Blood is not a Newtonian fluid. Its viscosity changes depending on how fast it flows, which makes it a non-Newtonian fluid with shear-thinning properties. In a Newtonian fluid like water, viscosity stays constant no matter how much force you apply. Blood breaks that rule: it flows more easily when pushed harder and thickens when it slows down. This behavior comes from the fact that blood is roughly 40–45% red blood cells by volume, and those cells interact with each other and with the surrounding plasma in ways that make the physics of blood flow far more complicated than a simple liquid.
Why Blood Doesn’t Behave Like Water
A Newtonian fluid has one viscosity at a given temperature. Honey is thick and water is thin, but neither changes its viscosity based on how quickly you stir it. Blood does. When blood moves slowly through a vessel, its viscosity climbs. When it speeds up, viscosity drops. This property, called shear thinning, means blood viscosity decreases exponentially as flow rates increase.1PubMed Central. Blood Rheology: Key Parameters, Impact on Blood Flow, Role in Sickle Cell Disease and Effects of Exercise The relationship is not subtle. Blood at a near-standstill can be several times more viscous than blood racing through a major artery.
The reason is red blood cells. At low flow rates, red blood cells clump together into stacks called rouleaux, held together partly by plasma proteins like fibrinogen. These stacks increase the internal friction of the blood, driving viscosity up.2PubMed. Blood viscosity: influence of erythrocyte aggregation As the flow speeds up and shear forces increase, those stacks break apart, individual cells align with the flow, and the blood becomes easier to push. The shear-thinning property is intimately tied to the dynamics and mutual interactions of red blood cells.3PubMed Central. Red cells’ dynamic morphologies govern blood shear thinning under microcirculatory flow conditions
Blood Has a Yield Stress
Blood doesn’t just thin when it flows faster. At very low forces, it resists flowing at all. This is because of those rouleaux structures: at rest, red blood cells form three-dimensional networks that give the blood a kind of structural integrity. You have to apply a minimum amount of force before blood starts to move. This minimum force is called the yield stress.4PubMed Central. A review on non-Newtonian fluid models for multi-layered blood rheology in constricted arteries
Think of it like ketchup in a bottle: nothing comes out until you shake it hard enough, and then it flows freely. Blood has a much lower yield stress than ketchup, but the principle is the same. Below that critical force, blood acts almost like a soft solid. Once the threshold is exceeded, shear-thinning behavior kicks in and flow begins. This detail matters medically. In regions of very sluggish circulation, blood can effectively stall, which is one of the conditions that promotes clot formation.
Blood Is Also Time-Dependent
Shear thinning isn’t the whole story. Blood also shows thixotropic behavior, meaning its viscosity depends not just on how fast it’s flowing right now, but on how long it has been flowing. If you suddenly start pumping stagnant blood, the rouleaux don’t break apart instantaneously. It takes time for the cell aggregates to disassemble and for viscosity to fall to its flowing-state level. And when flow stops, it takes time for the aggregates to reform.5PubMed. Pulsatile flow of thixotropic blood in artery under external body acceleration
This time dependence is relevant to the pulsatile nature of real blood flow. Your heart doesn’t push blood at a constant rate; it pumps in rhythmic surges. During each heartbeat, blood accelerates and then decelerates, and its viscosity is constantly adjusting to these changes, always lagging slightly behind. Computational models of blood flow in arteries increasingly account for this thixotropic behavior because treating blood as a simple fluid with a fixed viscosity gives inaccurate predictions, especially in vessels where flow slows or reverses direction during each cardiac cycle.
What Red Blood Cells Actually Do Under Flow
Individual red blood cells are remarkable little objects from a fluid-mechanics perspective. They are biconcave discs, roughly eight micrometers across, with no nucleus and a highly flexible membrane. When flowing through vessels, they don’t just tumble passively. Below a critical shear rate, red blood cells rotate end-over-end like tiny kayak paddles. Above that critical rate, they transition to a motion called tank-treading: the cell elongates, aligns with the flow, and its membrane rotates around the interior like the tread of a tank.6Journal of Fluid Mechanics. Threshold shear stress for the transition between tumbling and tank-treading of red blood cells in shear flow: dependence on the viscosity of the suspending medium This tank-treading motion persists even when cells are folded or deformed, as happens when they squeeze through narrow capillaries.7PubMed Central. Tank treading of optically trapped red blood cells in shear flow
This transition from tumbling to tank-treading is part of what drives shear thinning. Tumbling cells create more drag and disrupt the surrounding flow. Tank-treading cells are streamlined and move cooperatively with the plasma around them. As shear forces rise and more cells shift into the tank-treading mode, the effective viscosity of the whole blood drops.
Strange Things Happen in Small Vessels
Blood’s non-Newtonian behavior gets even more exotic when you zoom in to the smallest blood vessels. In capillaries and small arterioles with diameters roughly between 15 and 500 micrometers, two related phenomena emerge that don’t happen in larger vessels.
The first is the Fåhraeus effect: red blood cells in a small tube travel faster than the plasma around them, which means the concentration of red blood cells inside the tube is actually lower than the concentration in the blood entering or leaving it.8PubMed. Hematocrit fluctuations within capillary tubes and estimation of Fåhraeus effect The second is the Fåhraeus-Lindqvist effect: as the vessel diameter shrinks below about 0.3 millimeters, the apparent viscosity of blood also decreases.9PubMed Central. A continuum mechanics model for the Fåhræus-Lindqvist effect This is counterintuitive. You’d expect blood forced through a narrower tube to encounter more resistance, but the opposite happens, up to a point.
The explanation lies in how red blood cells distribute themselves in narrow tubes. They migrate toward the center of the vessel, leaving a thin layer of cell-free plasma hugging the vessel wall.10PubMed Central. Blood flow and cell-free layer in microvessels This cell-free layer acts as a lubricant, and because plasma by itself is much less viscous than whole blood, the effective resistance of the vessel drops. The cell-free layer is an important determinant of hydrodynamic resistance in microcirculatory vessels.11PubMed. A computer-based method for determination of the cell-free layer width in microcirculation No Newtonian fluid does this. The internal reorganization of blood’s solid components creates a self-lubricating flow that is uniquely suited to delivering oxygen through the tiniest corners of the vascular system.
When Blood Approximates a Newtonian Fluid
Despite all of this complexity, there are conditions under which treating blood as Newtonian is a reasonable simplification. In large arteries like the aorta and its major branches, shear rates are high enough that red blood cell aggregates have long since broken up. At these elevated shear rates, blood’s viscosity approaches a relatively stable value, and its behavior edges close to Newtonian. Many computational models of flow in major arteries therefore use a constant-viscosity approximation, and for certain questions, this works tolerably well.
But “tolerably” isn’t “perfectly.” Comparisons between Newtonian and non-Newtonian models of arterial and venous flow show meaningful differences. In patient-specific simulations, the Newtonian model underestimated wall shear stress by roughly 7 to 26% compared with non-Newtonian models, depending on the vessel location.12Nature / Scientific Reports. Effects of non-Newtonian viscosity on arterial and venous flow and transport That gap matters because wall shear stress is the mechanical signal that endothelial cells use to regulate vessel health. Getting it wrong by a quarter means getting the biology wrong.
In the carotid artery, where blood flow decelerates and recirculates around the bifurcation leading to the brain, the non-Newtonian character of blood becomes especially relevant. Red blood cell aggregation at low shear rates in these recirculation zones gives blood distinctly non-Newtonian characteristics. As the aggregates break apart in faster-flowing regions, blood’s Newtonian properties take over.13PubMed Central. CFD analysis of non-Newtonian blood flow through human carotid artery bifurcation: Carotid sinus susceptible to atherosclerosis The carotid sinus, where flow is slowest and most disturbed, is also where atherosclerotic plaques preferentially develop. That is not a coincidence.
Hematocrit, Fibrinogen, and What Pushes Blood Further From Newtonian
Two factors have an outsized influence on just how non-Newtonian any given person’s blood is. The first is hematocrit, the percentage of blood volume occupied by red blood cells. Higher hematocrit means more cells to aggregate and more internal friction. Blood viscosity is strongly dependent on hematocrit, and this relationship is one of the most important determinants of blood rheology in both healthy and sick people.14PubMed Central. Full Hematocrit-Viscosity Curve Identification Using Three-Dataset Krieger-Dougherty Regression
The second is fibrinogen, a plasma protein involved in clotting. Fibrinogen bridges red blood cells together and promotes rouleaux formation. Classic experiments showed that adding purified fibrinogen to red blood cell suspensions increased viscosity and promoted cell aggregation, with both effects growing more pronounced at higher fibrinogen concentrations and diminishing as shear rate increased.2PubMed. Blood viscosity: influence of erythrocyte aggregation This means conditions that raise fibrinogen levels, including inflammation, infection, and pregnancy, also push blood further from Newtonian behavior. During normal pregnancy, for instance, rising fibrinogen drives measurable increases in red blood cell aggregation.15PubMed. Erythrocyte aggregation during normal pregnancy
Diseases that alter red blood cell shape or stiffness amplify the departure from Newtonian behavior in different ways. In sickle cell disease, red blood cells are rigid and abnormally shaped, changing how they aggregate, deform, and interact with the vessel wall. The rheological abnormalities in sickle cell disease contribute directly to vascular complications, including painful crises and organ damage.1PubMed Central. Blood Rheology: Key Parameters, Impact on Blood Flow, Role in Sickle Cell Disease and Effects of Exercise
Why Engineers and Clinicians Care
The non-Newtonian nature of blood isn’t just an academic curiosity. It has direct consequences for how medical devices are designed and how cardiovascular disease is understood.
When engineers design artificial heart valves, ventricular assist devices, or extracorporeal membrane oxygenation circuits, they need to know how blood will behave inside those devices. If the fluid model assumes Newtonian behavior, the predicted shear stresses will be off, potentially in zones where red blood cells are being destroyed. Hemolysis, the mechanical rupture of red blood cells, is an unintended consequence of temporary and permanent intracardiac devices.16PubMed Central. Cardiac prostheses-related hemolytic anemia Getting the flow model right is part of minimizing that damage.
On the diagnostic side, blood viscosity is emerging as a biomarker for cardiovascular and metabolic health. It reflects the combined influence of hematocrit, plasma composition, and red blood cell deformability, and elevated whole blood viscosity has been linked to endothelial dysfunction and accelerated development of arterial plaques.17PubMed Central. Blood viscosity as a continuous marker of cardio-metabolic risk burden: a large-scale cross-sectional study of 38,574 adults Measuring viscosity is not yet routine in clinical practice, though diagnostic methods are evolving from traditional laboratory viscometers toward point-of-care and microfluidic technologies that could make viscosity testing much more accessible.18PubMed. Blood viscosity in clinical practice: A critical review of diagnostic utility, therapeutic implications, and evidence gaps
That said, the relationship between viscosity and tissue perfusion isn’t straightforward. Under normal conditions, the body’s autoregulatory mechanisms compensate for changes in blood viscosity by adjusting vessel diameter and cardiac output. This means that for a healthy person at rest, whole blood viscosity is largely negligible as a determinant of microvascular perfusion.19PubMed Central. Blood viscosity modulates tissue perfusion: sometimes and somewhere Where viscosity really starts to matter is when autoregulation fails, as in severe shock, during surgery under hypothermia, or in vessels blocked by spasm or clot. In those scenarios, the thickened, sluggish, maximally non-Newtonian blood at low flow rates becomes a serious clinical problem.
Cold Blood Flows Differently
Temperature is a powerful modifier of blood’s flow properties, and the effect goes well beyond what you’d expect from a simple liquid cooling down. When body temperature was lowered to 25°C in experimental settings, blood viscosity climbed to 173% of its value at normal body temperature. The increase came from three sources: cold plasma is inherently more viscous, cooling causes some plasma to shift out of the bloodstream and concentrate the remaining red cells, and the resulting low-flow state pushes blood into the high-viscosity, low-shear-rate part of its non-Newtonian curve. Much of the increased viscosity came from this last factor, the low-flow state itself.20PubMed. Hemodynamic functions and blood viscosity in surface hypothermia
This has real implications for patients undergoing deliberate hypothermia, such as during certain heart surgeries. The higher viscosity during hypothermia enhances the tendency of red blood cells to marginalize toward vessel walls, which can amplify shear-induced platelet aggregation. At the same time, slower blood flow reduces the forces that would normally wash forming clots away.21PubMed Central. Effect of hypothermia on haemostasis and bleeding risk: a narrative review So cold blood is simultaneously more viscous, more prone to clotting in some respects, and more non-Newtonian than warm blood. Surgical teams account for this by managing temperature carefully and adjusting anticoagulation strategies.
Not All Blood Is Created Equal Across Species
Mammalian red blood cells are unusual in biology. They lack a nucleus, which makes them exceptionally flexible. Most other vertebrates, including birds, reptiles, and fish, have nucleated red blood cells that are stiffer and less able to deform. This difference matters for blood rheology. Viscometric measurements comparing avian and human red blood cell suspensions show that nucleated avian cells create a more pronounced hydrodynamic disturbance during flow than mammalian cells do, because they are less able to adapt to local shear forces.22PubMed. Comparative rheology of nucleated and non-nucleated red blood cells. I. Microrheology of avian erythrocytes during capillary flow
In practical terms, bird blood is “more non-Newtonian” than human blood, and the cells are less cooperative in flow. The evolutionary loss of the red blood cell nucleus in mammals is thought to be an adaptation partly for improved oxygen-carrying capacity (more room for hemoglobin) and partly for exactly this rheological advantage: softer, more deformable cells that slip through capillaries more efficiently and create less resistance. The self-lubricating, cell-free-layer-forming, shear-thinning behavior of mammalian blood is, in a sense, an evolutionary achievement built on the unusual design of its most abundant cell.