The circulatory system
An arteriole /ɑrˈtɪəri.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. 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 circulation vasodilate 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.
