4.8 Mechanisms of Breathing and Ventilation/Perfusion Matching Flashcards
What is vital capacity
Vital capacity is the maximum amount of air you can exhale after a maximum inhalation
What is part of the upper respiratory system and what is part of the lower respiratory system
upper respiratory system - nasal cavity, pharynx, larynx
lower respiratory system - trachea, bronchi, bronchioles, alveoli
What is the functions of the upper respiratory tract
The upper respiratory tract functions to filter, humidify and warm the air
Describe the pathway the air takes through the lower respiratory tract.
The air travels through 28 sets of branches from the original trachea. The trachea and primary bronchi are held open by rings of cartilage. Then come the lower order airways where the bronchioles have a small diameter, they are made of smooth muscle wall but no cartilage. The job of these bronchioles is to move the gas towards the alveoli. Although they are small, there are many of them so collectively they have little resistance to move the air down towards the alveoli, the smooth muscle tone determines which areas the breath is directed towards. Gas exchange then occurs from the terminal bronchioles onwards.
What condition should the gas be in by the time it reaches the alveoli
-the gas needs to be at 37 degrees
-it needs to be saturated with water vapour so that it does not dry out the membranes
-foreign gasses and larger particles also need to be filtered out
Describe some properties of alveoli
-alveoli are very small, but there are many of them and collectively they have a large surface area (80-120m^2)
-they have very thin membranes (0.5um)
-alveolar macrophages act to remove any particles that have made it to the alveoli membrane
What are the muscles involved in moving air into and out of the body (ventilation)
diaphragm, external intercostal and internal intercostals
What does the diffusion of gas molecules across the membrane depend upon
molecular weight, concentrations and solubility
What does perfusion mean
Fluid movement
What is the long held physiological question that a ventilation/perfusion mismatch has answered
Why is there a difference between alveolar and arterial gas pressures. e.g Why is arterial pO2 lower than alveolar pO2
What is a shunt
A shunt describes when there is blood flow past the alveoli with no gas exchange, the blood will leave the lung with a low Po2 and a high Pco2. The alveoli is underventilated and overperfused
What is a dead space
A dead space is when there is gas exchange but no blood flow, causing the alveolar air to equilibrate with the atmosphere. The alveoli are overventilated and underperfused
What are the 2 types of dead space and shunt
There are physiological dead spaces and shunts and anatomical dead spaces and shunts
physiological dead space is greater than anatomical dead space
physiological shunt flow is greater than anatomical shunt flow
Describe the ventilation/perfusion matching continuem.
Every alveolus lies somewhere along the line between complete shunt and complete dead space, either being over-ventilated and under-perfused or under-ventilated and over-perfused. This is the case even in healthy lungs. Where an alveolus lies along this spectrum is due to factors e.g a minor blood clot (moves towards dead space) or a bit of mucus in airways (moves towards shunt).
How is ventilation/perfusion matching measured.
Using radioisotope gases to measure the air/blood distribution
When ventilation/perfusion matching is plotted, explain what it shows.
Generally, top of the lung has a little too much gas and not enough blood and bottom of the lung vice versa. The blood line is steeper and air line is shallower. A perfect v/q ratio (of 1) is found around the 3rd rib. V/Q > 1 over-ventilated and under-perfused (at the top of lung), V/Q < 1 under-ventilated and over-perfused (at the base of lung).
Explain why the top of the lung is over ventilated and under perfused and the bottom of the lung is under ventilated and over perfused.
for ventilation:
-the base of the lung inflates more than the apex because it is more compliant
-at start of inspiration, the basal regions are compressed due to weight of lung
-so base is easier to expand and hence more of the inspired gas is directed more to the base
for bloodflow:
-pulmonary blood pressures are low (typically 25/10mmHg) much lower than systemic pressures
-this is too low to ensure good perfusion of the top of lungs
What causes pulmonary oedema
If total pressure>plasma oncotic pressure then fluid will leave capillaries and enter tissue spaces/alveoli. This causes pulmonary oedema and is most likely to happen in the basal regions
How can a V/Q mismatch be reduced
Through exercise as pulmonary blood flow becomes more uniform but pulmonary blood pressure can not rise due to exercise.
