Airflow And Gas Exchange

Question 1

What is the normal respiratory rate for animals and then horses

Answer 1

Animals = 20-30brpm
Horses = 10-12brpm

Question 2

What do these terms mean
Eupnoea
Tachypnoea
Hyperpnoea
Dyspnoea
Apnoea

Answer 2

Eupnoea = normal breathing
Tachypnoea = increased RR
Hyperpnoea = increased respiratory depth
Dyspnoea = increased respiratory effort
Apnoea = absence of breathing

Question 3

What is inspiration and expiration

Answer 3

Inspiration = diaphragm and external intercostals contract
Expiration = usually passive. Can be active in some species and during exercise in this case internal intercostals and abdominal muscles contract

Question 4

What is compliance and what does it depend on

Answer 4

Degree to which a decrease in transpulmonary pressure leads to an increase in volume of the lung
C = change in volume/change in pressure
Depends on elasticity (of lungs and thoracic cage) and surface tension (in the alveoli)

Question 5

Why are alveoli lined with fluid

Answer 5

To facilitate dissolution and diffusion of gases

Question 6

How is surface tension created and and does it do

Answer 6

Water molecules form hydrogen bonds at water-air interface
Surface tension resists expansion of lungs
Higher surface tension = lower lung compliance

Question 7

What does surfactant do

Answer 7

Surfactant acts to reduce surface tension of the fluid film lining in the alveoli
O2 and CO2 do not dissolve in the surfactant they dissolve in water

Question 8

What does pressure inside the alveolus depend on

Answer 8

It’s radius and surface tension inside it
P=2T/R
The total amount of surfactant inside an alveolus is the same regardless of size.

Question 9

What does airway resistance depend on and what is the Pouseille’s equation

Answer 9

Resistance is dependent on the radius and length of a tube and the viscosity (n) of the substance travelling through it
Pouseille’s equation = 8Ln/Pie x r to the power of 4

Question 10

When and where is resistance present

Answer 10

Lower airways are always distended in inspiration resistance therefore greater during expiration
Upper airways resistance higher during inspiration
Most of resistance from upper airways

Question 11

What is smooth muscle in the walls of the airways innervated by

Answer 11

Autonomic nervous system.
Sympathetic B2 adrenoreceptors (relaxation of smooth muscle = dilation of airways)
Parasympathetic (contraction of smooth muscle = constriction of airways)
Disease states e.g. asthma

Question 12

When does turbulence increase

Answer 12

Increasing speed of flow

Question 13

What is tidal volume and minute ventilation

Answer 13

Tidal volume = the volume of air moved during a respiratory cycle 10ml/kg in a normal resting dog
Minute ventilation = Tidal volume x Respiratory rate
Must increase when oxygen requirement increases

Question 14

What is the fraction of gas in normal animals

Answer 14

FO2 in normal air is 21% (0.21)

Question 15

What is the normal partial pressure of air

Answer 15

PO2 in normal air is 0.21 x atmospheric pressure
Atmospheric pressure at sea level = 760mmHg
0.21x760mmHg = 160mmHg

Question 16

How does gas exert pressure

Answer 16

Gas molecules move around and collide with surfaces creating pressure.
Size of molecule is immaterial
Gas exchange along their own gradient

Question 17

What are the solubilities of O2 and CO2

Answer 17

CO2 is more soluble than O2
In contact with water gas molecules dissolve. More dissolve if the partial pressure of the gas increases.
Molecules can also come out of solution and re-enter the gas phase.
When the number of molecules of given gas entering and leaving solution during a given unit of time is equal this is a dynamic equilibrium.
At this point the partial pressure of the gas molecules in solution can be said to equal that of the molecule in gas phase

Question 18

What happens to air when it’s inhaled

Answer 18

Inhaled air is warmed and humidified as it moves through the upper airways. Water vapour is added
We must account for this in terms of the PO2 in the air by the time it reaches the distal airway.
PH2O at the alveolus is about 50mmHg
PO2 = (atmospheric pressure - PH2O) x 0.21
= (760-50) x 0.21 = 149mmHg

Question 19

What is the exchange of gases at the alveolus like

Answer 19

It is constant and ongoing
Only a small proportion of alveolar gas is exchanged with each breath.
PAO2 will always be lower than PO2 in the airways
PACO2 will always be higher than PCO2 in the airways

Question 20

What is the correct exchange of gases reliant on

Answer 20

The correct ventilation and perfusion of the alveolus

Question 21

What are alveoli with low ventilation to perfusion ratio

Answer 21

Overperfused and under ventilated

Question 22

What are alveoli with a high ventilation and perfusion ratio

Answer 22

Under perfused and over ventilated

Question 23

Can the ventilation and perfusion ratio alter

Answer 23

There is a ratio range in the normal lung
The caudal dorsal lung lobes are preferentially perfused in the dog for example changes to systemic arterial gas tensions will only occur if a significant proportion of alveoli have abnormal ratio

Question 24

What is hypocapnia

Answer 24

Hyperventilation will cause excess loss of CO2 leading to a drop in PaCO2

Question 25

What will cause reduced PAO2 and increase PACO2

Answer 25

Hypoventilation. In turn leading to reduced PaO2 (hypoxia) and increased PaCO2 (hypercapnia)
General anaesthesia

Question 26

What is dead space

Answer 26

Areas which are ventilated but don’t participate in gas exchange
Airways = anatomical dead space
Unperfused alveoli = functional dead space
During inspiration dead space gas fills alveoli before fresh air gets there shallow breathing (panting exacerbates this)
Equipment dead space = anaesthesia (circuits, ET tubes)