Arteriole
Encyclopedia
An arteriole is a small diameter blood vessel
in the microcirculation
that extends and branches out from an artery
and leads to capillaries
.
Arterioles have muscular
walls (usually only one to two layers of smooth muscle
) and are the primary site of vascular resistance
. The greatest change in blood pressure and velocity of blood flow occurs at the transition of arterioles to capillaries. This reduces the pressure and velocity of flow for gas and nutrient exchange to occur within the capillaries. Arterioles receive autonomic nervous system
innervation and respond to various circulating hormones in order to regulate their diameter. Further local responses to stretch, carbon dioxide, pH, and oxygen also influence tone. Generally, norepinephrine and epinephrine (hormones produced by sympathetic nerves and the adrenal gland medulla)are vasoconstrictive acting on alpha 1-adrenergic receptors. However, the arterioles of skeletal muscle, cardiac muscle, and pulmonary arterioles vasodilates in response to these hormones acting on beta-adrenergic receptors. Generally, stretch and high oxygen tension increase tone,and carbon dioxide and low pH promote vasodilation. Pulmonary arterioles are a noteworthy exception as they vasodilate to high oxygen. Brain arterioles are particularly sensitive to pH with reduced pH promoting vasodilation. A number of hormones influce arteriole tone such as angiotensin II (vasoconstrictive),endothelin (vasoconstrictive),bradykinin (vasodilation), atrial natruretic peptide (vasodilation), and prostacyclin (vasodilation).
Blood pressure
in the arteries supplying the body is a result of the work needed to pump the cardiac output
(the flow of blood pumped by the heart) through the vascular resistance, usually termed total peripheral resistance
by physicians and researchers. An increase in the media to lumenal diameter ratio has been observed in hypertensive arterioles (arteriolosclerosis
)as the vascular wall thickens and/or lumenal diameter decreases.
and determined by the interaction of the stroke volume
versus the volume and elasticity of the major arteries.
In a healthy vascular system the endothelium
lines all blood-contacting surfaces, including arterioles, arteries, veins, capillaries, and heart chambers. This healthy condition is promoted by the ample production of nitric oxide
by the endothelium, which requires a biochemical reaction regulated by a complex balance of polyphenol
s, various nitric oxide synthase
enzyme
s and L-arginine
. In addition there is direct electrical and chemical communication via gap junctions between the endothelial cells and the vascular smooth muscle.
Arteriolosclerosis is the term specifically used for the hardening of arteriole walls. This can be due to decreased elastic production from fibrinogen, associated with ageing, or pathological conditions such as atherosclerosis.
s), for example the dihydropyridines (nifedipine
and nicardipine
), which block the calcium conductance
in the muscular layer of the arterioles, causing relaxation.
This decreases the resistance to flow into peripheral vascular beds, lowering overall systemic pressure.
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...
in the microcirculation
Microcirculation
The microcirculation is a term used to describe the small vessels in the vasculature which are embedded within organs and are responsible for the distribution of blood within tissues; as opposed to larger vessels in the macrocirculation which transport blood to and from the organs...
that extends and branches out from an artery
Artery
Arteries are blood vessels that carry blood away from the heart. This blood is normally oxygenated, exceptions made for the pulmonary and umbilical arteries....
and leads to capillaries
Capillary
Capillaries are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste...
.
Arterioles have muscular
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
walls (usually only one to two layers of smooth muscle
Smooth muscle
Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...
) and are the primary site of vascular resistance
Vascular resistance
Vascular resistance is a term used to define the resistance to flow that must be overcome to push blood through the circulatory system. The resistance offered by the peripheral circulation is known as the systemic vascular resistance , while the resistance offered by the vasculature of the lungs...
. The greatest change in blood pressure and velocity of blood flow occurs at the transition of arterioles to capillaries. This reduces the pressure and velocity of flow for gas and nutrient exchange to occur within the capillaries. Arterioles receive autonomic nervous system
Autonomic nervous system
The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...
innervation and respond to various circulating hormones in order to regulate their diameter. Further local responses to stretch, carbon dioxide, pH, and oxygen also influence tone. Generally, norepinephrine and epinephrine (hormones produced by sympathetic nerves and the adrenal gland medulla)are vasoconstrictive acting on alpha 1-adrenergic receptors. However, the arterioles of skeletal muscle, cardiac muscle, and pulmonary arterioles vasodilates in response to these hormones acting on beta-adrenergic receptors. Generally, stretch and high oxygen tension increase tone,and carbon dioxide and low pH promote vasodilation. Pulmonary arterioles are a noteworthy exception as they vasodilate to high oxygen. Brain arterioles are particularly sensitive to pH with reduced pH promoting vasodilation. A number of hormones influce arteriole tone such as angiotensin II (vasoconstrictive),endothelin (vasoconstrictive),bradykinin (vasodilation), atrial natruretic peptide (vasodilation), and prostacyclin (vasodilation).
Blood pressure
Blood pressure
Blood pressure is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies...
in the arteries supplying the body is a result of the work needed to pump the cardiac output
Cardiac output
Cardiac output is the volume of blood being pumped by the heart, in particular by a left or right ventricle in the time interval of one minute. CO may be measured in many ways, for example dm3/min...
(the flow of blood pumped by the heart) through the vascular resistance, usually termed total peripheral resistance
Total peripheral resistance
Vasculature throughout the entire body can be thought of as two separate circuits - one is the systemic circulation, while the other is the pulmonary circulation. Total peripheral resistance is the sum of the resistance of all peripheral vasculature in the systemic circulation...
by physicians and researchers. An increase in the media to lumenal diameter ratio has been observed in hypertensive arterioles (arteriolosclerosis
Arteriolosclerosis
Arteriolosclerosis is a form of cardiovascular disease affecting the small arteries and arterioles.Types include hyaline arteriolosclerosis and hyperplastic arteriolosclerosis, both associated with vessel wall thickening and luminal narrowing that may cause downstream ischemic injury...
)as the vascular wall thickens and/or lumenal diameter decreases.
Natural fluctuation
The up and down fluctuation of the arterial blood pressure is due to the pulsatile nature of the cardiac outputCardiac output
Cardiac output is the volume of blood being pumped by the heart, in particular by a left or right ventricle in the time interval of one minute. CO may be measured in many ways, for example dm3/min...
and determined by the interaction of the stroke volume
Stroke volume
In cardiovascular physiology, stroke volume is the volume of blood pumped from one ventricle of the heart with each beat. SV is calculated using measurements of ventricle volumes from an echocardiogram and subtracting the volume of the blood in the ventricle at the end of a beat from the volume...
versus the volume and elasticity of the major arteries.
In a healthy vascular system the endothelium
Endothelium
The endothelium is the thin layer of cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. These cells are called endothelial cells. Endothelial cells line the entire circulatory system, from the heart...
lines all blood-contacting surfaces, including arterioles, arteries, veins, capillaries, and heart chambers. This healthy condition is promoted by the ample production of nitric oxide
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...
by the endothelium, which requires a biochemical reaction regulated by a complex balance of polyphenol
Polyphenol
Polyphenols are a structural class of natural, synthetic, and semisynthetic organic chemicals characterized by the presence of large multiples of phenol structural units...
s, various nitric oxide synthase
Nitric oxide synthase
Nitric oxide synthases are a family of enzymes that catalyze the production of nitric oxide from L-arginine. NO is an important cellular signaling molecule, having a vital role in many biological processes...
enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s and L-arginine
Arginine
Arginine is an α-amino acid. The L-form is one of the 20 most common natural amino acids. At the level of molecular genetics, in the structure of the messenger ribonucleic acid mRNA, CGU, CGC, CGA, CGG, AGA, and AGG, are the triplets of nucleotide bases or codons that codify for arginine during...
. In addition there is direct electrical and chemical communication via gap junctions between the endothelial cells and the vascular smooth muscle.
Pathology
Any pathogen which constricts blood flow, such as stenosis, will increase total peripheral resistance and lead to hypertension.Arteriolosclerosis is the term specifically used for the hardening of arteriole walls. This can be due to decreased elastic production from fibrinogen, associated with ageing, or pathological conditions such as atherosclerosis.
Medication
The muscular contraction of arterioles is targeted by drugs that lower blood pressure (antihypertensiveAntihypertensive
The antihypertensives are a class of drugs that are used to treat hypertension . Evidence suggests that reduction of the blood pressure by 5 mmHg can decrease the risk of stroke by 34%, of ischaemic heart disease by 21%, and reduce the likelihood of dementia, heart failure, and mortality from...
s), for example the dihydropyridines (nifedipine
Nifedipine
Nifedipine is a dihydropyridine calcium channel blocker...
and nicardipine
Nicardipine
Nicardipine hydrochloride is a medication used to treat high blood pressure and angina. It belongs to the class of calcium channel blockers....
), which block the calcium conductance
Calcium channel blocker
A calcium channel blocker is a chemical that disrupts the movement of calcium through calcium channels.CCB drugs devised to target neurons are used as antiepileptics. However, the most widespread clinical usage of calcium channel blockers is to decrease blood pressure in patients with...
in the muscular layer of the arterioles, causing relaxation.
This decreases the resistance to flow into peripheral vascular beds, lowering overall systemic pressure.