Venous return in CC and shock. Pathophys. Funk et al. 2013. CC med Flashcards
What percentage of blood volume do veins contain compared to arteries
- 70% of blood volumes in veins
- 18% of blood volumes in arteries
What percentage of blood volume does the venous circulation of the splanchnic bed hold?
20-33%
Describe the Hagen-Poiseuille’s law
The Hagen-Poiseuille’s law describes how fluid flow through a system depends on the pressure gradient between points and the resistance of the system
i.e., pressure difference of vascular structures and vascular resistance
How can you modify the Hagen-Poiseuille’s law to describe cardiac output?
cardiac output is the flow and MAP is the starting pressure point (P1) and RA pressure is the end pressure point (P2)
systemic vascular resistance is the resistance to flow through the circuit
Explain the modification of Hagen-Poiseuille’s law to describe venous return and how does this put the arterial circulation into perspective?
The venous return (i.e. flow) is determined by the upstream pressure of the venous circulation/mean systemic pressure (Pms) (i.e., P1) and the RA pressure (P2), the venous vascular resistance is R
The arterial circulation is not included in this calculation and implies that the aerterial pressure is unrelated to VR, other than maintaining to volume of the venous reservoir
Describe the calculation for vascular resistance
Resistance is calculation is
- directly proportional to the viscosity of blood, the length of the vessel
- indirectly proportional to the radius of the vessel
When assessing the calculation for vascular resistance, in what clinical settings does viscosity matter?
after administration of large amounts of low-viscosity fluids, i.e., hemodilution, the contribution of viscosity of vascular resistance may become more significant
How can alterations in cardiac function affect venous return?
decreased cardiac function may increase RA pressure, and therefore decrease the driving pressure/pressure gradient between venous circulation and RA for venous return
What is the normal pressure in venules compared to the RA?
8-12 mm Hg in the venules and 1-2 in the RA/vena cava
What calculation describes mean systemic pressure Pms?
the mean systemic pressure can be calculated with the stressed blood volume over the compliance of the venous reservoir (i.e., venosu vessels)
stressed blood volume = total blood volume - unstressed blood volume
Explain stressed and unstressed blood volume
unstressed blood volume is the volume of blood required to fill the circulatory system to capacity without an increase in transmural pressure
stressed blood volume is the volume of blood added on top of the unstressed blood volume to achieve the vascular transmural pressure
According to Pms = (Vt - V0)/C how can Pms be altered
Pms may be altered through 2 mechanisms:
- change in total blood volume
- change in the proportion of stressed and unstressed blood volume
in this model C (vessel compliance) is viewed as an aggregrate static mechanical proprty of the vessel wall
What may change the proportion of unstressed blood volume to stressed blood volume
- alteration in autonomic tone
- catecholamines
- administration of exogenous vasoactive substances
in this model C (vessel compliance) is viewed as an aggregrate static mechanical proprty of the vessel wall
What percentage of the human blood volume is normally stressed volume?
20-30%
What is the normal Pms in people?
8-10 mm Hg