Hypovolemia: An abnormally low volume of blood circulating through the body. It may result in hypovolemic shock (see SHOCK).Blood Volume: Volume of circulating BLOOD. It is the sum of the PLASMA VOLUME and ERYTHROCYTE VOLUME.Lower Body Negative Pressure: External decompression applied to the lower body. It is used to study orthostatic intolerance and the effects of gravitation and acceleration, to produce simulated hemorrhage in physiologic research, to assess cardiovascular function, and to reduce abdominal stress during childbirth.Shock: A pathological condition manifested by failure to perfuse or oxygenate vital organs.Plasma Volume: Volume of PLASMA in the circulation. It is usually measured by INDICATOR DILUTION TECHNIQUES.Dehydration: The condition that results from excessive loss of water from a living organism.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.Fluid Therapy: Therapy whose basic objective is to restore the volume and composition of the body fluids to normal with respect to WATER-ELECTROLYTE BALANCE. Fluids may be administered intravenously, orally, by intermittent gavage, or by HYPODERMOCLYSIS.Hypotension, Orthostatic: A significant drop in BLOOD PRESSURE after assuming a standing position. Orthostatic hypotension is a finding, and defined as a 20-mm Hg decrease in systolic pressure or a 10-mm Hg decrease in diastolic pressure 3 minutes after the person has risen from supine to standing. Symptoms generally include DIZZINESS, blurred vision, and SYNCOPE.Fluid Shifts: Translocation of body fluids from one compartment to another, such as from the vascular to the interstitial compartments. Fluid shifts are associated with profound changes in vascular permeability and WATER-ELECTROLYTE IMBALANCE. The shift can also be from the lower body to the upper body as in conditions of weightlessness.Vascular Capacitance: The measure of a BLOOD VESSEL's ability to increase the volume of BLOOD it holds without a large increase in BLOOD PRESSURE. The vascular capacitance is equal to the change in volume divided by the change in pressure.Diuretics: Agents that promote the excretion of urine through their effects on kidney function.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)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.Furosemide: A benzoic-sulfonamide-furan. It is a diuretic with fast onset and short duration that is used for EDEMA and chronic RENAL INSUFFICIENCY.Shock, Hemorrhagic: Acute hemorrhage or excessive fluid loss resulting in HYPOVOLEMIA.Thirst: A drive stemming from a physiological need for WATER.Hypernatremia: Excessive amount of sodium in the blood. (Dorland, 27th ed)Bed Rest: Confinement of an individual to bed for therapeutic or experimental reasons.Head-Down Tilt: Posture while lying with the head lower than the rest of the body. Extended time in this position is associated with temporary physiologic disturbances.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.Inappropriate ADH Syndrome: A condition of HYPONATREMIA and renal salt loss attributed to overexpansion of BODY FLUIDS resulting from sustained release of ANTIDIURETIC HORMONES which stimulates renal resorption of water. It is characterized by normal KIDNEY function, high urine OSMOLALITY, low serum osmolality, and neurological dysfunction. Etiologies include ADH-producing neoplasms, injuries or diseases involving the HYPOTHALAMUS, the PITUITARY GLAND, and the LUNG. This syndrome can also be drug-induced.Hemodynamics: The movement and the forces involved in the movement of the blood through the CARDIOVASCULAR SYSTEM.Intracranial Hypotension: Reduction of CEREBROSPINAL FLUID pressure characterized clinically by HEADACHE which is maximal in an upright posture and occasionally by an abducens nerve palsy (see ABDUCENS NERVE DISEASES), neck stiffness, hearing loss (see DEAFNESS); NAUSEA; and other symptoms. This condition may be spontaneous or secondary to SPINAL PUNCTURE; NEUROSURGICAL PROCEDURES; DEHYDRATION; UREMIA; trauma (see also CRANIOCEREBRAL TRAUMA); and other processes. Chronic hypotension may be associated with subdural hematomas (see HEMATOMA, SUBDURAL) or hygromas. (From Semin Neurol 1996 Mar;16(1):5-10; Adams et al., Principles of Neurology, 6th ed, pp637-8)Hemorrhage: Bleeding or escape of blood from a vessel.Blood Pressure: PRESSURE of the BLOOD on the ARTERIES and other BLOOD VESSELS.Cardiac Output: The volume of BLOOD passing through the HEART per unit of time. It is usually expressed as liters (volume) per minute so as not to be confused with STROKE VOLUME (volume per beat).Heart Rate: The number of times the HEART VENTRICLES contract per unit of time, usually per minute.Supine Position: The posture of an individual lying face up.Hypotension: Abnormally low BLOOD PRESSURE that can result in inadequate blood flow to the brain and other vital organs. Common symptom is DIZZINESS but greater negative impacts on the body occur when there is prolonged depravation of oxygen and nutrients.Hyponatremia: Deficiency of sodium in the blood; salt depletion. (Dorland, 27th ed)Drinking: The consumption of liquids.Stroke Volume: The amount of BLOOD pumped out of the HEART per beat, not to be confused with cardiac output (volume/time). It is calculated as the difference between the end-diastolic volume and the end-systolic volume.Water-Electrolyte Balance: The balance of fluid in the BODY FLUID COMPARTMENTS; total BODY WATER; BLOOD VOLUME; EXTRACELLULAR SPACE; INTRACELLULAR SPACE, maintained by processes in the body that regulate the intake and excretion of WATER and ELECTROLYTES, particularly SODIUM and POTASSIUM.Hydroxyethyl Starch Derivatives: Starches that have been chemically modified so that a percentage of OH groups are substituted with 2-hydroxyethyl ether groups.Spectroscopy, Near-Infrared: A noninvasive technique that uses the differential absorption properties of hemoglobin and myoglobin to evaluate tissue oxygenation and indirectly can measure regional hemodynamics and blood flow. Near-infrared light (NIR) can propagate through tissues and at particular wavelengths is differentially absorbed by oxygenated vs. deoxygenated forms of hemoglobin and myoglobin. Illumination of intact tissue with NIR allows qualitative assessment of changes in the tissue concentration of these molecules. The analysis is also used to determine body composition.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.Resuscitation: The restoration to life or consciousness of one apparently dead. (Dorland, 27th ed)Cardiovascular System: The HEART and the BLOOD VESSELS by which BLOOD is pumped and circulated through the body.