Lung Ventilation and Perfusion Flashcards
What is ventilation?
The movement of gases in and out of the lungs through airways.
How is ventilation measured?
Measured as change in volume per unit time
What is perfusion?
The blood flow through any organs through blood vessels
How is perfusion measured?
Measured as flow of blood per unit time
Why are the apical alveoli 4x larger than basal alveoli?
Due to effect of gravity
What is effect on ventilation of apical alveoli being 4x larger than basal alveoli?
More ventilation at bottom of lungs. Basal alveoli can expand more than apical alveoli so basal regions have better ventilation.
Describe blood circulation (in one complete pump)
Vena cava –> right atrium –> right ventricle –> pulmonary artery –> lungs (oxygenated) –> pulmonary vein –> left atrium –> left ventricle –> aorta –> rest of body
What is systemic circulation?
Moves blood between heart and rest of body
Carries oxygenated blood from the left ventricle, through the arteries, to the capillaries in the tissues of the body. From the tissue capillaries, the deoxygenated blood returns to the heart.
What is pulmonary circulation?
Moves blood between heart and lungs.
Carries deoxygenated blood from right ventricle to the lungs and returns oxygenated blood to the left atrium and ventricle.
What are the 2 subcategories of pulmonary circulation?
- Functional supply
2. Structural supply
What is functional supply?
Oxygenation of venous blood (main blood supply to lungs)
What is structural supply?
Nutrition to lung tissue (AKA bronchial circulation)
- Bronchial artery from thoracic aorta
- Bronchial vein to superior vena cava
- Only 2% of cardiac output
What is the definition of cardiac output?
The amount of blood pumped through the circulatory system in one minute.
Stroke volume x heart rate
What is stroke volume?
Volume of blood pumped out of left ventricle per beat
What is a typical full cardiac output?
3-5 L/min
What is the blood pressure on right side compared to left?
Pressure in left is much higher. In right, vessels provide less resistance
As pulmonary artery goes through hilum into the lung it starts to divide. What does it divide into?
Each branch keeps splitting into smaller branches. The vessels travelling through lung parenchyma are called extra-alveolar vessels
What are the pulmonary arterioles/venules
Extension of the pulmonary artery which exits the heart from the right ventricle and then bifurcates into right and left.
Extension of the pulmonary vein which exits the lungs and travels to the left atrium.
How does gas exchange occur?
Capillary network forms on surface of alveolus with large surface area.
What happens when air has been oxygenated in lungs?
Oxygenated blood flows in to pulmonary venules which unite to form 4 large veins emptying in to left atrium
Where does gas exchange start?
Beyond terminal bronchioles (before this is just ‘conducting’ portion)
Starts at smaller arterioles
How does emphysema lead to poor gas exchange?
Chronic inflammation leads to widespread destruction of alveolar and capillaries and dilation of distal airway.
Poor gas exchange –> hypoxia
What is hydrostatic pressure?
The force exerted by the weight of a fluid due to gravity (maximum pressure at bottom of glass of water)
How does hydrostatic pressure change in pulmonary circulation?
Different hydrostatic pressures in different areas of lungs in relation to position of right ventricle (above right ventricle will be less, below right ventricle will be more)
Where are extra-alveolar vessels found? Do they participate in gas exchange?
Running through lung parenchyma
No
What is the diameter of extra-alveolar vessels affected by?
Lung volume (breathe in –> diameter increases)
How does lung volume affect diameter of alveolar vessels?
Breathe in –> diameter decreases as vessels are squashed
What is Starling’s resistor?
Flow through collapsible segment is determined by the 2 largest pressure values (3rd not valid) and the difference between them
What does upstream pressure in Starling’s relate to in pulmonary circulation?
Arterial pressure (hydrostatic/blood pressure) at the arterial end of the capillary
What is the pressure in the box (outside the tube) in relation to pulmonary circulation?
Alveolar pressure –> gaseous pressure inside the alveoli
What is the downstream pressure in relation to pulmonary circulation?
Venous pressure –> hydrostatic/blood pressure at the venous end of the capillary
How many zones do lungs have?
3
Each zone has different pressure differentials which determines blood flow
What is Zone 1 called?
Alveolar dead space
Why does no gas exchange happen in Zone 1?
At top of lungs alveolar are almost fully open (squashes vessels). Pressure inside alveolus is higher than pressure of blood flowing through vessels.
P alveolar > P arterial > P venous (P out > P us > P ds)
What is ventilation/perfusion like in Zone 1?
Good ventilation but no perfusion
What is zone 1 like in healthy people?
Very small in healthy people
Where is apex found?
Higher than right ventricle (dead space)
What is zone 2 called? Where is it?
Recruitment zone. Lower down the lung than zone 1
Why is blood flow increased in zone 2?
Higher arterial pressure due to higher hydrostatic pressure. P arterial increases with respect to P alveolar.
P arterial > P alveolar > P venous (P us > P out > P ds)
What is blood flow in zone 2 determined by?
The difference in P arterial and P alveolar
What happens to blood pressure in a haemorrhage?
Huge drop in arterial pressure due to loss of blood. Area of lung is no longer participating in gas exchange so patients can get hypoxic.
What is positive pressure ventilation?
Mechanical ventilation as patient cannot generate negative intrathoracic
What is zone 3 called?
Distension zone
Where is zone 3?
Lung bases
Describe and explain the pressures in zone 3
Hydrostatic forces raise P arterial and P venous above P alveolar
P arterial > P venous > P alveolar (P us > P ds > P out)
Describe the blood flow in zone 3
Continuous blood flow which is determined by the difference between P arterial and P venous as they are the highest pressures
How is the ventilation-perfusion mismatch described?
V/Q ratio
What is the ideal ventilation-perfusion situation?
If the amount of ventilation to an area of lung is equal to the amount of perfusion, the ratio of Va/Q would be 1
How does the Va/Q ratio vary throughout the lungs?
Varies from 3.3 at apex to 0.6 in base of lungs
What is the normal average Va/Q ratio?
0.8
Describe the V/Q ratio in the alveolar dead space
High ventilation but almost no diffusion so ratio is high
What is the anatomical dead space?
Conducting airways where there is no gas exchange
What is the physiological dead space?
Part of each breath that doesn’t participate in gas exchange
Physiological dead space = Anatomic dead space + alveolar dead space
How can a pulmonary embolism affect dead space?
Blood clot from deep veins in leg/pelvis travels to pulmonary artery. Causes severe hypoxia causing lack of blood supply to that part of lung. This leads to ENLARGED ALVEOLAR DEAD SPACE
What is the pulmonary shunt?
Passage of deoxygenated blood that travels from right to left side of heart without participating in gas exchange. No/poor ventilation.
Describe the V/Q ratio in the shunt?
0 or very low
Low ventilation, good perfusion
What are examples of a shunt?
- Pneumothorax
- Age related changes
Where does the anatomical dead space lie?
Between the nose/mouth entrance and the respiratory bronchioles
