Derrick Garwood, Freelance Medical Writer and Editor

Have a guess at the total length of capillaries in the human body. Ten miles, a hundred, a thousand? No – the answer is approximately 25,000, or the equivalent of a trip around the equator. Here is another interesting statistic, particularly for anyone whose weight tends to fluctuate; according to this science radio show some 200 extra miles of these vessels develop for every pound of weight gained.

Capillaries lie between arteries and veins in the cardiovascular system and are about 5 – 10 mm in diameter, so narrow that red blood cells can only pass through in single file. The vessel walls (endothelium) consist of a single layer of cells, allowing oxygen, water and lipids to diffuse from the blood into the tissues, while waste products such as carbon dioxide and urea diffuse in the opposite direction to be eliminated from the body. This diagram summarises the fluid flow that results. The capillaries form a large branching network, as can clearly be seen in a photomicrograph of skin vessels. This microscope section shows a red blood cell within a (rather dilated) capillary, but gives an indication of relative size.

The circulatory system is often represented as passing from artery to arteriole to capillary to venule to vein, as in this diagram, but the real picture is more complex than this. Arterioles divide into metarterioles (10 – 20 mm in diameter) from which capillaries branch off through pre-capillary sphincters consisting of single muscle fibres. The state of contraction of these sphincters controls the flow of fluid through the capillaries. Eventually each metarteriole connects directly to a postcapillary venule, forming a 'preference channel' or shunt which can bypass the capillary bed. The number of capillaries per metarteriole varies with the individual tissue; for example, the widely varying oxygen requirements of skeletal muscle demand 8 – 10, but the stable mesenteric circulation requires only 2 or 3. This web site also has some interesting information on the different types of junction between endothelial cells – continuous, fenestrated and discontinuous – and the organs in which they are found.

Capillaries play an important role in the body's inflammatory response to injury or infection. The endothelial cells of the vessel wall retract to increase capillary permeability and allow the soluble mediators of immunity to escape and reach the site of inflammation. Leucocytes also migrate out into the surrounding tissues. There is a fairly comprehensive account of the mechanisms involved given at this website.

A collection of dilated blood capillaries is a telangiectasis, and where multiple examples are present the condition is termed telangiectasia. Such lesions often occur in the skin and may be relatively minor, as in this photograph, or quite extensive. The primary concern here is the patient's aesthetic appearance.

In other organs, however, telangiectasia may be associated with much more serious problems. Retinal telangiectasias comprise a group of rare, congenital vascular anomalies which affect the retinal capillaries, causing dilation, tortuosity and the formation of multiple aneurysms, with different degrees of leakage. There are three main types. Idiopathic juxtafoveolar retinal telangiectasia (JXT) causes gradual loss of vision in adulthood, and may be unilateral or bilateral. Patients are divided into 3 sub categories according to the perceived cause; capillary leakage, diffusion abnormalities or capillary occlusion. In cases of Leber's miliary aneurysms, patients often present in middle age with a reduction in visual acuity, and the condition represents a more severe form of telangiectasia. Cryotherapy or photocoagulation may be necessary to destroy the area of vascular abnormality.

The third type is Coats' Disease, a very severe unilateral condition that is progressive and most often seen in boys before the age of 10. It is characterised by large areas of dilated, tortuous, retinal blood vessels that overlie yellow exudate. Diagnosis is made from the appearance of the back of the eye (see the photograph here) and fluorescein angiography. The main symptom is deterioration in either central or peripheral vision and there may be strabismus (squint), but in many cases the disease halts of its own accord without treatment. There is evidence that this condition is caused by a somatic mutation in the NDP gene on chromosome 11 – if you are feeling adventurous you can tackle the original paper from the Human Molecular Genetics journal.

Hereditary haemorrhagic telangiectasia, or Osler rendu weber disease, is a rare autosomal dominant condition in which telangiectases occur not only in the skin and mucous membranes, but also in the brain and viscera, causing nosebleeds, haemoptysis and gastrointestinal bleeding. Multiple lesions in the lungs may lead to cardiac failure. This description of the condition and its genetic origin is very comprehensive, probably exhaustive and certainly exhausting!

Sufferers from ataxia telangiectasia (A-T) generally die from the condition by their twenties. It is a rare, progressive, recessive genetic disorder in which neurological problems and immunological abnormalities become apparent in infancy. A-T patients have a greatly increased risk of developing cancer (37 times that of the general population) and are particularly sensitive to ionising radiation. Telangiectases are characteristically present in the conjunctivae and skin. Clinical symptoms and the associated molecular genetics are described here.

Let us conclude with some potentially divisive research. It appears that sedentary men and women who begin physical exercise experience a similar increase in exercise capacity, but while the skeletal muscle of women demonstrates a statistically significant increase in capillary density, that of men does not. Bizarre!

This tour was submitted by Derrick Garwood, a Freelance Medical Writer and Editor.

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