Ventilation Flashcards
Explain how airflow works within the respiratory system
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
- 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.
What is the calculation for air flow?
F = ΔP/R
Explain the principles behind inspiration and expiration in relation to air flow
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.
List the inspiratory muscles
Diaphragm
Intercostal muscle (occupy intercostal space between ribs
External intercostal muscles (run caudoventrally) Internal intercostal muscles (run cranioventrally)
Describe the structure and function of the Diaphragm
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.
Describe the function of the inspiratory muscles
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.
Outline the structure and function of the pleural cavity
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
Explain how the pressures within the thorax assist ventilation
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.
Outline how volume, pressure and flow changes when during expiration
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
Explain the principles behind inspiration and expiration in relation to air flow
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.
Outline how volume, pressure and flow changes when an animal inhales
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.
Describe how locomotion is involved in ventilation
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.
List the factors which influence ventilation
- Resistance to flow in the airways
- Lung compliance
- Alveolar surface tension
Explain how the calibre of the airways results in resistance to flow
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.
What mechanisms are employed to overcome resistance in the airways
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.