Transportation of molecules and nutrients to the body by the vascular system includes extra-vascular plasma transportation. Perfusion of the tissues happens as blood is pumped from capillaries into the interstitial space, by hydrostatic pressure generated from the heart. Blood plasma leaves the arterial system from the capillaries, which are very leaky, and is spread throughout the tissue in the interstitial space. This fluid is partially reabsorbed into the capillaries again, by osmosis, in an equilibrium between hydrostatic and oncotic forces, defined by Ernest Starling in 1896. Osmosis, or endosmosis and exosmosis as Rene J. H. Dutrochet called it in 1828, is driven by an electrical gradient from polarization of water into a negatively charged gel-phase, (H3O2-)n, and positively charged protons bonded with water as hydronium (Pollack, 2013). Interstitial fluid is also reabsorbed via the "interstitial venous system" (the "lymphatic system") and transported with the rest of the venous system into vena cava superior. This interstitial venous system compensates for that perfusion of the body tissues is done by a process of internal bleeding, it circulates blood plasma back from the interstitium, preventing death. Absorption from interstitium can be likened to how plant roots absorb water and nutrients from soil.

The transition from aquatic environment of high ambient pressure to a terrestrial life with a lower ambient pressure shifts the hydrostatic component in Sterling's equations. Phyla that reflect a transition from aquatic to terrestrial life, such as amphibians like Anurans (frogs), have adapted to this increased elasticity of their interstitial space (Hendrick, 2013; Hillman, 2018), from lower ambient pressure, with a more developed interstitial venous system, with separate hearts just to improve reabsorption, so called "lymph hearts" (Muller, 1833; Hendrick, 2013; Hillman, 2018). These phyla have a net efflux from vascular to the interstitial space (Hendrick, 2013; Hillman, 2018). They are a good example of how the circulatory system involves constant internal hemorrhage, as they die from hypovolemia if their lymph hearts fail, they bleed to death inside their own bodies (Hendrick, 2013; Hillman, 2018) as a consequence of normal circulatory function.

Synapses

Dutrochet, R. (1828). Researches in Endosmosis and Exosmosis.

Muller, John and Horner, Leonard. (1837). On the existence of four distinct hearts, having regular pulsations, connected with the lymphatic system, in certain amphibious animals. Proc. R. Soc. Lond. http://doi.org/10.1098/rspl.1830.0094

Starling, E. H. (1896). On the Absorption of Fluids from the Connective Tissue Spaces. The Journal of Physiology, 19(4), 312–326. https://doi.org/10.1113/jphysiol.1896.sp000596

Pollack, G. (2013). The Fourth Phase Of Water. Seattle, WA. Ebner & Sons.

Hedrick, Michael & Hillman, Stanley & Drewes, Robert & Withers, Philip. (2013). Lymphatic regulation in nonmammalian vertebrates. Journal of applied physiology (Bethesda, Md. : 1985). 115. 10.1152/japplphysiol.00201.2013.

Hillman, S. S. (2018). Anuran amphibians as comparative models for understanding extreme dehydration tolerance: a unique negative feedback lymphatic mechanism for blood volume regulation. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 315(4), R790–R798. https://doi.org/10.1152/ajpregu.00160.2018