The respiratory system is extremely varied among the 25,000 species of fishes. This, once again, reflects the wide range of habitats to which fish have managed to adapt. The primary function of the respiratory system is to transport gases between the environment and the tissues of the body that consume oxygen as well as excrete ammonia and carbon dioxide. The vital components in this system are the water flow over the gills outside the fish and the circulatory system inside the body. The consumption of oxygen (02) as well as the excretion of carbon dioxide (C02) and ammonia (NH3 and NH4+) take place across the gill.
The fish gill has four gill arches on each side of the midline and two rows of primary or gill filaments per arch (Fig. 6). Arranged perpendicularly to the filament are rows of secondary lamellae on both side of each filament. The plates of secondary lamellae form narrow channels through which the water flows. This space is approximately 0.020.05 mm wide, 0.2-1.6 mm long, and 0.1-0.5 mm high. This width is particularly important in that one-half of that width is the minimum distance for gases and dissolved materials such as ions to diffuse between water and blood. The surface of the secondary lamellae is the primary surface across which gases, ions, and other dissolved materials pass between water and blood.
The water flow over the gills is directed in an anterior to posterior direction. More importantly, it flows in the opposite direction to the flow of blood through the secondary lamellae in the gills. This countercurrent flow maintains the maximum gradients between blood and water for the gases throughout the transit of water through the gills. The mouth or buccal pump is driven by skeletal muscles that control the mouth of the floor and opercular covers.
Water, therefore, flows from mouth, over gills, and out the operculum. When the floor of the mouth is lowered, negative pressure is created in the buccal cavity and water is sucked into the mouth. As the mouth is closed and the floor of the buccal cavity raised, the water is forced posteriorly, across the gills, through the opercular cavity. This cycle is repeated continuously and the result is a unidirectional flow of water through the mouth and gills.
While this rhythmic ventilation is the general rule, some fish are ram ventilators, that is, they ventilate the gills by keeping their mouths open and swimming forward through the water. Salmonids also do this at moderate to high swimming velocities. There are also other variations on how fish move water across the respiratory surface of the gill for respiration. Some of these variations such as ram ventilation may reduce the energetic costs of ventilation.
Blood is a suspension of various cells in a solution of proteins and electrolytes, which make up the plasma. It is the vehicle for transporting materials from different locations in the body, as discussed above. The volume of blood in teleosts ranges between 3 and 6% of wet body weight. Fish do not have bone marrow and the origin of the blood cells is the anterior kidney (anterior 30%) and the spleen. These are termed hematopoietic tissues. The production of blood cells is stimulated by conditions such as bleeding and inhibited by starvation. There is an erythropoesis stimulating factor, found in the plasma of fish that stimulates blood cell production. Very little is found in plasma of resting fish.
White blood cells or leukocytes, are involved in the clotting of blood and play an important part in the immune system. The red blood cell, or erythrocyte is important in the transport of oxygen in the body. The packed red cell volume, or hematocrit, is expressed in percent and varies between 20 and 35%. The erythrocyte contains the respiratory pigment, hemoglobin (Hb), which binds oxygen and enables the oxygen content of the blood to be higher than that of simple dissolved oxygen. The concentration of Hb in blood is approximately 7-12 g % (equal to grams/100 mL). The total oxygen content is about 9-14 vol %.
The proteins in the plasma serve to maintain the blood osmolarity. It is the source of colloid osmotic pressure. Some other functions are to buffer pH changes in the blood and to transport vitamins, hormones, and inorganic ions, that attach to these molecules in blood, to tissues. Another important function of special plasma proteins are the antibody proteins that play an important role in the specific immune system of the body. In carp, for example 4.15% of the blood weight are protein. Of that weight, 2.82 g is albumin, 0.79 g is globulin, and 0.23 g is fibrinogen.
Fish respond to environmental changes in different ways. Some fish are known as oxygen regulators because they maintain 02 consumption at lower 02 levels by increasing ventilation. Oxygen conformers, on the other hand, adjust 02 consumption according to ambient 02 levels. Salmonids are regulators. The following are some effects of environmental changes on the respiratory system and the response of fish.
Temperature increase presents a three-pronged problem for respiration. It increases the 02 demand with increased metabolism, lowers the solubility of 02 in water, and lowers the affinity of 02 to the haemoglobin molecule. Both heart rate and ventilation increase in response to an increase in temperature. The resistance of the peripheral circulation also decreases.
Hypoxia, or lowered environmental 02, causes an increase in the frequency and amplitude of ventilation in most fish. While there is a decrease in heart rate, there is an increase in the volume of each stroke of the heart (stroke volume), thus maintaining the output of blood from the heart. This results in a large increase in the ratio of water pumped over the gills to the blood perfused through the gills (ventilation-to-perfusion ratio). Oxygen uptake is, therefore, maintained in the face of lowered water 02 concentration.
Exercise increases cardiac output due to an increase in stroke volume. Ventilation increases and there is a fourfold to eightfold increase in 02 uptake. The ventilation-to-perfusion ratio remains relatively constant. Acidic conditions generally increase ventilation.
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