Pulmonary Circulation Flashcards
Summarize the features of pulmonary circulation”action
Entire CO goes through pulmonary circulation in series
Pulmonary artery: pressures very low (resistance pulmonary flow low)
Capillaries: large cross sectional area-low resistance, slow flow
Low pressures 25/8 (systolic vs diastolic)
- prevent edema in fragile/ thin alveoli
- thin walled arteries
Contrast pulmonary and systemic circulation
Pulmonary artery mean pressure- 15 mmHg
Aorta mean pressure- 100 mmHg
Left atrium - 8
Right atrium- 2 mmHg
Driving pressure of pulmonary circulation 15-8=7
Driv8ng pressure of system8c circulation= 100-2=98
How is the pulmonary circulation regulated by resistance?
Flow(Q)= pressure gradient(🔼P)=/resistance(R)
Primarily control flow with resistance-as the pressure gradients are small
Increases in resistance, leads to decrease in
Blood flow
Decrease in resistance leads to increase in …
Blood flow
Contrast pulmonary and systemic circulation
-accommodates entire cardiac output, hence, high capacity
Has to protect fragile and vast capillary bed—> hence, low pressure circulation (vs high pressure systemic circulation
Thin walls, less smooth muscle, highly distensible, highly compressible
Distribution of pressure drops…
Pulmonary circulation
Resistance is much more evenly distributed across the pulmonary circulation
No large pressure drop as occurs across systemic arterioles.
Note: pulmonary resistance 10x LESS systemic resistance
Why are pulmonary pressures low?
RV only has to pump blood to the apex of the lungs (10 cm above the heart). High pressure is needed.
Resistance: presence of capillaries bed rather than network
Pulmonary capillary bed- ‘sheet’ of blood- large total cross sectional area, lower resistance
Systemic capillary network- individual capillaries very small diameter, greater resistance
Describe dual circulation to lungs
Pulmonary (to respiratory zone) 100% CO
Bronchial (to conducting zone) 1% CO from bronchial artery
What are the two Types of vessels that run through lung tissue?
- Alveolar vessels
2. Extra-alveolar vessels
Describe alveolar vessels
- Capillaries and slightly larger vessels
- Surrounded by alveoli on all sides
- Alveolar air pressure (PA) affects their patency (openness)
- Expanded/inflated alveoli (increases PA)—> alveolar vessels crushed
- Collapsed alveoli (decreased PA)—> alveolar vessels open up
Describe extra-alveolar vessels
Outside the alveoli
Affected by lung volume inflation/radial tension
Pleural pressures (slightly negative(due to opposing forces- thorax expanding and lung recoil
Contrast alveolar and extra-alveolar vessels with increased lung volume end max inspiration
Alveolar vessels
-Expanded/inflated alveoli (increased PA)
- increased air pressure on blood vessels (BV)
- Decreased radius, alveolar vessels compressed
- increased resistance
- decreased flow
Extra-alveolar
- Pleural press more negative
- lung tissues expand outward pulling open extra-alveolar BV, increased radius
- decreased resistance
- increased flow
Contrast alveolar and extra-alveolar vessels with decreased lung volume end max expire
Alveolar
- alveolar smaller (decreased PA)
- decreased air pressure on blood vessels (BV)
- increase radius, alveolar vessels open up
- decreased resistance
- increase flow
Extra-alveolar
- pleural pressure less negative
- compressed lungs
- decreased radius, compressedextra-alveolar BV
- increased resistance
- decreased flow
How do alveoli change in changes in pressure?
When lung inflated maximally- alveoli distended
exert a pressure on blood vessels running between the alveoli-even stopping flow
-When lung is deflated - less pressure is exerted and blood flow resumes
Normally- lung in tidal breathing resistance increases on inspiration and decreases on expiration for ALVEOLAR VESSELS
Why can blood flow not stop during inspiration?
Lung also has Extra-alveolar vessels, I.e. not in the alveolar septa but running between alveoli in the lung paarenchyma
These vessels are sensitive to intrapleural pressure, I.e. as PIP becomes more negative radial tension increases and extra-alveolar vessels open up (somewhat analogous to airway opening during lung inflation)
Whaat are the differences in in response to changes in lung volume
Extr-alveolar volume- Radial traction forces keep them open. (High trans pulmonary pressures). Pulled open as alveoli expand
Radial Fraction forces> inflated lung is inflated. So their resistance falls as the lung inflates
Alveolar vessels- in alveolar walls. Stretching of the alveolar walls leads to stretching and pulls vessels closed.
What causes regional changes in pulmonary flow?
Gravity—>. Increased blood flow, pressures at base of lungs
What is lung zoning?
This is based on perfusion of lung- due to alveolar and vessel pressures
What are the 2 factors affecting lung zoning?
Alveoli:
Air pressure within alveoli - PA- relatively constant throughout lungs
Alveolar capillaries:
The arteriolar capillaries is ALWAYS > venular pressures (Pa> Pv)
This is the driving force of blood through the capillaries
How does the gravity affect blood pressures in lungs?
Pressure= pgH
p= density of blood
g=gravitational acceleration
H= height of cylinder
Not need to know equation
Take home message: weight of blood in different parts aff3cts blood pressure
Zone 1: PA> Pa> Pv no flow
Zone 2: Pa> PA> Pv restricted flow
Zone 3: Pa> Pv> PA
What is the perfusion gradient of the lung?
Perfusion to base of lungs is much > apex by about 5x
Depending on height relative to heart (blood pump) lung divided into 4 zones
- Alveolar vessels
- Sensitive to alveolar pressure (PA)
- Extra-alveolar vessels
- Sensitive to intrapleural pressure (PIP)
What is the significance of zone 1 in the perfusion gradient of the lung?
Extreme upper part of the lung. PA> Pa> Pv and vessels are crushed. Now, normally the vessels structure resist compression. This zone 1 occurs under NON PHYSIOLOGICAL conditions.
E.g. arteriolar pressure rapidly decreases-heamorrhage. Alveoli pressure increases rapidly-positive ventilation
What is the significance of zone 2 in the perfusion gradient of the lung?
-BV directly on basement membrane of alveoli + feel press (PA). Vessels slightly crushed by PA. Clothes pin pinching straw
What is the significance of zone 3 in the perfusion gradient in lung?
Pressures > the alveolar pressures. Thus, blood flow drive by arteriolar venous gradient
What is the significance of zone 4 in perfusion gradient?
Extreme bases of lung-intrapleural pressures (PIP) less negative (gravity effects on ventilation)
Effects of extra-alveolar vessels predominate
These vessels tend to collapse with positive PIP, blood flow here is less than max
Summarize the order of zones
During exercise, CO increases —> recruitment & distension occurs—> most of lung converts to zones 2 or 3–> zone boundaries & regional differences in blood flow become fuzzy
Wha t is the clinical significance of zone 1
Alveolar pressure exceeds arterial, therefore flow is limited by compressive force fo alveoli along length of pulmonary capillary. No flow and increase in zone 1 may occur with blood loss, hew or Hague , positive pressure ventilation (Low Pa or high PA)
What is the clinical significance of Zone 2?
Pulmonary arterial pressure exceeds alveolar pressure so the initial part of the capillary is patent. As capillary pressure falls below alveolar pressure along the length of the vessel compression occurs and increases capillary resistance
What is the clinical significance of zone 3?
Normal flow, arterial and venous pressure exceed alveolar so no compression along the length of the vessel
What is the significance of zone 4?
Very base of lung, where the lung structures are subject to small trans pulmonary pressure gradient (CPR lecture Regional Differences in Ventillation), extra-alveolar vessels have increased resistance and flow is reduced overall
What is the correlation of cardiac output and pulmonary vascular resistance ?
Unique placement of lung to receive 100% of cardiac output, must deal with entire CO at rest 5 L min-1 and during exercise 20-30 L min -1
The resistance falls further as CO increases!!!
This is due to capillaries
What is the chief mechanism of decreasing pulmonary vascular resistance ?
Chief mechanism for decrease in PVR with increasing arterial pressures from low levels
Recruitment- is the use of already available cross sectional area
Distention- vasodilation can open up new cross sectional area to further augment total cross sectional area
How does increasing cardiac output effect distention and recruitment ?
Results in recruitment-
-(vessels that were open but not conducting or vessels that were closed and now open up)
and
Results in distention-
-increasing cross sectional area and allowing more flow
Together-these changes allow the cardiac output to increase without causing an increase in resistance- in fact resistance falls
How does increased demand for O2 CO lead to in pulmonary arterial pressure?
- With increased demand for O2 CO must increase dramatically during exercise
- Must accommodate the increase CO with minimal increase in pulmonary vascular pressure to avoid edema
- Recruitment and distention allow the masssive increase in CO with minimal increase in pulmonary artery pressure due to adaptive decrease in pulmonary vascular resistance
CO increases by 4 fold for a 33% increase in pulmonary arterial pressure
Only 5 mmHg increase in pressure results in 4x greater flow!
What is the pulmonary response to increased CO?
- As CO increases
- Pulmonary vascular pressures increase
- Recruitment & distention of pulmonary vessels
- Pulmonary vascular resistance decreases
- Increased flow through pulmonary capillary bed
Increased CO is now accommodated by the pulmonary circulation without an increase in pressure
What is hypoxia pulmonary vasoconstriction?
- Pulmonary arteries- regulate tone based on O2 tension /hypoxia (innervation/endocrine independent)
- Response is [PO2] dependent, I.e. blood vessel tone inversely proportional to hypoxia
- Many mechanisms postulated - to date no consensus occurs (E.g. O2 sensitive K+ Channel, nitric oxide, endothelin etc)
What do pulmonary vessels do in response to hypoxia?
In response to hypoxia (low PO2), pulmonary blood vessels construct
- to divert blood flow to areas of the lung that are well oxygenated
- and prevent hypoxic blood entering the systemic supply
Hypoxic pulmonary vasoconstriction
It is the PO2 of the alveolar air (PAO2) that is relevant here
Hypoxia —> vasoconstriction—> increase in vascular resistance —> decrease in blood flow blood flow reduces
Describe vasoconstriction in response to local/ regional hypoxia
Regional decrease in PO2 e.g. local airway blockage shift blood flow away from hypoxic/hypercapnic regions
Gas exchange improved
Regional hypoxia
Blood derived to better perfumed alveoli - little effect on total PVR
What is the effect in global hypoxia(decrease in PO2)?
High altitudes /hypo ventilation, babies not breathing, lung disease
- whole lung hypoxic. Decrease PO2
- Vasoconstriction throughout the lung
- increase in pulmonary art pressure (Pa) - hypertension
What is the beneficial effect on global hypoxia on pulmonary vessels?
- Increasing Pa recruits previously under perfused pulmonary capillaries
- Increase surface area for gas exchange
- improves ventillation perfusion matching
What are the disadvantages of global hypoxia in pulmonary circulation?
- Increased workload of right ventricle (RV)
- high Pa may increase capillary hydrostatic pressure and cause edema
What the treatment of global hypoxia for pulmonary vessel?
Supplement oxygen, reduce physical activity, Nifedipine (calcium channel blocker) prior to altitude can help
Summarize pulmonary edema
Low pressure circulation in lungs—> imperative
High capillary hydrostatic pressure (>25 mmHg)
—> pulmonary capillary walls are very very thin - high pressure cause blood to be pushed into the alveoli
—> pulmonary edema
—> interferes with gas exchange
What are the forces affecting fluid movement?
Starling forces
Promote fluid filtration
Promote fluid absorption
What are the causing of pulmonary edema?
- over perfusion with saline alone (decreased albumin concentration)
- liver disease- loss of albumin
- Tumor or laryngitis
- ARDS- no surfactant - increased surface tension
- Over-ventilation with mechanical ventilator.Crushes alveolar capillaries (increased resistance)
- Inhalation of noxious substances. Affects capillary permeability
- Lymphatic dysfunction
What factors promote fluid filtration?
- increased capillary hydrostatic pressure
- decreased colloid oncotic pressure
- increased surface tension
- Decreased lymphatic drainage
- Increased capillary permeability
What factors promote fluid absorption?
Increase colloid oncotic pressure
Increase air pressure
Decreased capillary hydrostatic pressure
When does pulmonary edema develop?
Pulmonary edema will develop when the net balance of forces acting on water transport favors the movement of fluid from the vascular space into the Extracellular space or even the alveolar space.
What are the main factors influencing water transport across pulmonary capillary walls?
- Capillary. Hydrostatic pressure
- Oncotic pressure of the blood
- Alveolar pressure
- Surface tension
- Lymphatic drainage
How does capillary hydrostatic pressure contribute to edema?
Blood pressure in the capillaries- usually 8-10 mm Hg
Whaat is osmotic pressure of blood?
The osmotic pressure of the plasma proteins in blood- about 25 mmHg
What is alveolar surface tension?
Strong positive pressure push water into extracellular space from alveolar space (not usually an issue)
What is surface tension?
Surface tension pulls on the alveolus and decreases the hydrostatic pressure in the extracellular space - causing water to be sucked into that space. Can also. Act To pull water into the alveolar space
What is lymphatic drainage?
There is a net movement of fluid from the vascular to the extracellular space of 20 ml/he. This is cleared by the lymphatic system
What is the hydrostatic pressure threshold?
Threshold for edema is about 25 mmHg of hydrostatic pressure in the capillaries. (Exceeds oncotic pressures of about 20-30 mmHg
Hydrostatic pressure is high enough to push fluid out of blood into alveoli.
Adaptive mechanisms to reduce resistance and keep hydrostatic pressures below. 25 mmHg even with large increase in blood flow
Summarize fluid movement in lung edema
- Clearance by lymphatics. Earliest form of edema-perivasc/bronchial spaces
- Liquid rarely accumulate in healthy alveoli. Alveolar edema-only in later stages of pulmonary edemaa
