Healthy resting arterial pressures, are relatively low, mean systemic pressures typically being under 100 mmHg, about 1.8 lbf/in², above surrounding atmospheric pressure (about 760 mmHg or 14.7 lbf/in² at sea level).

To withstand and adapt to the pressures within, arteries are surrounded by varying thicknesses of smooth muscle, which have extensive elastic and inelastic connective tissues.

The pulse pressure, i.e. Systolic vs. Diastolic difference, is determined primarily by the amount of blood ejected by each heart beat, stroke volume, versus the volume and elasticity of the major arteries.

Over time, elevated arterial blood sugar (see Diabetes Mellitus), lipoprotein cholesterol, and pressure, smoking, and other factors are all involved in damaging both the endothelium and walls of the arteries, resulting in atherosclerosis. Diabetes Mellitus also leads to capillary damage.

Arteriole

An arteriole is a blood vessel that extends and branches out from an artery and leads to capillaries.

Arterioles have thick muscular walls and are the primary site of vascular resistance. The mean blood pressure in the arteries supplying the body is a result of the interaction between the cardiac output (the volume of blood the heart is pumping per minute) and the vascular resistance, usually termed total peripheral resistance by physicians and researchers.

The up and down fluctuation of the arterial blood pressure is due to the pulsatile nature of the cardiac output and determined by the interaction of the stroke volume versus the volume and elasticity of the major arteries.

The muscular contraction of arterioles is targeted by drugs that lower blood pressure (antihypertensives), for example the dihydropyridines (nifedipine and nicardipine), which block the calcium conductance in the muscular layer of the arterioles, causing relaxation. This decreases the resistance to flow into peripheral vascular beds, lowering overall systemic pressure.

Capillary

The word capillary is used to describe any very narrow tube or channel through which a fluid can pass. See capillary action for details.

Capillaries are the smallest of a body's blood vessels, measuring 5-10 μm. They connect arteries and veins, and most closely interact with tissues. Capillaries have walls composed of a single layer of cells, the endothelium. This layer is so thin that molecules such as oxygen, water and lipids can pass through them by diffusion and enter the tissues. Waste products such as carbon dioxide and urea can diffuse back into the blood to be carried away for removal from the body. Capillary permeability can be increased by the release of certain cytokines.

The endothelium also actively transports nutrients, messengers and other substances. Large molecules may be too big to diffuse across endothelial cells. In some cases, vesicles contained in the capillary membrane use endocytosis and exocytosis to transport material between blood and the tissues.

In an immune response, the endothelial cells of the capillary will upregulate receptor molecules, thus "catching" immune cells as they pass by the site of infection and aiding extravasation of these cells into the tissue.

The "capillary bed" is the network of capillaries supplying an organ. The more metabolically active the cells, the more capillaries it will require to supply nutrients. The capillary bed usually carries no more than 25% of the amount of blood it could contain, although this amount can be increased through autoregulation (e.g. active muscle cells) by constricting smooth muscle.

Venule

A venule is a small blood vessel that allows blood to return from the capillary beds to the larger blood vessels called veins. Venules have three layers: An inner endothelium composed of squamous epithelial cells that act as a membrane, a middle layer of muscle and elastic tissue and an outer layer of fibrous connective tissue. The middle layer is poorly developed so that venules have thinner walls than arterioles.

Vein

In biology, a vein is a blood vessel, which returns blood from the microvasculature to the heart. Veins form part of the circulatory system. The vessels carrying blood away from the heart are known as arteries.

Veins have one-way valves to prevent backflow caused by gravity.

In systemic circulation, de-oxygenated blood from the capillary blood vessels is taken by veins to the right part of the heart. Differently, in the pulmonary circulation oxygenated blood from the lungs is taken to the left part of the heart by pulmonary veins. Another special case is portal circulation where the portal vein transports blood rich in products of digestion from the intestines to the liver.

NAMES OF IMPORTANT VEINS:

Pulmonary veins
Portal vein
Superior vena cava
Inferior vena cava
Femoral vein
Great saphenous vein

Veins are used medically as points of access to the blood stream, permitting the withdrawal of blood specimens (venipuncture) for testing purposes, and enabling the infusion of fluid, electrolytes, nutrition, and medications. The latter is called intravenous delivery. It can be done by an injection with a syringe, or by inserting a catheter (a flexible tube).

If an intravenous catheter has to be inserted, for most purposes this is done into a peripheral vein (a vein near the surface of the skin in the hand or arm, or less desirably, the leg.) Some highly concentrated fluids or irritating medications must flow into the large central veins, which are sometimes used when peripheral access cannot be obtained. Catheters can be threaded into the superior vena cava for these uses: if long term use is thought to be needed, a more permanent access point can be inserted surgically.

The precise location of veins is much more variable from person to person than that of arteries.

VENAE CAVAE

The superior and inferior venae cavae are the veins that return the blood from the body into the heart. They both empty into the right atrium.

The inferior vena cava travels up alongside the abdominal aorta with blood from the lower part of the body.

The superior vena cava is above the heart, and forms from a convergence of the left and right brachiocephalic veins that contain blood from the head and the arms. The vena cava carries blood from the body to the right atrium of the heart.

PULMONARY ARTERIES

The pulmonary arteries carry blood from the heart to the lungs. They are the only arteries (other than umbilical arteries in the fetus) that carry deoxygenated blood.

In the human heart, the pulmonary trunk begins at the base of the right ventricle. It is short and wide - approximately 5 cm (2 inches) in length and 3 cm (1.2 inches) in diameter. It then branches into two pulmonary arteries, which connect to the base of each lung.

Role in disease

Pulmonary hypertension occurs alone and as a consequence of a number of lung diseases. It can be a consequence of heart disease (Eisenmenger's syndrome) but equally a cause (right-ventricular heart failure); it also occurs as a consequence of pulmonary embolism and scleroderma. It is characterized by reduced exercise tolerance. Severe forms, generally, have a dismal prognosis.

PULMONARY VEINS

The pulmonary veins carry oxygen rich blood from the lungs to the left atrium of the heart. They are the only veins in the adult human body that carry oxygenated blood.

There are four of them:

Right inferior

Right superior

Left inferior

Left superior

MAJOR BLOOD VESSELS

HEAD: ARTERIES: carotid - common carotid - internal carotid (ophthalmic, retinal, anterior cerebral, middle cerebral, posterior communicating) - external carotid (facial, maxillary, superficial temporal artery) - posterior cerebral - anterior communicating - basilar - circle of Willis - middle meningeal | VEINS: jugular - vein of Galen

ARMS: ARTERIES: axillary (superior thoracic, thoracoacromial, lateral thoracic, subscapular, anterior circumflex humeral, posterior circumflex humeral) - brachial - radial - ulnar - dorsal scapular | VEINS: axillary - brachial - radial - ulnar - median cubital - basilic - cephalic

THORAX: ARTERIES: aorta - brachiocephalic - bronchial - thoracic (lateral thoracic, internal thoracic) - subclavian - vertebral - axillary - pulmonary | VEINS: venae cavae (superior - inferior) - brachiocephalic - subclavian - portal - ductus venosus - azygos - pulmonary

ABDOMEN: ARTERIES: celiac artery - marginal - artery of Adamkiewicz - gastroduodenal - gastroepiploic - left gastric - umbilical - mesenteric (superior - inferior) | iliac (common - external - internal) - Internal pudendal - renal - hepatic - common hepatic - splenic | VEINS: mesenteric (inferior, superior) | iliac (common - external) - renal - hepatic - splenic

LEGS: ARTERIES: dorsalis pedis - femoral - peroneal - popliteal - profunda femoris - tibial (anterior, posterior) | VEINS: femoral - saphenous (great, small) - peroneal - popliteal - profunda femoris - tibial (anterior tibial, posterior tibial)

All text of this article available under the terms of the GNU Free Documentation License (see Copyrights for details).