Heart
<p >The <b>heart</b> is a <a href="http://en.wikipedia.org/wiki/Myogenic_contraction" title="Myogenic contraction" >myogenic</a> <a href="http://en.wikipedia.org/wiki/Muscle" title="Muscle" >muscular</a> <a href="http://en.wikipedia.org/wiki/Organ_(anatomy)" title="Organ (anatomy)" >organ</a> found in all <a href="http://en.wikipedia.org/wiki/Animal" title="Animal" >animals</a> with a <a href="http://en.wikipedia.org/wiki/Circulatory_system" title="Circulatory system" >circulatory system</a> (including all <a href="http://en.wikipedia.org/wiki/Vertebrate" title="Vertebrate" >vertebrates</a>), which pumps <a href="http://en.wikipedia.org/wiki/Blood" title="Blood" >blood</a> throughout the <a href="http://en.wikipedia.org/wiki/Blood_vessel" title="Blood vessel" >blood vessels</a> by repeated, rhythmic contractions. The term <i><b>cardiac</b></i> (as in <a href="http://en.wikipedia.org/wiki/Cardiology" title="Cardiology" >cardiology</a>) means "related to the heart" and comes from the <a href="http://en.wikipedia.org/wiki/Greek_language" title="Greek language" >Greek</a> καρδιά, <i>kardia</i>, for "heart".</p>
<p>
The vertebrate heart is principally composed of <a href="http://en.wikipedia.org/wiki/Cardiac_muscle" title="Cardiac muscle" >cardiac muscle</a> and <a href="http://en.wikipedia.org/wiki/Connective_tissue" title="Connective tissue" >connective tissue</a>. Cardiac muscle is an involuntary striated muscle tissue found only in this organ and responsible for the ability of the heart to pump blood. The average<a href="http://en.wikipedia.org/wiki/Human_heart" title="Human heart" >human heart</a>, beating at 72 beats per minute, will beat approximately 2.5 billion times during an average 66 year lifespan. It weighs approximately 250 to 300 grams (9 to 11 oz) in females and 300 to 350 grams (11 to 12 oz) in males.<sup id="cite_ref-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Heart#cite_note-0" >[1]</a></sup></p>
<p>
In <a href="http://en.wikipedia.org/wiki/Invertebrate" title="Invertebrate" >invertebrates</a> that possess a circulatory system, the heart is typically a tube or small sac and pumps fluid that
contains water and nutrients such as <a href="http://en.wikipedia.org/wiki/Protein" title="Protein" >proteins</a>, <a href="http://en.wikipedia.org/wiki/Fat" title="Fat" >fats</a>, and <a href="http://en.wikipedia.org/wiki/Sugar" title="Sugar" >sugars</a>. </p>
<p>
In <a href="http://en.wikipedia.org/wiki/Insect" title="Insect" >insects</a>, the "heart" is often called the <b>dorsal tube </b>and insect "blood" is almost always not oxygenated since they usually <a href="http://en.wikipedia.org/wiki/Respiration_(physiology)" title="Respiration (physiology)" >respirate</a> (breathe) directly from their body surfaces (internal and external) to air. </p>
<p>
However, the hearts of some other <a href="http://en.wikipedia.org/wiki/Arthropod" title="Arthropod" >arthropods</a> (including <a href="http://en.wikipedia.org/wiki/Spiders" title="Spiders" class="mw-redirect" >spiders</a> and <a href="http://en.wikipedia.org/wiki/Crustacean" title="Crustacean" >crustaceans</a>such as <a href="http://en.wikipedia.org/wiki/Crab" title="Crab" >crabs</a> and <a href="http://en.wikipedia.org/wiki/Shrimp" title="Shrimp" >shrimp</a>) and some other animals pump <a href="http://en.wikipedia.org/wiki/Hemolymph" title="Hemolymph" >hemolymph</a>, which contains the <a href="http://en.wikipedia.org/wiki/Copper" title="Copper" >copper</a>-based protein<a href="http://en.wikipedia.org/wiki/Hemocyanin" title="Hemocyanin" >hemocyanin</a> as an oxygen transporter similar to the <a href="http://en.wikipedia.org/wiki/Iron" title="Iron" >iron</a>-based <a href="http://en.wikipedia.org/wiki/Hemoglobin" title="Hemoglobin" >hemoglobin</a> in <a href="http://en.wikipedia.org/wiki/Red_blood_cells" title="Red blood cells" class="mw-redirect" >red blood cells</a> found in <a href="http://en.wikipedia.org/wiki/Vertebrates" title="Vertebrates" class="mw-redirect" >vertebrates</a>.</p>