Blood vessel between the blood and the tissues; and the veins, which carry blood from the capillaries back toward the heart. The word "vascular", meaning relating to the blood vessels, is derived from the Latin "vas", meaning vessel. A few structures (such as cartilage and the lens of the eye) do not contain blood vessels and are labeled. The arteries and veins have three layers. The middle layer is thicker in the arteries than it is in the veins: Capillaries consist of little more than a layer of endothelium and occasional connective tissue. When blood vessels connect to form a region of
Baleen whale a dense network of blood vessels (rete mirabile) which prevents heat-loss. Like in most mammals, heat is lost in their extremities, so, in baleen whales, warm blood in the arteries is surrounded by veins to prevent heat loss during transport. As well as this, heat inevitably given off by the arteries warms blood in the surrounding veins as it travels back into the core. This is otherwise known as countercurrent exchange. To counteract overheating while in warmer waters, baleen whales reroute blood to the skin to accelerate heat-loss. They have the largest blood corpuscles (red and white blood cells) of