Venule Health Dictionary

Tube through which blood is conducted from or to the heart. Blood from the heart is conducted via arteries and arterioles through capillaries and back to the heart via venules and then veins. (See ARTERIES and VEINS.)... blood vessel

The minute vessels which join the ends of the arteries to venules, the tiny commencement of veins. Their walls consist of a single layer of ?ne, ?at, transparent cells, bound together at the edges, and the vessels form a meshwork all through the tissues of the body, bathing the latter in blood with only the thin capillary wall interposed, through which gases and ?uids readily pass. These vessels are less than 0·025 mm in width.... capillaries

Any of the vessels that carry blood between the smallest arteries, or arterioles, and the smallest veins, or venules (see circulatory system). Capillaries form a fine network throughout the body’s organs and tissues. Their thin walls are permeable and allow blood and cells to exchange constituents such as oxygen, glucose, carbon dioxide, and water (see respiration). Capillaries open and close to blood flow according to the requirements of different organs. The opening and closing of skin capillaries helps to regulate temperature.

A direct blow to the body may rupture the thin capillary walls, causing bleeding under the surface of the skin, which in turn causes swelling and bruising. Increasing age, high doses of corticosteroid drugs, and scurvy (vitamin C deficiency) make capillaries more fragile; a tendency to purpura (small areas of bleeding under the skin) may develop.... capillary

The course of the circulation is as follows: the veins pour their blood, coming from the head, trunk, limbs and abdominal organs, into the right atrium of the HEART. This contracts and drives the blood into the right ventricle, which then forces the blood into the LUNGS by way of the pulmonary artery. Here it is contained in thin-walled capillaries, over which the air plays freely, and through which gases pass readily out and in. The blood gives o? carbon dioxide (CO2) and takes up oxygen (see RESPIRATION), and passes on by the pulmonary veins to the left atrium of the heart. The left atrium expels it into the left ventricle, which forces it on into the aorta, by which it is distributed all over the body. Passing through capillaries in the various tissues, it enters venules, then veins, which ultimately unite into two great veins, the superior and the inferior vena cava, these emptying into the right atrium. This complete circle is accomplished by any particular drop of blood in about half a minute.

In one part of the body there is a further complication. The veins coming from the bowels, charged with food material and other products, split up, and their blood undergoes a second capillary circulation through the liver. Here it is relieved of some food material and puri?ed, and then passes into the inferior vena cava, and so to the right atrium. This is known as the portal circulation.

The circle is maintained always in one direction by four valves, situated one at the outlet from each cavity of the heart.

The blood in the arteries going to the body generally is bright red, that in the veins dull red in colour, owing to the former being charged with oxygen and the latter with carbon dioxide (see RESPIRATION). For the same reason the blood in the pulmonary artery is dark, that in the pulmonary veins is bright. There is no direct communication between the right and left sides of the heart, the blood passing from the right ventricle to the left atrium through the lungs.

In the embryo, before birth, the course of circulation is somewhat di?erent, owing to the fact that no nourishment comes from the bowels nor air into the lungs. Accordingly, two large arteries pass out of the navel, and convey blood to be changed by contact with maternal blood (see PLACENTA), while a large vein brings this blood back again. There are also communications between the right and left atria, and between pulmonary artery and aorta. The latter is known as the ductus arteriosus. At birth all these extra vessels and connections close and rapidly shrivel up.... circulatory system of the blood

Common chronic in?ammation of the facial skin, this condition is seen in middle and late life. Redness, obvious dilatation of venules and crops of ACNE-like papules and pustules affect mainly the central forehead, cheeks, nose and chin. A keratoconjunctivitis (combined in?ammation of the cornea and conjunctiva of the EYE) may be associated. Subjects ?ush easily, especially after alcohol or hot drinks. Eventually the affected areas may become thickened and oedematous, and in men, proliferation of ?brous and sebaceous tissue may lead to gross thickening and enlargement of the nose (RHINOPHYMA).

Treatment Long-term, low-dose, oral tetracycline (see ANTIBIOTICS; TETRACYCLINES) is the treatment of choice. In mild cases, METRONIDAZOLE gel can be helpful. Potent topical CORTICOSTEROIDS are contraindicated and make rosacea worse.... rosacea

Abnormal dilatation of ARTERIOLES and venules (see VENULE). In the skin it is seen in spider NAEVUS and ROSACEA particularly.... telangiectasis

n. a *prostaglandin drug (PGE1) administered to improve lung blood flow in newborn babies with congenital heart defects who are awaiting surgery; it acts by preventing the closure of the blood vessel connecting the aorta to the pulmonary artery (see ductus arteriosus). Possible side-effects include diminished respiratory efforts. Alprostadil is also administered to treat *erectile dysfunction in men; it acts by relaxing smooth muscle and dilating the cavernosal arteries, so that blood is trapped when the venules become compressed against the *tunica albuginea. Side-effects may include dizziness and headache.... alprostadil

a thick-walled blood vessel that connects an arteriole directly with a venule, thus bypassing the capillaries. Arteriovenous anastomoses are commonly found in the skin of the lips, nose, ears, hands and feet; their muscular walls can constrict to reduce blood flow or dilate to allow blood through to these areas.... arteriovenous anastomosis

n. the act of passing through or across, e.g. the passage of blood cells through the intact walls of capillaries and venules (see diapedesis).... transmigration

The heart and blood vessels, which together maintain a continuous flow of blood throughout the body. The system provides tissues with oxygen and nutrients, and carries away waste products. The circulatory system consists of 2 main parts: the systemic circulation, which supplies blood to the whole body apart from the lungs; and the pulmonary circulation to the lungs. Within the systemic circulation, there is a bypass (the portal circulation), which carries nutrient-rich blood from the stomach, intestine, and other digestive organs to the liver for processing, storage, or re-entry into general circulation.

In the systemic circulation, oxygen-rich blood from the pulmonary circulation is pumped under high pressure from the left ventricle of the heart into the aorta, from where it travels through arteries and smaller arterioles to all parts of the body. Within body tissues, the arterioles branch into networks of fine blood vessels called capillaries. Oxygen and other nutrients pass from the blood through the capillaries’ thin walls into body tissues; carbon dioxide and other wastes pass in the opposite direction. Deoxygenated blood is returned to the heart via venules, veins, and the venae cavae.

Venous blood returns to the right atrium of the heart to enter the pulmonary circulation. It is pumped from the right ventricle through the pulmonary artery

to the lungs, where carbon dioxide is exchanged for oxygen. The reoxygenated blood then returns through the pulmonary veins to the heart and re-enters the systemic circulation.... circulatory system