venulae

Venulae are small veins in the circulatory system. They are the smallest venous vessels, measuring about 10 to 100 micrometers in diameter. These microscopic vessels collect deoxygenated blood from capillaries and begin the process of returning it to larger veins and ultimately to the heart.

Venulae are crucial for regulating blood flow and pressure. Their thin walls allow for some degree of nutrient and waste exchange, although this function is primarily associated with capillaries. They play a significant role in inflammation; during an inflammatory response, venulae become more permeable, allowing white blood cells and fluid to move from the bloodstream into the surrounding tissues to combat infection or repair damage.

There are two main types of venulae: postcapillary venules and muscular venules. Postcapillary venules are the smallest and are found immediately after the capillaries. They have a very simple structure. Muscular venules are slightly larger and begin to develop a thin layer of smooth muscle in their walls, which helps in controlling blood flow. As venulae merge and grow larger, they become venules and then veins. The collective function of venulae is essential for the continuous circulation of blood and the body's response to injury and infection.