Arterioles
Arteriolae
Also known as: arteriole, resistance vessels, metarterioles
Arterioles are the smallest arteries, about 10–100 µm in diameter, with one to three layers of smooth muscle. They are the main resistance vessels: they set total peripheral resistance and control flow into each capillary bed.
Overview
Blood pressure falls more steeply across the arterioles than anywhere else in the circulation. Mean pressure drops from about 85–90 mmHg in the small arteries to about 30–35 mmHg at the start of the capillaries. The arterioles account for roughly half of total peripheral resistance, and they largely remove the pulsatile swings in pressure.
By Poiseuille's law, resistance depends on the fourth power of the radius. A 20% narrowing of an arteriole therefore roughly doubles its resistance. Terminal arterioles lead into metarterioles, which have only scattered smooth muscle, and then into capillaries. Precapillary sphincters at the entrances to capillaries control which capillaries are open.
Arteriolar tone is set by two kinds of control. Extrinsic control comes from the sympathetic nerves, which constrict through α1 receptors and maintain resting tone, and from circulating hormones (angiotensin II, ADH, adrenaline). Adrenaline dilates skeletal muscle arterioles through β2 receptors. Intrinsic control comes from the myogenic response, local metabolites (adenosine, CO₂, H⁺, K⁺, lactate, low O₂) and endothelial nitric oxide. Extrinsic control serves blood pressure as a whole, while intrinsic control matches flow to the tissue's local needs.
Anatomy
- Histology
Endothelium, a thin or incomplete internal elastic lamina in larger arterioles, one to three layers of circular smooth muscle and a thin adventitia.
Function
- Set total peripheral resistance and therefore arterial blood pressure.
- Distribute cardiac output between organs according to need.
- Protect capillaries from high, pulsatile pressure.
Clinical significance
Hypertensive arteriolar disease
In benign hypertension and in diabetes, arteriolar walls become thickened and glassy (hyaline arteriolosclerosis). In malignant hypertension they develop "onion-skin" thickening (hyperplastic arteriolosclerosis) with fibrinoid necrosis. In the retina these changes appear as arteriovenous nipping, flame haemorrhages, cotton-wool spots and, in severe cases, papilloedema.
Distributive shock
In sepsis and anaphylaxis, the arterioles dilate inappropriately. Peripheral resistance and blood pressure fall even though cardiac output may be high. Vasopressors such as noradrenaline restore arteriolar tone.
Test yourself
Across which vessels does the largest drop in mean pressure occur in the systemic circulation?
- Aorta and large arteries
- Arterioles
- Capillaries
- Venules and veins
Answer: Arterioles
The arterioles are the resistance vessels. Mean pressure falls from about 85–90 mmHg to about 30–35 mmHg across them, and they account for roughly half of total peripheral resistance.
According to Poiseuille’s law, halving the radius of an arteriole (other factors unchanged) increases its resistance by a factor of about:
- 2
- 4
- 8
- 16
Answer: 16
Resistance is proportional to 1/r⁴, and 2⁴ = 16. This steep dependence on radius is why small changes in arteriolar tone have such large effects on blood flow and pressure.
Compiled from standard anatomy and physiology references — see the Cardiovascular System sources. For education only; not medical advice.