1 Physiology of gaseous exchange and mechanisms of ventilation

Which muscles are active during inspiration?

• Diaphragm

• External intercostals

• Accessory muscles:

  • Scalenes

  • Sternocleidomastoids

• Contraction increases thoracic capacity

What happens during normal expiration?

• Relaxation of inspiratory muscles

• Decreases thoracic capacity

• Due to elastic recoil of the lungs

Which muscles are active during forced expiration?

• Abdominal muscles:

  • Internal & external oblique

  • Rectus abdominis

  • Transversus abdominis

• Internal intercostals

• Some muscles of the back and neck

• Contraction further decreases thoracic capacity

What happens at the end of quiet expiration? respitory cycle 1

• Respiratory muscles are at rest

• Balance between:

  • Elastic recoil of the lungs (forces of collapse)

  • Elastic recoil of the chest wall (forces of expansion)

• Thoracic cavity is at equilibrium

What happens during inspiration? 2 respitory cycle

• Contraction of inspiratory muscles

• ↑ Thoracic volume

• ↓ Intrapleural pressure

• ↑ Lung volume

• ↓ Alveolar pressure

• Air moves into the lungs

What happens during expiration? respitory cycle 3

ck:

• Relaxation of inspiratory muscles

• ↓ Thoracic volume

• ↑ Intrapleural pressure

• ↓ Lung volume

• ↑ Alveolar pressure

• Air moves out of the lungs to functional residual capacity

What happens during forced expiration? 4th resipitory cycle

Back:

• Contraction of expiratory muscles

• ↓ Thoracic volume

• ↑ Intrapleural pressure

• ↓ Lung volume

• ↑ Alveolar pressure

• Air moves out of the lungs below functional residual capacity

What are the two opposing forces involved in breathing?

• Lung elasticity → wants to collapse the lungs

• Rib cage anatomy → wants to expand the lungs

What happens at the end of normal expiration? (forces )

• The two forces (lung collapse vs rib expansion) are balanced

• Negative pressure in pleural cavity holds lungs open

What happens during inspiration?(forces)

• Active process driven by the brain

• Rib cage expands

• ↑ Negative intrapleural pressure → pulls lungs open

• ↓ Alveolar pressure → air flows into lungs

What happens during normal expiration?

• Passive process due to lung elastic recoil

• Inspiratory muscles relax

• Lungs recoil inward

• Creates positive pressure → air flows out of lungs

What does lung compliance mean?

• How easily the lungs expand

• High compliance → lungs stretch easily

• Low compliance → lungs are stiff, harder to inflate

What keeps the lungs and chest wall “adhered” to each other?

• Negative pressure in the pleural cavity

• Acts like a vacuum between lungs and chest wall

• Allows lungs to follow chest movements during breathing

What is intrapleural pressure approximately equal to?

Intrapleural pressure ≈ intrathoracic pressure

Does intrapleural pressure remain constant?

• No, it is not constant

• Changes during the respiratory cycle

What is the pleural cavity and what does it contain?

• A virtual space between the two pleural membranes

• Contains a thin layer of fluid  acts as a lubricant for smooth lung movement

How does surface tension affect the pleural membranes and lung expansion?

• Surface tension “holds” the parietal and visceral pleura together

• When the chest expands, the parietal pleura moves outward

• Due to surface tension, the visceral pleura follows the parietal pleura

• This pulls the lung tissue, causing the lungs to expand

What are the two main types of airways?

• Conducting airways: no gas exchange (“dead space”)

• Respiratory airways: site of gas exchange

What structures make up the conducting airways?

• Trachea and bronchi: cartilage + smooth muscle

• Bronchioles: no cartilage, more smooth muscle (main site of obstruction in asthma)

What structures are involved in respiratory airways?

• Respiratory bronchioles

• Alveoli (major site of gas exchange)

What is the primary function of the airways?

• Deliver O₂ to the blood and remove CO₂

• Help maintain body pH by regulating arterial CO₂ (pCO₂)

Why do airways need defense systems?

• Airway is an opening into the body

• Must prevent harmful particles from entering the lungs

How do airways defend against particles?

• Warm and humidify inspired air

• Prevent particle entry

• Remove or neutralize particles that enter

• Mucociliary system and cough

• Phagocytic cells

• Inflammatory and immunologic responses

What are some other functions of the airways?

• Metabolic functions

• Vocalization

• Olfaction (sense of smell)

What is an important structural feature of bronchioles?

• No cartilage, so they can collapse

• More smooth muscle than larger airways

How do bronchioles behave during breathing?

• Inspiration: lungs expand → bronchioles pulled open

• Expiration: bronchioles get smaller

• At low lung volumes, bronchioles can close and trap gas

Why is bronchiole collapse clinically important?

• Volume at which they collapse increases in lung diseases

• Major site of smooth muscle contraction and obstruction in asthma

How close are alveoli to pulmonary capillaries?

• Air and blood are very close (~5 µm)

• Facilitates efficient gas exchange

What are the layers between alveolar air and blood?

• Alveolar epithelium

• Basement membrane

• Interstitium (thin)

• Basement membrane

• Capillary endothelium

What are the main types of alveolar cells and their functions?

• Type I cells: flat, primary lining, cover 95% of surface

• Type II cells: tall/granular, 60% of cells cover 5% of surface, produce surfactant

What provides elasticity in alveoli?

• Elastic fibers in interstitium

• Produced by fibroblasts → allow lung recoil and expansion

What is the role of alveolar macrophages?

• Scavenge particles

• Remove excess surfactant

Are alveoli individual isolated sacs?

• No, they form an interconnected honeycomb of cavities

• Connected by shared interconnecting walls

How is mechanical stress transmitted in alveoli?

• Stress on one alveolus is transmitted to neighboring alveoli

• Interconnected walls allow stress to be shared across lung parenchyma

How does alveolar interconnection contribute to lung function?

• Contributes to elastic recoil during lung distension

• Helps resist collapse of individual alveoli (prevents atelectasis)