology were cited as the source texts by the note-maker.