Invertebrate Coelomic fluid and hemolymph, Blood cells of invertebrates, Arteries, veins and capillaries


Invertebrate Coelomic fluid and hemolymph:
Some animals like Echinoderms, Annelids and Sipunculans are coelomic fluid as Supplementary or sole circulatory system. Coelomic fluid may be identified in composition to interstitial fluids or may differ particularly with respect to specific proteins and cells. Coelomic fluid transports gases, nutrients and waste products. It also functions in certain invertebrates (annelids) as a hydrostatic skeleton.

Hemolymph is circulatory fluid of animals with an open circulatory system. Most Arthropods, Ascidians and many Molluscs have hemolymph. In these animals heart pumps hemolymph at low pressure through vessels to tissue spaces (hemocoel) and sinuses. Generally hemolymph volume is high and the circulation slow. In the process of movement, essential gases, nutrients and wastes are transported.

Many times hemolymph has no circulatory functions. In insects, hemolymph pressure assists in molting of old cuticle and in inflation of the wings. In certain jumping spiders, hydrostatic pressure of hemolymph provides hydraulic mechanism for limb extension.

Blood cells of invertebrates:
blood of most animals contains circulating cells called blood cells or hemocytes. Some cells contain respiratory pigment such as haemoglobin and are called erythrocytes. These cells are present in high number to facilitate oxygen transport. Cells that do not contain respiratory pigments have other functions as blood clotting. Number and types of blood cells vary in different invertebrates.

Annelid blood contains Annelid blood contains hemocytes that are phagocytic. The coelomic fluid contains variety of coelomocytes that function in phagocytosis, glycogen storage, encapsulation, defense response and excretion. Hemolymph of Molluscs has two general types of hemocytes (amoebocytes and gametocytes) that have also nacrezation (pearl formation) in some bivalves. Insect hemolymph contains large number of various haemocyte types that function in phagocytosis, encapsulation and clotting.

Arteries, veins and capillaries:
Arteries: Arteries are elastic blood vessels that carry away blood from the heart to the organs and tissues of the body. Surrounding the lumen of an artery is a thick wall composed of three layers or tunicae (i) outer most layer consist of connective tissue. (ii) Middle layer has elastic and smooth muscle tissue (iii) Inner layer has single layer of smooth endothelial cells. Wall of artery is thicker than vein.

Veins: Most veins are inelastic large vessels carry blood from the body tissues to the heart. The wall of a vein contains same three layers as arterial walls but the middle layer is much thinner and one or more valves are present. The valves permit blood flow in only one direction which is important in returning the blood to the heart.

Capillaries: Arteries lead to terminal arterioles. The arterioles branch to form capillaries which connect to venules and the veins. Capillaries are generally composed of single layer of endothelial cells and are the most numerous blood vessels in animals’ body. An abundance of capillaries makes an enormous surface area for exchange of gases, fluids, nutrients and wastes between the blood and nearby cells.

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