
Vasodilation, also known as vasorelaxation, is the widening of
blood vessel
Blood vessels are the tubular structures of a circulatory system that transport blood throughout many Animal, animals’ bodies. Blood vessels transport blood cells, nutrients, and oxygen to most of the Tissue (biology), tissues of a Body (bi ...
s.
It results from relaxation of
smooth muscle
Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. It can also be found in invertebrates and is controlled by the autonomic nervous system. It is non- striated, so-called bec ...
cells within the vessel walls, in particular in the large
vein
Veins () are blood vessels in the circulatory system of humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those of the pulmonary and feta ...
s, large
arteries, and smaller
arteriole
An arteriole is a small-diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillary, capillaries.
Arterioles have vascular smooth muscle, muscular walls (usually only one to two layers of smoo ...
s.
Blood vessel walls are composed of endothelial tissue and a basal membrane lining the lumen of the vessel, concentric smooth muscle layers on top of endothelial tissue, and an
adventitia over the smooth muscle layers.
Relaxation of the smooth muscle layer allows the blood vessel to dilate, as it is held in a semi-constricted state by sympathetic nervous system activity.
Vasodilation 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. Vascular resistance is the amount of force circulating blood must overcome in order to allow
perfusion
Perfusion is the passage of fluid through the circulatory system or lymphatic system to an organ (anatomy), organ or a tissue (biology), tissue, usually referring to the delivery of blood to a capillary bed in tissue. Perfusion may also refer t ...
of body tissues. Narrow vessels create more vascular resistance, while dilated vessels decrease vascular resistance. Vasodilation acts to increase cardiac output by decreasing
afterload, −one of the four determinants of cardiac output.
By expanding available area for blood to circulate, vasodilation decreases
blood pressure
Blood pressure (BP) is the pressure of Circulatory system, 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 ...
.
The response may be
intrinsic (due to local processes in the surrounding
tissue) or
extrinsic (due to
hormone
A hormone (from the Ancient Greek, Greek participle , "setting in motion") is a class of cell signaling, signaling molecules in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physio ...
s or the
nervous system
In biology, the nervous system is the complex system, highly complex part of an animal that coordinates its behavior, actions and sense, sensory information by transmitting action potential, signals to and from different parts of its body. Th ...
). In addition, the response may be localized to a specific
organ (depending on the
metabolic
Metabolism (, from ''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 cellular processes; the ...
needs of a particular tissue, as during strenuous exercise), or it may be systemic (seen throughout the entire
systemic circulation
In vertebrates, the circulatory system is a organ system, system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. It includes the cardiovascular system, or vascular system, that consists of ...
).
Endogenous
Endogeny, in biology, refers to the property of originating or developing from within an organism, tissue, or cell.
For example, ''endogenous substances'', and ''endogenous processes'' are those that originate within a living system (e.g. an ...
substances and
drug
A drug is any chemical substance other than a nutrient or an essential dietary ingredient, which, when administered to a living organism, produces a biological effect. Consumption of drugs can be via insufflation (medicine), inhalation, drug i ...
s that cause vasodilation are termed vasodilators. Many of these substances are
neurotransmitter
A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a Chemical synapse, synapse. The cell receiving the signal, or target cell, may be another neuron, but could also be a gland or muscle cell.
Neurotra ...
s released by perivascular nerves of the
autonomic nervous system
The autonomic nervous system (ANS), sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates viscera, internal organs, smooth muscle and glands. The autonomic nervo ...
Baroreceptors sense blood pressure and allow adaptation via the mechanisms of vasoconstriction or vasodilation to maintain
homeostasis
In biology, homeostasis (British English, British also homoeostasis; ) is the state of steady internal physics, physical and chemistry, chemical conditions maintained by organism, living systems. This is the condition of optimal functioning fo ...
.
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
Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
,
lipid
Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...
s, or other
nutrients
A nutrient is a substance used by an organism to survive, grow and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...
. Vasodilation, both localized and systemic, also facilitates immune response.
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 build ...
, into the local
interstitial fluid, which diffuses to
capillary
A capillary is a small blood vessel, from 5 to 10 micrometres in diameter, and is part of the microcirculation system. Capillaries are microvessels and the smallest blood vessels in the body. They are composed of only the tunica intima (the inn ...
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 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 have not been found to be
mutually exclusive
In logic and probability theory, two events (or propositions) are mutually exclusive or disjoint if they cannot both occur at the same time. A clear example is the set of outcomes of a single coin toss, which can result in either heads or tails ...
.
Immune system
Vasodilation plays a major role in immune system function. Wider blood vessels allow more blood containing immune cells and proteins to reach the infection site. Vasodilation occurs as part of the process of
inflammation
Inflammation (from ) is part of the biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. The five cardinal signs are heat, pain, redness, swelling, and loss of function (Latin ''calor'', '' ...
, which is caused by several factors including presence of a pathogen, injury to tissues or blood vessels, and
immune complexes.
In severe cases, inflammation can lead to
sepsis
Sepsis is a potentially life-threatening condition that arises when the body's response to infection causes injury to its own tissues and organs.
This initial stage of sepsis is followed by suppression of the immune system. Common signs and s ...
or distributive shock.
Vasodilation is also a major component of
anaphylaxis
Anaphylaxis (Greek: 'up' + 'guarding') is a serious, potentially fatal allergic reaction and medical emergency that is rapid in onset and requires immediate medical attention regardless of the use of emergency medication on site. It typicall ...
.
Inflammation causes not only vasodilation but also causes increased
vascular permeability, allowing
neutrophil
Neutrophils are a type of phagocytic white blood cell and part of innate immunity. More specifically, they form the most abundant type of granulocytes and make up 40% to 70% of all white blood cells in humans. Their functions vary in differe ...
s,
complement proteins, and
antibodies
An antibody (Ab) or immunoglobulin (Ig) is a large, Y-shaped protein belonging to the immunoglobulin superfamily which is used by the immune system to identify and neutralize antigens such as bacteria and viruses, including those that caus ...
to reach the site of infection or damage.
Elevated vascular permeability can allow excess fluid to leave blood vessels and collect in tissues resulting in
edema; vasodilation prevents blood vessels from constricting to adapt to reduced volume in the vessels, causing low blood pressure and
septic shock.
In the case of inflammation, vasodilation is caused by
cytokine
Cytokines () are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling.
Cytokines are produced by a broad range of cells, including immune cells like macrophages, B cell, B lymphocytes, T cell, T lymphocytes ...
s.
Interferon gamma,
TNF-a,
interleukin 1 beta, and
interleukin 12 are a few examples of some inflammatory cytokines produced by immune cells such as
natural killer cell
Natural killer cells, also known as NK cells, are a type of cytotoxic lymphocyte critical to the innate immune system. They are a kind of large granular lymphocytes (LGL), and belong to the rapidly expanding family of known innate lymphoid cells ...
s,
B cell
B cells, also known as B lymphocytes, are a type of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or inserted into the plasm ...
s,
T cell
T cells (also known as T lymphocytes) are an important part of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell receptor (TCR) on their cell ...
s,
mast cell
A mast cell (also known as a mastocyte or a labrocyte) is a resident cell of connective tissue that contains many granules rich in histamine and heparin. Specifically, it is a type of granulocyte derived from the myeloid stem cell that is a p ...
s and
macrophage
Macrophages (; abbreviated MPhi, φ, MΦ or MP) are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris and foreign substances, which do not have proteins that ...
s.
Anti-inflammatory cytokines that regulate inflammation and help prevent negative results such as septic shock are also produced by these immune cells.
Vasodilation and increased vascular permeability also allow immune
effector cell
In cell biology, an effector cell is any of various types of cell that actively responds to a stimulus and effects some change (brings it about).
Examples of effector cells include:
* The muscle, gland or organ cell capable of responding to ...
s to leave blood vessels and follow
chemoattractants to the infection site via a process called
leukocyte extravasation.
Vasodilation allows the same volume of blood to move more slowly according to the flow rate equation Q = Av, where Q represents flow rate, A represents cross-sectional area, and v represents velocity. Immune effector cells can more easily attach to
selectins expressed on endothelial cells when blood is flowing slowly, enabling these cells to exit the blood vessel via
diapedesis.
Anaphylaxis
Anaphylaxis (Greek: 'up' + 'guarding') is a serious, potentially fatal allergic reaction and medical emergency that is rapid in onset and requires immediate medical attention regardless of the use of emergency medication on site. It typicall ...
is a severe allergic reaction characterized by elevated vascular permeability, systemic vasodilation, gastrointestinal dysfunction, and respiratory dysfunction.
Anaphylatoxin
Anaphylatoxins, or complement peptides, are fragments ( C3a, C4a and C5a) that are produced as part of the activation of the complement system. Complement components C3, C4 and C5 are large glycoproteins that have important functions in the immu ...
s, specifically
complement proteins C3a and C5a, bind to receptors on mast cells and basophils causing
degranulation.
Granules in these cells contain
histamine
Histamine is an organic nitrogenous compound involved in local immune responses communication, as well as regulating physiological functions in the gut and acting as a neurotransmitter for the brain, spinal cord, and uterus. Discovered in 19 ...
,
platelet-activating factor, and other compounds causing clinical manifestation of anaphylaxis- including systemic vasodilation causing dangerously low blood pressure.
Immunoglobulin E, an antibody produced by
plasma cell
Plasma cells, also called plasma B cells or effector B cells, are white blood cells that originate in the lymphoid organs as B cells and secrete large quantities of proteins called antibodies in response to being presented specific substances ca ...
s, also binds to receptors on mast cells and basophils causing degranulation.
Mechanism
A basic understanding of
cardiac output,
vascular resistance, and
blood pressure
Blood pressure (BP) is the pressure of Circulatory system, 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 ...
is necessary to understand the causes and impacts of vasodilation. Cardiac output is defined as the amount of blood pumped through the heart over 1 minute, in units of liters per minute, equal to
heart rate
Heart rate is the frequency of the cardiac cycle, heartbeat measured by the number of contractions of the heart per minute (''beats per minute'', or bpm). The heart rate varies according to the body's Human body, physical needs, including the nee ...
multiplied by
stroke volume.
It is directly related to
heart rate
Heart rate is the frequency of the cardiac cycle, heartbeat measured by the number of contractions of the heart per minute (''beats per minute'', or bpm). The heart rate varies according to the body's Human body, physical needs, including the nee ...
,
myocardial contractility
Myocardial contractility represents the innate ability of the heart muscle ( cardiac muscle or myocardium) to contract. It is the maximum attainable value for the force of contraction of a given heart. The ability to produce changes in force during ...
, and
preload, and inversely related with
afterload.
Elevated vascular resistance due to constricted blood vessels causes in increase in afterload, the amount of force against which the heart must contract.
Vasodilation therefore decreases vascular resistance, which decreases afterload, elevating cardiac output and allowing perfusion of tissues. Blood pressure measures how much pressure blood exerts on blood vessel walls;
systolic blood pressure measures pressure while the heart contracts (
systole
Systole ( ) is the part of the cardiac cycle during which some chambers of the heart contract after refilling with blood. Its contrasting phase is diastole, the relaxed phase of the cardiac cycle when the chambers of the heart are refilling ...
), and
diastolic blood pressure reflects pressure between contractions (
diastole).
Mean arterial pressure (MAP)is a weighted average of systolic and diastolic blood pressures, and is a better measurement of perfusion over the duration of the cardiac cycle. Vasodilation works to decrease vascular resistance and blood pressure through relaxation of smooth muscle cells in the
tunica media
The tunica media (Neo-Latin "middle coat"), or media for short, is the middle tunica (layer) of an artery or vein. It lies between the internal elastic lamina of the tunica intima on the inside and the tunica externa on the outside.
Artery
The ...
layer of large
arteries and smaller arterioles.
When vasodilation causes systolic blood pressure to fall below 90 mmHg,
circulatory shock is observed.
Vascular resistance 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 vascular resistance changing by the fourth power of the radius.
An increase in either of these physiological components (cardiac output or vascular resistance) causes a rise in MAP.
Arteriole
An arteriole is a small-diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillary, capillaries.
Arterioles have vascular smooth muscle, muscular walls (usually only one to two layers of smoo ...
s create the most vascular resistance of any blood vessel type, as they are very narrow and possess concentric layers of smooth muscle unlike
venule
A venule is a very small vein in the microcirculation that allows blood to return from the capillary beds to drain into the venous system via increasingly larger veins. Post-capillary venules are the smallest of the veins with a diameter of ...
s and
capillaries.
Vasodilation occurs in superficial blood vessels of
warm-blooded
Warm-blooded is a term referring to animal species whose bodies maintain a temperature higher than that of their environment. In particular, homeothermic species (including birds and mammals) maintain a stable body temperature by regulating ...
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 environment. The opposite physiological process is
vasoconstriction. These processes are naturally modulated by local
paracrine
In cellular biology, paracrine signaling is a form of cell signaling, a type of cellular communication (biology), cellular communication in which a Cell (biology), cell produces a signal to induce changes in nearby cells, altering the behaviour of ...
agents from
endothelial cells (e.g.,
nitric oxide
Nitric oxide (nitrogen oxide, nitrogen monooxide, 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 den ...
,
bradykinin
Bradykinin (BK) (from Greek ''brady-'' 'slow' + ''-kinin'', ''kīn(eîn)'' 'to move') is a peptide that promotes inflammation. It causes arterioles to dilate (enlarge) via the release of prostacyclin, nitric oxide, and endothelium-derived hyperpo ...
,
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 build ...
), and by the
autonomic nervous system
The autonomic nervous system (ANS), sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates viscera, internal organs, smooth muscle and glands. The autonomic nervo ...
and the
adrenal gland
The adrenal glands (also known as suprarenal glands) are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. They are found above the kidneys. Each gland has an outer adrenal corte ...
s, both of which secrete
catecholamines, such as
norepinephrine
Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic compound, organic chemical in the catecholamine family that functions in the brain and human body, body as a hormone, neurotransmitter and neuromodulator. The ...
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 a ...
, respectively.
Smooth muscle physiology
The
tunica media
The tunica media (Neo-Latin "middle coat"), or media for short, is the middle tunica (layer) of an artery or vein. It lies between the internal elastic lamina of the tunica intima on the inside and the tunica externa on the outside.
Artery
The ...
of the walls of arteries, arterioles, and veins is composed of
smooth muscle
Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. It can also be found in invertebrates and is controlled by the autonomic nervous system. It is non- striated, so-called bec ...
and causes vasodilation and vasoconstriction.
Contraction of smooth muscle cells causes vasoconstriction, and relaxation of smooth muscle causes vasodilation.
Smooth muscle is innervated by the
autonomic nervous system
The autonomic nervous system (ANS), sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates viscera, internal organs, smooth muscle and glands. The autonomic nervo ...
and is non-striated (does not contain sarcomeres).
Contraction is dependent on concentrations of Ca
2+ in the cytosol, either via Ca,Mg-ATPase from the
sarcoplasmic reticulum or voltage-gated calcium channels from the extracellular matrix.
Calcium ions bind with
calmodulin, activating
myosin light-chain kinase which
phosphorylates the myosin light-chain.
Phosphorylated light-chain myosin interacts with
actin
Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ...
filaments forming a
cross-bridge, allowing muscle contraction causing vasoconstriction.
Vasodilation is caused by
myosin-light-chain phosphatase, which
dephosphorylates the myosin light chain causing muscle relaxation.
Smooth muscle cells can remain contracted without use of ATP due to action of the myosin-binding subunit of myosin light-chain phosphatase. Phosphorylation of this subunit by
Rho-kinase prevents it from binding to and dephosphorylating the myosin light-chain, allowing the cell to remain contracted.
Vasodilation is the result of relaxation in
smooth muscle
Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. It can also be found in invertebrates and is controlled by the autonomic nervous system. It is non- striated, so-called bec ...
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 biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols:
:
This equation can be writ ...
of the light chain of the contractile protein
myosin
Myosins () are a Protein family, family of motor proteins (though most often protein complexes) best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are adenosine triphosphate, ATP- ...
. 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 and induction of calcium
symporters 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 inhibitor
A phosphodiesterase type 5 inhibitor (PDE5 inhibitor) is a vasodilating medication, drug that works by blocking the degradative action of cGMP-specific phosphodiesterase type 5 (PDE5) on cyclic GMP in the smooth muscle cells lining the blood vess ...
s and
potassium channel openers can also have similar results.
Compounds that mediate the above mechanisms may be grouped as
endogenous
Endogeny, in biology, refers to the property of originating or developing from within an organism, tissue, or cell.
For example, ''endogenous substances'', and ''endogenous processes'' are those that originate within a living system (e.g. an ...
and
exogenous.
Causes
Endogenous
The vasodilating action of activation of
beta-2 receptors (such as by adrenaline) appears to be
endothelium
The endothelium (: endothelia) is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the r ...
-independent.
Autonomic nervous system control
As referenced in the explanation of smooth muscle physiology, smooth muscle within the tunica media is innervated by the autonomic nervous system. The
autonomic nervous system
The autonomic nervous system (ANS), sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates viscera, internal organs, smooth muscle and glands. The autonomic nervo ...
(ANS) controls essential involuntary body functions and originates as nerves leaving the
brain stem
The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is co ...
or spinal cord; it contains both sensor and motor nerves.
The two divisions of the ANS, the
sympathetic nervous system
The sympathetic nervous system (SNS or SANS, sympathetic autonomic nervous system, to differentiate it from the somatic nervous system) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous sy ...
(SNS) and the
parasympathetic nervous system
The parasympathetic nervous system (PSNS) is one of the three divisions of the autonomic nervous system, the others being the sympathetic nervous system and the enteric nervous system.
The autonomic nervous system is responsible for regulat ...
(PSNS), impact blood vessels differently.
Traditionally we understand that these two divisions work against each other, the SNS producing "fight or flight" and the PSNS producing "rest and digest", but in the case of vascular innervation this line becomes blurred
ANS nerves do not directly innervate the vasculature via synapses with muscle cells; instead, they release
neurotransmitter
A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a Chemical synapse, synapse. The cell receiving the signal, or target cell, may be another neuron, but could also be a gland or muscle cell.
Neurotra ...
s that reach target cells and effect smooth muscle contraction or relaxation.
Physical characteristics of the SNS and PSNS cause the SNS to have a prolonged, systemic impact on blood vessels, while the PSNS causes short-lived, localized change.
SNS stimulation causes a base level of vasoconstriction often referred to as basal neural tone, maintaining blood pressure.
Often vasodilation is simply the result of insufficient neurotransmitter to maintain basal neural tone, without the presence of a compound directly causing vasodilation.
Neurotransmitters can act by binding directly to smooth muscle cells or by binding to endothelial cells mediating the effects of the neurotransmitter.
Below is a table summarizing major neurotransmitters involved in regulation of the vasculature.
Also worthy of mention when discussing neural control of vasodilation is the renin-angiotensin-aldosterone system, or RAAS.
The kidneys retain water by reabsorbing sodium ions, or eliminate water by eliminating sodium ions.
Sympathetic nervous system activity, reduced blood volume or reduced arterial pressure trigger β-adrenergic receptors in select kidney cells
to release
renin, which converts facilitates formation of angiotensin II from its substrate
angiotensin.
Angiotensin II triggers
adrenal gland
The adrenal glands (also known as suprarenal glands) are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. They are found above the kidneys. Each gland has an outer adrenal corte ...
s to secrete
aldosterone
Aldosterone is the main mineralocorticoid steroid hormone produced by the zona glomerulosa of the adrenal cortex in the adrenal gland. It is essential for sodium conservation in the kidney, salivary glands, sweat glands, and colon. It plays ...
, a potent vasoconstrictor.
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 a ...
, either exogenous or endogenous, is another vasoconstrictor released by the adrenal glands in response to stress.
It binds to α and β adrenergic receptors like
norepinephrine
Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic compound, organic chemical in the catecholamine family that functions in the brain and human body, body as a hormone, neurotransmitter and neuromodulator. The ...
, causing vasodilation and vasoconstriction in different body parts to redistribute circulation to critical areas.
Cold-induced
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 Chemical synapse, synapse. The cell receiving the signal, or target cell, may be another neuron, but could also be a gland or muscle cell.
Neurotra ...
s from the
sympathetic nerves to the muscular coat of the
arteriovenous anastomoses 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' 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. 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.
Miscellaneous
* 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 Sport, recreational activities.
Noise is frequently described as 'unwanted sound'. Within this context, envir ...
**
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 build ...
- adenosine agonist, used primarily as an anti-
arrhythmic
**
alpha blockers (block the
vasoconstricting effect of
adrenaline
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 a ...
)
**
atrial natriuretic peptide (ANP) - a weak vasodilator
**
ethanol
Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...
(alcohol) causes immediate vasodilation followed by increase in blood pressure
**
nitric oxide
Nitric oxide (nitrogen oxide, nitrogen monooxide, 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 den ...
inducers
***
l-arginine (a key amino acid)
***
citrulline
The organic compound citrulline is an α-amino acid. Its name is derived from '' citrullus'', the Latin word for watermelon. Although named and described by gastroenterologists since the late 19th century, it was first isolated from watermelon in ...
(causes increased levels of L-arginine in the body)
***
glyceryl trinitrate
Nitroglycerin (NG) (alternative spelling nitroglycerine), also known as trinitroglycerol (TNG), nitro, glyceryl trinitrate (GTN), or 1,2,3-trinitroxypropane, is a dense, colorless or pale yellow, oily, explosive liquid most commonly produced by ...
(commonly known as
nitroglycerin
Nitroglycerin (NG) (alternative spelling nitroglycerine), also known as trinitroglycerol (TNG), nitro, glyceryl trinitrate (GTN), or 1,2,3-trinitroxypropane, is a dense, colorless or pale yellow, oily, explosive liquid most commonly produced by ...
)
***
isosorbide mononitrate
Isosorbide mononitrate, sold under many brand names, is a medication used for heart-related chest pain (angina), heart failure and esophageal spasms. It can be used both to treat and to prevent heart-related chest pain; however, it is generally ...
and
isosorbide dinitrate
***
pentaerythritol tetranitrate
Pentaerythritol tetranitrate (PETN), also known as PENT, pentyl, PENTA (''ПЕНТА'', primarily in Russian), TEN (tetraeritrit nitrate), corpent, or penthrite (or, rarely and primarily in German, as ''nitropenta''), is an explosive material. ...
(PETN)
***
sodium nitroprusside
***
PDE5 inhibitor
A phosphodiesterase type 5 inhibitor (PDE5 inhibitor) is a vasodilating medication, drug that works by blocking the degradative action of cGMP-specific phosphodiesterase type 5 (PDE5) on cyclic GMP in the smooth muscle cells lining the blood vess ...
s: 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 hypertension, pulmonary arterial hypertension. It is also sometimes used off-label for the treatment of certain sym ...
(Viagra)
****
tadalafil (Cialis)
****
vardenafil (Levitra)
**
tetrahydrocannabinol
Tetrahydrocannabinol (THC) is a cannabinoid found in cannabis. It is the principal psychoactive constituent of ''Cannabis'' and one of at least 113 total cannabinoids identified on the plant. Although the chemical formula for THC (C21H30O2) de ...
(THC), the principal psychoactive constituent of cannabis
**
theobromine, the principal alkaloid found in ''
Theobroma cacao
''Theobroma cacao'' (cacao tree or cocoa tree) is a small ( tall) evergreen tree in the Malvaceae family. Its seedscocoa beansare used to make chocolate liquor, cocoa solids, cocoa butter and chocolate. Although the tree is native to the tropi ...
'', specifically in
cocoa solids (which is found in chocolate, especially
dark chocolate)
**
minoxidil
**
papaverine an alkaloid found in the opium poppy
papaver somniferum
''Papaver somniferum'', commonly known as the opium poppy or breadseed poppy, is a species of flowering plant in the family Papaveraceae. It is the species of plant from which both opium and poppy seeds are derived and is also a valuable orname ...
**
estrogen
Estrogen (also spelled oestrogen in British English; see spelling differences) is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three ...
Treatment
Direct vasodilation drugs
These drugs can keep vessels staying opened or help vessels refrain from being narrowed.
*
Angiotensin II receptor blockers
*
ACE inhibitors
*
Calcium channel blockers
Alpha-2A adrenergic receptor agonists
Drugs that appear to work by activating the
α2A receptors in the brain thereby decreasing
sympathetic nervous system
The sympathetic nervous system (SNS or SANS, sympathetic autonomic nervous system, to differentiate it from the somatic nervous system) is one of the three divisions of the autonomic nervous system, the others being the parasympathetic nervous sy ...
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 Heart, cardiac care in an effort to reduce disability ...
, Alpha-methyldopa may cause
Orthostatic syncope 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
*
guanabenz acetate
*
guanfacine hydrochloride
::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.
Blood vessel muscle relaxants
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 is not 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.
Therapeutic applications
Vasodilators are used to treat conditions such as
hypertension
Hypertension, also known as high blood pressure, is a Chronic condition, long-term Disease, medical condition in which the blood pressure in the artery, arteries is persistently elevated. High blood pressure usually does not cause symptoms i ...
, wherein the patient has an abnormally high blood pressure, as well as
angina,
congestive heart failure
Heart failure (HF), also known as congestive heart failure (CHF), is a syndrome caused by an impairment in the heart's ability to fill with and pump blood.
Although symptoms vary based on which side of the heart is affected, HF typically pr ...
, and
erectile dysfunction
Erectile dysfunction (ED), also referred to as impotence, is a form of sexual dysfunction in males characterized by the persistent or recurring inability to achieve or maintain a Human penis, penile erection with sufficient rigidity and durat ...
, 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 hypertension, pulmonary arterial hypertension. It is also sometimes used off-label for the treatment of certain sym ...
,
vardenafil and
tadalafil, work to increase blood flow in the penis through vasodilation. They may also be used to treat
pulmonary arterial hypertension (PAH).
See also
*
Arteriolar vasodilator
*
Nitrophorin
*
Vasodilatory shock
References
{{Authority control
Cardiovascular physiology