Vascular Tone: Function, Role & Diseases

Vascular tone corresponds to the general state of tension of the vascular musculature, which is responsible for blood flow. Higher regulation is the responsibility of the sympathetic nervous system, but local regulators are also available to the organism. Abnormal contractions of the vascular musculature are called vasospasms and can occur in the context of various diseases.

What is vascular tone?

Vascular tone is the basic state of tension that exists in vascular smooth muscle without any preceding excitation or contraction so caused. The human blood system is equipped with muscles also known as vascular smooth muscle. Vascular smooth muscle gives vessels their tone. Each muscle has a basic tone. This is the state of tension that a body structure maintains even without actual excitation. The basic tone is thus to be distinguished from the contraction that a muscle can actively undertake in response to excitation. This contraction increases the tone and thus causes it to rise above the fundamental tone. Vascular tone is the baseline state of tension that exists in vascular smooth muscle without any preceding excitation or contraction so induced. Changes in vascular tone automatically cause changes in vascular lumen. An increase in tone causes vasodilation. A decrease causes vasoconstriction of the lumen width. Thus, how much blood can pass through the blood vessels per unit time depends on the vascular tone. The tone of the vascular musculature thus affects vascular resistance and local blood flow. As a sum of serial resistances, vascular tone is the peripheral total resistance that significantly regulates blood pressure. Muscle control is via the autonomic nervous system.

Function and task

During vasoconstriction, the vascular muscles contract. This reduces the lumen of the vessels and decreases the amount of blood flow. Vasodilation, on the other hand, occurs when the vascular musculature relaxes. The lumen enlarges and blood flow increases. The tone of the vascular muscles thus determines the passage of blood in the individual vessels. The basic tone is between vasoconstriction and vasodilation. Thus, in the resting state, the vascular muscles are neither completely relaxed nor actively contracted, but exhibit a baseline tone. The vascular tone depends on various influencing variables that have a regulatory effect on blood pressure. One of these influencing variables is the tone of the sympathetic nervous system. This part of the autonomic nervous system determines the state of excitation of the nervous system. The sympathetic nervous system has a vasoconstrictive effect on the vessels and through this influence is responsible for their alkaline toning. When the sympathetic nervous system is inhibited, it affects the vascular tone. Inhibition of the permanently toning sympathetic nervous system leads to vasodilation. In addition to the tone of the sympathetic nervous system, hormones such as the stress hormone adrenaline and substances such as angiotensin II or vasopressin influence the tone of the vascular muscles. Furthermore, the vascular musculature has local regulatory mechanisms. These include the Bayliss effect and the Euler-Liljestrand mechanism. The Bayliss effect corresponds to a mechanism for maintaining constant organ perfusion despite changing blood pressure levels. The Euler-Liljestrand mechanism results in reflex vasoconstriction of pulmonary arterial vessels during hypoxia. In addition, endothelial factors influence vascular tone. These include, for example, NO, prostaglandin E2, and prostacyclin. In addition, tissue metabolites such as H+- ions or adenosine also show effects on vascular tone and thus automatically on the prevailing blood pressure.

Diseases and ailments

One of the best-known diseases with a pathological vascular tone is the so-called angina pectoris. This is an attack-like pain in the area of the chest, which is triggered by a short-lived circulatory disturbance in the heart and usually occurs in the context of coronary heart disease. In particular, the special form Prinzmetal’s angina is associated with pathological vascular tone. The associated ischemia of the myocardium results from a spasm of the coronary artery. The attack may last from seconds to minutes. Most frequently, the spastic constriction of the artery occurs in the context of more or less severe stress of the body or psyche.Basically, all vessels of the body can be affected by spastic contractions and their lumen can be greatly reduced during these seizure-like phenomena, resulting in reduced blood flow in the local tissues. All spasms of the vessels are summarized under the term vasospasm. This term is associated with a sudden onset of spasmodic constriction of blood-bearing vessels triggered by a specific stimulus. In the brain, such phenomena are called cerebral vasospasms and can sometimes have life-threatening consequences. They are often a complication of subarachnoid hemorrhage or are a symptom of neurological diseases. These pathological changes in vascular tone are also conceivable after certain intoxications, especially in connection with cocaine and methamphetamine use. When hemorrhage occurs, blood that has entered the subarachnoid space breaks down and releases vasoconstrictive substances. Since the consequence of cerebral vasospasm is the reduced supply of blood and oxygen to parts of the brain, a common consequence of the phenomenon is secondary stroke. Since the sympathetic nervous system regulates vascular tone, pathological vasospasm may in principle also be due to abnormalities in this brain region. An example of this connection is Raynaud’s syndrome. The condition causes patients’ fingers or toes to pale due to vasospasm. The body minimizes heat loss when exposed to cold by stimulating the autonomic nervous system. This process directs more blood to the deeper veins of the body. In Raynaud’s syndrome, this process is affected by a dysregulation that occurs primarily in the sympathetic part of the autonomic nervous system and results in excessive vasoconstriction of all end arteries through alpha-adrenoreceptors.