Blood vessel

• artery: late Middle English; from Latin arteria, from Greek artēria, probably from airein ("raise")
• vein: Middle English; from Old French veine, from Latin vena. The earliest senses were "blood vessel" and "small natural underground channel of water".
• capillary: mid 17th century; from Latin capillaris, from capillus ("hair"), influenced by Old French capillaire.

The arteries and veins have three layers. The middle layer is thicker in the arteries than it is in the veins:

• The inner layer, tunica intima, is the thinnest layer. It is a single layer of flat cells (simple squamous epithelium) glued by a polysaccharide intercellular matrix, surrounded by a thin layer of subendothelial connective tissue interlaced with a number of circularly arranged elastic bands called the internal elastic lamina. A thin membrane of elastic fibers in the tunica intima run parallel to the vessel.
• The middle layer tunica media is the thickest layer in arteries. It consists of circularly arranged elastic fiber, connective tissue, polysaccharide substances, the second and third layer are separated by another thick elastic band called external elastic lamina. The tunica media may (especially in arteries) be rich in vascular smooth muscle, which controls the caliber of the vessel. Veins don't have the external elastic lamina, but only an internal one. The tunica media is thicker in the arteries rather than the veins.
• The outer layer is the tunica adventitia and the thickest layer in veins. It is entirely made of connective tissue. It also contains nerves that supply the vessel as well as nutrient capillaries (vasa vasorum) in the larger blood vessels.

Capillaries consist of a single layer of endothelial cells with a supporting subendothelium consisting of a basement membrane and connective tissue.

When blood vessels connect to form a region of diffuse vascular supply it is called an anastomosis. Anastomoses provide critical alternative routes for blood to flow in case of blockages.

Leg veins have valves which prevent backflow of the blood being pumped against gravity by the surrounding muscles.[3]

Types

Transmission electron micrograph of a blood vessel displaying an erythrocyte (red blood cell, E) within its lumen, endothelial cells forming its tunica intima (inner layer), and pericytes forming its tunica adventitia (outer layer).

There are various kinds of blood vessels:

• Arteries
• Elastic arteries
• Distributing arteries
• Arterioles
• Capillaries (smallest blood vessels)
• Venules
• Veins
  • Large collecting vessels, such as the subclavian vein, the jugular vein, the renal vein and the iliac vein.
  • Venae cavae (the two largest veins, carry blood into the heart).
• Sinusoids
  • Extremely small vessels located within bone marrow, the spleen, and the liver.

They are roughly grouped as "arterial" and "venous", determined by whether the blood in it is flowing away from (arterial) or toward (venous) 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. This is because they are carrying the blood to and from the lungs, respectively, to be oxygenated.

Diagram of blood vessel structures

Blood vessels function to transport blood. In general, arteries and arterioles transport oxygenated blood from the lungs to the body and its organs, and veins and venules transport deoxygenated blood from the body to the lungs. Blood vessels also circulate blood throughout the circulatory system Oxygen (bound to hemoglobin in red blood cells) is the most critical nutrient carried by the blood. In all arteries apart from the pulmonary artery, hemoglobin is highly saturated (95–100%) with oxygen. In all veins apart from the pulmonary vein, the saturation of hemoglobin is about 75%.[4][5] (The values are reversed in the pulmonary circulation.) In addition to carrying oxygen, blood also carries hormones, waste products and nutrients for cells of the body.

Blood vessels do not actively engage in the transport of blood (they have no appreciable peristalsis). Blood is propelled through arteries and arterioles through pressure generated by the heartbeat.[6] Blood vessels also transport red blood cells which contain the oxygen necessary for daily activities. The amount of red blood cells present in your vessels has an effect on your health. Hematocrit tests can be performed to calculate the proportion of red blood cells in your blood. Higher proportions result in conditions such as dehydration or heart disease while lower proportions could lead to anemia and long-term blood loss.[7]

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, warmth and pain).

Vessel size

Constricted blood vessel.

Arteries—and veins to a degree—can regulate their inner diameter by contraction of the muscular layer. This changes the blood flow to downstream organs, and is determined by the autonomic nervous system. Vasodilation and vasoconstriction are also used antagonistically as methods of thermoregulation.[8]

The size of blood vessels is different for each of them. It ranges from a diameter of about 25 millimeters for the aorta to only 8 micrometers in the capillaries. This comes out to about a 3000-fold range.[9] 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).

Blood vessels play a huge 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.[13] Atherosclerosis, the narrowing of the blood vessels due to the buildup of plaque, and the coronary artery disease that often follows can cause heart attacks or cardiac arrest and is the leading cause of death worldwide resulting in 8.9 million deaths or 16% of all deaths.[14][15]

Blood vessel permeability is increased in inflammation. Damage, due to trauma or spontaneously, may lead to hemorrhage due to mechanical damage to the vessel endothelium. In contrast, occlusion of the blood vessel by atherosclerotic plaque, by an embolised blood clot or a foreign body leads to downstream ischemia (insufficient blood supply) and possibly infarction (necrosis due to lack of blood supply). Vessel occlusion tends to be a positive feedback system; an occluded vessel creates eddies in the normally laminar flow or plug flow blood currents. These eddies create abnormal fluid velocity gradients which push blood elements such as cholesterol or chylomicron bodies to the endothelium. These deposit onto the arterial walls which are already partially occluded and build upon the blockage.[16]

The most common disease of the blood vessels is hypertension or high blood pressure. This is caused by an increase in the pressure of the blood flowing through the vessels. Hypertension can lead to more serious conditions such as heart failure and stroke. To prevent these diseases, the most common treatment option is medication as opposed to surgery. Aspirin helps prevent blood clots and can also help limit inflammation.[17]

Vasculitis is inflammation of the vessel wall, due to autoimmune disease or infection.

• Circulatory system
• Heart
• List of bones of the human skeleton
• List of skeletal muscles of the human body
• List of nerves of the human body