capillary

Blood flows through 96,500 km/60,000 mi of arteries and veins, supplying oxygen and nutrients to organs and limbs. Oxygen-poor blood (blue) circulates from the heart to the lungs where oxygen is absorbed. Oxygen-rich blood (red) flows back to the heart and is then pumped round the body through the aorta, the largest artery, to smaller arteries and capillaries. Here oxygen and nutrients are exchanged with carbon dioxide and waste products and the blood returns to the heart via the veins. Waste products are filtered by the liver, spleen, and kidneys, and nutrients are absorbed from the stomach and small intestine.

The structure of a blood capillary. Capillaries are only one cell thick and white blood cells, which can move independently of the other blood components, can squeeze through them into tissue spaces.

In biology, narrowest blood vessel in vertebrates measuring 0.008–0.02 mm in diameter, barely wider than a red blood cell. Capillaries are distributed as beds, complex networks connecting arteries and veins. The function of capillaries is to exchange materials with their surroundings. For this reason, capillary walls are extremely thin, consisting of a single layer of cells through which nutrients, dissolved gases, and waste products can easily pass. This makes the capillaries the main area of exchange between the fluid (lymph) bathing body tissues and the blood. They provide a large surface area in order to maximize diffusion.

All body cells lie close to capillaries so they can receive the food and oxygen they require from the capillaries. Networks of capillaries are especially extensive in certain parts of the body. For example, in the lungs they surround the air sacs (alveoli), taking up oxygen and releasing carbon dioxide. They are also found in the lining of the gut – in little finger-like projections called villi – where they absorb the products of the digestion of food.

capillary

In physics, a very narrow, thick-walled tube, usually made of glass, such as in a thermometer. Properties of fluids, such as surface tension and viscosity, can be studied using capillary tubes.