Blood thickness, also known as blood viscosity, refers to how easily blood flows through the circulatory system. The thickness of blood is an important health indicator, as abnormal thickness can lead to medical complications. In this article, we will explore what makes blood thick or thin, normal ranges for blood viscosity, causes of abnormal thickness, health impacts, and treatment options.
What Determines Blood Viscosity?
Blood is made up of plasma, red blood cells, white blood cells, and platelets suspended in fluid. Plasma makes up about 55% of total blood volume, while red blood cells constitute around 45%. The concentration and characteristics of the cellular components in blood primarily determine its viscosity or thickness.
Here are the key factors that influence blood viscosity:
- Hematocrit – The percentage of red blood cells in the blood. A higher hematocrit corresponds to thicker blood.
- Red blood cell deformability – Stiff red blood cells resist deformation and flow less easily, increasing viscosity.
- Red blood cell aggregation – The tendency for red blood cells to stick together into stacks. More aggregation increases viscosity.
- Plasma viscosity – Determined by the concentration of plasma proteins. Higher protein content leads to thicker plasma.
- Temperature – Blood viscosity increases as temperature decreases.
- Cholesterol/lipids – Elevated cholesterol thickens the plasma.
- Inflammation – Inflammation releases proteins that thicken blood.
As you can see, both cellular and plasma factors influence the ultimate viscosity of blood. However, hematocrit and the behavior of red blood cells are the primary determinants.
Normal Range for Blood Viscosity
Blood viscosity is typically expressed using the measurement of centipoise or mPa.s (millipascal-second). The normal range can vary based on age, sex, which blood vessel is measured, and measurement methodology. However, healthy blood viscosity generally falls into these ranges:
| Blood Type | Normal Viscosity Range |
|---|---|
| Whole blood | 3.5-5.5 cP (at normal hematocrit of 40-45%) |
| Plasma | 1.10-1.35 cP |
Higher viscosity values above these normal ranges could indicate an increased risk of circulation problems or blood clots. Lower levels may be seen in anemia or overhydration.
What Causes Abnormally Thick Blood?
There are many medical conditions and factors that can lead to a thickening of the blood. Here are some of the most common causes of abnormally high blood viscosity:
- Polycythemia – A disorder causing an overproduction of red blood cells, which raises hematocrit. Polycythemia vera is an example.
- Congenital heart disease – Structural heart defects present from birth may elevate hematocrit.
- Chronic lung disease – Lung conditions like COPD lead to oxygen deprivation and increased red cell production.
- Sleep apnea – Interrupted breathing while sleeping can cause polycythemia from oxygen deprivation.
- Chronic dehydration – Water loss concentrates the blood, decreasing plasma volume relative to cells.
- High cholesterol – Elevated lipids thicken the plasma.
- Diabetes – Excess blood glucose thickens plasma and impairs red cell deformability.
- Hypertension – Hardening of arteries and elevated lipids associated with high blood pressure increase viscosity.
- Blood cancers – Diseases like leukemia may elevate white blood cell counts abnormally.
In many cases, thick blood arises from complex interactions between several factors that contribute to increased hematocrit or plasma viscosity. Chronic health conditions like hypertension, diabetes, and COPD often involve multiple causes of thicker blood.
Health Impacts and Complications
Abnormally high blood viscosity can negatively impact circulation and overall health. Some potential effects and complications include:
- Increased risk of blood clots and embolism – Thicker blood is more prone to clotting and reduced flow.
- Higher blood pressure – The heart must work harder to pump viscous blood, raising blood pressure.
- Tissue hypoxia – Thick blood delivers oxygen to tissues less efficiently, causing hypoxia.
- Organ damage – Impaired blood flow can damage organs like the heart, brain, and kidneys over time.
- Stroke risk – Thick blood and vessel blockages increase stroke risk.
- Retinopathy – Retinal damage from thick blood interfering with small vessels.
- Erectile dysfunction – Poor circulation inhibits blood flow required for erections.
- Fatigue, headaches, dizziness – Symptoms from inadequate oxygen delivery.
The health impacts are generally proportional to the degree of increased viscosity. Mildly thick blood may have minimal effects, while severely high viscosity can be life-threatening if untreated. Symptoms also depend on which areas of the body are most affected by poor circulation.
Thick Blood and Stroke
One of the most dangerous complications of high blood viscosity is an increased risk of stroke. Stroke risk rises because thick blood is more prone to form vessel-blocking clots and delivers less oxygen to the brain. One study found stroke patients had 12% higher blood viscosity compared to controls. Managing viscosity levels is an important part of stroke prevention.
Treatments to Lower Blood Viscosity
Treatment for overly thick blood aims to address the underlying cause while also directly lowering viscosity. Here are some common therapies:
- Therapeutic phlebotomy – Removing some blood lowers hematocrit and viscosity.
- Blood thinners – Anticoagulant medications like warfarin thin the blood.
- Antihypertension medications – Reduce blood pressure and plasma lipids.
- Hydration – Increasing fluid intake can lower viscosity in dehydration.
- Transfusions – May replace overly thick blood with normal blood.
- Chemotherapy – Used for blood cancers like leukemia to lower abnormal cell counts.
- Supplemental oxygen – Reverses polycythemia caused by chronic hypoxia.
- Diuretics – Removes excess fluid to concentrate overly diluted blood.
Dietary and lifestyle changes like quitting smoking, losing weight, and controlling cholesterol through diet may also help lower viscosity. In some cases, plasma exchange procedures are used to directly remove excess proteins thickening the plasma.
Conclusion
Blood thickness or viscosity is determined primarily by the concentration and behavior of the cellular blood components and plasma proteins. Normal viscosity ensures smooth blood flow, while high viscosity can impair circulation and oxygen delivery. Thick blood is most often caused by conditions that increase hematocrit, red cell aggregation, or plasma cholesterol and proteins. Treatments aim to both decrease viscosity and address the underlying condition. Maintaining blood viscosity within a healthy range is important to avoid potentially serious complications like strokes, organ damage, and high blood pressure.