Blood vessel
The
blood vessels are part of the
circulatory system and function to transport
blood throughout the body. The most important types,
arteries and
veins, are so termed because they carry blood away from or towards the
heart, respectively.
Blood vessels exist in varying calibers:
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Arteries**
Aorta (the largest artery, carries blood out of the heart)
**Branches of the aorta, such as the
carotid artery, the
subclavian artery, the
celiac trunk, the
mesenteric arteries, the
renal artery and the
ileac artery.
*
Arterioles
*
Capillaries (the smallest blood vessels)
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Venules
*
Veins
**Large collecting vessels, such as the
subclavian vein, the
jugular vein, the
renal vein and the
iliac vein.
**
Venae cavae (the 2 largest veins, carry blood into the heart)
They are roughly grouped as
arterial and
venous, determined by whether the blood in it is flowing
toward or
away from the
heart. The term "arterial blood" is nevertheless used to indicate blood high in
oxygen, although the
pulmonary artery carries "venous blood" and blood flowing in the
pulmonary vein is rich in oxygen.
All blood vessels follow the same
histological makeup. The inner lining is the
endothelium, followed by subendothelial
connective tissue. Then follows a muscular layer of
vascular smooth muscle, which is highly developed in arteries. Finally, there is a further layer of connective tissue termed the
adventitia, which contains
nerves that supply the muscular layer, as well as nutrient capillaries in the larger blood vessel.
Capillaries consist of little more than a layer of endothelium and occasional connective tissue.
In anatomy, the term for when a blood vessel
joins another to form a region of diffuse vascular supply is known as
anastamosis. This is important in several areas around the body, as blockages in one area can mean that anastamoses (plural of anastamosis) makes an alternative route for blood flow.
Stretched end to end, the blood vessels in an average human body will stretch approximately 62,000 miles.
Blood vessels do not actively engage in the transport of the blood (they have no appreciable
peristalsis), but arteries - and veins to a degree - can regulate their caliber by contraction of the muscular layer. This determines the blood flow to particular downstream organs, and is determined by the
autonomic nervous system. Vasodilation and vasoconstriction are also used antagonistically as a method of
thermoregulation in
homeotherms.
Oxygen (bound to
haemoglobin in
red blood cells) is the most critical nutrient carried by the blood. In all arteries apart from the pulmonary artery, haemoglobin is highly saturated (95-100%) with oxygen. In all veins apart from the pulmonary vein, the haemoglobin is desaturated at about 70%. (The values are reversed in the pulmonary circulation.)
The
blood pressure in blood vessels is traditionally expressed in
millimetres of mercury (1 mmHg = 133
Pa). In the arterial system, this is usually around 120 mmHg
systolic (high pressure wave due to contraction of the heart) and 80 mmHg
diastolic (low pressure wave). In contrast, pressures in the venous system are constant and rarely exceed 10 mmHg.
Vasoconstriction is the constriction of blood vessels (narrowing, becoming smaller in cross-sectional area) by contracting the
vascular smooth muscle in the vessel walls. It is regulated by
vasoconstrictors (agents that cause vasoconstriction). These include
paracrine factors (e.g.
prostaglandins), a number of
hormones (e.g.
vasopressin and
angiotensin) and
neurotransmitters (e.g.
epinephrine) from the nervous system.
Vasodilation is a similar process mediated by antagonistically acting mediators. The most prominent vasodilator is
nitric oxide (termed
endothelium-derived relaxing factor for this reason).
Permeability of the
endothelium is pivotal in the release of nutrients to the tissue. It is also increased in
inflammation in response to
histamine,
prostaglandins and
interleukins, which leads to most of the symptoms of inflammation (swelling, redness and warmth).
Blood vessels play a role in virtually every medical condition.
Cancer, for example, cannot progress unless the tumor causes
angiogenesis (formation of new blood vessels) to supply the malignant cells' metabolic demand.
Atherosclerosis, the formation of
lipid lumps (
atheromas) in the blood vessel wall, is the prime cause of
cardiovascular disease, the main cause of death in the Western world.
Blood vessel permeability is increased in
inflammation. Damage, due to
trauma or spontaneously, may lead to
haemorrhage. In contrast, occlusion of the blood vessel (e.g. by a ruptured atherosclerotic plaque, by an
embolised blood clot or a foreign body) leads to downstream
ischemia (insufficient blood supply) and
necrosis (tissue breakdown).
Vasculitis is
inflammation of the vessel wall, due to
autoimmune disease or
infection.
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vascular surgery*
vascular resistance*
anastomosis*
angioplasty
