Venous versus arterial
Arteries carry blood away from the heart, veins towards the heart. The proportion of the two types of vessels in the circulatory system is very different: compared to the veins, which make up the majority of blood vessels at around 75 percent, arteries are outnumbered at only around 20 percent (capillaries five percent). They are distributed throughout the body and are usually found in the vicinity of the veins.
Venous blood is often equated with oxygen-poor blood and arterial blood with oxygen-rich blood. However, this is not correct: it is true that most arteries transport oxygen-rich blood and most veins transport oxygen-poor blood. The pulmonary arteries carry deoxygenated blood from the heart to the lungs, where it absorbs new oxygen from the air we breathe. The now oxygen-rich blood flows back to the heart via the pulmonary veins.
Arteries: Structure
The diameter of the arteries ranges from 20 micrometers (µm) for arterioles (the smallest arterial vessels) to three centimeters for the aorta (the largest blood vessel in the body). The wall of all arteries consists of the classic three layers: Intima, Media, Adventitia.
The wall of an artery is characterized above all by the thick middle layer, which is hardly pronounced in the veins. The media contains smooth muscle and/or elastic connective tissue. The proportion of these two components varies, so that an elastic and a muscular type of artery can be distinguished (in addition to transitional forms between the two):
Arteries of the elastic type contain a particularly large number of elastic fibers in the media. This type of vessel includes above all the large vessels close to the heart, because they are particularly exposed to the high pressure fluctuations between contraction (systole) and relaxation (diastole) of the heart muscle and have to compensate for them. The wall of the muscular type arteries, on the other hand, has a middle layer with much more smooth muscle. Such vessels are mainly found in the organs. They can control the blood supply via the muscles in their walls.
Various arteries at a glance
Important arteries in the body are
- Aorta (main artery)
- Pulmonary artery (pulmonary artery)
- brachiocephalic artery (brachiocephalic trunk)
- Carotid artery (arteria carotis communis)
- Subclavian artery (subclavian artery)
- Hepatic-gastric artery (truncus coeliacus)
- Mesenteric artery (Arteria mesenterica)
- Renal artery (arteria renalis)
- Common iliac artery (Arteria iliaca communis)
- Upper arm artery (brachial artery)
Special arteries in terms of their form or function are
- Barrier artery: can cut off the blood supply through muscle contraction in its wall (bronchi, penis, clitoris)
- Helical artery (Arteria helicina): highly tortuous, can lengthen if necessary (in the penis during erection)
- Collateral artery (vas collaterale): Secondary vessel of an artery; serves as an alternative route if this main artery is blocked (bypass or collateral circulation)
- End artery: without collateral circulation
Arterioles
Finer vessels are required to supply the entire body with sufficient oxygen. The arteries therefore branch into smaller vessels, the arterioles, which then split further into the capillaries. The capillary network then forms the transition to the venous system.
The diameter of the arterioles varies between 20 and 100 micrometers (µm). The wall of the arterioles has little smooth muscle (thin media) and, at 40 to 75 mmHg, a slightly lower pressure than the larger arteries. These fine red vessels are clearly visible in the white sclera of the eyes.
Diseases of the arteries
Arterial vascular diseases are usually occlusive diseases resulting from advanced arteriosclerosis: deposits and inflammation on the inner walls can narrow a vessel (stenosis) or even block it completely, thus impairing the oxygen supply (as in the case of a stroke or heart attack).
This can also happen because blood clots can easily form on arteriosclerotically altered vessel walls, which can block a vessel in situ (thrombosis) or – after being carried away by the blood flow – elsewhere in the body (embolism).
Risk factors for arteriosclerosis and its secondary diseases include obesity, lack of exercise, high blood pressure, smoking and high blood lipid levels.
An abnormal sac- or spindle-shaped dilation of an artery is called an aneurysm. It can rupture suddenly, which can be life-threatening (e.g. if the abdominal aorta ruptures).
