Cardiovascular System

The cardiovascular system moves the blood around the body. It is basically composed of the heart (cardio-) and network of blood vessels (-vascular) throughout the body. The purpose of blood circulation is the transport of materials between certain locations within the body. Gases such as oxygen are taken up from the water and move from gills to the tissues and others such as carbon dioxide move from tissues back to the gills for excretion. End products of metabolism such as lactate, originate in the muscles and are transported to the liver for breakdown. Glucose made in the liver must be moved to the tissues where it will be used. Materials that are foreign to the body are excreted by gills, released in the urine, or engulfed and destroyed by specialized cells in the blood. Amino acids and other nutrients from digested materials must move from gut to the tissues. Blood cells that are produced in the anterior kidney and spleen must be distributed throughout the body. Because of the vast range of environments in which fish are found, the diversity in the form and function of the cardiovascular systems are as wide ranging. The basic conformity among salmonid fishes are emphasized here.


The teleost heart has four chambers: the sinus venosus, the atrium, the ventricle, and the bulbus (Fig. 4). The venous blood from the body is received in the sinus venosus. This is a rather flat bag anterior to the transverse septum separating the heart and head compartments from the visceral cavity. The atrium is a thin-walled bag that feeds the blood to the muscular ventricle. The ventricle is the powerhouse that contracts and propels the blood throughout the body. The bulbus is rather muscular as well and helps to dampen pressure waves as the blood pulses out of the ven-

Sinus (^""venosus


Figure 4. Cross section of a trout heart. Source: Ref. 1.

tricle. There are one-way valves that prevent the blood from flowing backward with each contraction. Cardiac output in teleosts ranges from 5 to 100 mL/kg per minute with a general mean about 15-30 mL/kg per minute.

Blood flow is accomplished primarily by the heart, which is a pump for the system. The basic route is from the heart to the gills, where gas and ionic exchange with the environment occurs, and then to the rest of the body (Fig. 5). The blood is then pumped back to the heart through a venous network where the cycle begins over again. About 4-6% of body weight is a reasonable approximation of the blood volume. Blood pressures in teleosts are in the range of 30-70 mm Hg. Because the resistance to blood flow increases as the diameter of the blood vessels decreases, blood flow declines from the point where it leaves the heart. For example, the blood vessel network in the gills reduces the pressure found in the ventral aorta by about 40-50%. That resistance may increase with stresses such as low oxygen levels in the water, whereas other conditions such as exercise or elevated levels of adrenaline dilate the blood vessels and decrease the resistance to blood flow.

Figure 5. General flow of blood in fish.

Figure 4. Cross section of a trout heart. Source: Ref. 1.

The Secondary Circulation

The primary circulation in fish comprises the blood vessels that contain red and white blood cells. There is a secondary circulation, sometimes known as the lymphatic or veno-lymphatic system, that is characterized by low blood cell content and by low pressure. There are direct connections between the primary and secondary circulation through small vessels, often at right angles to the blood vessel of the primary circulation. This physical configuration causes plasma to be skimmed out of the primary circulation and into the secondary circulation. The secondary circulation feeds this fluid, low in cells as a result of the skimming, and fluid from tissue beds back to the primary circulation Aria the heart. Salmonids have major ducts just under each lateral line and along the dorsal midline. These may serve to collect fluid from the secondary circulation and return it to the heart. Fin and trunk movements may play an important role in pumping venous fluids back to the heart. There is a tail heart in some fishes that aids the movement of this secondary fluid back to the heart.

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