The cardiovascular system is a closed network of vessels connected to the heart. It enables blood, oxygen and other nutrients to flow around the body. In this article, we will focus on the underlying principles of blood flow and the factors affecting it.
There are two ways in which blood flows within our vessels. In most straight blood vessels, the flow is laminar. Velocity is highest in the centre of the tube 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. Sometimes turbulent flow can be heard (known as a ‘bruit’) over arteries containing atherosclerotic plaques.
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
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 differing clinical situations.
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. Relating to the above equation, the value for pressure is normally calculated as the mean difference between the start and the end of the vessel.
Resistance is determined by Poiseuille’s Law:
R = 8ηl / πr4
Where: R – resistance, η – Viscosity, l – Length, r – Radius
Resistance is made up from 3 main components:
Small changes in the size of a blood vessel’s radius has a huge impact on the overall resistance – the 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 radius. This can then be used to calculate flow using the following equation:
Flow (cm3/s) = Cross Sectional Area (cm2) x Velocity (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. In fact, because capillary beds are connected in parallel, their collective cross section is vast. This gives capillaries a very slow flow overall.
Changing flow rate can also occur as a physiological response when a 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, so this variable doesn’t massively impact our blood flow. However, in some conditions such as with chronic smokers or dehydration, blood composition can change and subsequently flow can change.
The longer a vessel is, the higher its resistance. Again, this doesn’t affect a normally healthy person as they are able to maintain a high enough pressure to keep the blood flowing.
The different blood vessels throughout our bodies 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.
Virchow’s Triad consists of three factors which ultimately cause a decrease in blood flow and therefore, may lead to a clot (or “thrombosis”). These include:
- Stasis. A slowing down or stopping of blood flow which can occur for a variety of reasons. This includes long surgeries, sustained immobility and varicose veins.
- Hypercoagulability. A change in the constituents of our blood can occur in smokers, in sepsis, after a severe burn, during cancer or late into pregnancy. Old age is also an important risk factor. 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) could also lead to turbulent blood flow. As described above, clots commonly form in areas of turbulent flow. This is due to the interference with the natural antithrombotic properties of the innermost layer of vessel wall – the endothelium.