
Vasodilation is the widening of
blood vessels. It results from relaxation of
smooth muscle
Smooth muscle is an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit mus ...
cells within the vessel walls, in particular in the large
veins, large
arteries, and smaller
arterioles. The process is the opposite of
vasoconstriction, which is the narrowing of blood vessels.
When blood vessels
dilate, the
flow of blood is increased due to a decrease in
vascular resistance and increase in
cardiac output
In cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols Q, \dot Q, or \dot Q_ , edited by Catherine E. Williamson, Phillip Bennett is the volumetric flow rate of the heart's pumping output: t ...
. Therefore, dilation of arterial blood vessels (mainly the arterioles) decreases
blood pressure
Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressure" r ...
. The response may be
intrinsic (due to local processes in the surrounding
tissue) or
extrinsic (due to
hormones or the
nervous system). In addition, the response may be localized to a specific
organ
Organ may refer to:
Biology
* Organ (biology), a part of an organism
Musical instruments
* Organ (music), a family of keyboard musical instruments characterized by sustained tone
** Electronic organ, an electronic keyboard instrument
** Hammond ...
(depending on the
metabolic
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
needs of a particular tissue, as during strenuous exercise), or it may be systemic (seen throughout the entire
systemic circulation
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
).
Endogenous
Endogenous substances and processes are those that originate from within a living system such as an organism, tissue, or cell.
In contrast, exogenous substances and processes are those that originate from outside of an organism.
For example, es ...
substances and
drug
A drug is any chemical substance that causes a change in an organism's physiology or psychology when consumed. Drugs are typically distinguished from food and substances that provide nutritional support. Consumption of drugs can be via insuffla ...
s that cause vasodilation are termed vasodilators. Such
vasoactivity is necessary for
homeostasis (keeping the body running normally).
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 for oxygen but can occur when the tissue in question is not receiving enough
glucose,
lipids, or other
nutrients. Localized tissues have multiple ways to increase blood flow, including releasing vasodilators, primarily
adenosine
Adenosine ( symbol A) is an organic compound that occurs widely in nature in the form of diverse derivatives. The molecule consists of an adenine attached to a ribose via a β-N9-glycosidic bond. Adenosine is one of the four nucleoside building ...
, into the local
interstitial fluid, which diffuses to
capillary beds, provoking local vasodilation.
Some physiologists have suggested that it is the lack of oxygen itself that causes capillary beds to vasodilate by the smooth muscle
hypoxia
Hypoxia means a lower than normal level of oxygen, and may refer to:
Reduced or insufficient oxygen
* Hypoxia (environmental), abnormally low oxygen content of the specific environment
* Hypoxia (medical), abnormally low level of oxygen in the tis ...
of the vessels in the region. This latter hypothesis is posited due to the presence of
precapillary sphincters in capillary beds. These approaches to the mechanism of vasodilation are not
mutually exclusive.
Vasodilation and arterial resistance
Vasodilation directly affects the relationship between
mean arterial pressure,
cardiac output
In cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols Q, \dot Q, or \dot Q_ , edited by Catherine E. Williamson, Phillip Bennett is the volumetric flow rate of the heart's pumping output: t ...
, and
total peripheral resistance (TPR). Vasodilation occurs in the time phase of cardiac
systole
Systole ( ) is the part of the cardiac cycle during which some chambers of the heart contract after refilling with blood. The term originates, via New Latin, from Ancient Greek (''sustolē''), from (''sustéllein'' 'to contract'; from ''sun ...
, whereas vasoconstriction follows in the opposite time phase of cardiac
diastole.
Cardiac output
In cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols Q, \dot Q, or \dot Q_ , edited by Catherine E. Williamson, Phillip Bennett is the volumetric flow rate of the heart's pumping output: t ...
(blood flow measured in volume per unit time) is computed by multiplying the
heart rate (in beats per minute) and the
stroke volume
In cardiovascular physiology, stroke volume (SV) is the volume of blood pumped from the left ventricle per beat. Stroke volume is calculated using measurements of ventricle volumes from an echocardiogram and subtracting the volume of the blood i ...
(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) causes 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 to the atmosphere. The opposite physiological process is
vasoconstriction. These processes are naturally modulated by local
paracrine agents from
endothelial cells (e.g.,
nitric oxide
Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its che ...
,
bradykinin, potassium ions, and
adenosine
Adenosine ( symbol A) is an organic compound that occurs widely in nature in the form of diverse derivatives. The molecule consists of an adenine attached to a ribose via a β-N9-glycosidic bond. Adenosine is one of the four nucleoside building ...
), as well as an organism's
autonomic nervous system
The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that supplies viscera, internal organs, smooth muscle and glands. The autonomic nervous system is a control ...
and
adrenal glands, both of which secrete
catecholamines
A catecholamine (; abbreviated CA) is a monoamine neurotransmitter, an organic compound that has a catechol (benzene with two hydroxyl side groups next to each other) and a side-chain amine.
Catechol can be either a free molecule or a substi ...
such as
norepinephrine and
epinephrine
Adrenaline, also known as epinephrine, is a hormone and medication which is involved in regulating visceral functions (e.g., respiration). It appears as a white microcrystalline granule. Adrenaline is normally produced by the adrenal glands and ...
, respectively.
Examples and individual mechanisms
Vasodilation is the result of relaxation in
smooth muscle
Smooth muscle is an involuntary non-striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit mus ...
surrounding the blood vessels. This relaxation, in turn, relies on removing the stimulus for contraction, which depends on intracellular calcium ion concentrations and is tightly linked with
phosphorylation
In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, wh ...
of the light chain of the contractile protein
myosin
Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility.
The first myosin ...
. Thus, vasodilation works mainly either by lowering intracellular calcium concentration or by dephosphorylation (really substitution of ATP for ADP) of myosin.
Dephosphorylation by
myosin light-chain phosphatase
Myosin light-chain phosphatase, also called myosin phosphatase (EC 3.1.3.53; systematic name yosin-light-chainphosphate phosphohydrolase), is an enzyme (specifically a serine/threonine-specific protein phosphatase) that dephosphorylates the re ...
and induction of calcium
symporters
A symporter is an integral membrane protein that is involved in the transport of two (or more) different molecules across the cell membrane in the same direction. The symporter works in the plasma membrane and molecules are transported across the ...
and
antiporters that pump
calcium ions out of the intracellular compartment both contribute to smooth muscle cell relaxation and therefore vasodilation. 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
Endogenous substances and processes are those that originate from within a living system such as an organism, tissue, or cell.
In contrast, exogenous substances and processes are those that originate from outside of an organism.
For example, es ...
and
exogenous.
Endogenous
The vasodilating action of activation of
beta-2 receptor
The beta-2 adrenergic receptor (β2 adrenoreceptor), also known as ADRB2, is a cell membrane-spanning beta-adrenergic receptor that binds epinephrine (adrenaline), a hormone and neurotransmitter whose signaling, via adenylate cyclase stimulati ...
s (such as by adrenaline) appears to be
endothelium-independent.
Sympathetic nervous system vasodilation
Although it is recognized that the
sympathetic nervous system
The sympathetic nervous system (SNS) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of th ...
plays an expendable role in vasodilation, it is only one of the mechanisms by which vasodilation can be accomplished. The spinal cord has both vasodilation and
vasoconstriction nerves. The neurons that control vascular vasodilation originate in the hypothalamus. Some sympathetic stimulation of arterioles in skeletal muscle is mediated by epinephrine acting on β-adrenergic receptors of arteriolar smooth muscle, which would be mediated by cAMP pathways, as discussed above. However, it has been shown that knocking out this sympathetic stimulation plays little or 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 importance 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
Reflex syncope is a brief loss of consciousness due to a neurologically induced drop in blood pressure and/or a decrease in heart rate. Before an affected person passes out, there may be sweating, a decreased ability to see, or ringing i ...
.
[Guyton (2006) p. 208]
Cold-induced vasodilation
Cold-induced vasodilation (CIVD) occurs after cold exposure, possibly to reduce the risk of injury. It can take place in several locations in the human body but is observed most often in the extremities. The fingers are especially common because they are exposed most often.
When the fingers are exposed to cold,
vasoconstriction occurs first to reduce heat loss, resulting in strong cooling of the fingers. Approximately five to ten minutes after the start of the cold exposure of the hand, the blood vessels in the finger tips will suddenly vasodilate. This is probably caused by a sudden decrease in the release of
neurotransmitter
A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell.
Neuro ...
s from the
sympathetic nerves
The sympathetic nervous system (SNS) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of the ...
to the muscular coat of the
arteriovenous anastomoses
A circulatory anastomosis is a connection (an anastomosis) between two blood vessels, such as between arteries (arterio-arterial anastomosis), between veins (veno-venous anastomosis) or between an artery and a vein (arterio-venous anastomosis). ...
due to local cold. The CIVD increases blood flow and subsequently the temperature of the fingers. This can be painful and is sometimes known as the '
hot aches
Raynaud syndrome, also known as Raynaud's phenomenon, eponymously named after the physician Auguste Gabriel Maurice Raynaud, who first described it in his doctoral thesis in 1862, is a medical condition in which the spasm of small arteries cau ...
' which can be painful enough to bring on vomiting.
A new phase of vasoconstriction follows the vasodilation, after which the process repeats itself. This is called the
Hunting reaction The hunting reaction or hunting response is a process of alternating vasoconstriction and vasodilation in extremities exposed to cold. The term Lewis reaction is used too, named after Thomas Lewis, who first described the effect in 1930.
Vasocons ...
. Experiments have shown that three other vascular responses to immersion of the finger in cold water are possible: a continuous state of vasoconstriction; slow, steady, and continuous rewarming; and a proportional control form in which the blood vessel diameter remains constant after an initial phase of vasoconstriction. However, the vast majority of responses can be classified as the Hunting reaction.
Other possible causes of vasodilation
Other suggested vasodilators or vasodilating factors include:
* absence of high levels of
environmental noise
Environmental noise is an accumulation of noise pollution that occurs outside. This noise can be caused by transport, industrial, and recreational activities.
Noise is frequently described as 'unwanted sound'. Within this context, environmenta ...
* absence of
high levels of illumination
*
adenosine
Adenosine ( symbol A) is an organic compound that occurs widely in nature in the form of diverse derivatives. The molecule consists of an adenine attached to a ribose via a β-N9-glycosidic bond. Adenosine is one of the four nucleoside building ...
- adenosine agonist, used primarily as an anti-
arrhythmic
*
alpha blocker
Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', or ell, άλφα, álfa) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter aleph , which ...
s (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)
*
ethanol (alcohol)
* histamine-inducers
**
Complement proteins C3a, C4a, and C5a work by triggering histamine release from
mast cells and
basophil granulocytes.
*
nitric oxide
Nitric oxide (nitrogen oxide or nitrogen monoxide) is a colorless gas with the formula . It is one of the principal oxides of nitrogen. Nitric oxide is a free radical: it has an unpaired electron, which is sometimes denoted by a dot in its che ...
inducers
**
l-arginine (a key amino acid)
**
glyceryl trinitrate
Nitroglycerin (NG), (alternative spelling of nitroglycerine) also known as trinitroglycerin (TNG), nitro, glyceryl trinitrate (GTN), or 1,2,3-trinitroxypropane, is a dense, colorless, oily, explosive liquid most commonly produced by nitrating g ...
(commonly known as
nitroglycerin
Nitroglycerin (NG), (alternative spelling of nitroglycerine) also known as trinitroglycerin (TNG), nitro, glyceryl trinitrate (GTN), or 1,2,3-trinitroxypropane, is a dense, colorless, oily, explosive liquid most commonly produced by nitrating g ...
)
**
isosorbide mononitrate and
isosorbide dinitrate
**
pentaerythritol tetranitrate (PETN)
**
sodium nitroprusside
**
PDE5 inhibitors: these agents indirectly increase the effects of nitric oxide
***
sildenafil
Sildenafil, sold under the brand name Viagra, among others, is a medication used to treat erectile dysfunction and pulmonary arterial hypertension. It is unclear if it is effective for treating sexual dysfunction in women. It is taken by ...
(Viagra)
***
tadalafil (Cialis)
***
vardenafil (Levitra)
*
tetrahydrocannabinol (THC), the principal psychoactive constituent of cannabis.
*
theobromine, the principal alkaloid found in ''
Theobroma cacao'', specifically in
cocoa solids (which is found in chocolate, especially
dark chocolate).
*
minoxidil
*
papaverine an alkaloid found in the opium poppy
papaver somniferum
*
estrogen
*
apigenin: In rat small mesenteric arteries, apigenin acts on TRPV4 in endothelial cells to induce EDHF-mediated vascular dilation (Br J Pharmacol 2011 Nov 3)
Therapeutic uses
Vasodilators are used to treat conditions such as
hypertension
Hypertension (HTN or HT), also known as high blood pressure (HBP), is a long-term medical condition in which the blood pressure in the arteries is persistently elevated. High blood pressure usually does not cause symptoms. Long-term high bl ...
, wherein the patient has an abnormally high blood pressure, as well as
angina
Angina, also known as angina pectoris, is chest pain or pressure, usually caused by ischemia, insufficient blood flow to the Cardiac muscle, heart muscle (myocardium). It is most commonly a symptom of coronary artery disease.
Angina is typical ...
,
congestive heart failure, and
erectile dysfunction
Erectile dysfunction (ED), also called impotence, is the type of sexual dysfunction in which the penis fails to become or stay erect during sexual activity. It is the most common sexual problem in men.Cunningham GR, Rosen RC. Overview of male ...
, and where maintaining a lower blood pressure reduces the patient's risk of developing other cardiac problems.
Flushing may be a physiological response to vasodilators. Some
phosphodiesterase inhibitors such as
sildenafil
Sildenafil, sold under the brand name Viagra, among others, is a medication used to treat erectile dysfunction and pulmonary arterial hypertension. It is unclear if it is effective for treating sexual dysfunction in women. It is taken by ...
,
vardenafil and
tadalafil, work to increase blood flow in the penis through vasodilation. They may also be used to treat
pulmonary arterial hypertension (PAH).
Antihypertensives that work by opening blood vessels
;These drugs can keep vessels staying opened or help vessels refrain from being narrowed.
*
Angiotensin II receptor blockers
*
ACE inhibitors
*
Calcium channel blockers
Calcium channel blockers (CCB), calcium channel antagonists or calcium antagonists are a group of medications that disrupt the movement of calcium () through calcium channels. Calcium channel blockers are used as antihypertensive drugs, i.e., as ...
;Drugs that appear to work by activating the
α2A receptors in the brain thereby decreasing
sympathetic nervous system
The sympathetic nervous system (SNS) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous system and the enteric nervous system. The enteric nervous system is sometimes considered part of th ...
activity.
*
methyldopa
::According to
American Heart Association
The American Heart Association (AHA) is a nonprofit organization in the United States that funds cardiovascular medical research, educates consumers on healthy living and fosters appropriate cardiac care in an effort to reduce disability and death ...
, Alpha-methyldopa may cause
Orthostatic syncope Orthostatic syncope refers to syncope resulting from a postural decrease in blood pressure, termed orthostatic hypotension.
Orthostatic hypotension occurs when there is a persistent reduction in blood pressure of at least 20mmHg systolic or 10mmH ...
as it exerts a greater blood pressure lowering effect when one is standing upright which may lead to feeling weak or fainting if the blood pressure has been lowered too far. Methyldopa's prominent side effects include drowsiness or sluggishness, dryness of the mouth, fever or anemia. Additionally to these, male patients may experience impotence.
*
clonidine hydrochloride
Clonidine, sold under the brand name Catapres among others, is an Alpha-2 adrenergic receptor, α2-adrenergic agonist medication used to treat hypertension, high blood pressure, attention deficit hyperactivity disorder, ADHD, drug withdrawal (A ...
*
guanabenz acetate
*
guanfacine hydrochloride
Guanfacine, sold under the brand name Tenex among others, is an oral medication used to treat attention deficit hyperactivity disorder (ADHD) and high blood pressure. Guanfacine is FDA-approved for monotherapy treatment of ADHD as well as being ...
::Clonidine, guanabenz or guanfacine may give rise to severe dryness of the mouth, constipation or drowsiness. Abrupt cessation taking may raise blood pressure quickly to dangerously high levels.
;Directly relax the muscle in the walls of the blood vessels (especially the arterioles), allowing the vessel to dilate (widen).
*
hydralazine
*
minoxidil
::Hydralazine may cause headaches, swelling around the eyes, heart palpitations or aches and pains in the joints. In clinical setting, hydralazine isn't usually used alone.
::Minoxidil is a ''potent'' direct vasodilator used only in ''resistant'' severe high blood pressure or when
kidney failure is present. Noted adverse effects comprise fluid retention (marked weight gain) and excessive hair growth.
See also
*
Arteriolar vasodilator
Arteriolar vasodilators are substances or medications that preferentially dilate arterioles. When used on people with certain heart conditions, it causes a phenomenon known as the cardiac steal syndrome. Arteriolar vasodilators increase intracapi ...
*
Nitrophorin
*
Vasodilatory shock
Vasodilatory shock, vasogenic shock, or vasoplegic shock is a medical emergency belonging to Shock (circulatory), shock along with cardiogenic shock, septic shock, Anaphylaxis, allergen-induced shock and hypovolemic shock. When the blood vessels ...
References
{{Authority control
Cardiovascular physiology