Blood thickness, or viscosity, is determined by the concentration of red blood cells and proteins in your bloodstream, along with how well those red blood cells can bend and flow through narrow vessels. When any of these factors shifts, blood becomes harder to pump, moves more slowly through small capillaries, and raises the risk of clots. The causes range from simple dehydration to chronic medical conditions.
The Main Factors That Control Blood Thickness
Blood is essentially red blood cells suspended in a protein-rich liquid called plasma. Its thickness depends on four things: how many red blood cells are packed into a given volume (a measurement called hematocrit), the protein concentration in the plasma, how easily red blood cells can flex and squeeze through tiny vessels, and how much those cells clump together. A change in any one of these makes blood flow differently.
Hematocrit has the most straightforward relationship with viscosity. More red blood cells in the same volume of fluid means thicker blood. Normal hematocrit ranges from roughly 36% to 44% for women and 40% to 54% for men. Once it climbs above 45%, the risk of blood flow problems and clotting starts to increase, which is why doctors treating conditions that raise red blood cell counts aim to keep hematocrit below that threshold.
On the plasma side, proteins like fibrinogen and various globulins raise the liquid’s own viscosity. Fibrinogen is the most significant: it not only thickens plasma directly but also promotes red blood cell clumping. When red blood cells stick together in stacks (sometimes called rouleaux), they act like larger particles that resist flow, especially in slower-moving blood during the relaxation phase between heartbeats.
Red blood cell flexibility matters more than most people realize. Healthy red blood cells are soft, disc-shaped, and can fold nearly in half to pass through capillaries narrower than themselves. Anything that stiffens their membranes or changes their shape, like chronic high blood sugar or certain inherited conditions, forces the heart to work harder to push blood through the smallest vessels.
Dehydration: The Most Common Cause
When you lose fluid through sweat, illness, or simply not drinking enough water, the liquid portion of your blood shrinks while the number of red blood cells stays the same. This concentrates everything in the bloodstream, a process called hemoconcentration. The same thing happens during heat exposure or at high altitude, where the body loses water faster than usual. Rehydrating reverses this quickly in most cases, which is why drinking adequate fluids is one of the simplest ways to keep blood flowing well.
Living at High Altitude
Thinner air at elevation contains less oxygen per breath. Your body compensates by producing more red blood cells to carry what oxygen is available, gradually pushing hematocrit upward. This process reaches a moderately elevated steady state after a few weeks and stays there as long as you remain at altitude. For most people, this adaptation is harmless and even beneficial for oxygen delivery. But in some long-term high-altitude residents, hematocrit can climb high enough to cause headaches, fatigue, and circulation problems, a condition sometimes called chronic mountain sickness.
Smoking and Carbon Monoxide
Tobacco smoke contains carbon monoxide, which binds to hemoglobin roughly 200 times more tightly than oxygen does. This effectively kicks oxygen off red blood cells and creates a state of chronic low oxygen in the body. The kidneys respond by releasing a hormone that signals the bone marrow to produce more red blood cells, raising hematocrit over time. Long-term smokers can develop noticeably elevated red blood cell counts purely from this mechanism, independent of any lung disease. Quitting allows carbon monoxide levels to drop within days, and red blood cell production gradually normalizes.
Testosterone and Hormone Therapy
Testosterone stimulates red blood cell production, which is why men naturally have higher hematocrit than women. Testosterone replacement therapy amplifies this effect. In clinical studies, a substantial percentage of men on testosterone therapy see their hematocrit rise to levels that require monitoring. Older men appear more susceptible: in one study, 75% of older men on a moderate dose reached their peak hematocrit within 12 weeks, compared to 42% of younger men on the same dose. This is one of the most common side effects of testosterone therapy and the main reason blood work is checked regularly during treatment.
High Blood Sugar and Diabetes
Chronically elevated blood sugar changes red blood cells in ways that make blood harder to move through small vessels. Excess glucose attaches to proteins on the red blood cell membrane, stiffening it and reducing the cell’s ability to deform. Over time, red blood cells in people with poorly controlled diabetes become more irregularly shaped, with increased perimeters but decreased surface area. Some develop a rounder shape (spherocytosis), while others sprout spiky projections. These misshapen, rigid cells don’t slide through capillaries as easily, effectively raising blood viscosity at the tissue level even when hematocrit looks normal on a standard blood test.
People with type 2 diabetes also tend to have abnormal blood lipid profiles and higher levels of lipid oxidation byproducts, both of which further distort red blood cell membranes. The combined effect impairs microcirculation and contributes to the complications diabetes is known for: slow wound healing, kidney damage, and vision problems.
Bone Marrow Disorders
Polycythemia vera is a condition where the bone marrow produces too many red blood cells on its own, without an external trigger like low oxygen or testosterone. It’s driven by a genetic mutation that makes blood cell production run unchecked. Hematocrit can climb well above 50%, significantly increasing clot risk. The primary treatment is straightforward: periodic blood draws (therapeutic phlebotomy) to physically remove red blood cells and bring hematocrit below 45%. Most people with this condition need blood draws every few weeks to months, depending on how fast their counts rise.
Other Medical Conditions
Chronic lung diseases like COPD reduce oxygen levels in the blood, triggering the same red-blood-cell overproduction seen with altitude and smoking. Kidney tumors and certain other cancers can produce the hormone that drives red blood cell production, raising hematocrit even without low oxygen. Some autoimmune conditions and chronic infections raise fibrinogen and other inflammatory proteins in the plasma, thickening the liquid portion of blood independently of red blood cell count.
Inherited blood disorders like sickle cell disease don’t raise hematocrit, but they dramatically reduce red blood cell flexibility. Sickle-shaped cells are rigid and sticky, causing them to jam in small vessels and create flow problems that share some features with high-viscosity blood.
How Thick Blood Causes Problems
Thicker blood forces the heart to pump harder, which can raise blood pressure over time. More critically, sluggish flow through small vessels increases the chance that a clot will form. Clots in the legs (deep vein thrombosis), lungs (pulmonary embolism), or brain (stroke) are the most serious risks. Symptoms of overly thick blood can be subtle at first: headaches, blurred vision, dizziness, a reddish or flushed complexion, and unusual fatigue. Some people notice tingling or numbness in their hands and feet, reflecting reduced circulation in the smallest vessels.
A standard complete blood count will reveal an elevated hematocrit, which is typically the first clue. If doctors suspect the plasma itself is too thick, they can order a direct viscosity measurement. Identifying the underlying cause, whether it’s dehydration, a medication, smoking, or a bone marrow problem, determines how it’s managed. In most cases, addressing the root cause brings blood thickness back to a normal range.