venules
Summary
Venules are the smallest veins: thin-walled vessels that collect blood from capillaries and merge into larger veins. Pressure in them is low and they are very compliant, so venules and veins act as a blood reservoir. They are named by direction of flow toward the heart, not by oxygen content.
Detail
Meaning: Venules receive blood from capillary beds and join to form veins that return blood to the heart. Their walls are thin, with little smooth muscle and elastic tissue, so they stretch easily (high compliance) and hold blood at low pressure. Veins and venules together serve as capacitance vessels, holding a large share of total blood volume.
Equation: Q = vA, where Q is volume flow rate (m³/s), v is average flow speed (m/s), and A is the total cross-sectional area of all vessels at that level (m²). Q is the same at each level of the circuit, so as total area falls from capillaries to venules to veins, speed rises again.
On the MCAT: Expect to order the vessels (artery → arteriole → capillary → venule → vein) and predict trends. Blood pressure falls along the whole circuit, and speed is lowest in the capillaries, where total area is greatest. Questions may also ask how venoconstriction, such as from sympathetic stimulation, shifts blood toward the heart and raises venous return.
Common confusion: Students assume venules and veins carry deoxygenated blood. They are defined by flow toward the heart. Pulmonary venules and pulmonary veins carry oxygenated blood from the lungs to the left atrium.
Sources
- Campbell Biology
- Human Physiology: An Integrated Approach
- Principles of Anatomy and Physiology
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.