Open and closed circulation system

(a) The open vascular system: (most arthropods, some cephalad molluscs, tunicates). Blood is pumped by heart into an aorta which branches into a number of arteries.  These open into a series of blood spaces collectively called hemocoel.  Here cells are in contact with the blood and materials are exchanged by direct diffusion through the plasma membrane. Blood under low pressure, blood moves slowly between tissues gradually percolating back into the heart via open-ended veins.  The distribution of blood in tissue is poorly controlled. This limits the efficiency of the open system. Fortunately, gaseous exchange in insects takes place through the tracheal system.  The insect circulatory system is not therefore concerned with transporting oxygen and carbon dioxide.  Accordingly, it lacks oxygen-carrying pigment.  However, it plays an important role in distributing food substances and eliminating nitrogenous waste.

(b) The closed vascular system:  (echinoderm, cephalo, molluscs, annelids, vertebrate). Here blood is confined in a series of specific vessels and not permitted to touch the body tissues.  In animals with a closed system, the heart more muscular heart and blood are pumped by the heart rapidly around the body under sustained high pressure and back to the heart.  Material exchange occurs across the wall of blood capillaries, which ramify through the organs and come into close association will all cells. Animals with closed systems are generally larger and often more active. The disadvantage of the closed circulatory system is that the blood is contained in vessels whose walls form a barrier between the blood and the surrounding tissues. Oxygen and other materials have to cross this barrier. However, capillaries having very thin walls reduce this barrier. (i) The single closed circulatory system Here blood possesses through the heart once in every circuit of blood. In fish, for example, blood is pumped from the heart to the gills. After acquiring oxygen from the gills, blood flows to the body tissues and then back to the heart.

The problem with this arrangement is that blood has to pass through two capillary systems, the capillaries of the gills and then those of the body, before returning to the heart.  Capillaries offer considerable resistance to the flow of blood, and this means that in fishes there is a marked drop in blood pressure before the blood completes a circuit.  For this reason, the blood flow from tissues to the heart is sluggish. This is overcome to some extent by the fact that fishes have large sinuses, which offer minimum resistance to blood flow, in places of veins.  Nevertheless, the problem of getting blood back to the heart is an acute one and probably imposes a severe limitation on the activities of many species of fish. (b) Double Circulation In double circulation, blood passes through the heart twice in every complete circuit. Advantage

• Blood is pumped to the lungs at a much lower pressure than that at which it is pumped to the rest of the body.  In humans, the pressure in the pulmonary artery is about one-sixth of that in the aorta.
• Deoxygenated blood is separated from oxygenated blood and then pumped at a different pressure to the lung and the body respectively.

(i) Incomplete double circulation A frog’s heart has two atria and one ventricle, however, blood mixing in the ventricle is prevented by the spiral valve in the conus arteriosus a heart chamber immediately before blood is pumped into big arteries. (ii)  Separate hearts The squids and octopuses solved the pressure problem by having two hearts: blood is pumped from the main heart to various parts of the body. It flows through a system of sinuses to a pair of branchial hearts that pumps it to the gills.

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