Mechanics of Breathing

Question 0

What are the normal features of the ribs? Which ribs are atypical?

Answer 0

NORMAL:
Wedge-shaped head + two articular facets + tubercle + curve at costal angle + costal groove

ATYPICAL:

1st rib: short & broad body, only one articular facet (articulates with one vertebral body), grooves for subclavian vessels

2nd rib: tuberosity for serratus anterior

10th rib: only one articular facet (articulate with one vertebral body)

11th-12th rib: only one articular facet (articulate with one vertebral body), no neck or tubercle

Question 1

What are the differences between the rib attachments?

Answer 1

Ribs 1-7 = connected by costal cartilages to sternum

Ribs 8-10 = connected to the costal cartilage above

Ribs 11-12 = floating ribs ending free in the abdominal muscle

Question 2

What are the different types of intercostal muscles in the thorax? How are these muscles supplied?

Answer 2

Superficial -> Deep

External intercostal: fibres run anterior & inferior (elevate ribs)
Internal intercostal: fibres run posterior & inferior (depress ribs)
Innermost intercostal: depress ribs

Anterior rami of thoracic spinal nerves run between internal & innermost intercostal muscles (supply the muscles + parietal pleura + overlying skin)

Anterior (internal thoracic artery) & posterior (thoracic aorta) intercostal arteries of each space anastomose (supply the muscles + parietal pleura + overlying skin)

Question 3

How is the diaphragm innervated? What openings are present in the diaphragm?

Answer 3

Phrenic nerve (C3, C4, C5) + margins of diaphragm innervated by intercostal nerves

Openings:
T8 = vena cava
T10 = oesophagus
T12 = aortic hiatus

Question 4

What muscles are used in inspiration?

Answer 4

External intercostal muscles elevate the ribs

Diaphragm contracts to descend

Sternocleidomastoid splints 1st rib

Pectoral muscles & latissimus dorsi
note: this is why breathless patients rest arms on the back of the chair/sit on the edge of the bed gripping the sides/rest hands on knees (fixes the pectoral girdle)

Question 5

What muscles are used in expiration?

Answer 5

Quiet respiration:
Passive - elastic recoil of chest wall & lungs

Forced respiration:
Internal intercostal muscles (depress ribs)
Innermost intercostal muscles (depress ribs)
Abdominal wall muscles

Question 6

Where is the mediastinum? What are the contents of the mediastinum?

Answer 6

Between the right & left pleural sacs

Contents:

• heart
• great vessels
• trachea
• oesophagus

Question 7

How is the pleural cavity formed? What are its contents?

Answer 7

Visceral pleura adheres to surfaces of horizontal & oblique fissures
Parietal pleura adheres to thoracic wall, mediastinum, & diaphragm

Pleura continuous at hilum —> forms pleural cavity

Pleura contains the costodiaphragmatic recess (surrounds the upward convexity of the diaphragm)

Question 8

What is a bronchopulmonary segment?

Answer 8

Area of lung supplied by its own segmental bronchus & segmental branches of the pulmonary artery & vein

Can be isolated and removed surgically with little damage to others

Question 9

What is the significance of the pleural seal?

Answer 9

Lungs held at a larger volume by the pleural seal (force of surface tension ensures that the lungs change volume as the thorax does)

Question 10

What aspect of ventilation requires the most effort?

Answer 10

Stretching the lungs, unless the diaphragm cannot easily move into the abdomen
e.g. pregnancy, obesity, corsets (increased abdominal pressure)

Question 11

What is the definition of compliance? What is the definition of surface tension?

Answer 11

COMPLIANCE = stretchiness of the lungs = volume change per unit pressure change

SURFACE TENSION = interactions between molecules at surface of a liquid which resists stretching (increased area of surface tension -> increased force of surface tension)

note: compliance is not reduced for liquid v.s. lining

Question 12

What is the definition of surfactant? What is the evidence that surfactant exists?

Answer 12

Mixture of detergents produced by type II alveolar cells; molecules disrupt interactions between surface water molecules by inserting between them

At low lung volumes, the force of surface tension is lower than expected.

Reduces surface tension when the lungs are deflated but not when fully inflated (allows an increase in surface tension when alveoli inflate so that the pressure stays high)

Question 13

What is hysteresis? Why is this significant?

Answer 13

Force depends on direction - the energy put into stretching a film of surfactant is not all recovered when the film recoils

Loss greatest when the tidal volume is maximal (less efficient breathing)

Question 14

What is Laplace’s law?

Answer 14

Bubbles form when a film of fluid surrounds gas (film shrinks until there is an equilibrium between tension and pressure)

Therefore larger bubbles have lower pressure

So, alveoli (i.e. interconnecting bubbles) should have smaller bubbles (high pressure) collapse into larger bubbles (low pressure)

Surfactant prevents this from happening by becoming less effective as alveoli increase in size (allows an increase in surface tension)

Question 15

What is respiratory distress syndrome?

Answer 15

Reduction in surfactant compromises gas exchange

• Premature babies: few, large alveoli & stiff lungs (give mothers expecting premature delivery steroids to quicken lung development/pump artificial surfactant into lungs as an aerosol)
• Adults: post-trauma (unknown aetiology)

Question 16

How does airway resistance change during normal breathing and with obstructive airway disease?

Answer 16

Normal:
INSPIRATION: highest in nose and lowest in small airways (overall)

FORCED EXPIRATION: resistance in alveoli increases (trapped air) but cartilage helps keep patent

Obstructive airway disease: increased resistance earlier in expiration (at higher lung volumes) causes difficulty expiring (abdominal muscles used)

Question 17

Describe the arterial and venous drainage of the intercostal muscles.

Answer 17

Anterior intercostal arteries supplying upper 6 intercostal spaces originate from internal thoracic arteries; supplying lower 6 originate from musculophrenic arteries

Posterior intercostal arteries originate from thoracic aorta (apart from 1st, which originates from a branch of the subclavian artery)

Intercostal veins all drain centrally into the azygos system of veins or internal thoracic veins (connect to brachiocephalic veins)

Question 18

How does a pneumothorax cause a collapsed lung?

Answer 18

Negative pressure in pleural cavity (relative to atmospheric pressure) —-> air enters the pleural space until pressure equilibrates —> absence of negative pressure in pleura to counteract elastic recoil of lungs —> collapsed lung

Question 19

Why do we tend to breathe through our mouths in exercise?

Answer 19

Reduced resistance

Reduced anatomical dead space (?)

Question 20

What is the mechanism of coughing?

Answer 20

Cough reflex coordinated by medulla oblongata

Initiated by irritation of cough receptors in upper airway

Glottis closes + strong contraction of expiratory muscles (abdominal muscles & internal intercostals)
= increased intra-pulmonary pressure

When glottis opens, an explosive discharge of air results