Question

Briefly describe respiration in fishes. What adaptive mechanism increases efficiency? Diagram may help What is meant by "countercurrent flow" as it applies to fish gills? Compare the osmotic problem and mechanism of osmotic regulation in freshwater and marine bony fishes. Again diagrams may help.

Answer 1

Respiration in fish or in that of any organism that lives in the water is very different from that of human beings. Organisms like fish, which live in water, need oxygen to breathe so that their cells can maintain their living state. To perform their respiratory function, fish have specialized organs that help them inhale oxygen dissolved in water.

Respiration in fish takes with the help of gills. Most fish possess gills on either side of their head. Gills are tissues made up of feathery structures called gill filaments that provide a large surface area for gas exchange. A large surface area is crucial for gas exchange in aquatic organisms as water contains very little amount of dissolved oxygen. The filaments in fish gills are arranged in rows in the gill arch. Each filament contains lamellae, which are discs supplied with capillaries. Blood enters and leaves the gills through these small blood vessels. Although gills in fish occupy only a small section of their body, the immense respiratory surface created by the filaments provides the whole organism with an efficient gas exchange.

Fish take in oxygen-rich water through their mouths and pump it over their gills. As water passes over the gill filaments, blood inside the capillary network picks up the dissolved oxygen. The circulatory system then transports the oxygen to all body tissues and ultimately to the cells. While picking up carbon dioxide, which is removed from the body through the gills. After the water flows through the gills, it exits the body of the fish through the openings in the sides of the throat or through the operculum, a flap, usually found in bony fish, that covers and protects the fish gills.

Adaptive Mechanism =While a large surface area combined with short diffusion distances make fish gills well suited for gas exchange, these properties leads to costly water and ion fluxes and exposure to toxic substances and pathogens. Thus, gill morphology is likely to be a compromise between opposing demands. It has become clear that some fishes have the ability to modify gill structure in response to environmental parameters such as oxygen levels and temperature. Maybe the most dramatic example of gill plasticity is the adaptive and reversible changes in gill surface area displayed by crucian carp (Carassius carassius) and goldfish

COUNTER CURRENT FLOW =

The rows of gill filaments have many protrusions called gill lamellae. The folds are kept supported and moist by the water that is continually pumped through the mouth and over the gills..Fish also have an efficient transport system within the lamellae which maintains the concentration gradient across the lamellae.....The arrangement of water flowing past the gills in the opposite direction to the blood (called countercurrent flow) means that they can extract oxygen at 3 times the rate a human can.(Figure 1)

To understand countercurrent flow, it is easiest to start by looking at concurrent flow where water and blood flow over and through the lamellae in the same direction.When the blood first comes close to the water, the water is fully saturated with oxygen and the blood has very little.There is therefore a very large concentration gradient and oxygen diffuses out of the water and into the blood.As you move along the lamella, the water is slightly less saturated and blood slightly more but the water still has more oxygen in it so it diffuses from water to blood.This continues until the water and the blood have reached equal saturation....After this the blood can pick up no more oxygen from the water because there is no more concentration gradient. The maximum saturation of the water is 100% so the maximum saturation of the blood is 50%.(Figure 2)

As the blood flows in the opposite direction to the water, it always flows next to water that has given up less of its oxygen...This way, the blood is absorbing more and more oxygen as it moves along. Even as the blood reaches the end of the lamella and is 80% or so saturated with oxygen, it is flowing past water which is at the beginning of the lamella and is 90 or 100% saturated....Therefore, even when the blood is highly saturated, having flowed past most of the length of the lamellae, there is still a concentration gradient and it can continue to absorb oxygen from the water.(Figure 3)

2) The osmotic problem in freshwater fishes is the gain of water and loss of salt across thin membranes of the gills because external water has far less salt concentration than internal water. Freshwater fishes (hyperosmotic regulators) have several defenses: 1. Opisthonephric kidney pumps out excess water by forming a very dilute urine. 2. Salt absorbing cells in the gill epithelium move salt ions. This and salt in the food counters diffusive salt loss. Marine fishes (hyposmotic regulators) actually have lower internal salt concentrations than external so they tend to gain salt and lose water. This puts them at risk to dry out. 1. Drink sea water and dispose of the extra salt in two ways: Major sea salt ions (Na, Cl, K) are carried by the blood to the gills where the salts are secreted by salt secretory cells. The remaining ions (Mg, S, Ca) leave the body through feces or kidney excretion.

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