What Is A Shark?

WHAT IS A SHARK?
Many people think that the sharks are dangerous creatures, with their sharped teeth, with their fixed eyes and with their deadly look. In fact they are great hunters, but almost does not offer danger to human being. There are currently more than 375 species of sharks known by human, each one with its different, they can measure between 20cm of length up to 12m of length, however the majority only reaches up to 1m.
There are more than 375 species of sharks, all divided in 8 groups, this division depends on the presence or absence of certain internal or external characteristics, such as for example, format of intestinal valve, anal fin, spines on fins, etc. Not always it is possible to determine in which group a shark can be. These classifications may change with the discovery of a new species or new features, revealing new relations of kinship.
SHARK'S TEETH:
Thousands of teeth: The sharks are never without teeth. When the teeth of the front row start to crack or become worn, fall out to be replaced by new row, some sharks exchange 1 or 2 teeth, however other sharks exchange the complete row.

(Cut side of a jaw) As a treadmill the new teeth move forward making the old fall, so they can take the old one place.

SHARK'S SKELETAL SYSTEM:

Obs:Because of its rigid fins, sharks can not swim backwards or stop suddenly.

Unlike other fish, sharks have no bones in their body. They belong to a group called Elasmobranchs. The Elasmobranchs skeleton is made ​​of cartilage. The Elasmobranchs are also called Cartilaginous Fish.Curiosity: Chondroitin, a component of shark cartilage, has an important use in medicine.

The skull, jaws, gill arches and spine are made of a  firm cartilage, but flexible. The upper jaw is hanging in the skull. This system makes that the shark design your jaw forward so they can give big and powerful bites in it's prey.

CARDIOPULMONARY SYSTEM:

The shark breathes oxygen. However, that oxygen is dissolved in the sea water. Delicate organs called gills remove the oxygen molecules from the water and take them to the bloodstream, where the heart pumps in to a network of arteries and capillaries, to the cells, which use that oxygen to turn into nutrients and energy.

1) Efferent spiracula artery 2) Stapedial artery 3) Afferent spiracular artery 4) Afferent branchial artery 5) Ventral aorta 6) Efferent branchial artery 7) Dorsal aorta 8) Testicular artery 9) Hepatic artery 10) Celiac artery 11) Internal carotid 12) Hyoidean artery 13) Paired dorsal aorta 14) Conus arteriosus 15) Coronary artery 16) Ventricle 17) Atrium 18) Sinus venosus 19) Transverse septum 20) Subclavian artery 21) Brachial artery 22) Ventrolateral artery 23) Gastric artery 24) Duodenal artery 25) Pyloric artery 26) Anterior intestinal artery 27) Anterior mesenteric artery 28) Gastrosplenic artery 29) Posterior mesenteric artery 30) Inferior jugular vein 31) Anterior cardinal sinus 

32)Subclavian vein 33) Hepatic portal vein 34) Brachial vein 35) Posterior cardinal sinus 36) Hepatic vein 37) Posterior cardinal vein 38) Lateral abdominal vein 39) Renal portal vein 40) Cloacal vein 41) Iliac vein.