Venule
Venula
Definition
Venules are small veins that drain blood from capillary networks (capillary plexuses) and merge to form larger veins. They lie immediately downstream of capillary beds and possess very thin walls, consisting primarily of an endothelial tube with minimal surrounding smooth muscle or connective tissue.
Because of their delicate structure, often limited to a tunica intima and scant connective tissue, venules appear flattened in cross-section. Their permeability allows fluid and leukocytes to exit into surrounding tissues, playing a key role in inflammation and immune responses.
Multiple venules converge into progressively larger veins, ultimately returning deoxygenated blood from tissues back toward the heart.
Feature | Venules | |
Location | Upstream of capillary beds (deliver blood) | Downstream of capillary beds (drains blood) |
Wall structure | Thick walls with smooth muscle (tunica media) | Very thin walls, mainly endothelium + scant connective tissue |
Function | Regulate blood flow and pressure into capillaries | Collect blood from capillaries, allow fluid and leukocyte exchange |
Pressure | Relatively high (close to arterial pressure) | Low (close to venous pressure, ~10 mmHg) |
Appearance (histology/imaging) | Round lumen, maintains shape due to muscular wall | Flattened, irregular lumen when not distended |
Clinical relevance | Major site of vascular resistance, target for antihypertensive drugs | Key site of inflammation, leukocyte migration, and edema formation |
References
Saladin KS. Anatomy & Physiology: The Unity of Form and Function. 9th ed.
Guyton AC, Hall JE. Textbook of Medical Physiology. 14th ed.
