Open circulatory system in the context of "Vasculature"

Arthropods (/ˈɑːrθrəˌpɒd/ AR-thrə-pod) are invertebrates in the phylum Arthropoda. They possess an exoskeleton with a cuticle made of chitin, often mineralised with calcium carbonate, a body with differentiated (metameric) segments, and paired jointed appendages. In order to keep growing, they must go through stages of moulting, a process by which they shed their exoskeleton to reveal a new one. They form an extremely diverse group of up to ten million species.

Haemolymph is the analogue of blood for most arthropods. An arthropod has an open circulatory system, with a body cavity called a haemocoel through which haemolymph circulates to the interior organs. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. They have ladder-like nervous systems, with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, and their brains are formed by fusion of the ganglia of these segments and encircle the esophagus. The respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong.

In vertebrates, the circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. It includes the cardiovascular system, or vascular system, that consists of the heart and blood vessels (from Greek kardia meaning heart, and Latin vascula meaning vessels). The circulatory system has two divisions, a systemic circulation or circuit, and a pulmonary circulation or circuit. Some sources use the terms cardiovascular system and vascular system interchangeably with circulatory system.

The network of blood vessels are the great vessels of the heart including large elastic arteries, and large veins; other arteries, smaller arterioles, capillaries that join with venules (small veins), and other veins. The circulatory system is closed in vertebrates, which means that the blood never leaves the network of blood vessels. Many invertebrates such as arthropods have an open circulatory system with a heart that pumps a hemolymph which returns via the body cavity rather than via blood vessels. Diploblasts such as sponges and comb jellies lack a circulatory system.

Hemolymph or haemolymph is a body fluid that circulates inside arthropod bodies transporting nutrients and oxygen to tissues, comparable with the blood in vertebrates. It is composed of a plasma in which circulating immune cells called hemocytes are dispersed in addition to many plasma proteins (hemoproteins) and dissolved chemicals. It is the key component of the open circulatory system characteristic of arthropods such as insects, arachnids, myriopods and crustaceans. Some non-arthropod invertebrates such as molluscs and annelids also possess a similar hemolymphatic circulatory system.

In insects, the largest arthropod clade, the hemolymph mainly carries nutrients but not oxygen, which is supplied to the tissues separately by direct deep ventilation through an extensive tracheal system. In other arthropods, oxygen is dissolved into the hemolymph from gills, book lungs or across the cuticle and then distributed to the body tissues via the hemocoel.