Physio. Ch.16 Pulmonary Ventilation

Respiration

• The process of gas exchange in the body

• Subdivided

- Internal respiration

- External respiration

Internal respiration

• Also known as cellular respiration

• Use of O2 in ATP production

- Produces CO2 as byproduct

External respiration

• Exchange of oxygen and carbon dioxide between the atmosphere and body tissues

• Requires respiratory & circulatory systems

• Multi-step process

- Step 1: Pulmonary ventilation

- Step 2: Exchanging of O2 and CO2 between air spaces of the lung and blood diffusion

- Step 3: Transport of O2 and CO2 through pulmonary circulation by the blood

- Step 4: Exchange of O2 and CO2 between blood and tissues

Pulmonary ventilation

• Movement of air into the lungs and out of the lungs by bulk flow

Conducting zone

• Division of the respiratory system

• Upper part of the respiratory tract that conducts air to lungs

- Humidifies and brings air to body temperature

• Referred to as “dead space”

- Named this because no conduction is done here

Respiratory zone

• Division of the respiratory system

• Site of gas exchange within the lungs

• Made of simple squamous epithelium cells

• Structures

- Type 1 alveolar cells

- Type 2 alveolar cells

- Alveolar macrophages

Pleural sac

• A membrane surrounding each lung

• Subdivided

- Parietal pleura

- Visceral pleura

Parietal pleura

• Lines the thoracic cavity

• Attaches to the chest wall

Visceral pleura

• Outer surface of the lungs

Intrapleural space

• Thin space between the visceral and parietal pleura

• Filled with intrapleural fluid

Bulk flow

• The mass movement of air into and out of cells

• Powered by pressure gradients between alveoli and outside air

- The pressure gradient is produced by muscular pumps

• Rate of bulk flow: How fast air flows in to out of the lungs

- Calculate: Flow = (Patm - Palv) / R

Airway resistance

• Mechanism used to control pressure gradients

• Controlled by diameter of respiratory airways

- Inversely related

Atmospheric pressure (Patm)

• Pressure of the outside air

• 760 mm Hg (1 atm) at sea level

- Decreased as altitude increases

• All other lung pressures are expressed relative to atmospheric pressure

Intra-alveolar/intrapulmonary pressure (Palv)

• Pressure within the alveoli relative to the atmosphere

• During inspiration value is -3 (less than atmospheric so air will flow to it)

• During rest value is 0 (same as the atmospheric so air won’t move either way)

• During exhalation value is 3 (more than atmospheric so air will away from it)

• The difference between intra-alveolar pressure and atmospheric pressure (Palv and Patm) is the pressure gradient that drives ventilation

Intrapleural pressure (Pip)

• Pressure of the fluid in the pleural sac

• Caused by the opposite directions of pull from the chest wall and lung wall

• Always negative

- If not the lung will collapse

• At rest is -4mmHg

• During inspiration is -6mmHg

Transpulmonary pressure

• Pressure difference between the intra- alveolar pressure and intrapleural pressure

- Calculation: Palv – Pip

• Increase in transpulmonary pressure → increases distending pressure across lungs → Causes lungs (alveoli) to expand

• At rest 4mmHg

Functional residual capacity (FRC)

• Volume of air in the lungs between breaths

Pneumothorax

• Air enters the pleural space, raising intrapleural pressure

• Without negative intrapleural pressure, lung collapses due to its elastic recoil

• Subdivided

- Spontaneous

- Tension

• Typically only happens in one lung at a time

- Promotes survivability because the body still has one lung to function

Spontaneous pneumothorax

• Type of pneumothorax

• Air leaks due to damage from inside

- Broken rib

- lung disorders: COPD, cystic fibrosis, or rupture of a lung blister)

Tension

• Type of pneumothorax

• Caused by trauma/open chest wound

Boyle’s Law

• Inverse relationship between pressure and volume

• Inspiration: Increased volume, decreased intra-alveolar pressure → Air flows in

• Expiration: Decreased volume, increased intra-alveolar pressure → Air flows out

Mechanics of breathing

• The muscles of respiration change the volume of the lungs → create pressure gradients → drives air flow into and out of lungs

Respiratory Muscles

• Inspiratory muscles: diaphragm + external intercostals

- Often active

• Expiratory muscles: internal intercostals + abdominal muscles

- Often passive

Alveoli

• Changes in alveoli volume are produced by changes in the volume of the thoracic cavity

Process of Inspiration

• Stage 1: Neural stimulation of inspiratory muscles causing Diaphragm contraction (flattens & moves downward) and External intercostals contraction (ribs pulled up & out)

• Stage 2: Thoracic cavity volume increases & parietal pleural pulls on visceral pleura

• Stage 3: Intrapleural pressure decreases, which increases transpulmonary pressure

• Stage 4: Greater distending force across the lungs causes alveoli to expand with the chest wall → Decreases the intra-alveolar pressure to below atmospheric pressure

• Stage 5: Air flows into alveoli due to pressure gradient

Quiet breathing

• Passive process with no muscle contraction

• Step 1: Relaxation of inspiratory muscles → recoil of chest

  wall and lungs to resting positions

• Step 2: Visceral pleura pulls on parietal pleura

• Step 3: Volume of the thoracic cavity decreases

• Step 4: Alveolar pressure rises above atmospheric pressure

• Step 5: Air flow out due to pressure gradient

Active expiration

• Contraction of expiratory muscles causes a greater pressure gradient

Lung Compliance

• Change in lung volume that results from a given change in transpulmonary pressure

- Very high compliance

• Dependent on

- Elasticity of the lungs

- Surface tension of the fluid lining the alveoli

Elasticity

• Factor that affects lung compliance

Surface Tension

• Factor that affects lung compliance

• Resistance to distension created by the thin film of water that lines the alveoli

• Greater tension means less compliance

• Pulmonary surfactant: Decreases alveolar surface tension by interfering with the hydrogen bonding between water molecules

Pulmonary surfactant

• Decreases alveolar surface tension by interfering with the hydrogen bonding between water molecules

Airway Resistance

• Resistance of the entire airway system in the respiratory tract

• Determined by differences in the diameter of different individual airways

• Larger resistance requires larger pressure gradient overcome

- Low resistance under normal conditions

• Effected by

- Elasticity, mucus secretion, and smooth muscle activity in the bronchioles

Bronchodilation

• Dilation of the bronchioles

• Caused by sympathetic stimulation

- Epinephrine is released in response to high CO2 levels

• Epinephrine binds to B2

• Norepinephrine binds to B2

Bronchoconstriction

• Constriction of the bronchial

• Triggered by parasympathetic stimulation and histamine releases

- Parasympathetic: ACh binds to M3

- Intrinsic: histamine released

Respiratory Distress Syndrome (RDS)

• Pathophysiology when surfactant production is low

• Causes septic shock in adults

• Causes alveolar collapse in babies

Surfactant

• Hydrophobic protein and phospholipids that reduces surface tension

• Prevents collapse and allows a residual volume of air to remain in lungs

• Production begins in week 24 - 28 of pregnancy

Asthma

• Pathophysiology

• Increased airway resistance, spastic contractions of bronchiole smooth muscle and increased mucus secretion and inflammation of bronchiole walls

Chronic obstructive pulmonary disease (COPD)

• Chronic increases in airway resistance

• Subdivided

- Emphysema

- Chronic bronchitis

Emphysema

• Type of COPD

• Destruction of airway walls & elastic CT

Chronic bronchitis

• Type of COPD

• Inflammation and thickening of airway lining, high mucus production, destruction of normal tissue & fibrosis