Arteriole

Arterioles are the smallest arteries, with diameters ranging from about 30 to 300 micrometers. Their walls consist of an intima (endothelium), a media with one or two layers of smooth muscle cells, and an adventitia of connective tissue. The smooth muscle is innervated by sympathetic nerves and responds to circulating hormones and local metabolites. By contracting or relaxing, arterioles adjust their diameter, thereby controlling the resistance to blood flow. This resistance is a major determinant of mean arterial pressure and the distribution of cardiac output to different organs. The pressure drop across arterioles is substantial, from about 80–100 mmHg in arteries to about 35–40 mmHg at the capillary entrance, protecting delicate capillaries from high pressure.

Arteriolar tone is regulated by multiple mechanisms, including myogenic autoregulation, metabolic factors, and endothelial-derived substances. Myogenic response: arterioles constrict when stretched and dilate when pressure falls, maintaining constant flow over a range of pressures. Metabolic regulation: increased tissue activity produces metabolites like CO2, H+, and adenosine, which cause vasodilation, matching flow to demand. Endothelial cells release nitric oxide (NO) in response to shear stress, causing relaxation of smooth muscle. Sympathetic stimulation causes vasoconstriction via alpha-adrenergic receptors, while parasympathetic and some hormonal signals (e.g., atrial natriuretic peptide) cause vasodilation. These integrated responses ensure precise control of capillary perfusion.

Arteriolar dysfunction is central to several diseases. In hypertension, increased arteriolar resistance raises blood pressure, and chronic hypertension leads to arteriolar remodeling and thickening. In diabetes, arteriolar damage contributes to microvascular complications such as retinopathy and nephropathy. Arteriolosclerosis, the hardening of arterioles, is associated with aging and hypertension. In shock, arteriolar constriction can compromise tissue perfusion, leading to organ failure. Conversely, excessive vasodilation can cause hypotension. Pharmacological agents that target arteriolar smooth muscle, such as calcium channel blockers and nitrates, are used to treat hypertension and angina.

Arterioles exhibit rhythmic oscillations in diameter known as vasomotion, which may enhance tissue oxygenation. The precapillary sphincters, located at the origin of capillaries, are not true arterioles but are often considered together with them. In the kidney, the afferent and efferent arterioles have distinct roles in glomerular filtration; the efferent arteriole has a smaller diameter, creating high pressure in the glomerulus. Arterioles are also involved in thermoregulation: in the skin, arteriovenous anastomoses (shunts) bypass capillaries to regulate heat loss. The study of arterioles has been advanced by intravital microscopy, allowing direct observation in living tissues. Some arterioles have fenestrated endothelium in certain organs, facilitating rapid exchange.

Microcirculation

The circulation of blood in the smallest blood vessels, including arterioles, capillaries, and venules.

Vascular resistance

The resistance to blood flow offered by the blood vessels, primarily determined by arteriolar diameter.

Myogenic autoregulation

The intrinsic ability of arterioles to constrict or dilate in response to changes in intraluminal pressure.

Precapillary sphincter

A ring of smooth muscle at the origin of a capillary that regulates blood flow into the capillary bed.