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So, I have been wondering and couldn't find texts explaining this, but why is the diastolic volume of the circulation ~80, while that of the ventricles is almost 0. To phrase it another way, why does the ventricular pressure oscillate between 120 (systole) and ~0 during diastole, while that in the arteries, it is 120/80 (just assuming ideal case)?

I get that the diastolic pressure in the vessels is caused by the volume of blood that is present there, pushing- so to say- the arteries, but isn't there some volume of blood in the ventricle as well (End diastolic volume) that can cause some pressure to the walls of the ventricles. Till now, I have pictured it something like this:

In the systemic circulation, there is a pressure gradient, from the arteries to the veins so blood tends to flow downstream, however, there is the TPR, because of which the blood exerts the pressure (the diastolic pressure) on the arteries. However, in the ventricles, there is no pressure gradient within the ventricle- as it is one chamber. Thus, there is no flow within the ventricles, so there is minimal pressure exerted during diastole.

I have not read it from anywhere and it is just some intuitive explanation. I would love it if anyone could verify this as true, or if not, point me towards some resources where I might get the answer. Thanks :)

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First, I’ll cover some anatomy and physiology of the heart.

Cardiac anatomy

Cardiac anatomy

The heart is divided into two sides. The right side (on the left of the image) receives blood from the body and pumps it to the lungs. The left side (on the right of the image) receives blood back from the lungs and pumps it to the body.

Cardiac physiology

Blood entering the heart first enters an atrium. This contracts to fill the ventricle (the main chamber), which then contracts to expel blood from the heart. This cycle is divided into two main parts: diastole (when the atrium contracts and the ventricle is relaxed and filling) and systole (when the ventricle contracts).

Left ventricular systolic pressure must exceed the aortic diastolic pressure for blood to be ejected from the ventricle.

Blood pressure

The cardiac output (CO - measured in litres per minute) depends on the stroke volume (SV - blood pumped with each cardiac cycle) multiplied by the heart rate (HR).

Systemic vascular resistance (SVR - often also called total peripheral resistance) is equal to the difference between the mean arterial pressure (MAP) and central venous pressure (CVP) divided by the cardiac output. CVP is usually close to zero, so this is often shortened to:

Equation for SVR

Blood pressure (BP) is a measure of the force being exerted on the walls of arteries as blood is pumped out of the heart. It is proportional to cardiac output (CO) and systemic vascular resistance (SVR). It is usually expressed as the systolic blood pressure over the diastolic blood pressure.

Importantly, with regard to your question, while peripheral systolic blood pressure is largely due to ventricular contraction in the heart, diastolic pressure in peripheral arteries is due to the walls of the arteries contracting passively after having been expanded during the pressure wave of systole.

This diagram shows the atrial, ventricular and aortic pressures during the cardiac cycle.

Cardiac cycle

This is a pressure volume loop showing the relationship of ventricular pressure and volume during a single cardiac cycle.

Pressure volume loop

You can see from both of these images that there is a point in early diastole where left ventricular pressure drops close to zero, before rising slowly. Later in diastole, the pressure rises gradually and the left ventricular end-diastolic pressure (LVEDP) is commonly measured as a metric of cardiac performance.

Why the low left ventricular diastolic pressure?

This low diastolic ventricular pressure is primarily because a well-functioning ventricle will now be isolated from the systemic circulation by the closed aortic valve. In addition, it will have expelled a significant amount of the blood contained within (a normal ejection fraction (EF) is 55-70%), it is now relaxing and expanding in size and it has not yet received much blood from the atrium.

In heart failure with a impaired systolic function, the ejection fraction (EF) drops, but there is a phenomenon of diastolic dysfunction, where the EF is normal and the problem is impaired relaxation and compliance of the left ventricle. This is commonly linked to hypertension, as displayed in this paper.

This pressure volume loop (in red) demonstrates this effect in diastolic dysfunction. ESPVR / EDPVR = end systolic / diastolic pressure volume relationship (a gradient of the graph or first derivative of pressure with respect to volume).

Diastolic dysfunction PV graph

Also, there is evidence of a link between a higher LVEDP and increased mortality when measured during coronary angiography for acute myocardial infarction.


In summary, the left ventricular diastolic pressure being low is normal and several factors are responsible for that. It is higher where there is ventricular diastolic dysfunction.


Sources:

Source for images

CV Physiology

Planer et al

Lelande and Johnson

Bagai et al

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    Thanks a lot. I read a lot of physiology for this (Guyton, Ganong, etc), but couldn't find a convincing answer. I just needed an intuitive mind picture of the phenomenon and I loved this particular statement: "diastolic pressure is due to the walls of arteries contracting passively after having been expanded during the pressure wave of systole." – abacus143 Nov 10 '19 at 7:15
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    @abacus143 Thanks. From your question I thought it sounded like you were familiar with the physiology. The background will hopefully be helpful for other people reading. Ganong is the physiology text I remember, but it’s been a few years! – Chris Nov 10 '19 at 8:06
  • Pressure in the cardiac chambers is a function of volume and compliance. The walls of the ventricles do not contract during diastole. The walls relax using energy dependent and passive mechanisms. In addition, when the mitral valve opens after isovolumetric relaxation, volume enters a healthy heart until onset of isovolumetric contraction. – Todd D Nov 10 '19 at 15:16
  • @ToddD Thanks for this. That particular part was referring to maintenance of diastolic blood pressure in the peripheral circulation, which is partly maintained by elastic recoil of the vessels. I agree that the ventricular walls do not contract during diastole. While a doctor I am not a cardiologist and it has been a while since I studied cardiac physiology. Could you recommend any improvements to the answer? – Chris Nov 10 '19 at 15:22
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    I believe the difference between pressure in the systemic circulation and LV during diastole is best explained by the normal function of the aortic valve. – Todd D Nov 10 '19 at 19:15

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