Lecture 2 -- Airflow and Gas Exchange

What is the normal resting respiratory rate for most domestic species?

20-30 brpm

What are the normal breathing rate of horses?

10-12 brbpm

What do we call normal, resting breathing?

Eupnoea.

What is tachypnoea?

Increased respiratory rate.

Define hyperpnoea.

Increased respiratory depth.

What is dyspnoea?

Laboured breathing with increased effort.

What pressure must be generated in the alveoli for inhalation to occur?

Alveolar pressure less than atmospheric pressure.

What pressure must be generated in the alveoli for exhalation to occur?

Alveolar pressure must be greater than atmospheric pressure.

What do we call the absence of breathing?

Apnoea.

Describe the process of inhalation

Step 1: External intercostal muscle contract
Step 2: Diaphragm contracts and moves downwards
Step 3: Ribs goes outward and cranially
Step 3: Increase thoracic cavity volume
Step 4: Alveolar pressure < atmospheric pressure
Step 5: Air goes in

Describe the process of inactive exhalation

Elastic properties of lung causes them to recoil → Decrease the size of thoracic cavity → Alveolar pressure > atmospheric pressure → Air goes out

Describe the process of active exhalation

E.g. exercise
Step 1: Internal intercostal muscle contracts → Ribs move inward and caudally
Step 2: Abdominal muscle contracts → Push the abdominal content upwards + Diaphragm move upwards and back to dome shape
Step 3: Decrease the volume of thoracic cavity
Step 4: Alveolar pressure > atmospheric pressure
Step 5: Air is expelled from the lungs.

What are the factors that affect compliance of the lungs?

1. Surface tension

2. Elasticity of lung tissue (Disease may lead to deterioration of lung tissue)

What are the surface tension of alveoli?

Alveoli are lined with fluid to facilitate dissolution and diffusion of gases → Water molecules form hydrogen bond = Surface tension
Increase surface tension → Decrease lung compliance

What are surfactant?

Phospholipid molecule (Hydrophobic head; hydrophilic tail)
→ Hydrophobic head sits on the water molecule
→ Hydrophilic tail help break down the surface tension in alveoli

What are the factors that affect the pressure inside the alveoli?

1. Radius → Bigger radius → Decrease pressure

2. Surface tension → Increase surface tension → Increase pressure

What is the role of surfactant in the alveoli according to their distribution?

→ Smaller alveoli have a smaller radius, which means they have a greater pressure; larger alveoli have a larger radius, which means they have a smaller pressure → Pressure gradient created → Gas move from smaller alveoli to larger alveoli → Collapse
→ To prevent the collapse of alveoli, smaller alveoli tends to have more surfactant to minimise the surface tension and, consequently, the pressure, allowing for equalization of pressure and preventing collapse.

What factors that affect the resistance of airway?

1. Length: Longer length → Larger resistance

2. Radius: Larger radius → Smaller resistance
   Bronchovascular bundle: Bronchus, arteries and veins are wrapped by connective tissue and connected way up to the visceral pleura → When the lungs stretch, the lower airways is also distended → Open up the airway = Larger radius → Smaller resistance

3. Turbulence: Speed of flow in upper airway (Trachea + larger bronchi) is highest → Higher turbulence → Higher resistance; The airflow of lower airway (smaller bronchi + bronchioles) is laminar → Lower turbulence → Lower resistance

What is tidal volume?

The volume of air moved during a respiratory cycle.

What is functional residual capacity?

Expiratory reserve volume + residual volume
= Total amount air left in the lungs after a normal expiration at rest

What is expiratory reserve volume and inspiratory reserve volume?

Expiratory reserve volume = Using internal intercostal muscle and abdominal muscle to fully exhale the air out of the lungs
Inspiratory reserve volume = Using additional muscle e,g, neck muscle to full inhale the air inside the lungs

What is vital capacity?

Expiratory reserve volume + tidal volume + inspiratory reserve volume, representing the maximum amount of air a person can exhale after a deep inhale.

What is residual volume?

The volume of air that remains after full expiration

What is the atmospheric pressure at sea level?

760 mmHg

What is the pressure of oxygen by the time it reaches the distal airways?

149 mmHg
Because the inhaled air is warmed and humidified as the air passes through the upper airway, water vapour is added
→ (Atmospheric pressure - pH2O) x 0.21 = 149mmHg

What does high V:Q ratio means?

Can be caused by over-ventilated/ under-perfused
E.g. dead space

What does low V:Q ratio means?

Can be caused by under-ventilated/ over-perfused
→ pCO2 and PO2 stay the same (before and after the air reaches the air sac)

What happens to PAO2 and PACO2 during hypoventilation?

PAO2 decreases → PaO2 = Hypoxia and PACO2 increases → PaCO2 = Hypercapnia

What happens to PAO2 and PACO2 during hyperventilation?

PACO2 decrease → PaCO2 decrease = Hypocapnia

What is dead space?

Alveoli that are ventilated but not perfused.

What is the minute ventilation?

Tidal volume x RR

What is the consequence of insufficient surfactant in premature neonates?

Lack of surfactant = Increase compliance of the lungs + Increase resistance of the airway → Respiratory distress syndrome with severe dyspnoea.

What can cause atelectasis (collapse of alveoli) in adults?

Inadequate surfactant release during prolonged general anaesthesia

Does CO2 have a lower solubility to O2?

No. CO2 is more soluble than O2

Solubility vs Partial pressure

If the partial pressure of that gas is higher, there will be more gases dissolved

Relationship between PAO2 and PO2; PACO2 and PCO2

PAO2 is always lower than PO2 → Alway taken up by the blood
PACO2 is always higher than PCO2

Which part of the lung lobes are preferentially perfused in dogs?

Caudodorsal lung lobe

Give an example of alveolar dead space

Airway (Ventilation protion e.g. trachea; bronchi; bronchioles)

Give an example of functional dead space

Unperfused alveoli

Give an example of equipmental dead space

ET tube/ Circuits via having anaesthesia

What shallowing breathing e.g. panting may exacerbates the dead space?

Shallow breathing, such as panting, can increase dead space
-> Because less volume of air is inspired to displace the dead space gas -> Gases only moves in and out of the dead space without fresh gas getting in

Why alveolar PO2 is lower than in room air?

• Ongoing gas exchange

• Water vapour is added