Ventilation

Question 1

Explain how airflow works within the respiratory system

Answer 1

There are 2 parts to lungs:
1. The conducting zone which extends from the trachea to bronchi and then the bronchioles
a. gaseous exchange doesn’t take part in the conducting zone

2. The Respiratory zone which extends from respiratory bronchioles and goes down to alveolar ducts
   and at the end are alveolar sacks.
   a. Gaseous exchanges takes part in the respiratory zone.

Air flows from areas of high pressure to areas of low and flow rate (or F in the equation above) depends on the difference between the areas of pressure (ΔP) and the resistance to the flow (R). So in order for air to flow there needs to be a pressure difference.

Question 2

What is the calculation for air flow?

Answer 2

F = ΔP/R

Question 3

Explain the principles behind inspiration and expiration in relation to air flow

Answer 3

This pressure difference is governed by Boyles law. P1V1= P2V2, which means if there is an increase in volume, there’s a proportional decrease in pressure.

So as the thoracic cage expands due to the inspiratory muscles, the lung volume expands too and there is a decrease in pressure and air is drawn in from the outside.

Expiration is the opposite, when the muscles contract, the pressure inside the alveoli increases and is higher than the pressure in the atmosphere, so air is forced out until the pressure inside the alveoli is equal to atmospheric pressure.

Question 4

List the inspiratory muscles

Answer 4

Diaphragm

Intercostal muscle (occupy intercostal space between ribs

               External intercostal muscles (run caudoventrally)
              
               Internal intercostal muscles (run cranioventrally)

Question 5

Describe the structure and function of the Diaphragm

Answer 5

Diaphragm is a major muscle in the thorax

Outer part of diaphragm is skeletal muscle, so it’s supplied by a motor nerve – phrenic nerve, 2 phrenic nerves running down each side.

Runs down from cervical region of spine down into thorax to diaphragm. If the neck is broken in the cervical region the diaphragm will cease to function.

Question 6

Describe the function of the inspiratory muscles

Answer 6

Inspiration
During inspiration, the diaphragm contracts, the dome flattens and moves caudally.

          The external intercostal muscles contract during inspiration, pulling the ribs cranially and 
          outwards.

Expiration
Expiration particularly at rest, is a mainly passive process.

          When the diaphragm and the external intercostal muscles relax back, expiration is driven by 
          elastic recoil of the lungs and thoracic cage.

          This is not true during exercise, during exercise both inspiration and expiration become active 
          processes.

Question 7

Outline the structure and function of the pleural cavity

Answer 7

The pleural cavity is the space between the parietal pleura and the visceral pleura and it’s the pleura that allows the volume of the lungs to change with the volume of the thoracic cavity. The pleura are thin, moist membranes

Question 8

Explain how the pressures within the thorax assist ventilation

Answer 8

Surface tension and the pleura help to “stick” the lungs to the wall so the lungs can change volume with the volume of the thoracic cavity.

The different pressures found with the thorax are what allows the lungs to expand.

In physiological conditions (i.e. in a live animal) The intrapleural pressure in always negative, the alveolar pressure changes during inspiration and expiration and the transpulmonary pressure is the difference between alveolar pressure and the intrapleural pressure.

Question 9

Outline how volume, pressure and flow changes when during expiration

Answer 9

o Relaxation of inspiratory muscles, ribs recoil back to original position
o Thoracic cavity contracts
o Pressure within intrapleural cavity becomes less negative
o Transpulmonary pressure decreases
o Expiratory lung muscles contract
o Alveolar pressure increases above atmospheric pressure
o Air flows out until alveolar pressure becomes equal to atmospheric pressure

Question 10

Explain the principles behind inspiration and expiration in relation to air flow

Answer 10

This pressure difference is governed by Boyles law. P1V1= P2V2, which means if there is an increase in volume, there’s a proportional decrease in pressure.

So as the thoracic cage expands due to the inspiratory muscles, the lung volume expands too and there is a decrease in pressure and air is drawn in from the outside.

Expiration is the opposite, when the muscles contract, the pressure inside the alveoli increases and is higher than the pressure in the atmosphere, so air is forced out until the pressure inside the alveoli is equal to atmospheric pressure.

Question 11

Outline how volume, pressure and flow changes when an animal inhales

Answer 11

o the inspiratory muscles contract,
o the thoracic cavity expands,
o the intrapleural pressure becomes more negative,
o increase in transpulmonary pressure, which creates a suction that pulls the lungs outwards,
o the lungs distend,
o alveolar pressure falls below that of atmospheric pressure
o air can flow into the lungs until the brief pause between inspiration and expiration that means
alveolar pressure is equal to atmospheric pressure.

Question 12

Describe how locomotion is involved in ventilation

Answer 12

Movement of limbs is linked to expiration and inspiration. I.e. the movements can aid expiration and inspiration

The weight of the abdominal cavity can help to force the air out and if you think of a bigger animal such as a horse, there’s quite an additional force that would come from the abdominal cavity that can assist in expiration.

Question 13

List the factors which influence ventilation

Answer 13

1. Resistance to flow in the airways
2. Lung compliance
3. Alveolar surface tension

Question 14

Explain how the calibre of the airways results in resistance to flow

Answer 14

This refers to the size of the airways

In fact resistance to flow is inversely proportional to the 4th power of the radius
An inversely proportional relationship; As the radius decreases, the resistance increases.

Quite a lot of resistance comes from the nasal cavities, hence many species breath through their mouths during exercise.

Question 15

What mechanisms are employed to overcome resistance in the airways

Answer 15

As the tube gets smaller the resistance increases, but in fact at each bifurcation there is an increase in the number of parallel branches which more than compensates for the increase in resistance meaning that total resistance actually declines.

Another mechanism of reducing resistance comes from the elastic fibres within the lung. As the lung volume increases, the elastic fibres within the lung pull more strongly on tube walls to increase the diameter and again reduce the resistance.

Finally, the smooth muscle is another mechanism for altering the resistance to flow. Smooth muscle wraps around the respiratory bronchioles and is innervated by the autonomic nervous system. Relaxation of the smooth muscles, can increase the tube diameter and again reduce resistance.

Question 16

Explain how the turbulence within the airways results in resistance to flow

Answer 16

Air flow in the lungs is either turbulent or laminar.

In the trachea and larger bronchi the turbulent airflow is characterised by the formation of vortices, you see a rise in turbulent air flow as flow rate increases.

In comparison Laminar air flow is parallel movement of air, indicated at the bottom of the diagram, there is minimal friction between air molecules, which means the resistance is least in laminar air flow and you see laminar airflow in the smallest bronchi and bronchioles which decreases resistance to flow.

Question 17

Explain how lung compliance affects influence ventilation

Answer 17

Circumstances when an animal is in good health, the lungs are very distensible or elastic and this is due to a number of different properties.
Firstly, coiled collagen fibres, which straighten upon lung expansion, they provide the maximum
limit for distension.

       The lung’s have their own elastic fibres principally made up of elastin, a rubbery fibrous 
       connective tissue protein which can stretch during inspiration.

      Lastly the thoracic cage itself is elastic the intercostal muscles become stretched during 
      inspiration and recoil during expiration, the joints of the intercostal muscles and pleura also 
      provide elastic forces which can work together to reduce lung volume during expiration.

Question 18

Outline pathology of lung compliance

Answer 18

Reduced lung compliance can occur, due to scar tissue in the lungs or pleura or a disease which affects the mobility of thoracic cage joints. The ventilation rate is generally increased in this situations as less air is inhaled during inspiration.

Increased lung compliance can also occur pathologically due to excessive coughing in chronic bronchitis, this can reduce the elasticity of the lungs and affect expiration causing the animal to engage expiratory muscles even at rest.

Question 19

Explain how alveolar surface tension influences ventilation

Answer 19

When the air in the lumen of the alveoli meets the inner surface of the alveoli filled with fluid, surface tension arises which resists the expansion of the alveoli.

To reduce the surface tension, alveoli secrete surfactant (a phospholipid) from type 2 alveolar cells. which disrupts the surface tension and allows the alveoli to still expand.

they do this by disrupting the formation of hydrogen bonds in the water molecules and effectively inhibiting surface tension.

Question 20

Outline how the body counters pressure increases in smaller alveoli to stop air from flowing to a larger alveoli of lower pressure

Answer 20

If air was to flow from the small alveoli to the large alveoli, it would cause the large alveolus to increase in size and the small alveoli to empty and eventually collapse.

The lungs can counteract this by having different concentrations of surfactant in different sized alveoli. So smaller alveoli will have a greater concentration of surfactant, which reduces the surface tension meaning the pressure is the same in both large and small alveoli allowing air flow to go back and forth between small and large alveoli