Subcapsular Splenic Microcirculation Quantitatively Analyzed By In Vivo Microscopy

The objective of this work was to improve in vivo microscopy of the splenic microcirculation to obtain quantitative data such as velocity of RBCs, diameters and volumes of vessels, and to study possible mechanisms controlling redistribution of blood flow.;Illumination limitations necessitated using small spleens. Young rats (27-89 g) were the predominant species studied, although mice were occasionally used. Inverted microscopy and oblique illumination were key elements in obtaining high quality video images for quantitative analysis. Still photographs were obtained from video images by exposing film to several successive fields.;The splenic volume occupied by capillaries, reticular meshwork and sinuses is 0.16%, 84% and 15% respectively. Mean diameters ({dollar}\mu{dollar}m) of, and RBC velocities ({dollar}\mu{dollar}m/s) in, the above channels are: 5.3 and 296; N/A and 0.3; 13.2 and 87 respectively. Sinusal surface area per splenic volume is 365 sq.cm/ml. Mean rate of RBC passage through the interendothelial slits (IESs) is 15.5 RBCs/slit/min. Thus, a maximum of 3.6% of the splenic blood flow can pass through the slits. Flow balancing shows that roughly one-half of the flow from the capillaries bypasses the reticular meshwork and suggests that the subcapsular region of the rat spleen is not representative of deeper structures.;Statistical analysis of in vivo spontaneous cyclic contractions of the capillary wall demonstrates that endothelial cells are responsible for 18-77% of the total variance in luminal diameter, whereas pericytes contributed only 0-4%. In 50% of contractions the luminal diameter decreased to less than 1{dollar}\mu{dollar}m, sometimes for longer than one minute often stopping the flow of RBCs. This phenomena may be one mechanism responsible for blood flow redistribution in the spleen. Analysis of 1500 RBCs squeezing through IES have shown that RBCs invariably flow from reticular meshwork to venous sinuses at mean rates of 5.6 to 28 cells/min. The RBCs flow through the IES in brief discontinuous bursts, sometimes as fast as 10 cells/s. Transit times of the RBCs range from 0.02 to 60.5s. The varying rate through IESs suggests that the IESs are not passive, as previously believed, but are capable of changing their caliber and may constitute another mechanism of flow redistribution