Abstract
Oxygen delivery to cells is the basic prerequisite of life. Within the human body, an ingenious oxygen delivery system, comprising steps of convection and diffusion from the upper airways via the lungs and the cardiovascular system to the microvascular area, bridges the gap between oxygen in the outside airspace and the interstitial space around the cells. However, the complexity of this evolutionary development makes us prone to pathophysiological problems. While those problems related to respiration and macrohemodynamics have already been successfully addressed by modern medicine, the pathophysiology of the microcirculation is still often a closed book in daily practice. Nevertheless, here as well, profound physiological understanding is the only key to rational therapeutic decisions. The prime guarantor of tissue oxygenation is tissue blood flow. Therefore, on the premise of intact macrohemodynamics, the microcirculation has three major responsibilities: 1) providing access for oxygenated blood to the tissues and appropriate return of volume; 2) maintaining global tissue flood flow, even in the face of changes in central blood pressure; and 3) linking local blood flow to local metabolic needs. It is an intriguing concept of nature to do this mainly by local regulatory mechanisms, impacting primarily on flow resistance, be this via endothelial or direct smooth muscle actions. The final goal of microvascular blood flow per unit of time is to ensure the needed exchange of substances between tissue and blood compartments. The two principle means of accomplishing this are diffusion and filtration. While simple diffusion is the quantitatively most important form of capillary exchange activity for the respiratory gases, water flux across the blood-brain barrier is facilitated via preformed specialized channels, the aquaporines. Beyond that, the vascular barrier is practically nowhere completely tight for water, with paracellular filtration giving rise to generally low but permanent fluid flux outwards into the interstitial space at the microvascular high pressure segment. At the more leaky venular aspect, both filtration and diffusion allow for bidirectional passage of water, nutrients, and waste products. We are just beginning to appreciate that a major factor for maintaining tissue fluid homeostasis appears to be the integrity of the endothelial glycocalyx.
Highlights
Single cell eucaryotes communicate directly with their aqueous environment to extract oxygen (O2) and nutrients, and to excrete carbon dioxide (CO2) and waste products, via the cell surface
Jacob et al Critical Care (2016) 20:319 for cells within complex organ systems: oxygenation at the cellular level is the basic prerequisite for maintaining aerobic metabolism, enabling the maximal possible gain of adenosine triphosphate (ATP) per molecule of glucose while avoiding large-scale production of lactic acid
Where Adenosine diphosphate (ADP) = adenosine diphosphate, CO2 = carbon dioxide; P = phosphate, H2O = water. This equation holds true allowing that the 2 molecules of GTP initially formed in the citrate cycle per molecule of glucose are converted to ATP
Summary
Single cell eucaryotes communicate directly with their aqueous environment to extract oxygen (O2) and nutrients, and to excrete carbon dioxide (CO2) and waste products, via the cell surface. Maldistributive shock resulting from a problem with directing the blood flow towards the tissues with high metabolic levels might contribute to the clinically well known oxygen extraction deficit [14].
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