In this article we will discuss about the Circulation System in Invertebrate:- 1. Introduction to Circulation System in Invertebrate 2. Types of Circulatory System in Invertebrate.
Introduction to Circulation System in Invertebrate:
Circulatory system is an essential means of transporting materials like respiratory gases, nutrients, hormones, intermediate metabolic products and metabolic end products, to and from all body tissues, even those that are remote from the source of supply.
Not all animals do possess a circulatory system. In smaller animals, transporting materials diffuse readily through the intercellular spaces, so no circulatory system is required for their transport. Even larger animals may lack a definite circulatory system because of their primitive organization or low rate of metabolism.
For example, coelenterates and flatworms have achieved such a digestive system that performs the functions related to both digestion as well as circulation; on the other hand, the echinoderms have such a low rate of metabolism that an active circulation does not appear to be essential.
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However, the majority of multicellular animals because of their body size, various activities and associated metabolic demands require a continuous and reliable circulation of body fluids carrying different essential materials.
In invertebrates, the circulatory system is a network of channels and spaces in the connective tissue, which is continuous around and between all epithelial layers of the body. Invertebrate blood or haemal channels range widely in size.
Small tubular channels are called vessels; large sac like spaces are sinuses; small spaces are lacunae; and in case where the connective tissue compartment enlarges to form a voluminous, blood filled cavity, is called haemocoel.
Blood vessels that pass through mesenteries and septa are overlain -by mesothelium, which is often composed of myocytes bearing circularly arranged fibres. When the myocytes are distributed widely over the surface of a blood vessel, their rhythmic contraction produces peristalsis, which pushes the blood forward in waves.
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Often, however, the myocytes are localized and thickened into one or more specialized organs called heart.
In general, circulatory system includes a pumping device or heart, vessels like arteries (vessels that carry the blood away from heart), veins (vessels that carry the blood towards heart), capillaries (interconnecting vessels between artery and vein around cells), sinus, lacunae and the fluid like blood, lymph and haemolymph.
Types of Circulatory System in Invertebrate:
In organized animals, two distinct arrangements of circulatory channels are found:
Open Circulatory System:
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Arthropods, most molluscs and some lesser known animals have an open circulatory system where there are no capillaries connecting the veins with the arteries. They do not form a closed network and the arterial blood sooner or later, passes into sinuses or lacunae, so that it bathes the major organs and tissues.
From these tissue spaces, the blood slowly goes back into the open end of the veins or the ostia opening into the heart. In this open system, different organs lie directly in the fluid or blood filled haemocoel or in the primary body cavity.
Closed Circulatory System:
In this type of circulatory system, the animals like annelids, a few other groups of invertebrates and vertebrates have a completely closed system of vascular channels. A network of minute capillaries unite the small arteries with the veins. Fluid or blood, filters through the wall of the capillaries into the tissue spaces and transfers the nutrient to the cells.
Animals with a closed circulatory system, maintain a distinct separation between the blood and the tissue fluid; whereas in the open circulatory system, the distinction between blood and lymph or tissue-fluid breaks down completely. The same fluid moves through vascular channels and tissue spaces, and hence the circulating fluid is called haemolymph.
Open and closed circulatory systems require distinctly different pumping devices. In the open circulatory system, blood is dissipated and not available for refilling of the pump or heart. So during the relaxation of the heart, the pressure within the vessels that return the blood to the heart is equal to the internal body pressure.
This operation system depends on the suction of the haemolymph into the heart and comparable to that of a suction pump which provides an intermittent flow of haemolymph to the tissues. In the closed circulatory system, a force pump or pressure pump maintains a steady flow throughout a continuous network of pipes.
In this case, an elastic reservoir is present to maintain the positive pressure within the system while the pump is not contracting.