Lecture 22: Pulmonary Circulation And Gas Exchange Flashcards Preview

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Flashcards in Lecture 22: Pulmonary Circulation And Gas Exchange Deck (22)
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Describe the two circulations within the lung.

* High pressure; low flow:
- Thoracic aorta → bronchial arteries →
-- Trachea, bronchial tree, adventitia, CT
* Low pressure; high flow:
- Pulmonary artery and branches → alveoli
-- Wall thickness of arterial artery is 1/3 that of the aorta.
--- Therefore, pulmonary arterial tree has a larger compliance:
--- 7 ml/mm Hg
---- Similar to that of entire systemic arterial tree
---- Allows pulmonary arteries to accommodate stroke volume output of right ventricle


Describe the pulmonary system pressures in both the heart, and the lungs (in mmHg)

* Pressures in Heart (mm Hg)
- Right Ventricle Systolic: 25 mm Hg
- Right Ventricle Diastolic: 0-1
- Left Ventricle Systolic 120-125
- Left Ventricle Diastolic 0-5
* Pressures in Lung (mm Hg)
- Pulmonary Artery Systolic 24-25
- Pulmonary Artery Diastolic 8-9
- Mean Pulmonary Arterial 15
- Mean pulmonary Capillary 7

- See Slide 8-9


Describe Pulmonary System Pressures

- Pulmonary arterial pressure = 24/9 mm Hg
- Mean pulmonary arterial pressure = 15 mm Hg
- Left atrium pressure = 8 mm Hg (diastole)
- Pressure gradient in pulmonary system = 7 mm Hg
- Mean pressure in left atrium is about 2 mm Hg
-- Measure using pulmonary wedge pressure:
- Refer to text for how this is done (Page 510).
- Refer to Figures 39-1 and 39-2.


Describe the lung's blood volume

* 450 ml blood within pulmonary circulation:
- 9% of total blood volume in body
- About 70 ml is in pulmonary capillaries
* Failure of left side of heart can cause pressure to build up in pulmonary circulation:
- Increases blood volume as much as 100%
- Increases blood pressure
- Mild systemic effect because systemic blood volume is 9 times that of the pulmonary system.


Describe a physiological shunt

* Physiologic shunt:
- About 2% of the blood in the systemic arteries is blood that has bypassed the pulmonary capillaries:
- This is blood coming from the lung parenchyma and left side of the heart (Blood from the wall of the left atrium that dumps directly into the left atrium).
- Therefore:
-- Blood in the systemic arteries contains less oxygen per deciliter than blood that has equilibrated with the alveolar air.


Describe Blood Distribution through the lungs

* When oxygen concentration in alveoli is 70% (73 mm Hg PO2) or more below normal:
- Adjacent blood vessels constrict
- Caused by unknown vasoconstrictor:
-- Possibly released by alveolar epithelial cells
* Those alveoli that are poorly ventilated get even less blood while those with adequate ventilation get more blood.
* In the standing position, there is little blood flow to top of lungs but about 5x as much to bottom of lungs.
* In the standing position, there is little blood flow to top of lungs but about 5x as much to bottom of lungs.
* Lungs can be divided into three zones (Refer to Figures 39-3 and 39-4):
- Zone 1 No blood flow; local alveolar capillary pressure never rises higher than alveolar air pressure.
- Zone 2 Intermittent blood flow (only during systole)
- Zone 3 Continuous blood flow
* Normally:
- Apices have zone 2 flow
- Lower areas have zone 3 flow
- Exercise can convert apices from zone 2 to zone 3 flow.

- See Slide 14

* Distensibility of pulmonary veins makes them an important blood reservoir.
- Pulmonary blood volume increases by up to 400 ml.
* This is released to general circulation when person stands up.
* Results of obstructing blood supply to one normal lung:
- Blood flow through other lung is doubled.
- Because of passive dilation of pulmonary vessels, the pulmonary pressure in the other lung is only slightly increased.


What are some agents that affect pulmonary vessels and nerve fibers

* Agents that constrict pulmonary arterioles:
- Norepinephrine
- Epinephrine
- Angiotensin II
- Some prostaglandins
* Agents that dilate pulmonary arterioles:
- Isoproterenol
- Acetylcholine
* Agents that constrict pulmonary venules:
- Serotonin
- Histamine
- E. coli endotoxin
* Sympathetic vasoconstrictor nerve fibers:
- Outflow from cervical sympathetic ganglia
- Decrease pulmonary blood flow by as much as 30%
- Mobilize blood from pulmonary reserve

- See Slide 17


Describe pulmonary blood flow during exercise

* During heavy exercise blood flow through lungs increases 4x to 7x.
- Increases number of open capillaries up to 3x
- Distends all capillaries and increases flow rate up to 2x
- Increases pulmonary arterial pressure
* Because of 1 and 2 above, pulmonary arterial pressure rises little even during maximum exercise.
- Conserves energy of right side of heart
- Prevents significant rise in pulmonary capillary pressure.

- See Slide 19


Describe left sided heart failure

* Left atrial pressure normally never above +6 mm Hg.
* In left heart failure:
- Blood begins to dam up in left atrium
- Left atrial pressure rises from 1-5 mm Hg to 40-50 mm Hg
- Increases above 8 mm Hg cause equal increases in pulmonary arterial pressure
- Above 30 mm Hg, pulmonary edema is likely to develop


Describe Lung Capillary Fluid Exchange

* Pulmonary capillary pressure = 7 mm Hg
* Peripheral tissue capillary pressure = 17 mm Hg
* Interstitial fluid pressure in lung is slightly more negative than that in peripheral subcutaneous tissue.
* Pulmonary capillaries are relatively leaky to protein molecules:
- Colloid osmotic pressure in pulmonary interstitial tissue is about 14 mm Hg compared to less than 7 mm Hg in peripheral tissues.
* Alveolar walls are extremely thin
* Alveolar epithelium can be ruptured by any positive pressure in the interstitial spaces greater than alveolar air pressure (less that 0 mm Hg)
* Hydrostatic and osmotic forces (page 514):
- Capillaries → pulmonary interstitium forces:
-- Hydrostatic pressure
-- Interstitial fluid osmotic pressure
-- Interstitial fluid hydrostatic pressure
-- Total outward force +7 (-)14 (-)8 29
- See Slide 23
* Hydrostatic and osmotic forces (page 514) (cont.):
- Pulmonary interstitium → capillaries forces:
- Capillary osmotic pressure 28
- Total inward force 28
* Mean filtration pressure:
- +29 –28 = 1 mm Hg
* Excess fluid can be carried away by pulmonary lymphatics.


Describe Pulmonary Edema

* Occurs when pulmonary capillary pressure > 25 mm Hg.
* Most common cause:
- Left-sided heart failure or mitral valve disease
- Damage to pulmonary blood capillary membranes:
-- Infections
-- Breathing noxious substances
--Refer to Figure 39-7.
- Lethal pulmonary edema can occur within hours or minutes.
* What might happen when capillary pressure remains chronically elevated for two weeks or more?


Describe Pleural Effusion

* Pumping of fluid from the pleural space by the lymphatics creates a normal pressure in the pleural space of -7 mm Hg.
* If this pressure becomes more positive (-4 mm Hg) the lungs tend to collapse.
* Pleural effusion is edema of the pleural cavity.
* Causes of pleural effusion:
- Blockage of lymphatic drainage from pleural cavity
- Cardiac failure
- Considerably reduced plasma colloid osmotic pressure
- Infection/inflammation


Describe Hypoxia and Pulmonary Blood Flow

* Hypoxia increases pressure in the pulmonary artery:
- Possibly because of the release of a prostaglandin.
* Results of bronchial obstruction:
- Constriction of vessels supplying the poorly ventilated alveoli:
-- Due locally to low alveolar PO2 effect on the vessels
-- Drop in pH due to accumulation of CO2
-- Decline in pH produces vasoconstriction in pulmonary vessels.
-- Decline in pH produces vasodilation in other tissues.
* Reduction of blood flow to a portion of the lung
- Lowers alveolar PCO2, resulting in a constriction of the bronchi supplying that portion of the lung.


What are Factors that affect rate of gas diffusion in a fluid?

* Solubility of gas in the fluid
* Cross-sectional area of the fluid
* Distance through which the gas must diffuse
* Molecular weight of gas
* Temperature of fluid (remains reasonably constant)


What is the Gas Diffusion Constant

D is equivalent to (deltaP * A * S)/(d * (sqrt(MW))

D = Diffusion Rate
P = Partial Pressures between two ends of the pathway
A = Area of the Pathway
S = solubility of gas
d = distance of diffusion
MW = Molecular Weight of Gas

- See Slide 32


Describe Alveolar Air Replacement

* The functional residual capacity of the lungs of the average man is 2300 ml.
* But only 350 ml of new air is brought into the alveoli with each normal inspiration, and this same amount of old alveolar air is expired.
* Therefore, the volume of alveolar air replaced by new atmospheric air with each breath is only one seventh of the total, so multiple breaths are required to exchange most of the alveolar air.
- See Slide 34-35


Describe Partial Pressures and Alveolar Ventillation

* Oxygen concentration in the alveoli, as well as its partial pressure, is controlled by:
- Rate of absorption of oxygen into the blood
- Rate of new oxygen entry into the lungs (alveolar ventilation)
* Review Figure 40-4:
- Note that to quadruple oxygen consumption, alveolar ventilation must also quadruple.
- Explain why alveolar ventilation cannot increase PO2 above 149 mm Hg under normal conditions.
* Carbon dioxide concentration in the alveoli, as well as its partial pressure, is controlled by:
- Rate of carbon dioxide excretion: Alveolar PCO2 increases in direct proportion to rate of excretion.
- Alveolar ventilation: Alveolar PCO2 decreases in inverse proportion to alveolar ventilation.

- See Slide 37-41


- Your text discusses the structure of the respiratory unit and the respiratory membrane on pages 521-522.
- Review this information on your own. It is testable.

Also review slides 43-45


List factors that determine how rapidly a gas will pass through the respiratory membrane.

* Membrane thickness
* Membrane surface area
* Diffusion coefficient of gas in the substance of the membrane
* Partial pressure difference of gas between the two sides of the membrane

- Any factor that increases membrane thickness (i.e., edema or fibrosis) to more than 2x or 3x normal can interfere with gas exchange.


What is the Va/Q Ratio?

* Ventilation-perfusion ratio:
= Va/Q = alveolar ventilation/blood flow
* Va/Q is normal when both factors are normal for a given alveolus
Va/Q = 0.8: (pulmonary = 5 L/min; Ventilation ≈ 4 L/min)
* Va/Q = 0 when Va= 0 but there is still perfusion:
- Due to airway obstruction (i.e., mucus plug)
- Blood gas composition remains unchanged.
* Va/Q = ∞ when Q = 0 but there is sll venlaon (no gas exchange):
- Due to vascular obstruction (i.e., pulmonary embolism)
- Alveolar gas composition remains unchanged because there is no blood contact. This creates a physiologic shunt.

AtmO2= 150
Atm CO2= 0
Alv O2 = 100
Alv CO2 = 40
Cap O2= 40-100
Cap CO2= 45-40

- See Slide 50-54


Describe what happens when Va/Q ratios are infinite, and during normal alveolar perfusion

* When Va/Q = ∞:
- Alveoli partial pressures:
-- PO2 = 149 mm Hg
-- PCO2 = 0 mm Hg
* Normal alveolar perfusion:
- PO2 = 104 mm Hg;
- PCO2 = 40 mm Hg


Summarize Va/Q Ratios and terms

* Whenever Va/Q is below normal a certain fraction of the venous blood passing through the pulmonary capillaries does not become oxygenated.
* This is called shunted blood.
* Some blood is also shunted through bronchial vessels rather than through alveolar capillaries.
* The greater the physiologic shunt, the greater the amount of blood that fails to be oxygenated.
* When ventilation of some of the alveoli is great but alveolar blood flow is low, there is far more available oxygen in the alveoli than can be transported away from the alveoli by the flowing blood.
* The ventilation of these alveoli is wasted.
* This sum plus the sum of the anatomic dead space = physiologic dead space.

See Slide 57