Hematocrit: The volume of packed RED BLOOD CELLS in a blood specimen. The volume is measured by centrifugation in a tube with graduated markings, or with automated blood cell counters. It is an indicator of erythrocyte status in disease. For example, ANEMIA shows a low value; POLYCYTHEMIA, a high value.Hemodilution: Reduction of blood viscosity usually by the addition of cell free solutions. Used clinically (1) in states of impaired microcirculation, (2) for replacement of intraoperative blood loss without homologous blood transfusion, and (3) in cardiopulmonary bypass and hypothermia.Blood Viscosity: The internal resistance of the BLOOD to shear forces. The in vitro measure of whole blood viscosity is of limited clinical utility because it bears little relationship to the actual viscosity within the circulation, but an increase in the viscosity of circulating blood can contribute to morbidity in patients suffering from disorders such as SICKLE CELL ANEMIA and POLYCYTHEMIA.Erythrocyte Count: The number of RED BLOOD CELLS per unit volume in a sample of venous BLOOD.Plasma Volume: Volume of PLASMA in the circulation. It is usually measured by INDICATOR DILUTION TECHNIQUES.Hemoglobins: The oxygen-carrying proteins of ERYTHROCYTES. They are found in all vertebrates and some invertebrates. The number of globin subunits in the hemoglobin quaternary structure differs between species. Structures range from monomeric to a variety of multimeric arrangements.Erythropoietin: Glycoprotein hormone, secreted chiefly by the KIDNEY in the adult and the LIVER in the FETUS, that acts on erythroid stem cells of the BONE MARROW to stimulate proliferation and differentiation.Anemia: A reduction in the number of circulating ERYTHROCYTES or in the quantity of HEMOGLOBIN.Polycythemia: An increase in the total red cell mass of the blood. (Dorland, 27th ed)Erythrocyte Volume: Volume of circulating ERYTHROCYTES . It is usually measured by RADIOISOTOPE DILUTION TECHNIQUE.Blood Volume: Volume of circulating BLOOD. It is the sum of the PLASMA VOLUME and ERYTHROCYTE VOLUME.Exchange Transfusion, Whole Blood: Repetitive withdrawal of small amounts of blood and replacement with donor blood until a large proportion of the blood volume has been exchanged. Used in treatment of fetal erythroblastosis, hepatic coma, sickle cell anemia, disseminated intravascular coagulation, septicemia, burns, thrombotic thrombopenic purpura, and fulminant malaria.Erythrocyte Indices: ERYTHROCYTE size and HEMOGLOBIN content or concentration, usually derived from ERYTHROCYTE COUNT; BLOOD hemoglobin concentration; and HEMATOCRIT. The indices include the mean corpuscular volume (MCV), the mean corpuscular hemoglobin (MCH), and the mean corpuscular hemoglobin concentration (MCHC).Bloodletting: Puncture of a vein to draw blood for therapeutic purposes. Bloodletting therapy has been used in Talmudic and Indian medicine since the medieval time, and was still practiced widely in the 18th and 19th centuries. Its modern counterpart is PHLEBOTOMY.Erythrocytes: Red blood cells. Mature erythrocytes are non-nucleated, biconcave disks containing HEMOGLOBIN whose function is to transport OXYGEN.Hematologic Tests: Tests used in the analysis of the hemic system.Hemoglobinometry: Measurement of hemoglobin concentration in blood.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.Blood Cell Count: The number of LEUKOCYTES and ERYTHROCYTES per unit volume in a sample of venous BLOOD. A complete blood count (CBC) also includes measurement of the HEMOGLOBIN; HEMATOCRIT; and ERYTHROCYTE INDICES.Plasma Substitutes: Any liquid used to replace blood plasma, usually a saline solution, often with serum albumins, dextrans or other preparations. These substances do not enhance the oxygen- carrying capacity of blood, but merely replace the volume. They are also used to treat dehydration.Iron: A metallic element with atomic symbol Fe, atomic number 26, and atomic weight 55.85. It is an essential constituent of HEMOGLOBINS; CYTOCHROMES; and IRON-BINDING PROTEINS. It plays a role in cellular redox reactions and in the transport of OXYGEN.Erythrocyte Transfusion: The transfer of erythrocytes from a donor to a recipient or reinfusion to the donor.Blood Chemical Analysis: An examination of chemicals in the blood.Reticulocyte Count: The number of RETICULOCYTES per unit volume of BLOOD. The values are expressed as a percentage of the ERYTHROCYTE COUNT or in the form of an index ("corrected reticulocyte index"), which attempts to account for the number of circulating erythrocytes.Erythropoiesis: The production of red blood cells (ERYTHROCYTES). In humans, erythrocytes are produced by the YOLK SAC in the first trimester; by the liver in the second trimester; by the BONE MARROW in the third trimester and after birth. In normal individuals, the erythrocyte count in the peripheral blood remains relatively constant implying a balance between the rate of erythrocyte production and rate of destruction.Blood Pressure: PRESSURE of the BLOOD on the ARTERIES and other BLOOD VESSELS.Oxygen: An element with atomic symbol O, atomic number 8, and atomic weight [15.99903; 15.99977]. It is the most abundant element on earth and essential for respiration.Blood Volume Determination: Method for determining the circulating blood volume by introducing a known quantity of foreign substance into the blood and determining its concentration some minutes later when thorough mixing has occurred. From these two values the blood volume can be calculated by dividing the quantity of injected material by its concentration in the blood at the time of uniform mixing. Generally expressed as cubic centimeters or liters per kilogram of body weight.Hemorheology: The deformation and flow behavior of BLOOD and its elements i.e., PLASMA; ERYTHROCYTES; WHITE BLOOD CELLS; and BLOOD PLATELETS.Blood Transfusion: The introduction of whole blood or blood component directly into the blood stream. (Dorland, 27th ed)Phlebotomy: The techniques used to draw blood from a vein for diagnostic purposes or for treatment of certain blood disorders such as erythrocytosis, hemochromatosis, polycythemia vera, and porphyria cutanea tarda.Blood Proteins: Proteins that are present in blood serum, including SERUM ALBUMIN; BLOOD COAGULATION FACTORS; and many other types of proteins.Renal Dialysis: Therapy for the insufficient cleansing of the BLOOD by the kidneys based on dialysis and including hemodialysis, PERITONEAL DIALYSIS, and HEMODIAFILTRATION.Kidney Failure, Chronic: The end-stage of CHRONIC RENAL INSUFFICIENCY. It is characterized by the severe irreversible kidney damage (as measured by the level of PROTEINURIA) and the reduction in GLOMERULAR FILTRATION RATE to less than 15 ml per min (Kidney Foundation: Kidney Disease Outcome Quality Initiative, 2002). These patients generally require HEMODIALYSIS or KIDNEY TRANSPLANTATION.Anemia, Neonatal: The mildest form of erythroblastosis fetalis in which anemia is the chief manifestation.Erythrocyte Deformability: Ability of ERYTHROCYTES to change shape as they pass through narrow spaces, such as the microvasculature.Isotonic Solutions: Solutions having the same osmotic pressure as blood serum, or another solution with which they are compared. (From Grant & Hackh's Chemical Dictionary, 5th ed & Dorland, 28th ed)Altitude: A vertical distance measured from a known level on the surface of a planet or other celestial body.Dehydration: The condition that results from excessive loss of water from a living organism.Serum Albumin: A major protein in the BLOOD. It is important in maintaining the colloidal osmotic pressure and transporting large organic molecules.Platelet Count: The number of PLATELETS per unit volume in a sample of venous BLOOD.Anemia, Hypochromic: Anemia characterized by a decrease in the ratio of the weight of hemoglobin to the volume of the erythrocyte, i.e., the mean corpuscular hemoglobin concentration is less than normal. The individual cells contain less hemoglobin than they could have under optimal conditions. Hypochromic anemia may be caused by iron deficiency from a low iron intake, diminished iron absorption, or excessive iron loss. It can also be caused by infections or other diseases, therapeutic drugs, lead poisoning, and other conditions. (Stedman, 25th ed; from Miale, Laboratory Medicine: Hematology, 6th ed, p393)Hydroxyethyl Starch Derivatives: Starches that have been chemically modified so that a percentage of OH groups are substituted with 2-hydroxyethyl ether groups.Time Factors: Elements of limited time intervals, contributing to particular results or situations.Blood Substitutes: Substances that are used in place of blood, for example, as an alternative to BLOOD TRANSFUSIONS after blood loss to restore BLOOD VOLUME and oxygen-carrying capacity to the blood circulation, or to perfuse isolated organs.Capillaries: The minute vessels that connect the arterioles and venules.Hemodynamics: The movement and the forces involved in the movement of the blood through the CARDIOVASCULAR SYSTEM.Body Weight: The mass or quantity of heaviness of an individual. It is expressed by units of pounds or kilograms.Blood Transfusion, Autologous: Reinfusion of blood or blood products derived from the patient's own circulation. (Dorland, 27th ed)Polycythemia Vera: A myeloproliferative disorder of unknown etiology, characterized by abnormal proliferation of all hematopoietic bone marrow elements and an absolute increase in red cell mass and total blood volume, associated frequently with splenomegaly, leukocytosis, and thrombocythemia. Hematopoiesis is also reactive in extramedullary sites (liver and spleen). In time myelofibrosis occurs.Ferritins: Iron-containing proteins that are widely distributed in animals, plants, and microorganisms. Their major function is to store IRON in a nontoxic bioavailable form. Each ferritin molecule consists of ferric iron in a hollow protein shell (APOFERRITINS) made of 24 subunits of various sequences depending on the species and tissue types.Blood Flow Velocity: A value equal to the total volume flow divided by the cross-sectional area of the vascular bed.Blood Loss, Surgical: Loss of blood during a surgical procedure.Microcirculation: The circulation of the BLOOD through the MICROVASCULAR NETWORK.Eagles: Large members of the FALCONIFORMES order of birds, family Accipitridae, most especially the genera Aquila, Haliaeetus, Harpia, and Circaetus. They are characterized by their powerful talons, which carry long, curved, pointed claws and by their opposable hindtoe.Rheology: The study of the deformation and flow of matter, usually liquids or fluids, and of the plastic flow of solids. The concept covers consistency, dilatancy, liquefaction, resistance to flow, shearing, thixotrophy, and VISCOSITY.Cerebrovascular Circulation: The circulation of blood through the BLOOD VESSELS of the BRAIN.Reference Values: The range or frequency distribution of a measurement in a population (of organisms, organs or things) that has not been selected for the presence of disease or abnormality.Blood Specimen Collection: The taking of a blood sample to determine its character as a whole, to identify levels of its component cells, chemicals, gases, or other constituents, to perform pathological examination, etc.