The cardiovascular system is a network of vessels connected to the heart. Like many other fluids blood flow through these vessels is determined by laws of fluid dynamics and by factors such as pressure and resistance. This article focuses on principles of blood flow in vessels, as well as the factors affecting it.
For more information on cardiac output and the factors affecting it, please read our article on flow in peripheral circulation.
Flow is defined as the volume of fluid passing a given point per unit of time (e.g. cm3/s). At any point within the cardiovascular system:
Flow = Pressure / Resistance
Pressure is normally calculated as the mean difference between the start and the end of the vessel.
It is widely accepted that the flow of blood will be the same at any two points within the cardiovascular system. However, the value of flow can vary throughout the day and in different clinical situations.
Laminar and turbulent flow
There are two ways in which blood flows within our vessels. In most straight blood vessels, the flow is laminar. Velocity (rate of blood flow) is highest in the centre of the vessel and decreases closer to the vessel wall. This decreasing velocity gradient is due to increasing resistance closer to the vessel wall.
However, if the blood vessels branch off or become constricted, the flow becomes turbulent. This is where the flow is no longer unidirectional and each cell takes a slightly different route, leading to ‘messier’ flow. Sometimes, turbulent flow (known as a ‘bruit’) can be heard over arteries with atherosclerotic plaques.
Liquids flow down their concentration gradients from areas of high pressure to areas of lower pressure. In practice, this means blood will flow from the arterial end of a vessel to the venous end. This pressure gradient is primarily created by the pumping action of the heart.
Resistance is the force that opposes the flow of blood. Different blood vessels throughout the body have varying levels of resistance to blood flow.
For example, our veins have very little resistance due to their ability to distend; this enables a vein’s resistance to fall in response to increasing pressure and thus keeps flow constant.
Resistance is determined by Poiseuille’s Law:
R = 8ηl / πr4
Where: R – resistance, η – Viscosity, l – Length, r – Radius
Resistance is dependent on 3 main factors:
Small changes in the radius of a blood vessel have a huge impact on the overall resistance – r4 means that a 2x change in radius equals a 16x (24x) change in resistance. From the equation, we can see the smaller the radius, the larger the resistance.
This can be taken one step further. We can calculate the cross-sectional area (CS) of the vessel using the equation CS = πr2 where r is the radius. This can then be used to calculate flow using the following equation:
Flow = Cross Sectional Area x Velocity
Flow is measure in cm3/s, cross-sectional area is is measured in cm2 and velocity is measured in cm/s.
It is important to note the slight difference between flow and velocity at this point. Velocity (which measures the rate at which fluid particles move) is proportional to flow (which measures the volume of fluid moving).
If we assume the flow is always constant, we can say: As a vessel’s cross-sectional area decreases, the average velocity of blood increases.
Therefore, we could consider that a capillary should have a high velocity because of its very small cross-section. However, because capillary beds (a network of capillaries) are connected in parallel, their collective cross-section is large. This gives capillaries a slower flow overall, allowing for the exchange of nutrients and waste.
A change in flow rate can also occur as a physiological response. The blood vessel’s smooth muscle relaxes or contracts to change the cross-sectional area, thus altering the flow rate appropriately.
Blood viscosity is relatively consistent day-to-day, therefore, this variable does not have a significant impact on our blood flow. However, in some conditions such as with chronic smokers, blood composition can change. Due to chronic hypoxia, an increase in red blood cells (known as polycythaemia) occurs in an attempt to deliver more oxygen to the tissues. Thus, the viscosity of the blood increases and subsequently, blood flow decreases.
The length of a blood vessel is directly proportional to its resistance The longer a vessel is, the higher its resistance. The greater the resistance, the higher the blood pressure and the lower the blood flow. Again, this does not affect a normally healthy person as they are able to maintain a high enough pressure to keep blood flowing.
Virchow’s triad consists of three factors that ultimately cause a decrease in blood flow and therefore, lead to a venous clot (or “thrombosis”) such as a deep vein thrombosis in the leg. These include:
- Stasis of blood flow. A slowing down or stopping of blood flow can occur for a variety of reasons. This includes long surgeries, sustained immobility and varicose veins.
- Hypercoagulability. A change in the components of blood can occur in smokers, in sepsis, after a severe burn or during cancer. This can be due to the activation of the coagulation system, or a change in the level of clotting factors in the plasma. Old age is also an important risk factor. Moreover, an increase in viscosity leads to an increase in resistance and can lead to a clot.
- Vessel wall injury. A bifurcation of blood vessels or marked damage (e.g. atherosclerosis) can lead to turbulent blood flow and vessel damage. Turbulence interferes with the anti-thrombotic substances released from the innermost layer of the vessel wall – the endothelium. This results in clot formation.