Una arteriola es un vaso sanguíneo de diámetro menor de 40 o 100 micras según la bibliografía consultada, dependiendo su diámetro del grado de contracción arteriolar, que resulta de ramificaciones de las arterias y conduce la sangre hacia los capilares. La resistencia que presentan las arteriolas al paso sanguíneo está determinada por el grado de contracción o relajación de la capa muscular de dichas arteriolas.

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An arteriole /ɑːrˈtɪəri.oʊl/ 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. The decreased velocity of flow in the capillaries increases the blood pressure, due to Bernoulli's principle. This induces gas and nutrients to move from the blood to the cells, due to the lower osmotic pressure outside of the capillary. The opposite process occurs when the blood leaves the capillaries and enters the venules, where the blood pressure drops due to an increase in flow rate. Arterioles receive autonomic nervous system innervation and respond to various circulating hormones in order to regulate their diameter. Retinal vessels lack a functional sympathetic innervation.Further local responses to stretch, carbon dioxide, pH, and oxygen also influence arteriolar 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 circulation vasodilate in response to these hormones when they act 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 in response to high oxygen. Brain arterioles are particularly sensitive to pH with reduced pH promoting vasodilation. A number of hormones influence 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.

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