Pulmonary Artery Blood

[seminoma]

Costanzo makes a point of saying that pulmonary arterial blood is "true venous blood because the coronary arteries drain into the right atrium and the blood doesn't get mixed up until it reaches the pulmonary artery".

I get that pulmonary arterial blood is mixed venous blood, but what is Costanzo getting at by emphasizing it? And what does the part about coronary artery drainage have to do with it? It's all still mixed venous blood once it enters any venous structure, right?


IVC = mixed venous blood
RA = mixed venous blood
Coronary Sinus = mixed venous blood (potentially different SaO2 than IVC because coronary utilization)
PA = mixed venous blood

---

Members don't see this ad.

 

[Mad Jack]

He's just making sure that you understand a basic concept that a lot of people mix up early on in med school. Don't overthink it.

 

[seminoma]

He's just making sure that you understand a basic concept that a lot of people mix up early on in med school. Don't overthink it.

Ok thanks.

Can you also maybe explain something about the cardiac/vascular function curve graphs? I understand that venous return (vascular function curve) will be zero when right atrial pressure equals mean systemic pressure. However, if venous return is zero, shouldn't cardiac output also be zero? The graph shows that cardiac output continues to increase even as venous return decreases (up to the pressure where CO plateaus)...

 

[Mad Jack]

Ok thanks.

Can you also maybe explain something about the cardiac/vascular function curve graphs? I understand that venous return (vascular function curve) will be zero when right atrial pressure equals mean systemic pressure. However, if venous return is zero, shouldn't cardiac output also be zero? The graph shows that cardiac output continues to increase even as venous return decreases (up to the pressure where CO plateaus)...

You'll need somebody a bit better at hemodynamics to explain that. As I understand it, decreased venous return should decrease cardiac output. I don't fully understand the relationship on the graph you posted, it feels completely counterintuitive to me.

 

[seminoma]

You'll need somebody a bit better at hemodynamics to explain that. As I understand it, decreased venous return should decrease cardiac output. I don't fully understand the relationship on the graph you posted, it feels completely counterintuitive to me.

Hey, thanks for taking a look at it. I think I figured it out. Looking at the graph I posted, if you relabel the x-axis as "pressure (mm Hg)" then it makes complete sense. The x-intercept is mean systemic pressure, and the point at which the two curves intersect is RA pressure. With that in mind, any change in RA pressure will necessitate a shift in one or both curves since RA pressure is the intersection point (rather than any point as in the graph with x-axis labeled "RA pressure").

Your graph is a much better representation of the relationship between CO, MSP, and RA pressure.

 

[dfib slim]

Ok thanks.

Can you also maybe explain something about the cardiac/vascular function curve graphs? I understand that venous return (vascular function curve) will be zero when right atrial pressure equals mean systemic pressure. However, if venous return is zero, shouldn't cardiac output also be zero? The graph shows that cardiac output continues to increase even as venous return decreases (up
the pressure where CO plateaus)...

When looking at a point on the graph imagine the two curves connected at that point and moving as one. If you move the point to venus return (RA pressure) = 0, then the cardiac output curve drops flat.