Vasodilation

Vasodilation refers to the widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls, particularly in the large arteries, smaller arterioles and large veins. The process is essentially the opposite of vasoconstriction, or the narrowing of blood vessels. When vessels dilate, the flow of blood is increased due to a decrease in vascular resistance. Therefore, dilation of arterial blood vessels (mainly arterioles) leads to a decrease in blood pressure. The response may be intrinsic (due to local processes in the surrounding tissue) or extrinsic (due to hormones or the nervous system). Additionally, the response may either be localized to a specific organ (depending on the metabolic needs of a particular tissue, as during strenuous exercise), or systemic (seen throughout the entire systemic circulation). Factors that result in vasodilation are termed vasodilators.

Function
The primary function of vasodilation is to increase blood flow in the body to tissues that need it most. This is often in response to a localized need of oxygen, but can occur when the tissue in question is not receiving enough glucose or lipids or other nutrients. Localized tissues utilize multiple ways to increase blood flow including releasing vasodilators, primarily adenosine, into the local instersitial fluid which diffuses to capillary beds provoking local vasodilation. Some physiologists have suggested it is the lack of oxygen itself which causes capillary beds to vasodilate by the smooth muscle hypoxia of the vessels in the region. This latter hypothesis is posited due to the presence of precapillary sphincters in capillary beds. Neither of these approaches to the mechanism of vasodilation is mutually exclusive of the other.

Vasodilation and arterial resistance
Vasodilation directly affects the relationship between mean arterial pressure, cardiac output and total peripheral resistance (TPR). Vasodilation occurs in the time phase of cardiac systole while vasoconstriction follows in the opposite time phase of cardiac diastole. Mathematically, cardiac output (blood flow measured in volume per unit time) is computed by multiplying the heart rate (in beats per minute) and the stroke volume (the volume of blood ejected during ventricular systole). TPR depends on several factors, including the length of the vessel, the viscosity of blood (determined by hematocrit) and the diameter of the blood vessel. The latter is the most important variable in determining resistance, with the TPR changing by the fourth power of the radius. An increase in either of these physiological components (cardiac output or TPR) cause a rise in the mean arterial pressure. Vasodilation works to decrease TPR and blood pressure through relaxation of smooth muscle cells in the tunica media layer of large arteries and smaller arterioles.

Vasodilation occurs in superficial blood vessels of warm-blooded animals when their ambient environment is hot; this process diverts the flow of heated blood to the skin of the animal, where heat can be more easily released into the atmosphere. The opposite physiological process is vasoconstriction. These processes are naturally modulated by local paracrine agents from endothelial cells (e.g. nitric oxide, bradykinin, potassium ions and adenosine), as well as an organism's autonomic nervous system and adrenal glands, both of which secrete catecholamines such as norepinephrine and epinephrine, respectively.

Examples and individual mechanisms
Vasodilation is the result of relaxation in smooth muscle surrounding the blood vessels. This relaxation, in turn, relies on removing the stimulus for contraction, which depends on intracellular calcium ion concentrations and, consequently, phosphorylation of the light chain of the contractile protein myosin. Thus, vasodilation mainly works either by lowering intracellular calcium concentration or the dephosphorylation of myosin. This includes stimulation of myosin light chain phosphatase and induction of calcium symporters and antiporters that pump calcium ions out of the intracellular compartment. This is accomplished through reuptake of ions into the sarcoplasmic reticulum via exchangers and expulsion across the plasma membrane. There are three main intracellular stimuli that can result in the vasodilation of blood vessels. The specific mechanisms to accomplish these effects vary from vasodilator to vasodilator.

PDE5 inhibitors and potassium channel openers can also have similar results.

Compounds that mediate the above mechanisms may be grouped as endogenous and exogenous.

Endogenous
The vasodilating action of activation of beta-2 receptors (such as by noradrenaline) appears to be endothelium-independent.

Sympathetic nervous system vasodilation
Whereas it is recognized that that the sympathetic nervous system plays an expendable role in vasodilation, it is one of the mechanisms by which it can be accomplished. The spinal cord has both vasodilation and vasoconstriction nerves. The neurons that control vascular vasodilation originate in the hypothalmus. Some sympathetic stimulation of capillaries in skeletal muscle is mediated by epinephrine acting on β-adrenergic receptors of capillary smooth muscle which would be mediated by cAMP pathways as mentioned above. However, it has been shown that knocking out this sympathetic stimulation plays little to no role in whether skeletal muscle is able to receive sufficient oxygen even at high levels of exertion, so it is believed that this particular method of vasodilation is of little import to human physiology.

In cases of emotional distress, this system may activate, resulting in fainting due to decreased blood pressure from vasodilation, which is referred to as vasovagal syncope.

Other mechanisms of vasodilation
Other suggested vasodilators or vasodilating factors include:
 * absence of high levels of environmental noise
 * absence of high levels of illumination
 * Adenocard - adenosine agonist, primarily used as an antiarrhythmic
 * alpha blockers (block the vasoconstricting effect of adrenaline)
 * amyl nitrite and other nitrites are often used recreationally as a vasodilator, causing lightheadedness and a euphoric feeling
 * atrial natriuretic peptide (ANP) - a weak vasodilator
 * capsaicin (chili) [not necessarily safe for diabetics]
 * ethanol (alcohol)
 * histamine-inducers
 * Complement proteins C3a, C4a and C5a work by triggering histamine release from mast cells and basophil granulocytes.
 * nitric oxide inducers
 * glyceryl trinitrate (commonly known as nitroglycerin)
 * isosorbide mononitrate and isosorbide dinitrate
 * pentaerythritol tetranitrate (PETN)
 * sodium nitroprusside
 * PDE5 inhibitors: these agents indirectly increase the effects of nitric oxide
 * sildenafil (Viagra)
 * tadalafil
 * vardenafil
 * tetrahydrocannabinol (THC) - the major active chemical in marijuana, its mild vasodilating effects redden the eyes of cannabis users.
 * theobromine
 * papaverine an alkaloid found in the opium poppy papaver somniferum
 * estrogen

Therapeutic uses
Vasodilators are used to treat conditions such as hypertension, where the patient has an abnormally high blood pressure, as well as angina and congestive heart failure, where maintaining a lower blood pressure reduces the patient's risk of developing other cardiac problems. Flushing may be a physiological response to vasodilators. Viagra, a phosphodiesterase inhibitor, works to increase blood flow in the penis through vasodilation. It may also be used to treat pulmonary arterial hypertension (PAH).