Lecture 3: Pulmonary And Bronchial Circulations Flashcards
Blood flow varies throughout the lung as a result of (3 things):
- Low vascular pressures
- Gravity
- Distensible pulmonary vessels
Right ventricle ejection fraction of blood is distributed into ___ and equals ___
Lungs and equals LV cardiac output
Why is pulmonary blood pressure lower than systemic blood pressure?
Because the resistance to flow in the pulmonary system is one tenth of systemic circulation
Resistance to flow in the pulmonary system is ___ of systemic circulation
1/10th
Describe the wall of the pulmonary artery
- Thin walled (1/3 thickness of aorta)
- Very compliant
Pulmonary vessels are divided into ___ and ___
Alveolar and extra-alveolar vessels
Alveolar vessels are closely related to ___
Acini
What is acini?
Alveolocapillary network involved in gas exchange
Alveolar vessels are directly affected by…
Alveolar pressure
What collapses alveolar vessels during lung expansion?
High positive pressure
Alveolar capillaries can be ___ so that they contain ___
Compressed, no blood
What are the extra-alveolar vessels?
Arteries and veins which convey blood to-and-from the respiratory units
How does the structure of extra-alveolar vessels differ from alveolar vessels?
Larger vessels with thicker walls and substructure connective tissue
Are extra-alveolar vessels directly affected by pressures in the lung?
No
Do extra-alveolar vessels become compressed during positive pressure?
No
What pulls extra-alveolar vessels open during lung volume increases?
Surrounding lung tissue
What are the bronchial vessels?
- Vessels that contain oxygenated blood from systemic circulation
- Comprise 1-2% of cardiac output
- Empty into left atrium
Alveolar vessels provide ___ ___ to flow
Longitudinal resistance to flow
What resists pulmonary blood flow in the alveolar vessel network?
Alveolar vessel network dimensions and distensibility
Alveolar network dimensions are not regulated by ___ or ___
Autonomic or hormone control
Alveolar capillary walls contribute ___% of total resistance
40%
Alveolar arterioles contribute ___% of resistance
50%
In the body, arterioles are…
Major resistance vessels—75% of total systemic circulation resistance!
What does the resistance of capillary vessels depend on?
Lung conditions
Reduced resistance at ___ lung volumes
Low
Reduced resistance at ___ blood flow rates
High
Greater resistance at ___ blood pressures or ___ vascular distending pressures
Low, less
___ regulation of blood flow through capillaries occurs in response to changes in cardiac output
Passive
What accommodates increases in alveolar blood flow?
Recruitment and distention
What do recruitment and distention of alveoli prevent?
Prevents rise in pulmonary driving pressure with increase in blood flow
What does recruitment mean?
Opening of previously closed vessels
What does distention mean?
Increase in caliber of vessels
What are two mechanisms for decreasing pulmonary vascular resistance?
- Recruitment
- Distention
What are the pulmonary capillaries?
Extensive network within alveolar walls
___% of alveolar surface area is covered by capillary bed
70-80%
Total capillary surface area almost equals…
Alveolar surface area
What increases capillary volume?
Opening closed segments (recruitment)
What is the normal functional capillary volume (FCV) at rest?
70 ml (1 ml/kg body weight)
What is the maximal functional capillary volume (FCV)?
200 ml
What is the alveolocapillary network?
Continuous network over several alveoli
What is the average distance RBC travels through the alveolocapillary network?
600 to 800 micrometers
Capillary network blood volume =
RV stroke volume
How long does the RBC remain in the alveolocapillary network?
One cardiac cycle (0.75 sec)
How long does the RBC require for gas exchange equilibrium?
Less than 0.25 seconds for gas exchange equilibrium
What is pulmonary circulation volume?
Total blood volume from the main pulmonary artery to the left atrium
How much is the pulmonary circulation volume?
500 ccs
Lung is ___% blood by weight
40-50%
This volume fraction is > than any other organ
What is the capacitance reservoir for the left atrium?
The idea that the pulmonary vasculature acts as a reservoir and can alter its volume from 50% to 200% of resting volume
What does the capacitance reservoir prevent?
Prevents changes in blood return to the right ventricle from affecting the left ventricular diastolic filling pressures over 2-3 cardiac cycles
What is pulmonary capillary volume?
Approximately equal to stroke volume of the right ventricle
How much blood does the capillary bed contain at rest?
70 ml
What is the maximum volume of blood in the capillary bed during exercise?
200 ml
What is recruitment?
Process of increasing capillary volume by opening closed vessels
Increased CO ___ pulmonary vascular pressures, BUT ___ pulmonary vascular resistance
Raises pulmonary vascular pressures, decreases pulmonary vascular resistance
When does recruitment occur?
Occurs during periods of stress and increased tissue oxygen demand
Recruitment ___ pulmonary vascular resistance
Decreases—chief mechanism for fall in PVR!
Recruitment ___ pulmonary vascular pressures
Raises
What is distention?
When internal vessel pressures raise and open capillary beds
What is the effect of elevated left atrial pressure on distention of pulmonary capillary beds?
It increases distention (i.e.: mitral regurgitation, LV failure)
What does elevated left atrial pressure lead to long term?
Lung congestion; ultimately heart failure
Distention is seen at ___ vascular pressures
High vascular pressures
How does strenuous exercise affect cardiac output?
Increases cardiac output significantly
Increased CO during exercises ___ pulmonary arterial pressure
Raises
How does increased CO during exercise affect recruitment of alveolar wall capillaries?
Increased CO = more recruitment of alveolar wall capillaries
What happens to capillary volume during exercise? Why?
Capillary volume doubles to give time for adequate gas exchange during increased blood flow
Inspiration pleural pressure
More negative than -5 mm H2O
Pressure gradient for blood flow into thorax during inspiration is ___
Increased
How does inspiration affect blood flow in RV/LV?
- RV receives greater blood volume in diastole
- Increase of venous blood return into thorax
- LV ejects less blood secondary to increased pressure gradient between LV and systemic pressures
Expiration pleural pressure
Lower pleural pressure gradient—pleural pressure less negative than -5 mm H2O
How does thoracic pressure during expiration affect venous blood return?
More positive thoracic pressure decreases venous blood return
Decreased pressure gradient during expiration prevents…
Venous blood return to RV
What is the RV ejection pressure during expiration?
Less
The reduced gradient between LV and systemic arteries during expiration allows…
Increased stroke volumes
What alters pulmonary vascular resistance?
Changes in lung volumes during breathing
PVR is minimal when…
Lung volume is close to FRC
PVR is increased with…
Higher and lower lung volumes
What dilates during inspiration?
Extra-alveolar vessels
How does inspiration affect the diameter and flow resistance of extra-alveolar vessels?
- Increases diameter
- Reduces flow resistance
How much blood volume do the extra-alveolar vessels get during inspiration, and what is the effect on the alveolar vessels?
There is increased blood volume to the extra-alveolar vessels during inspiration; higher alveolar pressure compresses alveolar vessels
What happens to the alveolar vessels during inspiration?
Alveolar vessels compress
How do elevated alveolar pressures affect capillary resistance?
Elevated alveolar pressures increase capillary resistance
Pulmonary capillaries are vessels of…
Major vascular resistance
Alveolar pressure is ___ by mechanical positive pressure ventilation
Artificially increased
Mechanical ventilation increases amount of zone ___ lung volume relative to pulmonary venous pressure
2
The rise in alveolar pressure increases resistance to…
Blood flow in zone 2
Positive-pressure ventilation can ___ cardiac output or ___ V/Q imbalance
Decrease cardiac output; increase V/Q imbalance
What is the bronchial circulation?
Circulation of oxygenated blood from the aorta returning to nourish the lung
What lung structures are perfused by the bronchial circulation?
- Conducting airways to terminal bronchioles
- Parenchyma supporting structures: pleura, interlobar septal tissues, pulmonary arteries and veins
Bronchial blood flows at ___ and is ___% cardiac output
Systemic pressures, 1-2% of cardiac output
___% of bronchial blood circulation returns to right atrium via azygos vein
50%
The rest of the bronchial blood exits lung by…
Small anastomoses with pulmonary veins contributing to normal venous admixture (right-to-left shunt)
What does the pulmonary lymphatic system do?
- Keeps alveoli free of fluid moving from capillaries
- Hydrostatic starling forces tend to move fluid out at 20 ml/hr
- Numerous lymphatics drain fluid from interstitium
Alveolar edema interferes with…
Pulmonary gas exchange
The pulmonary interstitium is kept at a slight ___ pressure
Negative
Effect of gravity on systemic blood pressure—in upright position, arterial pressure is higher…
In the feet than the head (postural dependent relationship with gravity)
Effects of gravity on pulmonary circulation—___ alterations in flow occur because pulmonary circulation pressures are ___
Greater alterations; pulmonary circulation pressures much lower
Distribution of blood flow in the lung is affected by ___
Gravity
What affects distribution of blood flow over the height of the lung?
Changes in pulmonary arterial pressure
What is hydrostatic pressure?
The pressure effect gravity has on a column of fluid
What are considered zero reference points?
The right atrium level and middle of the lung
What positions minimize hydrostatic pressures?
- Supine
- Prone
___ affects the perfusion of blood in the different zones of the lungs
Gravity
What part of the lungs receives a greater portion of the RV ejection fraction?
Lung base receives a greater portion than the apex of the lung
Hydrostatic pressure causes ___ and ___ of pulmonary capillaries in base of lung
Distention and recruitment
Blood flow distribution over the height of the lung is divided into ___ functional zones
3
Blood flow depends on pressure in ___ vessels relative to ___ pressure
Pulmonary vessels, relative to alveolar pressure
Pressures in pulmonary vessels/alveoli are dependent on (4 things):
- Hydrostatic pressure
- Gravity
- Transmural compressive pressure
- Lung volume
The perfusion zones of the lung depend on the relationship between what two things?
The relationship between alveoli and blood pressure in pulmonary arteries and veins
Zone 1
- Region does not receive blood flow
- Alveolar pressure > regional pulmonary blood pressure (PA > Pa)
- Pulmonary capillaries are collapsed by higher PA
Zone 2
- Occurs where blood flow driving force is arterial-Alveolar pressure gradient
- Pa > PA > PV
- Intermittent blood flow
- ‘Water fall’ effect—downstream venous pressure changes do not alter blood flow
Zone 3
- Conditions exist in lung base because of effect of gravity
- Hydrostatic gravity pressures cause distention and recruitment of pulmonary capillaries
- Pa > PV > PA
- Distention and recruitment decrease resistance to blood flow (increased flow in zone 3)
Zone 4
- An abnormal condition of reduced blood flow
- High pulmonary venous pressures (i.e.: LV failure or mitral stenosis)
- Pulmonary edema creates fluid accumulation and vascular cuffing
- Increased vascular resistance and reduced local blood flow
___ affects the regional distribution of blood flow in the different lung zones
Gravity
Transmural distending pressure of vessels at ___ of lung > apex
Bottom
Describe blood flow in the base of the lung (3 things)
- Blood vessels are more distended in base
- Decreased resistance to flow in base
- Greater blood flow in lower lung
___ pressure affects blood flow
Alveolar
What pressures are higher in the apex of the lung?
Alveolar pressures are higher than capillary pressures in the apex of the lung
Pressure differences in the apex of the lung results in ___ perfusion in apex
Decreased perfusion
What creates differences in lung zones? (3 things)
Pulmonary arterial, venous, and alveolar pressure
What 3 factors expand zone 1 perfusion?
- Decreased pulmonary artery pressure (i.e.: shock, hypovolemia)
- Increased alveolar pressure (i.e.: PEEP)
- Occlusion of blood vessels (i.e.: pulmonary embolism)
What 2 factors reduce zone 1 perfusion?
- Increased pulmonary artery pressure (i.e.: infusion of fluid or blood)
- Reduced hydrostatic effect (i.e.: change patient position from standing to supine)
___ PA pressures with increased CO (i.e.: during exercise)
Increased
PVR =
The change in pressure over cardiac output
Active regulation of blood flow in the pulmonary vasculature occurs by…
Altering vascular smooth muscle tone in pulmonary vessels (arterioles)
Pulmonary capillary smooth muscle alters ___
PVR
Most active regulation of pulmonary vessels is mediated by…
Local metabolic influences
What two factors affect vasomotor tone/pulmonary vascular resistance?
- Vasoconstrictors
- Vasodilators
What are 5 vasoconstrictors that affect pulmonary vascular resistance?
- Reduced PaO2
- Increased PCO2
- Thromboxane A2
- Histamine
- Prostaglandins
What are 3 vasodilators that affect pulmonary vascular resistance?
- Increased PaO2
- Prostacyclin
- Nitric oxide
Thromboxane A2 is a ___
Potent vasoconstrictor—constrictor of pulmonary arterial and venous smooth muscle
What is thromboxane A2 made of?
It is a product of cell membrane arachidonic acid metabolism
When is thromboxane A2 produced?
Produced during acute lung tissue damage by macrophage, leukocytes, and endothelial cells
Where does thromboxane A2 exert its effect?
Effect is localized to the injured region of the lung because the half-time of thromboxane inactivation is only seconds
Prostacyclin (Prostaglandin I2) is a ___
Potent vasodilator
Prostacyclin inhibits…
Platelet activation
What produces prostacyclin?
Endothelial cells—product of arachidonic acid metabolism
Nitric oxide (NO) is a ___
Epithelial vasodilator
Where does NO work?
Strictly localized effect to the vascular site of production
How does NO work?
- NO activates guanylyl cyclase and increases cGMP
- It causes smooth muscle relaxation through synthesis of cyclic GMP
- How nitroglycerin and sodium nitroprusside work
What is the clinical use of NO?
Selective pulmonary vasodilation delivered by inhalation technique
NO at high concentrations is…
-Very toxic!
NO binds irreversibly to…
Heme iron in hemoglobin
NO binds to hemoglobin ___ times more than oxygen
200,000x
Partial pressure of ___ in alveoli is a critical factor governing pulmonary circulation
Oxygen (PaO2)
PaO2 in alveoli is more important than…
Oxygen tension in mixed venous blood
Oxygen diffusing into pulmonary arteriole walls causes…
Smooth muscle dilation
As alveolar oxygen tension decreases, surrounding arterioles ___
Constrict
Low alveolar PaO2 causes increase in…
Local vascular resistance
Low alveolar PaO2 causes blood flow to shift to areas of the lung with…
Higher PaO2
Global reduction in alveolar oxygen tension ___ total PVR by…
Increases, by constriction of arterioles and small arteries
Alveolar hypoxia produces…
Hypoxic pulmonary vasoconstriction (HPV)
What is hypoxic pulmonary vasoconstriction (HPV)?
- Localized response of pulmonary arterioles
- Caused by hypoxia and enhanced by hypercapnia and acidosis
- Contraction of smooth muscle in small arterioles in hypoxic region
- Opposite reaction than systemic circulation to hypoxia
HPV is an important mechanism of balancing what ratio?
-V/Q ratio—shift flow to better ventilated pulmonary regions
What does HPV result from?
-Decreased formation and release of nitric oxide by pulmonary endothelium in hypoxic region
What is pulmonary hypertension?
Increased resistance to blood flow in the lung
Pulmonary vascular resistance and pulmonary artery pressures are both ___ in pulmonary HTN
Elevated
What are effects of pulmonary HTN?
- Generalized alveolar hypoxia increases total pulmonary resistance
- Hypoventilation
- Low inspired PO2
- Increased PCO2
- Pain
- Histamine release
- High altitudes
Pulmonary hypertension causes increased work for the ___, leading to
Right ventricle, leading to:
- right ventricular hypertrophy
- tricuspid regurgitation
- right heart failure (cor pulmonale)
Pulmonary hypertension is a…
SERIOUS PULMONARY VASCULAR CONDITION
In pulmonary HTN, small muscular pulmonary arteries ___
Narrow
In pulmonary HTN, pulmonary arterial pressure ___
Increases
In pulmonary HTN, right ventricle pressures ___
Rise to compensate until RV failure occurs
What is the only effective treatment for pulmonary HTN?
Lung transplant
What is the V/Q ratio?
Ventilation : perfusion ratio
V/Q distribution is regulated to maintain systemic oxygen partial pressure range between ___ to ___ mm Hg
85-100
Distribution of V/Q ratios among the terminal respiratory units are ___, even in a normal lung
Not uniform
A diseased lung’s V/Q ratios may be very ___
Nonuniform
Alveolar-arterial (A-a) PO2 differences express…
The unequal match of V/Q ratio
Normal (A-a) PO2 differences are…
10 to 15 mm Hg
Larger (A-a) PO2 gradients indicate…
Intrinsic pulmonary disease —> shunting
Hypoxemia with normal (A-a) PO2 gradient indicates…
Hypoventilation
What is the most common cause of inefficient O2 and CO2 exchange?
Mismatched V/Q ratio
What are two causes of abnormal (A-a) PO2 differences?
- Wasted ventilation
- Venous admixture
Shunt =
Perfused but NOT ventilated
A right-to-left heart shunt is AKA as…
Pulmonary venous admixture
What happens in a right-to-left heart shunt?
- A portion of cardiac output (CO) flowing through pulmonary circulation does not participate in gas exchange
- A fraction of blood flow bypasses the lung to enter the systemic arteries without becoming oxygenated, leading to venous admixtures
- Reduction of systemic arterial oxygen tension and concentration
- Reduce efficiency of gas exchange
Are small shunts normal?
Yes—because some venous blood bypasses the lung to enter the left heart
Examples of true anatomical shunts (4)
- bronchopulmonary venous anastomoses
- intracardiac thebesian veins
- mediastinal veins
- pleural veins
Venous admixture is the blood flow equivalent of…
Wasted ventilation
A left-to-right heart shunt is AKA…
Pulmonary venous recirculation
What happens in a left-to-right heart shunt?
- A portion of CO returns to the right heart without flowing through the body
- This type of shunt does not affect systemic arterial oxygen tension
- The oxygen tension in the right heart is increased
What indicates the site of a L-to-R shunt?
- The location of increased oxygen concentration
- Amount of change in PO2 allows estimate of the shunt
What are two types of heart shunts?
- Right-to-left heart shunt (pulmonary venous admixture)
- Left-to-right heart shunt (pulmonary venous recirculation)
