The results indicate that there is a significant effect of increasing concentrations of THC and CBD on blood viscosity. The observed increase is significant at all concentrations except at 2.5 ng/ml. This effect is even more pronounced with the extracts containing both THC and CBD. Two extracts were used with ratios of 4THC:1CBD and 1THC:1CBD. The viscosity values obtained for both extracts were significantly higher than the control sample, but they were not significantly different from each other indicating that regardless of ratio, THC and CBD have a stronger/potentiative effect on each other in relation to whole blood viscosity, when used in combination than when used separately.

The concentrations of THC and CBD used in the study were chosen based on concentrations of THC in plasma that were observed to give a psychoactive effect in previous studies (Watson et al. 2000) where it was shown that on administration of approximately 0.32 mg/kg (range: 0.22 to 0.50 mg/kg) of THC, the resulting plasma concentration ranged from 50 to 100 ng/ml with subjects indicating that they were approximately 50% high when the plasma concentration was 50 ng/ml. As these dosages were based on concentrations observed in the blood following the smoking of cannabis cigarettes with varying percentages of THC, taking into consideration that some THC may have been left in the butt or lost in the side stream smoke, they represent actual dosages that may be consumed by persons ingesting cannabis. The range of concentrations from 0 to100 ng/ml was therefore chosen to reflect the effect of THC and CBD both at low and high concentrations.

The fact that THC and CBD when used in combination have a stronger effect than when they are used individually is another example of the entourage effect which has also been observed in previous studies (Russo 2019). Studies have shown that depending on the ratio in which they are administered, CBD may have either an antagonistic or potentiative effect on THC (Varvel et al. 2006). In this study, however, there were no significant differences in the values obtained for the two extracts indicating that the difference in ratios used did not seem to play a role. The combined effect of THC and CBD was however found to be stronger than the individual effect.

The findings of the present study which indicates that blood viscosity increased with increasing cannabinoid concentrations are also indicative of a concomitant decreased deformability of the red blood cells as the concentrations of THC and CBD increased. This is supported by the results obtained for elasticity which though not significant when THC and CBD are used separately indicate that elasticity is highest when the highest concentration of either cannabinoid is used. The extracts, however, which contain different ratios of both cannabinoids show a significantly increased elasticity in comparison to the control used. Greater membrane elasticity indicates greater storage of elastic energy which therefore means that the red blood cell is more rigid and therefore less deformable as greater energy will be required in order for deformation to take place (Baskurt et al. 2007).

The cannabinoids are believed to exert their physiological effects primarily through two cannabinoid receptors which are referred to as CB1 and CB2 receptors. CB1 receptors are mainly expressed in the CNS, but they are also abundantly expressed in the PNS and other peripheral tissues and organs to include cardiac muscle, hepatic tissue, the gastrointestinal tract and vascular endothelium. The CB2 receptors on the other hand are primarily expressed in cells and tissues of the immune system although they are expressed to a lesser extent in the brain and other peripheral tissues (Subramaniam et al. 2019; Zou and Kumar 2018). Therefore, while cannabis has an effect on numerous systems throughout the body, increasing attention has been focussed on the adverse effects of cannabis on the cardiovascular system which include myocardial infarction, sudden death, peripheral arteritis and stroke (Wolff et al. 2015).

According to the literature, cannabis induces increased production of reactive oxygen species (ROS). This leads to increased oxidative stress which has been implicated in the occurrence of ischemic stroke and possibly other adverse cardiovascular events. It is believed that the production of the reactive oxygen species is primarily mediated by CB1 receptors (Han et al. 2009). The presence of reactive oxygen species can have deleterious effects on the proper functioning of erythrocytes whose main function in the circulatory system is the transport of oxygen and carbon dioxide to and from the lungs and tissues and also to maintain acid base equilibria. In this way, they play a very important role in cardiovascular homeostasis (Red blood cell function and dysfunction 2017). Increased production of ROS can have a profound impact on the integrity of the red blood cell membrane with the possibility of haemoglobin degradation, decreased red cell deformability and haemolysis, all of which have been implicated in serious pathological effects on the cardiovascular system.

One recent study (Ballas 2017) indicated that there was increased vaso-occlusive crises (VOC) in sickle cell patients who utilized cannabis resulting in more frequent hospitalizations. Based on the results obtained in this study, the observed increased incidence of VOC from that study was likely due to consumption of cannabis which could have led to increased viscosity, elasticity and impaired membrane integrity and therefore decreased erythrocyte deformability. Impaired erythrocyte deformability directly affects the ability of red blood cells to pass through small arterioles or capillaries (Baskurt and Meiselman 2003). The decreased deformability could further lead to blockage in the blood vessels, thereby resulting in increased VOCs. This can ultimately prevent oxygen delivery to the tissues and may result in hypoxia.

The increased viscosity and elasticity observed in this study indicate that as concentrations of THC and CBD in the blood increases, more energy is required for deformation and disaggregation of red blood cells. The red cells are therefore less likely to respond quickly to changes in circulation including the ability to change shape under stress imposed by high rates of flow through in the microcirculation. This will directly affect their ability to deliver oxygen to the tissues (Baskurt et al. 2007).

Since red cell deformability is influenced by three distinct cellular components namely cell shape geometry, viscosity of the cell cytoplasm and stability of the membrane (Baskurt et al. 2007; Lester et al. 2014), it would appear that THC and CBD are able to interact with these factors, thereby impairing red blood cell deformability and resulting in the increased viscosity and elasticity observed with increasing concentrations of both substances.

In conjunction with the increased viscosity and elasticity observed, there were increased morphological changes in the membrane of red blood cells with increasing concentrations of THC and CBD both separately and in combination. The results obtained therefore indicate that there is a dose dependent effect of THC and CBD on red blood cells. The cells were normal in appearance when 2.5 ng/ml and 25 ng/ml of THC and CBD were administered separately; however, at 50 ng/ml, there was evidence of impaired membrane integrity which worsened at 100 ng/ml. In the extract containing both THC and CBD in a 1:1 ratio, there was marked red cell agglutination along with degeneration, while in the extract containing THC and CBD in a 4:1 ratio, there was complete disintegration of red blood cells.

Red blood cells (RBCs) play a significant role in blood rheology due to the fact that they are the major cellular components of blood. As such, they are also the predominant factor that affects blood viscosity due to inherent characteristics of the red blood cells to include their ability to orient themselves with flow, haematocrit, the ability to elastically deform in response to mechanical forces and the capability to form rouleaux when there is low flow. Of note here is the fact that red blood cells are able to significantly increase blood viscosity when they lose their deformability which can significantly affect blood flow (Red blood cell function and dysfunction 2017).

While a search of the literature did not reveal specific studies linking RBC disintegration or haemolysis in human cannabis users with cannabis usage, studies have indicated that the psychological effects of THC were highest at the highest concentrations used in the study (Watson et al. 2000).

Changes in red cell morphology will affect the deformability of the red blood cells which in turn will affect their ability to enter microcirculation (Baskurt et al. 2007). The changes observed in red cell morphology therefore concur with the increased viscosity and elasticity observed with increasing concentration.

As previously mentioned, since there is a paucity of information in regard to the effect of cannabis on blood viscosity, elasticity and RBC morphology, this study helps to fill the gaps in knowledge that are not specifically covered in other studies. The results also give an indication that the lower concentrations of cannabinoids, more specifically THC and CBD may be safer and therefore more beneficial. Considering the increased utilization of cannabis for medicinal and recreational purposes, considering the increasing reports of adverse cardiovascular effects associated with cannabis use and considering the importance of blood viscosity, elasticity and RBC integrity in the maintenance of cardiovascular health, this study provides beneficial information which can help inform usage of cannabis.

The study was limited in that it was conducted in vitro rather than in vivo and therefore there is the possibility of variations arising in living systems. Limited quantities of reagents were also a limiting factor which prevented the conduction of the study on a wider scope such as the inclusion of a group of smokers for comparison purposes.