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SHARKS IN GENERAL


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Anatomy and Physiology

Skeleton

The skeleton of elasmobranchs is made of cartilage. It may be partially calcified to some extent with calcium phosphates and carbonates, particularly in the vertebral column. A cranium is a single compact cartilaginous block which encloses the brain, olfactory, and auditory capsules, and jaws loosely attached to it. There is no support from the vestigial ribs.

Muscles

There are two kinds of muscles - Red and White. Red muscle is used for cruising. It needs oxygen to function (aerobic) and therefore contains an oxygen-carrying pigment (myoglobin). White muscle is used for sudden bursts of speed. It does not use oxygen (anaerobic).

Digestive tract

Uper portion of the digestive tract is esophagus, which is short and wide and barely discernible from the stomach. Many species have a spiral valve after an U-shaped stomach. Lower portion of the digestive tract is a spiral valve. In order to increase the surface area and thus increases nutrient absorptionis spiral valve is internally twisted or coiled. End of the digestive tract is comprised of rectum and cloaca. The cloaca is a common opening for the urinary, digestive, and reproductive systems.

Circulatory system

A shark's heart is a two-chambered S-shaped tube, small in proportion to body size. Blood flows from the heart to the gills and then to body tissues. Due to the modified circulatory system associated with the red muscle, fast-swimming sharks (e.g. Great Whites, Makos, etc,) have a body temperature that can be higher than the surrounding water  (even 8°C or 14.4°F higher).

Red muscle generates the heat by working and warms the blood circulating through it. Blood returning trough veins from the muscle to the heart is warmer than blood traveling from the heart to the muscle trough arteries. Due to the nearness of arteries and veins, heat passes from warmer veins to cooler arteries within the shark's body, rather than dissipating to the cooler environment. This modified circulatory system retains heat in the red muscle.

Sharks have low blood pressure. The walls of the membranous sac that encloses the heart (pericardium) are rigid, creating a suction within the pericardium to maintain the flow of blood. To circulate blood throughout their bodies, many sharks must swim continuously.

Gills and respiration

It was assumed that all sharks must swim to move water into their mouth and over their gills to respire, but sharks can respire by pumping water over their gills by opening and closing their mouths. It is true that many sharks do have to swim continuously, but it is because of their low blood pressure, as muscular contractions are needed to circulate their blood.

Respiration is effected as follows:

Water enters the gill chambers through the mouth or spiracles. Blood in the gill filaments absorbs oxygen from the incoming water. Gill rakers, cartilaginous projections on the gill support structure, protect the delicate gill filaments from particles in the water that might damage them. Water exits through the gill slits. In species where they are present, spiracles supply oxygenated blood directly to the brain and eyes, through a separate blood vessel which is reduced or absent in active, fast-swimming sharks.

Liver

A shark's liver store energy and helps buyoyancy. Its relatively large, making up 5% to 25% of its total body weight. It is made of 2 large lobes that concentrate and store fatty acids and oils.


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