Hemodynamics describes the flow behavior of blood. It deals with the physical principles of blood circulation and the factors that affect blood flow, such as blood pressure, blood volume, blood viscosity, flow resistance, and vascular architecture and elasticity.
What is hemodynamics?
Hemodynamics describes the flow behavior of blood. It deals with the physical principles of blood circulation and the factors that affect blood flow. The fluid mechanics of blood is influenced by various parameters. This regulates the blood flow to organs and body regions and adjusts it to their needs. The most important parameters for regulation are: Blood pressure, blood volume, cardiac output, viscosity of blood, and vascular architecture and elasticity, which in medicine is called the lumen of a blood vessel. It is controlled by the autonomic nervous system as well as by the endocrine system with the help of hormones. Hemodynamics not only determines the flow of blood, but also has an influence on the function of the endothelium and vascular smooth muscle. The arterial blood vessels have a certain extensibility due to their wall structure, which means that they can increase or decrease their radius. If high blood pressure is registered, vasodilatation, i.e. vasodilatation, can be induced. Via the release of vasodilatory substances, such as nitric oxide, the radius of the blood vessel increases and thus the blood pressure and flow velocity decrease. This works the same way in reverse with low blood pressure and vasoconstriction, the narrowing of the vessels.
Function and purpose
The complex interplay of this system is of great importance to humans to ensure adequate blood supply to the organs when any of the parameters change. Under physiological conditions, laminar flow is present almost everywhere in the vascular system. This means that the fluid particles in the center of the vessel have a much higher velocity than the fluid particles at the edge. As a result, the cellular components, especially the erythrocytes, move in the center of the blood vessel, while the plasma flows closer to the wall. The erythrocytes travel faster through the vasculature than the blood plasma. The resistance to flow in laminar flow is most effectively affected by changing the radius of the vessel. This is described by the Hage-Poiseuille law. According to this, the current strength is proportional to the 4th power of the inner radius, which means that when the diameter is doubled, the current strength increases by a factor of 16. Under certain conditions, tubular flow can also occur. Turbulence causes an increase in flow resistance, which means extra workload for the heart. In addition, the viscosity of the blood also affects the flow resistance. As viscosity increases, so does resistance. Since the composition of the blood varies, the viscosity is not a constant variable. It depends on the viscosity of the plasma, the hematocrit value and the flow conditions. The viscosity of the plasma is in turn determined by the plasma protein concentration. If these parameters are taken into account, the viscosity is referred to as apparent viscosity. In comparison, the relative viscosity exists, here the blood viscosity is given as a multiple of the plasma viscosity. The hematocrit influences the blood viscosity in that an increase in cellular components causes the viscosity to increase. Since erythrocytes are deformable, they can adapt to different flow conditions. In strong flow with high shear stress, erythrocytes assume a low-resistance shape and apparent viscosity decreases dramatically. Conversely, it is possible for erythrocytes to aggregate into money roll-like aggregates during slow flow. In extreme cases, this can lead to hemostasis, or stasis. Apparent viscosity is also affected by vessel diameter. The erythrocytes are forced into the axial flow in small blood vessels. A thin layer of plasma remains at the edge, allowing faster movement. The apparent viscosity decreases with smaller vessel diameter, resulting in minimal blood viscosity in the capillaries. This is the so-called Fåhraeus-Lindqvist effect.
Diseases and disorders
Pathologic changes in blood vessels can disrupt hemodynamics.This is the case, for example, with arteriosclerosis. The disease develops slowly and often goes unnoticed for years because patients do not notice any symptoms. Deposits of blood fats, thrombi and connective tissue form in the blood vessels. So-called plaques develop, which narrow the vessel lumen. This restricts blood flow and leads to secondary diseases. Another danger is that cracks form in the vessel wall due to the increased stress, leading to hemorrhage and thrombus formation. In addition to the restriction of the lumen due to the deposits, the blood vessels, which are actually stretchable, become rigid and there is increasing hardening. Arteriosclerosis leads to various secondary diseases due to the circulatory disturbance, depending on the localization. The effect in the cerebral vessels is particularly threatening, since a disturbance of the brain function is the consequence. Complete occlusion of arteries leads to stroke. Coronary artery disease can develop in the coronary arteries. Its spectrum ranges from an asymptomatic form to angina pectoris and myocardial infarction. Smokers in particular often develop peripheral arterial disease (PAVD). Leg or pelvic arteries are affected and, with increasing severity, the walking distance that affected persons can cover becomes shorter. This is why PAVD is colloquially known as “shop window disease.” However, the danger of arteriosclerosis does not only come from lumen narrowing. The detachment of arteriosclerotic plaques or thrombi can also lead to life-threatening complications, such as pulmonary embolism or stroke. Risk factors for atherosclerosis include smoking, high blood pressure, diabetes mellitus, and high blood lipid levels.
