Vascular function curve (of Guyton)

[Ref: KB2:p103-104, PK1:p135-141;BL8:p200-215]

Shows the effect of increases in cardiac output (independent variable) on right atrial pressure (dependent variable)

• Dependent on the characteristics of the vascular system
  * Peripheral resistance
  * Arterial and venous compliance
  * Blood volume
• Independent of the heart
• Note that the axes are swapped when drawn together with cardiac function curve, such that right atrial pressure is on the x-axis.

Mean systemic pressure [KB2]

aka static pressure [BL8]

aka mean circulatory pressure [BL8]

aka mean systolic filling pressure [PK1]

[KB2:p103] ... a theoretical pressure that is said to represent the force promoting venous return.

[PK1:p136] MSFP is (proposed by Guyton)... the average of all the pressures in the different vessels weighted according to their relative compliances.

--> MSFP approximates to mean venous pressure

Normal value ~ 7mmHg

Basically, my understanding is:

Mean systemic pressure is the mean pressure exerted by the volume of blood alone, at a given vascular system (and vessel compliances) without any contribution from the heart, or any activation of compensatory reflexes.

Changes in mean systemic pressure

Mean systemic pressure increases with:

• Venoconstriction
• Increase in blood volume

Not significantly affected by total peripheral resistance
* Because arterioles which contribute the most to resistance only contain very little blood (1-2% of total blood volume)

 

NB:

• At some critical maximum value of cardiac output, sufficent fluid will be translocated from the venous to the arterial side of the circulation to reduce the pressure in venous side to below ambient pressure, causing collapse and impedient to venous return.

Slope of vascular function curve

Compliance of arteries to veins
~ 1:19

When cardiac output increase, blood is moved from venous circulation to arterial circulation

Increase in arterial blood volume
= Decrease in venous blood volume

However, because of the differences in compliance

--> Increase in arterial BP is 19 times the decrease in venous BP

Mathematical expression

[BL8:p203-205]

[##20050614(01)]

dPa = -19 x dPv

R = (Pa-Pv)/Qr

--> Pv = - [R.Ca/(Ca+Cv)]Qr + Pmc

• Pv = Venous pressure
• Pa = Arterial pressure
• R = Resistance
• Ca = Arterial compliance
• Cv = Venous compliance
• Qr = Cardiac output
• Pmc = mean circulatory pressure (i.e. static pressure)

Thus, slope of the curve is only dependent on:

• Resistance
• Arterial compliance
• Venous compliance

 

Shifts in vascular function curve

Increase in blood volume

--> MSP increased, gradient unchanged

--> Vascular function curve moves up

 

Vasoconstriction

--> MSP unchanged, gradient decreases

--> Vascular function curve rotate downwards

 

Sympathetic stimulation

--> MSP increased (due to venoconstriction), gradient decreased (due to vasoconstriction)

--> Vascular function curve shifts upwards and slope flatter

 

Heart failure

--> MSP increased (due to fluid retention), gradient unchanged

--> Vascular function curve shifts upwards