Fish have single circuit circulation. That is, blood leaves the heart, travels throughout the body, and then returns to the heart. Mammals (and reptiles and birds) have dual circuit circulation: blood is pumped by the heart to the lungs and is oxygenated and then returns to the heart to be pumped again to the other tissues. The two circuit (mammalian) model maintains higher blood pressure throughout the circulatory system. The only place that mammals have two capillary beds (high resistance areas that reduce pressure) in series is in the hepatic portal system where arterial blood in the capillaries surrounding the gut picks up nutrients and coalesces in the hepatic vein, breaking back up into sinusoids (analogous to capillaries) in the liver that processes the nutrients before the blood flows again into larger veins that return it to the heart. In fish, the lacunar space of the lamellae is a high resistance system, analogous to a capillary bed. By the time the blood coalesces in the dorsal aorta, blood pressure is reduced. It must then flow through the capillary beds of the tissues. Fish blood always has to go through two high resistance areas, i.e. it always goes through the gills and the systemic circulation. Moreover, fish have two portal systems: a renal portal and a hepatic portal. The fish kidney, unlike the mammalian kidney which is entirely supplied by the arterial system, is largely fed from the venous system. So, fish blood must always pass through two, and sometimes more, high resistance areas during circulation. The consequence is relatively low blood pressure compared to mammals.
Mammals are subject to gravity, while the effect of gravity on fish is essentially zero. Gravity requires that mammals expend more energy when stationary or during slow movement than fish and the blood itself must be pumped upwards against gravity, as well. The regulation of body temperature in mammals also requires a higher metabolic rate compared to poikilothermic fish. These two differences mean that a high pressure, rapid circulation is necessary in homeotherms to replenish the cells with oxygen and nutrients. Homeotherms have lower affinity oxygen dissociation curves than fish for the same reason. On the other hand, fish have lower metabolic rates than homeotherms and, in some cases, they must, because the partial pressure of oxygen in the aquatic environment is often less than air (see Ch. II), and with less available oxygen fish cannot maintain as high a metabolic rate.
While circulation patterns vary widely in the fishes, the following is the general pattern in teleosts. Unoxygenated blood flows from the bulbous arteriosus to the ventral aorta and branches off into the four afferent branchial arteries on each side of the fish's head. Blood becomes oxygenated at the gill and collects in the four efferent branchial arteries on each side of the fish, which then coalesce into the dorsal aorta. Left and right carotid arteries run forward to the front of the head. The dorsal aorta runs back through the hemal arch of the spine to the tail with several major arterial branches. The paired subclavian arteries feed the region around the pelvic girdle. The next major branch is the coeliacomesenteric artery that supplies the gut, spleen, and gonads. It is also the beginning of the hepatic portal system. The renal artery supplies the kidney with arterial blood. The dorsal aorta runs on to the tail, with branches at each myotome.
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The blood collects from the myotomes and flows forward from the tail in the caudal vein. The caudal vein splits with one branch breaking into renal capillaries that comprise the renal portal system. The other branch, the hepatic portal vein, collects blood from the gut, spleen, and gonad capillaries before breaking into liver sinusoids and coalescing again into the hepatic vein that returns blood to the sinus venosus of the heart. The dorsal and anterior part of the fish is drained by a system of paired veins, the cardinals. The left and right postcardinal veins drain the kidney and flow forward, joining the precardinals to form the common cardinals that feed into the sinus venosus. The precardinals along with jugular veins drain the head and the subclavian veins drain the pectoral area. Venous return in fish, like in mammals, is aided by muscular contraction, especially so in fish since they have such a low pressure system. The veins in fish also have valves, like in mammals, to prevent backflow of blood. Some fish have accessory hearts (valved sacs) in the caudal region that are weakly contractile to aid venous return.
In lungfish, there is a pulmonary vascular circuit. Blood returning from the body flows into the sinus venosus and then into a divided atrium. Blood returning from a separate circuit to the lung flows into the other side of the atrium. The ventricle is partially divided and only partial mixing of the oxygenated and unoxygenated blood occurs. A special bulbous further maintains this separation, sending the oxygenated blood into tissue circulation and unoxygenated blood to the gills. The pulmonary artery takes off of one of the efferent branchial arteries, sending the blood to the lung and then the pulmonary vein takes it back to the heart.
Lampreys have an unusual circulatory trait that is unique among vertebrates: the artery that feeds the gut runs inside the vein that drains the gut. Also, lampreys lack a hepatic portal system.
Muscle in fish is segregated into poorly vascularized white muscle and richly vascularized red muscle. A skinned fish shows a large bulk of the muscle to be white, while a stripe of red muscle runs along the lateral line. Red muscle is highly vascularized because it is the muscle that is used for continuous swimming, the white muscle is used only in bursts of activity. Red muscle relies on the efficient, aerobic respiration of oils to generate ATP, while white muscle uses much less efficient anaerobic fermentation of glycogen to create ATP. Glycogen has about half the calories per gram of oils and anaerobic glycolysis harvests only a tiny fraction of that energy. The rest is tied up in lactic acid (lactate) which can only be further metabolized aerobically. Since white muscle is poorly vascularized the lactate is oxidized very slowly. While, white muscle is an inefficient producer of mechanical energy from food, it does provide a mechanism for fish to be highly active for brief periods despite a low-pressure circulatory system.
There is a strong correlation between routine swimming activity and amount of red muscle in fish species. Sedentary fishes have very little red muscle while fish that swim continually have much more, though all fish have more white than red. Constant fast swimmers such as tuna have a large mass of red muscle, moreover, a countercurrent system of blood vessels feeding the red muscle conserves the heat generated by muscle contractions and allows the muscle to reach a temperature much higher than the surrounding ocean water, allowing for a faster rate of metabolism. Since red muscle is oily and oils retain a "fishy" flavor, red muscle is routinely discarded when fish is prepared for food. For example, tuna red muscle is used in canned cat food and the white in canned tuna for human consumption.
Like other vertebrates, fish have red (erythrocytes) and white (leukocytes) blood cells. Unlike mammals, but like birds and reptiles, fish erythrocytes retain their nucleus. While the density of erythrocytes in fish blood varies greatly among species, a typical hematocrit in fish is about 30%, much less than the 50% usually found in mammals. Fewer red blood cells are needed in fish because they have a lower metabolism and thus need to move less oxygen. In very active fish, like tuna, the hematocrit approaches 50%. Like other vertebrates the leukocytes in fish have a variety of types which function in different ways from immune reaction to clotting.
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