Vascular system

In vertebrates, it is a ductal system that, together with the lymphatic system, constitutes the circulatory system, but in invertebrates, there is no distinction, so it is used synonymously with the circulatory system. It consists of the heart and blood vessels, and the vascular systems of vertebrates and annelids are closed systems in which blood does not come into direct contact with tissue cells, whereas the vascular systems of arthropods and mollusks are open systems in which blood is pushed from the open ends of the arteries into tissue spaces, circulates throughout the body, and then returns directly to the heart.

The heart is an organ that pumps fluids in blood vessels, i.e. blood, in a certain direction. In annelids and other animals, it pumps blood by peristaltic movement of the ampulla of the dorsal blood vessel. In arthropods and mollusks, muscles and valves are developed in specific parts of the blood vessels, and blood flow is created by their pulsation. The hearts of mollusks are more developed than those of arthropods, and many of them have differentiated atria and ventricles. In the vascular system of vertebrates, the heart is differentiated into four parts: the sinus venosus, atria, ventricles, and bulbus arteriosus. Fish have a gill-breathing circulatory system that creates a single circulation path from bulbus arteriosus → branchial artery → gill → aorta → whole body → sinus venosus. However, in the pulmonary-breathing circulatory system of amphibians and above, there are two circulation paths: the pulmonary circulation (small circulation) and the systemic circulation (large circulation). In connection with the appearance of the lungs, the atria of amphibians differentiate into the right atrium, which connects to the sinus venosus, and the left atrium, which receives the pulmonary vein. In reptiles, a partition develops inside the ventricle, while in birds and mammals, the left and right ventricles are independent and the pulmonary circulation and systemic circulation are completely separated.

The main role of the circulatory system is to transport oxygen, carbon dioxide, nutrients, waste products, hormones, antibodies, etc., and in warm-blooded animals it also helps maintain the temperature of various parts of the body.

[Seiichiro Kawashima]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

脊椎(せきつい)動物ではリンパ系とともに循環系を構成する管系で、無脊椎動物ではこの区別がないので循環系と同義に用いられる。心臓と血管とからなり、脊椎動物や環形動物の血管系は血液と組織細胞が直接に接触しない閉鎖血管系であるが、節足動物や軟体動物の血管系は開放血管系で、血液は動脈の開放末端から組織間隙(かんげき)に押し出され、体内を循環したのちに直接心臓に戻る。

　心臓は血管内の体液、すなわち血液を一定方向に送る器官で、環形動物などでは背側血管の膨大部の蠕動(ぜんどう)運動により血液を送る。節足動物と軟体動物では血管の特定部分に筋肉と弁が発達し、その拍動により血流をつくる。軟体動物の心臓は節足動物のものよりよく発達し、心房と心室とが分化しているものが多い。脊椎動物の血管系では、静脈洞、心房、心室、動脈球の4部分からなる心臓が分化している。魚類はえら呼吸型循環系で、動脈球→鰓(さい)動脈→えら→大動脈→全身→静脈洞という単一の循環路をつくる。しかし両生類以上の肺呼吸型循環系では、肺循環（小循環）と体循環（大循環）の2循環路がある。肺の出現と関連して、両生類の心房は、静脈洞に接続する右心房と、肺静脈を受ける左心房とに分化する。爬虫(はちゅう)類になると心室内に隔壁が生じ、鳥類と哺乳(ほにゅう)類では左右の心室が独立して肺循環と体循環は完全に分離する。

　血管系の役目は、酸素、二酸化炭素、栄養分、老廃物、ホルモン、抗体などの運搬がおもで、恒温動物では体の各所の温度維持にも役だっている。

［川島誠一郎］

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例