The function
Blood circulation aims to bring oxygen and nutrients to the entire body to reach cells, tissues and organs. Subsequently, the blood collects carbon dioxide as well as toxins and cellular debris that are eliminated in the liver and kidneys. The blood is carried through the veins and from there to the lungs, where the carbon dioxide is removed and oxygen is taken up again.
Circulation is carried out by an effective system of blood vessels (arteries, arterioles, veins, venules and capillaries) that allow reaching all points of the body.
The circulatory system is essential for many functions of the body. An optimal blood circulation is therefore essential for our well-being.
When circulation is disrupted, organs, muscles, and the brain no longer receive enough oxygen or nutrients, leading to various disorders.
Habits that can affect circulation are tobacco, high alcohol consumption, excess medication, lack of physical exercise, poor sleep, a diet low in nutrients (vitamins, minerals, fatty acids, antioxidants, … ), stress, anxiety and many others. And of course, aging.
Many factors are related to circulation and its possible dysfunctionalities.
Protects endothelial dysfunction caused by excessive oxidative stress as a consequence of genetic, metabolic and external factors.
Blood vessels
Blood vessels are all made of three layers. Within each layer, the amount of muscle and collagen fibrils varies, depending on the size and location of the vessel. (Anatomy, Blood Vessels; William D. Tucker et al.; Statpearls 2020-06-17).
Arteries play a major role in nourishing organs with blood and nutrients. Arteries are always under high pressure. To accommodate this stress, they have an abundance of elastic tissue and less smooth muscle. The presence of elastin in the large blood vessels enables these vessels to increase in size and alter their diameter.
There are two main types of arteries found in the body: the elastic arteries and the muscular arteries. Muscular arteries include the anatomically named arteries like the brachial artery, the radial artery, and the femoral artery for example. Muscular arteries contain more smooth muscle cells in the tunica media layer than the elastic arteries.
Elastic arteries are those nearest the heart (aorta and pulmonary arteries) that contain much more elastic tissue in the tunica media than muscular arteries. This feature of the elastic arteries allows them to maintain a relatively constant pressure gradient despite the constant pumping action of the heart.
Arterioles provide blood to the organs and are chiefly composed of smooth muscle. The autonomic nervous system influences the diameter and shape of arterioles. They respond to the tissue’s need for more nutrients/oxygen. Arterioles play a significant role in the systemic vascular resistance because of the lack of significant elastic tissue in the walls.
Capillaries are thin-walled vessels composed of a single endothelial layer. Because of the thin walls of the capillary, exchange of nutrients and metabolites occurs primarily via diffusion. The arteriolar lumen regulates the flow of blood through the capillaries.
Venules are the smallest veins and receive blood from capillaries. They also play a role in the exchange of oxygen and nutrients for water products. There are post-capillary sphincters located between the capillaries and venules. The venule is very thin-walled and easily prone to rupture with excessive volume.
Veins: blood flows from venules into larger veins. Just like the arterial system, three layers make up the vein walls. But unlike the arteries, the venous pressure is low. Veins are thin walled and are less elastic. This feature permits the veins to hold a very high percentage of the blood in circulation. The venous system can accommodate a large volume of blood at relatively low pressures, a feature termed high capacitance. At any point in time, nearly three-fourths of the circulating blood volume is contained in the venous system.
The arteries are not inert and rigid tubes, but they modify the course of the blood by virtue of two properties that owe to the anatomical elements of his tunic, by virtue of its elasticity and its contractibility.
Arterial elasticity
The arterial elasticity acts as a spring, which yields to the cardiac impulse, later recovering and returning to the blood impulse part of the cardiac force expended on its distension. The result of this mechanism is to make the blood flow continue when it reaches the capillary networks, since it would be intermittent as the impulses it receives if the conduction tubes were rigid. Furthermore, circulation is favored because liquids circulate in greater quantity by elastic tubes with equal diameter.
Arterial elasticity depends fundamentally on the intrinsic elastic properties of the arterial wall: this is on the relative content of collagen (which provides resistance) and elastin (which makes them elastic) and the role of vascular smooth muscle. It is convenient to avoid excess rigidity of the vascular walls, which, together with the narrowing of the arteries, are two of the risk factors to take into account to avoid possible damage. Its elasticity allows you to maintain an almost constant blood pressure between each heartbeat.
The arteries (which are strong, flexible, and elastic) carry blood from the heart and withstand the highest pressures. Due to their elasticity, the arteries passively contract (recoil) when the heart relaxes between beats and thus helps maintain blood pressure.
The walls of large arteries, especially the aorta, lose elasticity over time, and this process results in increased arterial stiffness. Arterial stiffening, at least in part, reflects gradual fragmentation and loss of elastin fibers and accumulation of stiffer collagen fibers in the arterial wall (Wagenseil JE, “Elastin in large artery stiffness and hypertension”. J Cardiovasc Transl Res. 2012)
In recognizing the clinical importance of arterial stiffness, the American Heart Association also published a scientific statement to encourage further improvement and standardization of arterial stiffness measurements for clinical use and vascular research (Townsend RR, et al. Recommendations for improving and standardizing vascular research on arterial stiffness: a scientific statement from the American Heart Association. Hypertension. 2015).
Arterial stiffness, also known as the loss of arterial elasticity, represents the mechanical property of artery resistant to deformation. Inflammation, haemodynamic or genetic factors could break the synthesis/degradation balance and raise the ratio of collagen/elastin, thus increasing arterial stiffness.
Elastin is an elastic connective tissue protein with structural functions that, unlike collagen, which mainly provides resistance, gives elasticity to tissues. It is part of the wall of the arteries, providing them with the necessary elasticity for their extension and relaxation due to variations in blood pressure
Collagen is another protein. Its fibers have great rigidity and resistance to traction, helping to strengthen the blood vessels in the middle and adventitious layers.
With age, the elasticity of the vessels is lost due to the loss of elastic fibers and changes in their quality and internal organization, causing collagen wear that gives resistance to the wall. The coronary arteries undergo significant stiffening and decreased resistance during aging.