Chapter 158

### INTRODUCTION

Microcirculation can be defined structurally as blood vessels less than 150 μm in diameter and encompasses the arterioles, capillaries, and venules. Structurally, capillaries consist of a single layer of epithelium along with a basement membrane. The parenchymal cells are arranged in close proximity to at least one capillary to ensure an adequate oxygen supply through passive diffusion.

Physiologically, microcirculation can be defined as the part of circulation where oxygen, nutrients, and waste products are exchanged, and vessels respond to changes in internal pressure with changes in lumen diameter. This physiologic definition highlights the critical role that microcirculation plays in oxygenation and hemodynamic stability.

### NORMAL FUNCTIONS

The two primary functions of microcirculation are: (1) to optimize nutrient and oxygen supply and (2) to reduce large hydrostatic pressure fluctuations. Microcirculation is the important interface between supply that the circulation provides and demands of the parenchymal cells. The pathway for oxygen begins after release from oxyhemoglobin, through the interstitium, into parenchymal cells, and ending with mitochondrial oxidative phosphorylation in which cellular ATP is generated. The cardiovascular system must provide sufficient blood flow to the capillaries of an organ to support the diffusional fluxes of solutes across the capillary walls to meet metabolic needs. Autoregulation and vasomotor changes occur in the microvasculature to maintain adequate and stable blood flow. Thus, from the end-organ perspective, the main determinants of tissue perfusion are oxygen concentration and capillary blood flow.

### FICK’S PRINCIPLE

The transcapillary flux of solutes can be calculated using Fick’s equation, which is based on the Law of Conservation of Mass. The arteriovenous difference of a solute multiplied by the blood flow through the capillary gives the flux of that solute across the capillary wall. Thus, increasing the oxygen concentration or blood flow will influence solute flux to meet the metabolic demands of the cells.

Transcapillary Flux of X (Fx) = BF × ([X]a – [X]v) Fx = BFa × [X]a – BFv × [X]v

Fx = flux of solute X across the capillary wall (mass/min)

BFa = blood flow entering capillary (mL/min)

BFv = blood flow leaving capillary (mL/min)

[X]a = concentration of X in arterial blood (mass/mL)

[X]b = concentration of X in venous blood (mass/mL)

### METHODS TO ASSESS MICROCIRCULATION FLOW

To accurately assess microcirculation, information regarding the oxygen tension and blood flow is needed. Microelectrodes are used to study oxygen tension in the interstitial fluid and mitochondria. Optical technologies such as the orthogonal polarization spectral and sidestream dark-field imaging methods are used to determine microcirculatory network through detecting erythrocyte movements. Clinically, mixed venous saturation and cardiac output are used as surrogates to determine adequacy of oxygen ...

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