Greta Donisi Pecchiari Lesson 11 - 23/03/2015

Mechanical coupling between the heart and the vessels

this man here is Guyton, the author of the book.

(slide 1, above) What you see here is an electrical circuit consis

(slide 2, above) Before we go on, we need to summarise all we know about cardiac func

to a given situa

wer cardiac output rela

distance between the total PV curve and the x axis in the rst graph drawn). the two structures are mechanically in parallel and therefore the pressure dierence between the inside and the outside is given by the sum of the pressure dierence across each individual part.

This occurs in normal condi

(slide 4) Here we see the eects of changing the heart rate on cardiac func

(slide 6) Now please ignore these lines that are va-scular func

(slide 7, above) Consider a normal cardiovascular system (panel A); this is however a model, the elements of these model are made by: the pump including not only the two parts of the heart but also the lungs, so you can imagine this is a pump- oxygenator and its equivalent to the heart-lung prepara

in the arterial compartment decreases and the V in the venous compartment increases, this goes on un

(Slide 8)But this things have been veried also in animal experiments and the heart was replaced by an ar

(slide 11, above) the next step is to understand which are the determinants of this cardiac curve. In order to do so we have to assume a model, you see the model I've chosen which is one of the se-veral models presented by guyton himself. This is an electrical representa

se to use this model simply because it will highlight the dependence of the CO on the resistance of the terminal part of the venous compartment. This is maybe not of interest for physiologists be-cause most of the control of peripheral resistances is made at level of arteriolar resistance. But in several clinical situa

(slide 15) More algebra and we obtain the equa

(Slide 16) Now compare the rela

should have a P dierence between the arterial and venous compartment and therefore you have to transfer blood from the venous to the arterial compartment, if the values of venous and arterial compliance are unaected by the degree of lling, for a certain volume change you will have a cer-tain pressure dierence that will correspond to a give ow through the peripheral res, and this P dierence will be independent on the state of lling and therefore increasing the cardiac output we'll have a certain decrease of the central venous pressure which will be independent on the star-

(Slide 19, above) It is important to realize that we have a poten

To try to understand what this graph means from a physical point of view we can imagine to do an ideal experiment and it's described in Berne and Levy for who is reading it. We can imagine to do the following, very quickly, instantaneously we take an amount of blood from the arterial com-partment and we transfer to the venous compartment so that the total intravascular volume is not changed, when you take a volume of blood from the arterial to the venous compartment you have that the volume of the venous compartment is increased and therefore the pressure of the venous compartment should be increased too, this is shown here from point D to point A; we can imagine that the transfer is instantaneous so we have not change of the CO with the change in central ve-nous pressure. But we know according to the CFC for this level of preload this level of CO should correspond so that if you do this opera

(slide 21) We can also see the eect of a transfusion. This is the CFC(green line) this is the VFC in control situa

Q: in the model in which we have also the resistances, why Rv is only due to big veins?

A: because you can if you want think about the resistance of the small veins but the fact is that Rv occurs aTer the compliance soma least in the model should be aTer the capacitance vessel. Guy-ton made several of these models with dierent subdivisions of the venous compartment of the arterial compartment and so on and each of this compartment was represented by a compliance and a resistance. I choose this one because it is rather simple and gives you an idea of what hap-pens when you change arteriolar resistances and when you compress the veins.

You can imagine that if the venules and small veins were the big capacitance vessels you could stretch the model and say that Rv is the resistance aTer these vessels.

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