Blood Vessels

Blood is carried around the body in a closed system of vessels. Blood leaves the heart in arteries, which branch into smaller arterioles. Arterioles lead into networks of tiny capillaries, where substances are exchanged with the tissues. Capillaries join to form venules, which join to form veins that return blood to the heart.The space in the middle of a vessel through which blood flows is the lumen. Arteries, arterioles, venules and veins all have walls made of three layers:Inner layer (tunica intima). A single layer of flattened cells called the endothelium. It is very smooth, which reduces friction with the flowing blood.Middle layer (tunica media). Contains smooth muscle, which can contract to narrow the lumen, and elastic fibres, which can stretch and recoil.Outer layer (tunica externa). Mainly tough collagen fibres, which resist overstretching and protect the vessel.The thickness of each layer differs between vessel types, and this matches the pressure of the blood inside them. Vessels near the heart receive blood in high-pressure surges, while vessels returning blood to the heart carry it at low pressure.Capillaries are different: their wall is only the endothelium, one cell thick. This gives the short diffusion pathway needed for exchange. The next lessons look at the structure of each type of vessel in turn and how it is related to its function.