Why Are There Three Kinds of Tube? Each One Solves a Different Problem

An artery carries blood away from the heart at high and fluctuating pressure, so it must resist bursting and must smooth the pulse into steadier flow. A capillary exists to allow exchange between blood and tissue, so it must present an enormous surface and a barrier thin enough for molecules to cross. A vein returns blood at low pressure, frequently upwards against gravity, so it must hold a large volume and must prevent backflow. Those three jobs are essentially incompatible, which is why the vessels differ in wall thickness, in composition, in diameter and in what they contain, rather than being one kind of tube in different sizes.

The wall of each reflects its function directly:

• •Arteries have thick walls with elastic fibres and muscle, and narrow channels
• •Elastic recoil in the largest ones smooths the pulse between beats
• •Muscle in the smaller ones adjusts diameter and therefore where blood goes
• •Capillaries are one cell thick and just wide enough for a red cell to pass
• •Veins have thin walls, wide channels and hold most of the body's blood
• •Veins contain one-way valves preventing flow backwards

Returning blood from the feet to the heart against gravity is a genuine problem, since venous pressure there is very low and the heart is pulling rather than pushing. Three mechanisms do the work. Contracting leg muscles squeeze the deep veins running between them, which forces blood along, and the valves ensure it can only move upwards, so walking acts as a pump. Breathing lowers the pressure in the chest on each inhalation, which draws blood towards the heart. And the pulsation of adjacent arteries compresses veins rhythmically. Standing still for long periods disables the first of these, which is why soldiers on parade faint and why moving the feet prevents it.

The scale of the network is worth stating because the figures are hard to believe. Capillaries are so numerous that no cell in the body sits more than a fraction of a millimetre from one, and estimates of the total length of all vessels in an adult run to tens of thousands of kilometres, with the great majority of that being capillaries. Their combined cross-sectional area is hundreds of times greater than that of the aorta, which is why blood slows almost to a crawl on entering them, and that slowing is essential since exchange takes time. Most capillaries are closed at any given moment, with rings of muscle at their entrances opening and closing to direct blood towards whichever tissue is working.

The characteristic failures follow from the structures. Arteries accumulate fatty deposits in their walls, which narrows them and stiffens them, reducing flow and raising pressure, and a deposit that ruptures triggers a clot that blocks the vessel entirely, which causes heart attacks and most strokes. A weakened arterial wall can balloon into an aneurysm that may burst. Venous valves that fail allow blood to pool, which stretches the vein further and produces varicose veins and, over time, skin damage at the ankle. Blood in the deep veins of the leg can clot when flow is slow, and that clot can travel to the lungs. Capillaries leak when inflamed, which produces swelling.

Arteries resist high fluctuating pressure with thick elastic muscular walls, capillaries allow exchange with a wall one cell thick, and veins return blood at low pressure through wide channels fitted with one-way valves. Contracting leg muscles squeezing deep veins is the main pump returning blood upwards, which is why standing still causes fainting. The failures follow from the structures, with deposits blocking arteries and failed valves pooling blood in veins.