Lecture 18 - Special Circulations: Pulmonary and Fetal

Question 1

How much blood in pulmonary circulation?

Answer 1

0.5 L

Question 2

Average transit time of blood particle in systemic circulation?

Answer 2

1 min

Question 3

What can be said of pulmonary arteries compared to pulmonary veins?

Answer 3

Hard to distinguish because both are thin walled due to the low pressure low resistance system

Question 4

What pulmonary structures provide efficient gas exchange?

Answer 4

Sheet blood flow around alveoli and through alveolar vessels

Question 5

What are the 2 main types of vessels of the pulmonary circulation? Describe each.

Answer 5

1. Alveolar: analogous to capillaries and have as sole function to surround alveoli with a thin layer of blood for gas diffusion
2. Extra-alveolar: analogous to small arterioles and venules

Question 6

What can affect blood flow through the alveolar capillaries?

Answer 6

Alveolar increased pressure and increased volume will impede flow through alveolar capillaries

Question 7

What can affect blood flow through the extra alveolar vessels?

Answer 7

Increased total lung volume with lung expansion causes expansion and increased blood flow through extra alveolar vessels

Question 8

Functions of the lung?

Answer 8

1. Respiration
2. Blood reservoir
3. Blood filtration
4. Metabolism
5. Defense

Question 9

What is the defense function of the lung?

Answer 9

Lung produces Ig and stores WBCs

Question 10

Describe the function of the lung as a blood reservoir.

Answer 10

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

Question 11

What can the lung acting like a blood reservoir prevent?

Answer 11

Orthostatic hypotension

Question 12

Describe the function of the lung as a blood filter.

Answer 12

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)

Question 13

Describe the dual blood supply to the lungs.

Answer 13

1. 98% from the pulmonary circulation

2. 1-2% from bronchial systemic circulation

Question 14

Describe the relationship between pulmonary arterial pressure and pulmonary vasculature resistance. Is this a passive or active mechanism? Purpose?

Answer 14

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!)

Question 15

Describe the recruitment mechanism in the lungs.

Answer 15

Increase in pulmonary pressure => blood flow directed to normally closed pulmonary vessels

Question 16

Describe the distension mechanism in the lungs.

Answer 16

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

Question 17

Which circulation is more affected by environmental and passive factors: pulmonary or systemic circulation? Why?

Answer 17

Pulmonary because they have much lower pressures

Question 18

Describe the relationship between pulmonary venous pressure and pulmonary vasculature resistance.

Answer 18

Same relationship, just dealing with much lower increases in venous pressures

Question 19

Does the pulmonary circulation have the myogenic reflex?

Answer 19

NOPE, quite the opposite actually

Question 20

Describe the extra-alveolar vessels at low lung volume. How does this affect resistance?

Answer 20

Slightly collapsed because they do not have the expanded alveoli pulling them open => decreased diameter => increased resistance

Question 21

Describe the curve between lung volume and vascular resistance. What is it due to?

Answer 21

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

Question 22

What are the environmental and passive factors affecting pulmonary blood flow?

Answer 22

1. Pulmonary arterial pressure

2. Gravity

Question 23

Describe the effect of gravity on pulmonary blood flow. Is this a passive or active mechanism?

Answer 23

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

Question 24

Where is the highest blood flow in the lung location wise?

Answer 24

5 cm up the lung

Question 25

Alveolar pressure vs hydrostatic pressure at the bottom of the lung? Result?

Answer 25

Hydrostatic P > alveolar P => vessels distend

Question 26

Alveolar pressure vs hydrostatic pressure at the top of the lung? Result?

Answer 26

Hydrostatic P < alveolar P => vessels collapse

Question 27

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?

Answer 27

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

Question 28

How do we know we are talking about systemic vs pulmonary O2 pressures?

Answer 28

Systemic: Pv and Pa Pulmonary: PA

Question 29

Describe the effects of breathing in a low PO2 atmosphere. Short term? Long term? Potential problem?

Answer 29

1. 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 2. 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)

Question 30

What are the 3 regulations of pulmonary blood flow. Is each active or passive?

Answer 30

1. Pulmonary arterial pressure - passive 2. Gravity - passive 3. Alveolar O2 partial pressure - active

Question 31

Is the fetal circulation in parallel or in series? Explain.

Answer 31

In parallel with mixing of blood between the right and left heart and oxygenated and deoxygenated blood

Question 32

What 6 fetal circulation structures will FUNCTIONALLY disappear postnatally? Describe the function of each.

Answer 32

1 and 2. 2 umbilical arteries: carry low O2 blood TO the placenta 3. Umbilical vein: carries oxygenated blood from the placenta 4. Ductus venosus: connects umbilical vein to fetal venous system, BYPASSING the liver 5. Foramen ovale: shunts oxygenated blood from RA to LA 6. 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

Question 33

How do the 2 umbilical arteries affect the fetal systemic circulation?

Answer 33

Decreased systemic resistance compared to postnatally

Question 34

What 2 options does the blood have once it exits the umbilical vein in the fetal circulation?

Answer 34

Umbilical vein => ductus venosus OR liver circulation to process nutrients

Question 35

What happens to the fetal blood once it reaches the RA?

Answer 35

Most of it is shunted to the LA through the foramen ovale

Question 36

How do the foramen ovale and ductus arteriosus function to bypass the fetal lungs?

Answer 36

Pressures in RA (due to blood flow from ductus venosus) and PA (due to collapsed lung) are higher than in LA and aorta

Question 37

How do the fetal ventricles differ from postnatal ventricles?

Answer 37

Have the same size muscle walls because are pumping against similar resistances

Question 38

Why do the fetal lungs have high resistance?

Answer 38

1. Smallest lung volume => increased resistance (tip of J curve) 2. Low PO2 => vessel constriction

Question 39

In what portion of the fetal circulation is there mixing of oxygenated and deoxygenated blood?

Answer 39

LA

Question 40

Describe the changes to the CV system upon being born.

Answer 40

1. Placenta is delivered => stress of birth => vasospasm of the umbilical arteries to functionally close 2. 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 3. RA pressure decreases => foramen ovale functionally closes 4. Baby takes first breath => increased PO2 in lungs => decreased pulmonary vascular resistance => increased pulmonary blood flow => decreased PAP 4. Ductus arteriosus functionally closes because aortic pressure is higher than PAP and oxygenated which causes spasms and closure

Question 41

Describe how the ductus arteriosus closes.

Answer 41

Prostaglandins are keeping it open in the fetus => O2 causes free radical destruction of the enzymes producing the prostaglandins => vasospasms

Question 42

How to treat a patent ductus arteriosus after birth?

Answer 42

NSAIDs to block prostaglandin production keeping the DA patent

Question 43

Are the pressures in the ductus arteriosus, ductus venosus, and foramen ovale 0 mmHg right at birth?

Answer 43

No, it takes weeks or months for them to fully fully close anatomically

Question 44

% complete closure of foramen ovale in adults?

Answer 44

A little over 60%

Question 45

Describe the vasoconstriction of the pulmonary circulation compared to the systemic circulation.

Answer 45

Much more diffuse in the pulmonary circulation vs more discrete in systemic

Question 46

Do fetuses also have a 95 to 100 PO2?

Answer 46

YUP