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Lecture 13: Physiology of Pulmonary Circulation
Cardiorespiratory
43
Medical
Professional
08/28/2011

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Term
What advantages do the short, thin-walled, large diameter pulmonary arterial vessels convey?
Definition
low resistance to blood flow-->low BP in pulmonary vessels

high compliance, allowing pulmonary arteries to act as volume "buffers" for output from right heart
Term
quanitit of blood flowing through lungs
Definition
5 L/min at rest (same as systemic)
Term
bronchial vessels
Definition
keep pulmonary tissue alive-->nutritive circulation of non-gas-exchanging parts of lung

like rest of systemic circ, it's a high P system
Term
muscle coat in arterial wall
Definition
extends w/age until ~20 y.o., where it reaches arterioles adjacent to alveoli
Term
fate of blood in bronchial circulation
Definition
1. drains back to right heart directly

2. drains to left atrium of heart
a) some gets reoxygenated, joining pulmonary blood before passing through alveolar capillaries
b) some does not get reoxygenated, joining pulmonary blood after alveoli-->that's why systemic arterial blood slighly lower O2 content than pulmonary venous blood
Term
venous admixture
Definition
when venous blood mixes directly w/arterialized blood in lung
Term
blood volume of lungs
Definition
450 ml, roughly 1/10 total in circ system
Term
usual pressure unit of lung
Definition
mm Hg
Term
15 mm Hg = ___ cm Hg
Definition
15 mm Hg = 1.5 cm Hg
Term
1 mm Hg = ___ mm H2O
Definition
1 mm Hg = 13.6 mm H2O
Term
normal ventilation:perfusion ratio
Definition
~ 1
Term
driving force for blood flow through the alveolar capillaries is about...
Definition
5 mm Hg in recumbent human at rest

**this pressure difference sufficient ot drive full CO through pulmonary capillaries-->lot of flow from a little P
Term
2 adaptations that maximize gas exhanging capability of lung
Definition
large area and short diffusion path
Term
diminished alveolar oxygen ____ local alveolar blood flow
Definition
decreases
Term
When concentration of O2 in alveoli decreases below normal, adjacent blood vessels ______, and vascular R ______ as much as fivefold.
Definition
When concentration of O2 in alveoli decreases below normal, adjacent blood vessels CONSTRICT, and vascular R INCREASES as much as fivefold.

**this is opposite of what's seen in systemic vessels

**when they vessels constrict, the blood will through areas of lung that are better aerated, providing an automatic control system for distributing blood flow to different pulmonary areas in proportion to their degree of ventilation
Term
it's believed that the low O2 concentration causes some vasoconstrictor substance to be released from lung tissue, which causes constriction of small arteries. What's this mechanism referred to as?
Definition
hypoxic vasoconstriction
Term
autonomic nervous contorl of pulmonary blood flow through lungs
Definition
weak - vagal (parasympathetic) fibers cause slight decrease in pulmonary vascular resistance and vv for sympathetic
Term
autonomic contorl of pulmonary blood volume in lungs
Definition
strong -->has great effect in constricting the large pulmonary capacitative vessels, esp. the veins

**gravity also exerts a strong influence over distribution of blood through pulm. circ
Term
CAPILLARY FLUID DYNAMICS

1. pulmonary capillary pressure is...
Definition
1. very low, about 7 mm Hg (peripheral about 17)
Term
interstitial fluid hydrostatic P is...
Definition
slightly more negative than epripheral tissues about -5 to -8 (caused largely by lymphatic pumping)
Term
3. pulmonary capillaries are relatively ____ to protein molecules
Definition
pulmonary capillaries are relatively LEAKY to protein molecules, so that interstitial fluid has more protein and a colloid osmotic P of about 14 mm Hg (less than half that in peripheral subcutaneous tissues)
Term
4. alveolar walls are extremely...
Definition
thin, and alveolar epithelium covering alveolar surfaces is so weak that if ruptured by any positive P in interstitial space greater than atmospheric, it allows dumping of fluid from iterstitial spaces into alveoli
Term
forces driving fluid from capillaries into interstitium (3)
Definition
capillary BP, interstitial fluid colloid osmotic pressure, negative hydrostatic P in interstitium

**total outward force = 29 mm Hg
Term
force driving absorption of fluid into capillaries
Definition
plasma colloid osmotic pressure, which is ~ 28 mm Hg

**normally net outward force, so extra is picked up by lymphatic ducts and pumped back into systemic circ
Term
pulmonary edema usually caused by what 2 things?
Definition
1. damage to pulmonary capillary membrane caused by infections or breathing noxious substances

2. left heart failure or mitral valve disease
Term
initial rise in left atrial pressure up to about _____ mmHg has virtually no effect on pulmonary capillary (peak) pressure
Definition
7-8 mm Hg

**any increase in left atrial P beyond this increases the capillary pressure almost as much, which begins to drive fluid out of the capillaries
Term
before positive interstitital fluid P can occur and cause overt edema, what 3 factors must be overcome?
Definition
1. normal negativity of interstitial fluid P of lungs

2. lymphatic pumping of fluid out of interstitial spaces

3. increased osmosis of fluid into pulmonary capillaries caused by decreased protein in interstitial fluid when lymph flow increases
Term
pulmonary capillary P must normally rise to a value at least equal to _____ before significant pulmonary edema will occur
Definition
the plasma colloid osmotic P
Term
the pulmonary capillary P must rise from teh normal level of ____ to over ____ to cause acute pulmonary edema, meaning there is a safety factor of about _____
Definition
the pulmonary capillary P must rise from the normal level of 7 mm Hg to over 28 mm Hg to cause acute pulmonary edema, meaning there is a safety factor of about 21 mm Hg
Term
in chronic pulmonary edema, safety factor against pulmonary edema can rise to as high as _____, compared to normal value of 21 mm Hg in acute conditions
Definition
35-45 mm Hg

**when pulmonary capillary P remains elevated chronically (2+ wks), the lungs become more resistant to pulmonary edema bc the lymph vessels expand greatly, increasing their capability for carrying fluid away from interstitial spaces as much as tenfold
Term
ventilation ratio =
Definition
VA/Q

VA = alveolar ventilation (~5250 ml/min)
Q = CO (~5000 ml/min)
Term
2 reasons why local values for VA/Q can be considerably different from 1, even in a healthy lung
Definition
1. not all parts of lung get same amount of fresh air (bottom gets more and top gets less when standing)

2. local pulmonary blood flow also varies considerably from bottom to top of lungs bc pressure from bottom to top of lungs later amount of local pulmonary blood flow
Term
in the upper part of the lung, pulmonary blood flows for _____ and for part of time _______
Definition
in the upper part of the lung, pulmonary blood flows for PART OF CARDIAC CYCLE and for part of time IT DOES NOT FLOW

**high-pressure bonrchial circ still keeps tissue alive
Term
when you reach points only ____ mm Hg above teh heart, then diastolic pulmonary pressure is sufficient to keep blood moving and so blood continues flowing throughout cardiac cycle in these lower parts of lung
Definition
8 mm Hg
Term
ZONES OF WEST ("zones of the lungs")

Zone 1 blood flow
Definition
systolic P never sufficient to force blood to upper parts of lungs, so NO pulmonary blood flow there-->only found when air pressure is high in lungs (trumpet, deep sea diving, right heart failure)
Term
zone 2 blood flow
Definition
intermittent through cardiac cycle (during diastole, P usually not enough during systole)-->present in normal lung from pt 8 mm Hg above heart to top of lungs
Term
Zone 3 blood flow
Definition
continuous, has greatest pulmonary flow in lungs
Term
Zone 4
Definition
still continuous but reduced bc teh shear wt of lung tissue above crushes down on this part and compresses the pulmonary capillaries, whereby reducing their blood flow

Zone 4 is a special type of Zone 3 blood flow
Term
What happens when you turn the lungs sideways, as when a subject lies down?
Definition
no part of lung is very far above or below heart, so essentially whole lung has Zone 3, continuous flow
Term
these zones refer not to _____ parts of lung but to ____ patterns of pulmonary blood flow
Definition
these zones refer not to ANATOMICAL parts of lung but to FUNCTIONAL patterns of pulmonary blood flow
Term
in normal lung, ventilation per volume and pulmonary blood flow are greatest where?
Definition
at base and least at apex
Term
Where is the VA/Q ratio lowest and greatest in lung? Why?
Definition
lowest at bottom and greatest at top

bc blood flow drops off proportionately more as you move toward the apex - bc blood flow is so low at top of lung
Term
What happens to VA/Q ratio when ventilation overbalances perfusion (as in pulmonary embolism)? Body's response?
Definition
VA/Q greater than 1 due to too much ventilation or too little perfusion body compensates by bronchiolar constriction and arteriolar dilation opposite is true for perfusion overbalancing ventilation (body responds by bronchiolar dilation and arteriolar constriction)
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