Hemoconcentration describes a condition where the cellular components of the blood become densely packed within the bloodstream. This occurs due to a reduction in the fluid portion of the blood known as plasma, not because the body produced too many blood cells. The concentration of cells and other solid elements rises because the overall volume of water has decreased. This physiological change is common in clinical settings and can impact the accuracy of diagnostic tests.
The Biological Mechanism of Hemoconcentration
Blood is a fluid mixture composed primarily of plasma (roughly 92% water) and formed elements (red blood cells, white blood cells, and platelets). Normally, the ratio between these formed elements and the plasma volume is maintained for smooth circulation. Hemoconcentration disrupts this balance by selectively reducing the plasma component.
The mechanism involves water shifting out of the vascular space into surrounding body tissues or exiting the body entirely. When fluid volume within the blood vessels drops, the remaining cells are contained within a smaller total volume, making the blood appear thicker and more concentrated.
Fluid movement out of the capillaries is often triggered by changes in hydrostatic pressure or osmotic forces. For example, a rise in blood pressure can make capillary walls more permeable, effectively squeezing plasma water into the interstitial space. This fluid shift leaves behind larger cellular elements and proteins, causing them to concentrate in the blood vessels.
This relative increase in cellular elements does not mean the absolute number of red cells has changed. Instead, the cell-to-fluid ratio measurement is skewed because the plasma volume is low. This mechanism of fluid displacement is foundational to interpreting resulting changes in a patient’s laboratory values.
Common Triggers and Causes
The primary clinical scenarios leading to hemoconcentration involve significant losses of body water that shrink the plasma volume. Severe dehydration is the most frequent cause, resulting from insufficient fluid intake or excessive fluid loss. This includes conditions such as prolonged, heavy sweating without rehydration, persistent vomiting, or diarrhea.
Certain medical conditions also lead to plasma loss. Severe burns cause massive plasma leakage from damaged blood vessels onto the skin surface, resulting in a direct loss of fluid from circulation. Another trigger is diabetic ketoacidosis, where high blood sugar levels lead to osmotic diuresis, pulling excessive water out through the kidneys.
Fluid can also be sequestered, or trapped, in a third space outside the bloodstream, such as in the abdomen during severe inflammation or shock. This process effectively removes plasma from the circulation. These situations prevent the body from maintaining its normal blood volume, forcing the remaining blood components into a concentrated state.
Temporary hemoconcentration can occur during routine medical procedures. For instance, leaving a tourniquet on a limb for an extended period during a blood draw forces plasma out of the vein into the tissue. This momentarily concentrates the blood sample before collection.
How Hemoconcentration Is Measured
Hemoconcentration is primarily quantified using the hematocrit (Hct) test. This test measures the volume percentage of red blood cells in the total blood volume. An elevated hematocrit value is the clinical hallmark of hemoconcentration, reflecting the higher proportion of cells relative to the reduced plasma volume.
The measurement is often performed by centrifuging a blood sample, which separates the blood into its components. These components include packed red blood cells, a thin buffy coat of white cells and platelets, and plasma. The height of the red cell column is measured against the total sample height to yield the percentage.
Hemoglobin concentration is another related measurement that appears falsely elevated in a hemoconcentrated state. Because hemoglobin is contained within red blood cells, a higher concentration of red cells automatically increases the measured hemoglobin level. Clinicians must account for the patient’s fluid status when interpreting these results to differentiate hemoconcentration from a true increase in red cell mass.
The urine specific gravity (USG) test is a secondary tool for assessing underlying dehydration. This test measures the concentration of solutes in the urine. A high USG indicates that the kidneys are attempting to conserve water by producing highly concentrated urine, often correlating with a hemoconcentrated state.
Health Consequences and Reversal
The immediate consequence of hemoconcentration is increased blood viscosity, often described as “thicker blood.” This higher viscosity forces the heart to work harder to pump the resistant blood through the circulatory system, placing strain on the cardiac muscle. The increased resistance can impair circulation, particularly in the microvasculature.
The thickening of the blood significantly elevates the risk of thromboembolism, or the formation of blood clots. Less plasma means cellular elements, red cells, and platelets have a greater chance of interacting and aggregating. This heightened clotting risk can lead to serious events such as deep vein thrombosis, stroke, or heart attack.
Reversing hemoconcentration requires addressing the primary cause of fluid loss and restoring the depleted plasma volume. Management involves aggressive fluid replacement, achieved through oral intake for mild cases or intravenous (IV) administration for severe depletion. The goal of this fluid therapy is to dilute the blood back to its proper concentration ratio.
As fluid is replaced, the plasma volume expands, the concentration of cells decreases, and the hematocrit value returns toward the normal range. Monitoring the patient’s symptoms, urine output, and laboratory values guides the rate and total amount of fluid needed. Successful treatment normalizes blood viscosity, reducing strain on the heart and mitigating the risk of clot formation.