Artery

Arteries are blood vessels that travel from the heart to the peripheral organs, and carry blood that is expelled from the heart. Generally, arterial blood that flows through arteries is blood that is sufficiently oxygenated. However, this relationship is reversed for the pulmonary artery (the artery that travels from the heart to the lungs). The blood in the pulmonary artery is venous blood that has absorbed unnecessary carbon dioxide from the body, and the heart accepts this blood and sends it out to the lungs. In other words, the heart sends venous blood to the lungs via the pulmonary artery, exchanging carbon dioxide for oxygen.

Arteries begin with the aorta (approximately 3 centimeters in diameter) which is the largest artery that leaves the heart, and as they are distributed to tissues and organs throughout the body, they branch and gradually become thinner. In the process of branching, the walls of the blood vessels also become thinner. Once inside an organ, the arteries branch even more finely to become arterioles (less than 0.5 mm in diameter), and finally to become capillaries surrounded by endothelial cells arranged in a single layer. A capillary network is a network of thin tubes. Eventually, the capillaries join together again to become venules. There are various forms in which arteries run through the body, but when blood vessels connecting the proximal and distal parts of the affected part expand due to a circulatory disorder in the original ductus arteriosus, they are called "collateral vessels" and promote blood circulation. In addition, arterial branches that communicate with each other are called "communicating branches" or "anastomotic branches." A network of blood vessels that branches into many branches and connects to each other in a mesh-like pattern is called an "arterial network," and the three-dimensional structure of this arterial network is called an "arterial plexus." A network of blood vessels in which arterioles suddenly branch to form a mesh-like structure before dividing into capillaries is called an "arterial plexus." An example of this is the vascular network of the glomeruli in the kidneys. In the arteries of the cerebral gray matter, lungs, liver, spleen, kidneys, and thyroid gland, the arterioles do not anastomose with each other before they become capillaries. These are called "terminal arteries." If blood circulation is impaired in a terminal artery, the tissue in the area in which the blood vessels are distributed will degenerate.

The walls of capillaries consist of only one layer of endothelial cells, and their average diameter is about 8 micrometers. Some capillaries have holes (windows) in the walls of the endothelial cells. These are called fenestrated or perforated capillaries. These holes are usually closed by a septum that is thinner than the cell membrane, but are commonly found in tissues that require rapid exchange of materials between tissue fluid and blood, such as the stomach, intestines, and endocrine glands. The surface area of ​​capillaries is said to be 6,000 square meters, and their total diameter is said to be about 800 times that of the aorta. The tissue that makes up the walls of large arterial ducts generally consists of three layers, which are distinguished from the inside out: the tunica intima, tunica media, and tunica adventitia. The tunica intima has a single layer of endothelial cells that contacts the vascular lumen, and on the outside it has homogeneous elastic fibers (internal elastic lamina) made of elastin. The tunica media is an extremely thick membrane, and is made up of a large number of smooth muscle fibers and elastic fibers that run in circles. The larger the blood vessel, the more developed and thick the tunica media is. The adventitia is made up of longitudinal connective tissue fibers, but in large blood vessels, collagen fibers and elastic fibers are mixed together. In medium-sized arteries or aortas, the autogenous blood vessels (the blood vessels of the blood vessels) that nourish the blood vessel itself run within the adventitia. Autogenous blood vessels are particularly common in large blood vessels. In the blood vessel wall, unmyelinated autonomic nerve fibers form a reticular structure in the adventitia and further penetrate between the striated muscle fibers of the tunica media, where they are involved in the movement of the blood vessels. Lymphatic vessels are also distributed in the blood vessel wall.

[Kazuyo Shimai]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

心臓から末梢(まっしょう)器官に向かう血管が動脈であり、心臓から排出される血液を流している。一般に動脈の中を流れる動脈血は、酸素を十分に含んだ血液である。しかし、肺動脈（心臓から肺臓に向かう動脈）に限ってはこの関係が逆になる。肺動脈の中の血液は、体内の不要な炭酸ガス（二酸化炭素）を吸収してきた静脈血であり、これを心臓が受け入れて肺臓に送り出している。つまり、心臓は肺動脈によって静脈血を肺臓へ送り、炭酸ガスと酸素の交換を行うわけである。

　動脈は心臓から出るもっとも太い大動脈（直径約3センチメートル）から始まり、体中の組織や器官に分布するため、分枝しながらしだいに細くなる。この分枝の過程で、血管壁も同時に薄くなっていく。器官の中に入ると、動脈はさらに細かく分枝して細動脈（直径0.5ミリメートル以下）となり、ついには1層に配列した内皮細胞に囲まれた毛細血管となる。毛細血管網とは、細い管が網状構造となったものである。やがて、毛細血管はふたたび合して細静脈に移行する。動脈が体内を走る形式にはいろいろあるが、本来の動脈管に血行障害が生じたため、障害部分の近位部と遠位部とをつないでいる血管が拡張し、血行を促すようになるものを「側副血管」という。また、動脈の枝が互いに連絡交通しているのを「交通枝」あるいは「吻合枝(ふんごうし)」とよぶ。多数の枝に分枝して、互いに網目状に連絡する血管網を「動脈網」といい、この動脈網の立体的構造を「動脈叢(そう)」とよぶ。細動脈が毛細血管に分かれる前に、急に分枝して網状構造となる血管網を「怪網」という。腎臓(じんぞう)内の糸球体の血管網がこの例である。脳髄灰白質、肺臓、肝臓、脾臓(ひぞう)、腎臓、甲状腺(こうじょうせん)の動脈では、毛細血管になる前の細動脈は、互いに吻合をつくらない。これを「終動脈」とよぶ。終動脈に血行障害が生じると、その血管の分布区域の組織は変性をおこすこととなる。

　毛細血管の壁は1層の内皮細胞層だけからなり、その直径の平均値は8マイクロメートルほどである。毛細血管のなかには、内皮細胞の壁に孔（窓）をもつものがある。これを有窓型または有孔型毛細血管という。この孔は、普通は細胞膜よりも薄い隔膜で閉ざされているが、胃、腸、内分泌腺などのように、組織液と血液との間において急速な物質交換が必要とされる組織にはよくみられる。毛細血管の表面積は6000平方メートルとされ、また、その直径総計は大動脈の約800倍といわれる。太い動脈管の壁を構成する組織は一般に3層からなり、内側から内膜・中膜・外膜を区別する。内膜は、血管内腔(ないくう)に接する1層の内皮細胞層と、その外側にエラスチンからなる等質性の弾性線維（内弾性板）をもつ。中膜はきわめて厚い膜で、輪走する多量の平滑筋線維と弾性線維からなる。太い血管ほど中膜が発達していて厚くなる。外膜は縦走する結合組織線維からなるが、太い血管では、膠原(こうげん)線維と弾性線維が混在する。中等大の動脈あるいは大動脈では、自己の外膜の中に血管自体を養う自養血管（血管の血管）が走る。自養血管は、とくに大血管で多い。血管壁では、無髄の自律神経線維が外膜のところで網状構造をつくり、さらに中膜の横紋筋線維の間に入り込んで血管の運動にかかわっている。このほか、血管壁にはリンパ管も分布している。

［嶋井和世］

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