Pulmonary perfusion, ventilation and VQ mismatch Flashcards
ventilation definition
the process of moving air into and out of the lungs to facilitate gas exchange with the internal environment
what does ventilation depend upon?
diffusion
diffusion definition
the net movement of molecules from a region of higher concentration to a region of lower concentration until equilibration
what occurs in the respiratory zone?
carbon dioxide diffuses out of the venous blood and oxygen diffuses into the venous blood across the blood-air barrier
blood-air barrier definition
capillary endothelium, alveolar epithelium and their fused basement membranes and associated structures that form a very thin diffusion pathway
respiratory zone definition
region where gaseous exchange takes place
10% in bronchioles and alveolar ducts
90% in alveoli
diffusion capacity definition
the measurement of the transfer of gas from the air in the lung to the red blood cells in lung blood vessels
what normally impacts the diffusion capacity?
a misnomer, as most commonly diffusion is not the limiting factor but instead it is perfusion
Fick’s Law of diffusion
Ficks Law= DxAx∆P/d
D= diffusion constant
A= surface area
∆P= partial pressure difference
d= diffusion distance
What does Fick’s law measure?
the diffusion flux, of which the dimension is amount of substance per unit area per unit time
what is nick’s law proportional to?
the gas’s solubility in the liquid
how is the diffusion speed increased in the lungs?
increase in surface area of alveoli
thin blood-air barrier
good blood supply, maintaining concentration gradient
pathologies that affect diffusion + why
pulmonary fibrosis- thicker alveoli, longer diffusion distance
pulmonary oedema- fluid in lungs increases effective thickness of alveoli
emphysema- alveoli and lung tissue destroyed, smaller surface area available for exchange
explain perfusion limitation
the blood comes into equilibrium with the alveolar gas
doubling the diffusing capacity won’t alter the gas exchange
doubling the blood flow will double the gas exchanged
explain diffusion limitation
Partial pressure gradient essentially constant along the capillary
doubling blood flow will not change gas exchange
doubling diffusing capacity will double gas exchange
whether there is diffusion or perfusion capacity depends upon?
- diffusing capacity, the bigger this is the more likely it is to get equilibration
- speed of chemical reaction with blood
- the solubility in blood, including the gas reacting with the blood
- blood flow, faster flow less time to equilibrate
where does pulmonary circulation begin?
at the main pulmonary arteries which receive mixed venous blood from the right ventricle
where do the pulmonary arteries accompany?
accompany the airways as far as the terminal bronchioles
what happens to the blood vessels at the terminal bronchioles?
arterioles form a capillary network to supply the alveoli
adaptations of the pulmonary capillaries
very large surface area
short diffusion distance formed by the close apposition of the alveoli and the vessels
what is the mean pressure in the main pulmonary artery?
15mmHg
Systolic and diastolic pressure in the main pulmonary artery
25mmHg and 8mmHg
what type of circulation is pulmonary?
low pressure circulation
why is the pressure likely to be lower in pulmonary compared to systemic?
reduced height through which the blood is required to ascend, so not pumped as much
what is a consequence of the low pressure?
pulsatile
structure of pulmonary artery wall
very thin, containing relatively little smooth muscle, often mistaken for veins
two types of pulmonary vessels
alveolar vessels- capillaries and slightly larger vessels in the corners of the alveolar walls, which are exposed to alveolar pressure
extra-alveolar vessels- larger pulmonary vessels that run through the lung parenchyma
adaptations and structures of alveolar vessels
virtually surrounded by gas
thing, single cel endothelial layer with little support
caliber definition
the diameter of the vessels
what controls the caliber in alveolar vessels?
caliber is determined by the pressure difference between the alveoli and capillaries
if the alveolar pressure rises above the pressure in the capillaries they become compressed and collapsed, negative transmural pressure
what controls the caliber of extra alveolar vessels? + explained
lung volume
as the lung expands the vessels are pulled open by radial traction of the elastic parenchyma
results in low pressure surrounding the extra alveolar vessels, causing them to increase in diamater
experiment to show the distribution of blood flow
radioactive xenon can be used to demonstrate regional blood flow
xenon dissolved in saline is injected into the peripheral vein, when it reaches the pulmonary capillaries its evolved into alveolar gas due to its low solubility
the distribution of radioactvity, and thus perfusion, can be measured by counters over the chest during breath holding
how lung perfusion changes in an upright position
decreases linearly from the base to the apex
how lung perfusion changes in a supine position
apical zone blood flow increases but the basal zone remains unchanged, meaning the distribution becomes almost uniform
blood flow in posterior exceeds anterior regions
how lung perfusion changes during exercise
both upper and lower zone blood flow increases upon mild exercise and the regional diffrences become less
what is the major determinant in perfusion of the lung?
gravity
how is the lung divided?
into 4 zones
explain perfusion and ventilation in zone 1
region at the top of lung where pulmonary atrerial pressure falls below alveolar pressure, capillaries are squashed and there is no flow
perfect ventilation
what is zone 1 often referred to as? + WHY
alveolar dead space, the area is ventilated but not perfused, no exchange despite the alveoli working fine
when does alveolar dead space occur?
arterial pressure is decreased during a haemorrhage
alveolar pressure is increased due to positive pressure ventilation and deep inspiration
explain perfusion and ventilation in zone 2
pulmonary arterial pressure increases because of the hydrostatic effect and exceeds alveolar pressure
venous pressure is still much lower than alveolar, blood flow determined by arterial-alveolar pressure differences, not the venous pressure
what effect takes place in zone 2 explained
starling resistor/waterfall effect
capillaries collapse downstream when the alveolar pressure exceeds the pressure in the capillaries
explain perfusion and ventilation in zone 3
hydrostatic pressure increases both the arterial and venous pressure, so flow is determined by the arterial venous pressure difference
perfusion increases due to the distension of capillaries
explain perfusion and ventilation in zone 4
region of reduced blood flow at the base of the lungs due to extra alveolar vessels narrow from reduced radial traction
ventilation perfusion relationships definition
concerns the matching of ventilation and blood flow within various regions in the lung which is critical for adequate gaseous exchange
what does mismatching of V and Q cause?
responsible for most defective gas exchange in pulmonary diseases
what is alveolar PO2 determined by?
the continual replenishment by alveolar ventilation
the removal of O2 by pulmonary capillaries
explain replenishment
alveolar ventilation is not continuous, and fluctuates between each breath
tidal volume is small compared to volume of gas in the lung so the process is regarded as continuous
explain removal
rate of removal of o2 is governed by the O2 consumption of the tissues
in resting conditions consumption is rarely steady so alveolar PO2 is largely determined by alveolar ventilation
hypoventilation explained + causes
alveolar ventilation is abnormally low, alveolar PO2 levels fall, alveolar CO2 rises
fall in arterial PO2 and rise in PCO2
damage to the chest wall, paralysis of respiratory muscles, high resistance to breathing
diffusion limitation explained
alveolar PO2 and pulmonary artery PO2 is not the same, however ideally should be
very rarely happens, however can be due to a low O2 mixture being inhaled reducing the PO2 gradient or due to exercise with faster blood flow and less time to equilibrate
Shunt definition
refers to blood which enters the arterial system without passing through ventilated areas of the lung
two examples of lung shunt + explained
bronchial artery blood- supplies lung parenchyma and drains into the pulmonary veins after it has perfused the bronchi
coronary venous blood- drains directly into the left ventricle ia the cardiac veins
explain the shunt
there is perfectly fine ventilation, yet little perfusion due to a pulmonary arteriovenous fistula
how can a shunt be tested?
inspire 100 O2, which would raise arterial PO2, so ventilation is fine
if arterial PO2 does not rise then there is a problem with perfusion
what is a common cause of hypoxaemia?
VQ mismatch
what is used to measure VQ mismatch?
VQ ratio
what happens if V/Q is reduced by obstructing ventilation?
PO2 in the lung will decrease
PCO2 will rise
what is V/Q=0?
No ventilation, arterial blood will have the same pO2 as venous blood
V/Q is increased by obstructing blood flow explained
PO2 will increase
alveolar PO2 and CO2 becomes that of inspired gas
where are both ventilation and perfusion greater in the lung?
at the bottom than the apex
rate of increase
ventilation increases slowly
blood flow increases rapidly
why does perfusion vary to a greater extent?
density of blood id greater than the density of air so gravity has a greater effect
explain V/Q ratios near apex
abnormally high, as blood flow is minimal
explain V/Q at base
perfusion is greatest so V/Q is smaller
ventilation top and borrom
bottom of lung is under ventilated relative to increased blood flow
top of lung is over ventilated relative to the blood flow
explain V/Q=1
well ventilated and well perfused alveoli, blood equilibrates with alveolar air
explain V/Q <1
Poorly ventilated, well perfused
alveolar PO2 and PCO2 tend towards the levels found in mixed venous blood
found in the lower lung regions
explain V/Q >1
well ventilated, poorly perfused
alveolar PO2 and PCO2 tend towards levels found in inspired gas
found in higher lung regions
what V/Q does deadspace have?
infinity
importance of the shape of the oxyhaemoglobin dissociation curve
the sigmoidal shape means that increasing alveolar PO2 cannot further increase Hb saturation, so increasing oxygen when V/Q is very small is unlikely to help
explain hypoxic pulmonary vasoconstriction
physiological phenomenon where small pulmonary arteries constrict in the presence of alveolar hypoxia
what does hypoxic pulmonary vasoconstriction allow + explained
better V/Q matching
blood floe becomes redirected from badly ventilated regions to well ventilated regions so more oxygen can be moved into the vlood
molecular mechanism of hypoxic pulmonary vasocontriction
hypoxia sensitive voltage gated potassium channels in pulmonary artery smooth muscle cells leads to depolarisation
activates voltage gated calcium channels, increases intracellular calcium and activates smooth muscle contractile machinery
explain acute pulmonary hypertension
condition where there is an increased blood pressure within the arteries of the lungs
occurs where perfusion is low to increase V/Q to 1
due to thromobosis, emboli clots
