Your muscles alone may contain 9,000 to 19,000 kilometres of capillaries
August Krogh once estimated that the capillaries in a heavily muscled human body might stretch about 100,000 kilometres. Later studies, using better density figures and average adult muscle mass, trimmed that to between 9,000 and 19,000 kilometres, still a staggering length for tubes only about 5 micrometres wide.
Blood travels through five kinds of vessel. Arteries carry it away from the heart and branch into arterioles, then into capillaries where water and chemicals pass between blood and tissue, then into venules, and finally into veins that bring it back. Sizes run from roughly 25 to 30 millimetres across for the aorta down to a few micrometres for a capillary. The names are old: vascular comes from the Latin vas, vessel, and capillary from capillus, hair. A few tissues, including cartilage and the lens and cornea of the eye, have no vessels at all.
Arteries and veins share three layers. The thin inner lining is a single sheet of flat cells; the middle layer, thickest in arteries, holds elastic fibres and smooth muscle that can narrow or widen the tube; the outer layer, thickest in veins, is connective tissue carrying nerves and, in large vessels, tiny capillaries that nourish the vessel wall itself. Veins often have valves to stop blood sliding backward, while arteries lack them except for the two leaving the heart's ventricles.
Vessels do not pump; the heartbeat drives the flow. Arterial pressure typically swings between about 120 and 80 mmHg, while venous pressure rarely tops 10. Haemoglobin in most arteries is 95 to 100 percent saturated with oxygen and about 75 percent in most veins, except in the lung circuit, where the pulmonary artery carries oxygen-poor blood and the pulmonary vein oxygen-rich blood. Narrowing and widening, steered by the autonomic nervous system, redirects flow and helps control body temperature, with nitric oxide the most prominent widening agent. Resistance climbs as blood thickens and as vessels get longer or narrower.
Blood vessels play a part in nearly every disease. A tumour cannot grow without sprouting new vessels to feed it, a process called angiogenesis. Atherosclerosis, the build-up of plaque in artery walls, lies behind around 85 percent of deaths from cardiovascular disease, which killed about 17.9 million people worldwide in 2019.
Source: Blood vessel
