Figure 3.

Stressed and unstressed vascular volume. The volume inside a vessel at near zero transmural pressure is termed ‘unstressed volume’ (blue). It fills the system without exerting tension in the vessel wall. The blood volume that creates positive transmural pressure via the elastic recoil of the vessel wall is termed ‘stressed volume’ (red). Mean circulatory filling pressure (MCFP) is a function of stressed volume and vascular compliance; compliance is the slope of the pressure–volume curve above the unstressed volume. (A) Cross section of a blood vessel. (B) The relationship between blood volume and MCFP. (C) The venous system contains approximately 70% of the blood volume. The splanchnic vascular bed serves as a reservoir and will adjust the amount of venous return based on signalling from the autonomic nervous system. In chronic HF, the increased sympathetic tone associated with activation of the renin–angiotensin–aldosterone system will also activate the splanchnic circulation to increase venous return, referred to SBV. This will increase the CVP and the pulmonary capillary wedge pressure. Q, cardiac output; R, systemic vascular resistance; RA: right atrium. Panels (A) and (B) freely adapted from Grübler et al. ⁵⁹ Panel (C) is freely adapted from Noel‐Morgan and Muir. ⁶⁰