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The second form of NST occurs in skeletal muscle. While eutherians use both BAT and skeletal muscle NST for thermogenesis, birds only use the latter form. This process has also been shown to occur in rare instances in fish. [17] In skeletal muscle NST, Calcium ions slip across muscle cells to generate heat. [17]
The body of a fish is denser than water, so fish must compensate for the difference or they will sink. Many bony fishes have an internal organ called a swim bladder, or gas bladder, that adjusts their buoyancy through manipulation of gases. In this way, fish can stay at the current water depth, or ascend or descend without having to waste ...
Muscle contraction is one such metabolic process generating heat energy, [6] and additional heat results from friction as blood circulates through the vascular system. All organisms metabolize food and other inputs, but some make better use of the output than others.
Muscles can also receive messages from the thermoregulatory center of the brain (the hypothalamus) to cause shivering. This increases heat production as respiration is an exothermic reaction in muscle cells. Shivering is more effective than exercise at producing heat because the animal (includes humans) remains still.
The resting human body generates about two-thirds of its heat through metabolism in internal organs in the thorax and abdomen, as well as in the brain. The brain generates about 16% of the total heat produced by the body. [8] Heat loss is a major threat to smaller creatures, as they have a larger ratio of surface area to volume.
In some fish, capillary blood flows in the opposite direction to the water, causing countercurrent exchange. The muscles on the sides of the pharynx push the oxygen-depleted water out the gill openings. In bony fish, the pumping of oxygen-poor water is aided by a bone that surrounds the gills called the operculum. [6]
A fish's hypoxia tolerance can be represented in different ways. A commonly used representation is the critical O 2 tension (P crit), which is the lowest water O 2 tension (P O 2) at which a fish can maintain a stable O 2 consumption rate (M O 2). [2] A fish with a lower P crit is therefore thought to be more hypoxia-tolerant than a fish with a ...
Stage three, the rest phase, cause the fish to return to normal steady-state swimming and the body undulations begin to cease. Large muscles located closer to the central portion of the fish are stronger and generate more force than the muscles in the tail. This asymmetry in muscle composition causes body undulations that occur in Stage 3.