CIRCULATORY SYSTEMS

            The circulatory system functions, primarily, as the major transport system of the vertebrate body.  Oxygen and food are carried from sites of absorption to all tissues and carbon dioxide and other wastes are carried from the tissues to sites of removal.  Hormones are also transported by the circulatory system. 

            The closed circulatory system of vertebrates consists of the heart, arteries, veins, capillaries and blood.  Arteries carry blood away from the heart to the sites of exchange in the tissues, in the capillaries.  Veins carry blood from tissue capillaries back to the heart. 

            The circulatory system of the perch is a typical low pressure single type system in which the heart is a single pump and there is a single circuit of blood flow.  Venous (deoxygenated) blood from the body is pumped through the heart forward to the gills.  From the gills, where it is oxygenated, the blood goes directly to the body.  Thus the blood makes a single circuit during which it is pumped, oxygenated, and distributed to the body, before it returns to the heart.  In this pattern of circulation the heart pumps only deoxygenated blood.

            The circulatory system of the rat typifies the high-pressure double circulatory system of mammals in which the heart is a double pump and there are two circuits of blood flow, systemic and pulmonary.  In this type of system, blood is pumped through the heart twice.  Deoxygenated blood from the body is pumped to the lungs where it is oxygenated (pulmonary circulation).  Oxygenated blood from the lungs returns to the heart whereupon it is pumped to the rest of the body (systemic circulation).  Thus the heart pumps both oxygenated and deoxygenated blood.

          Perch Circulatory System

 The Heart

            To expose the pericardial (heart) cavity extend the ventral incision approximately 2 cm anterior to the pelvic fins.  Extend this incision dorsally under the gills to the lateral line.  The pericardial membrane should adhere to this flap of body wall you have cut.  Gently free the membrane as you raise the cut flap.

            You will notice a heavy membrane separating the pericardial and abdominal cavities.  It is termed the transverse septum.  The fish heart consists of four distinct parts.  Posterior to anterior (in the same direction as blood flow) these are the sinus venosus, atrium, ventricle and bulbus arteriosus (Figure 17, 18). 

             

Figure 17: Heart of the perch (ventral view).

Figure 18:  Heart of the Perch  (right lateral view).

            These parts are not arranged in a straight line, but have folded over one another to produce a S-shaped organ.  The top of the S, the sinus venosus, receives blood from two common cardinal veins and the hepatic sinus.  The sinus venosus is thin walled and opens directly into the atrium.  The atrium is equivalent to the paired atria of higher vertebrates.  It is thicker walled and larger than the sinus venosus.  The ventricle itself is a thick muscular structure.  Like the atrium, it has a single internal chamber.  Backflow of blood from the ventricle during contraction is prevented by a valve.  The last portion of the perch heart is the bulbus arteriosus.  The bulbus arteriosus is really an enlarged, very muscular portion of the ventral aorta, the vessel in which blood flows away from the heart and toward the gills.  Backflow of blood from the bulbus arteriosus is also prevented by the presence of valves.  Figure 19 traces the flow of blood through the perch heart.

Figure 19: Longitudinal section through the heart of the perch (Lateral view). Arrows indicate the direction of  the blood flow.