Lecture 18 - Special Circulations: Pulmonary and Fetal Flashcards
How much blood in pulmonary circulation?
0.5 L
Average transit time of blood particle in systemic circulation?
1 min
What can be said of pulmonary arteries compared to pulmonary veins?
Hard to distinguish because both are thin walled due to the low pressure low resistance system
What pulmonary structures provide efficient gas exchange?
Sheet blood flow around alveoli and through alveolar vessels
What are the 2 main types of vessels of the pulmonary circulation? Describe each.
- Alveolar: analogous to capillaries and have as sole function to surround alveoli with a thin layer of blood for gas diffusion
- Extra-alveolar: analogous to small arterioles and venules
What can affect blood flow through the alveolar capillaries?
Alveolar increased pressure and increased volume will impede flow through alveolar capillaries
What can affect blood flow through the extra alveolar vessels?
Increased total lung volume with lung expansion causes expansion and increased blood flow through extra alveolar vessels
Functions of the lung?
- Respiration
- Blood reservoir
- Blood filtration
- Metabolism
- Defense
What is the defense function of the lung?
Lung produces Ig and stores WBCs
Describe the function of the lung as a blood reservoir.
It can act to buffer changes in cardiac output from beat to beat
If the PA is occluded the lungs will ensure that the LV CO remains unchanged for 2 beats and then gradually decreases
What can the lung acting like a blood reservoir prevent?
Orthostatic hypotension
Describe the function of the lung as a blood filter.
Can filter out blood clots if they are not too large because if one alveolar vessel gets blocked, there are MANY to take over (better for it to get lodged there rather than other organs)
Describe the dual blood supply to the lungs.
- 98% from the pulmonary circulation
2. 1-2% from bronchial systemic circulation
Describe the relationship between pulmonary arterial pressure and pulmonary vasculature resistance. Is this a passive or active mechanism? Purpose?
As pulmonary arterial pressure increases the vasculature resistance DECREASES due to recruitment and distension = passive mechanism
Purpose = to be able to accommodate increases in CO (for example during exercise!)
Describe the recruitment mechanism in the lungs.
Increase in pulmonary pressure => blood flow directed to normally closed pulmonary vessels
Describe the distension mechanism in the lungs.
Walls of the pulmonary vessels are much more compliant than the walls of the systemic vessels: when pressure increases the walls expand to allow a greater volume of blood
Which circulation is more affected by environmental and passive factors: pulmonary or systemic circulation? Why?
Pulmonary because they have much lower pressures
Describe the relationship between pulmonary venous pressure and pulmonary vasculature resistance.
Same relationship, just dealing with much lower increases in venous pressures
Does the pulmonary circulation have the myogenic reflex?
NOPE, quite the opposite actually
Describe the extra-alveolar vessels at low lung volume. How does this affect resistance?
Slightly collapsed because they do not have the expanded alveoli pulling them open => decreased diameter => increased resistance
Describe the curve between lung volume and vascular resistance. What is it due to?
J shaped:
- As long volume first increases, the vascular resistance decreases because extra-alveolar vessels are pulled open
- Past a certain point, over-expansion of the lungs will cause overall vascular resistance to increase because resistance in alveolar vessels is increased
What are the environmental and passive factors affecting pulmonary blood flow?
- Pulmonary arterial pressure
2. Gravity
Describe the effect of gravity on pulmonary blood flow. Is this a passive or active mechanism?
Lungs are 25 cm long and blood flow enters them at about the middle of their height, so with gravity:
- Blood flow is the greatest at the bottom of the lung due to distension and recruitment causing lower resistance at higher hydrostatic pressure due to gravity
- Blood flow at the top of the lung is lowest due to collapse of vessels causing high resistance at low hydrostatic pressure
Where is the highest blood flow in the lung location wise?
5 cm up the lung
Alveolar pressure vs hydrostatic pressure at the bottom of the lung? Result?
Hydrostatic P > alveolar P => vessels distend
Alveolar pressure vs hydrostatic pressure at the top of the lung? Result?
Hydrostatic P < alveolar P => vessels collapse
What is the most important active control of pulmonary blood flow? Explain how this works. How does this compare to metabolic regulation of the systemic regulation?
Alveolar PO2 (oxygen partial pressure):
- Low PO2 => vasoconstriction => blood shunted to areas of better ventilation with greater alveolar PO2
- High PO2 => vasodilation => blood flow increases to allow for gas exchange
COMPLETE opposite of systemic metabolic regulation
How do we know we are talking about systemic vs pulmonary O2 pressures?
Systemic: Pv and Pa
Pulmonary: PA
Describe the effects of breathing in a low PO2 atmosphere. Short term? Long term? Potential problem?
- Short-term: High altitude => decreased O2 content of air => constriction of pulmonary vessels => increased pulmonary pressure => distention + recruitment => pulmonary vascular resistance decreases => gas exchange increases + SNS increases so CO increases
- Long-term: increased production of RBCs = polycythemia
Potential issue: pulmonary hypertension due to excess vasoconstriction or pulmonary edema due to increased PAP (more filtration than absorption)
What are the 3 regulations of pulmonary blood flow. Is each active or passive?
- Pulmonary arterial pressure - passive
- Gravity - passive
- Alveolar O2 partial pressure - active
Is the fetal circulation in parallel or in series? Explain.
In parallel with mixing of blood between the right and left heart and oxygenated and deoxygenated blood
What 6 fetal circulation structures will FUNCTIONALLY disappear postnatally? Describe the function of each.
1 and 2. 2 umbilical arteries: carry low O2 blood TO the placenta
- Umbilical vein: carries oxygenated blood from the placenta
- Ductus venosus: connects umbilical vein to fetal venous system, BYPASSING the liver
- Foramen ovale: shunts oxygenated blood from RA to LA
- Ductus arteriosus: anastomosis from the pulmonary trunk to the aorta to allow more blood to go from right to left heart to bypass the lungs
How do the 2 umbilical arteries affect the fetal systemic circulation?
Decreased systemic resistance compared to postnatally
What 2 options does the blood have once it exits the umbilical vein in the fetal circulation?
Umbilical vein => ductus venosus OR liver circulation to process nutrients
What happens to the fetal blood once it reaches the RA?
Most of it is shunted to the LA through the foramen ovale
How do the foramen ovale and ductus arteriosus function to bypass the fetal lungs?
Pressures in RA (due to blood flow from ductus venosus) and PA (due to collapsed lung) are higher than in LA and aorta
How do the fetal ventricles differ from postnatal ventricles?
Have the same size muscle walls because are pumping against similar resistances
Why do the fetal lungs have high resistance?
- Smallest lung volume => increased resistance (tip of J curve)
- Low PO2 => vessel constriction
In what portion of the fetal circulation is there mixing of oxygenated and deoxygenated blood?
LA
Describe the changes to the CV system upon being born.
- Placenta is delivered => stress of birth => vasospasm of the umbilical arteries to functionally close
- Umbilical vein vasospasms too and functionally closes off a little later to allow for the fetal blood to self-transfuse at birth
=> increased systemic vascular resistance + decreased CVP
- RA pressure decreases => foramen ovale functionally closes
- Baby takes first breath => increased PO2 in lungs => decreased pulmonary vascular resistance => increased pulmonary blood flow => decreased PAP
- Ductus arteriosus functionally closes because aortic pressure is higher than PAP and oxygenated which causes spasms and closure
Describe how the ductus arteriosus closes.
Prostaglandins are keeping it open in the fetus => O2 causes free radical destruction of the enzymes producing the prostaglandins => vasospasms
How to treat a patent ductus arteriosus after birth?
NSAIDs to block prostaglandin production keeping the DA patent
Are the pressures in the ductus arteriosus, ductus venosus, and foramen ovale 0 mmHg right at birth?
No, it takes weeks or months for them to fully fully close anatomically
% complete closure of foramen ovale in adults?
A little over 60%
Describe the vasoconstriction of the pulmonary circulation compared to the systemic circulation.
Much more diffuse in the pulmonary circulation vs more discrete in systemic
Do fetuses also have a 95 to 100 PO2?
YUP
