Venules: The minute vessels that collect blood from the capillary plexuses and join together to form veins.Leukocytes: White blood cells. These include granular leukocytes (BASOPHILS; EOSINOPHILS; and NEUTROPHILS) as well as non-granular leukocytes (LYMPHOCYTES and MONOCYTES).Endothelium, Lymphatic: Unbroken cellular lining (intima) of the lymph vessels (e.g., the high endothelial lymphatic venules). It is more permeable than vascular endothelium, lacking selective absorption and functioning mainly to remove plasma proteins that have filtered through the capillaries into the tissue spaces.Arterioles: The smallest divisions of the arteries located between the muscular arteries and the capillaries.Microcirculation: The circulation of the BLOOD through the MICROVASCULAR NETWORK.Mesenteric Veins: Veins which return blood from the intestines; the inferior mesenteric vein empties into the splenic vein, the superior mesenteric vein joins the splenic vein to form the portal vein.Leukocyte Rolling: Movement of tethered, spherical LEUKOCYTES along the endothelial surface of the microvasculature. The tethering and rolling involves interaction with SELECTINS and other adhesion molecules in both the ENDOTHELIUM and leukocyte. The rolling leukocyte then becomes activated by CHEMOKINES, flattens out, and firmly adheres to the endothelial surface in preparation for transmigration through the interendothelial cell junction. (From Abbas, Cellular and Molecular Immunology, 3rd ed)L-Selectin: Cell adhesion molecule and CD antigen that serves as a homing receptor for lymphocytes to lymph node high endothelial venules.Cell Adhesion: Adherence of cells to surfaces or to other cells.Mesentery: A layer of the peritoneum which attaches the abdominal viscera to the ABDOMINAL WALL and conveys their blood vessels and nerves.P-Selectin: Cell adhesion molecule and CD antigen that mediates the adhesion of neutrophils and monocytes to activated platelets and endothelial cells.Capillary Permeability: The property of blood capillary ENDOTHELIUM that allows for the selective exchange of substances between the blood and surrounding tissues and through membranous barriers such as the BLOOD-AIR BARRIER; BLOOD-AQUEOUS BARRIER; BLOOD-BRAIN BARRIER; BLOOD-NERVE BARRIER; BLOOD-RETINAL BARRIER; and BLOOD-TESTIS BARRIER. Small lipid-soluble molecules such as carbon dioxide and oxygen move freely by diffusion. Water and water-soluble molecules cannot pass through the endothelial walls and are dependent on microscopic pores. These pores show narrow areas (TIGHT JUNCTIONS) which may limit large molecule movement.Splanchnic Circulation: The circulation of blood through the BLOOD VESSELS supplying the abdominal VISCERA.Microscopy, Video: Microscopy in which television cameras are used to brighten magnified images that are otherwise too dark to be seen with the naked eye. It is used frequently in TELEPATHOLOGY.Endothelium, Vascular: Single pavement layer of cells which line the luminal surface of the entire vascular system and regulate the transport of macromolecules and blood components.Capillaries: The minute vessels that connect the arterioles and venules.Cheek: The part of the face that is below the eye and to the side of the nose and mouth.Lymphatic System: A system of organs and tissues that process and transport immune cells and LYMPH.Lymph Nodes: They are oval or bean shaped bodies (1 - 30 mm in diameter) located along the lymphatic system.Receptors, Lymphocyte Homing: Cell surface glycoproteins on lymphocytes and other leukocytes that mediate adhesion to specialized blood vessels called high endothelial venules. Several different classes of lymphocyte homing receptors have been identified, and they appear to target different surface molecules (addressins) on high endothelial venules in different tissues. The adhesion plays a crucial role in the trafficking of lymphocytes.Endothelium: A layer of epithelium that lines the heart, blood vessels (ENDOTHELIUM, VASCULAR), lymph vessels (ENDOTHELIUM, LYMPHATIC), and the serous cavities of the body.Corrosion Casting: A tissue preparation technique that involves the injecting of plastic (acrylates) into blood vessels or other hollow viscera and treating the tissue with a caustic substance. This results in a negative copy or a solid replica of the enclosed space of the tissue that is ready for viewing under a scanning electron microscope.Veins: The vessels carrying blood away from the capillary beds.Mucoproteins: Conjugated proteins in which mucopolysaccharides are combined with proteins. The mucopolysaccharide moiety is the predominant group with the protein making up only a small percentage of the total weight.Erythrocyte Aggregation: The formation of clumps of RED BLOOD CELLS under low or non-flow conditions, resulting from the attraction forces between the red blood cells. The cells adhere to each other in rouleaux aggregates. Slight mechanical force, such as occurs in the circulation, is enough to disperse these aggregates. Stronger or weaker than normal aggregation may result from a variety of effects in the ERYTHROCYTE MEMBRANE or in BLOOD PLASMA. The degree of aggregation is affected by ERYTHROCYTE DEFORMABILITY, erythrocyte membrane sialylation, masking of negative surface charge by plasma proteins, etc. BLOOD VISCOSITY and the ERYTHROCYTE SEDIMENTATION RATE are affected by the amount of erythrocyte aggregation and are parameters used to measure the aggregation.Cell Movement: The movement of cells from one location to another. Distinguish from CYTOKINESIS which is the process of dividing the CYTOPLASM of a cell.Selectins: Transmembrane proteins consisting of a lectin-like domain, an epidermal growth factor-like domain, and a variable number of domains that are homologous to complement regulatory proteins. They are important cell adhesion molecules which help LEUKOCYTES attach to VASCULAR ENDOTHELIUM.Cell Adhesion Molecules: Surface ligands, usually glycoproteins, that mediate cell-to-cell adhesion. Their functions include the assembly and interconnection of various vertebrate systems, as well as maintenance of tissue integration, wound healing, morphogenic movements, cellular migrations, and metastasis.Retinal Vessels: The blood vessels which supply and drain the RETINA.Chemokine CCL21: A CC-type chemokine with specificity for CCR7 RECEPTORS. It has activity towards DENDRITIC CELLS and T-LYMPHOCYTES.Blood Flow Velocity: A value equal to the total volume flow divided by the cross-sectional area of the vascular bed.Mice, Inbred C57BLE-Selectin: Cell adhesion molecule and CD antigen that mediates neutrophil, monocyte, and memory T-cell adhesion to cytokine-activated endothelial cells. E-selectin recognizes sialylated carbohydrate groups related to the Lewis X or Lewis A family.Sialomucins: A subcategory of mucins that contain SIALIC ACID.Integrin alpha4: An integrin alpha subunit that is unique in that it does not contain an I domain, and its proteolytic cleavage site is near the middle of the extracellular portion of the polypeptide rather than close to the membrane as in other integrin alpha subunits.Glycocalyx: The carbohydrate-rich zone on the cell surface. This zone can be visualized by a variety of stains as well as by its affinity for lectins. Although most of the carbohydrate is attached to intrinsic plasma membrane molecules, the glycocalyx usually also contains both glycoproteins and proteoglycans that have been secreted into the extracellular space and then adsorbed onto the cell surface. (Alberts et al., Molecular Biology of the Cell, 3d ed, p502)Capillary Resistance: The vascular resistance to the flow of BLOOD through the CAPILLARIES portions of the peripheral vascular bed.Microvessels: The finer blood vessels of the vasculature that are generally less than 100 microns in internal diameter.Lymphocytes: White blood cells formed in the body's lymphoid tissue. The nucleus is round or ovoid with coarse, irregularly clumped chromatin while the cytoplasm is typically pale blue with azurophilic (if any) granules. Most lymphocytes can be classified as either T or B (with subpopulations of each), or NATURAL KILLER CELLS.