blood vessel
Meaning of blood vessel: Wall thickness tracks the pressure a vessel must contain, which is why an artery and a vein of equal width behave nothing alike.
Definition of blood vessel
A blood vessel is a tube that carries blood through the body. The vessels form one continuous, branching network: arteries leaving the heart, arterioles narrowing from them, capillaries thin enough for exchange with tissue, then venules and veins gathering the blood back again. Each type has a wall built for the pressure it holds and the job it does.
How do the types differ?
| Vessel | Wall | What it does |
|---|---|---|
| Artery | Thick, muscular and elastic | Carries blood away from the heart and absorbs the surge of each beat |
| Arteriole | Mostly muscle, small bore | Widens or narrows to steer blood toward the tissue that needs it |
| Capillary | A single layer of cells | Hands over oxygen and nutrients and takes up waste |
| Vein | Thin, floppy, often valved | Returns blood to the heart and holds most of the body's volume in reserve |
Arteries and veins are defined by direction of travel, not by what the blood is carrying. The vessels running from the heart to the lungs are arteries even though their blood is oxygen poor, and the vessels returning from the lungs are veins even though their blood is freshly loaded. Veins also work against far lower pressure, which is why they need valves and the squeeze of surrounding muscle to keep blood moving upward against gravity.
Where does the actual exchange happen?
Only in the capillaries. Everything before and after them is plumbing. A capillary wall is one cell thick, and no cell in the body sits far from one, because oxygen has to cross the remaining distance by diffusion alone. Different organs then tailor that wall to their purpose. In the kidney it is perforated so that fluid can be filtered out in bulk. In the liver and bone marrow the gaps are wide enough for whole cells to pass. In the brain the cells are sealed tightly together to form the blood brain barrier, which admits very little without a dedicated transporter, and that selectivity is as much an obstacle to treatment as a protection.
What goes wrong with vessels?
Four broad patterns. A vessel can narrow, usually through fatty plaque building in the wall, starving the tissue beyond it. It can block outright, from a clot forming on that plaque or arriving from elsewhere. It can weaken and balloon outward, forming an aneurysm that risks tearing. Or it can leak, whether from injury, from inflammation of the wall, or from the fragile disordered vessels a tumor builds for itself. Sustained blood pressure above normal accelerates several of these at once by thickening and stiffening the wall, and because the circulation is a single connected system, disease found in one region is a reasonable predictor of disease elsewhere.
Used in a sentence
Imaging identified narrowing of the blood vessel supplying the affected limb.
Good to know
Arteries and veins differ in wall structure and the pressure they carry, which is why a blockage or injury affecting one produces different effects than the same problem affecting the other.