Analysis of the hematocrit distribution in the mesenteric microcirculation

Area of Science:

• Physiology
• Biophysics
• Microcirculation Research

Background:

• Hematocrit (Hsys) represents the overall red blood cell concentration in systemic circulation.
• Microcirculation hematocrit (Hmicro) can vary significantly from Hsys, particularly in smaller vessels.
• The Fahraeus effect describes the reduction in blood viscosity and hematocrit in narrow tubes.

Purpose of the Study:

• To determine the hematocrit in rat mesenteric microvessels.
• To investigate the relationship between vessel diameter, hematocrit, and red blood cell distribution.
• To reconcile observed microhematocrit values with conservation laws using mathematical modeling.

Main Methods:

• Hematocrit determination via optical density and red cell counts from microvessel photographs.
• In vitro experiments to correct for the Fahraeus effect using centrifuged blood in glass tubes.
• Mathematical modeling incorporating flow rate dependence and shunt vessels.

Main Results:

• Mean Hmicro decreased with decreasing vessel diameter, with capillary Hmicro/Hsys averaging 0.36.
• Even after Fahraeus effect correction, data suggested a 'loss' of red cells in microcirculation.
• A model combining flow-dependent hematocrit and a shunt component best described the experimental data.

Conclusions:

• Microcirculatory hematocrit is significantly lower than systemic hematocrit, particularly in capillaries.
• Red blood cell distribution in microvessels is influenced by vessel diameter, flow rate, and potential shunt pathways.
• Vasodilation can increase average capillary hematocrit by promoting more homogeneous flow distribution.