• A Thin double walled sac that surrounds the heart and contains the roots of the great vessels it characterized by two layers.
  • First layer is an outer fibrous pericardium which anchors the heart to the surrounding structures.
  • Second layer is the inner serous pericardium, consisting of an outer parietal layer and inner visceral layer.

Heart wall

Chambers:

• Atria (Atrium)
  
  • Thin-walled chambers located superiorly
  • The anterior part of each atrium is a wrinkled, flap-like extension called the auricle
  • Separated from ventricles externally by deep coronary sulcus that extends around the heart.
    • All sulci house blood vessels that supply the heart
      
  • Receives blood returning to the heart from both the circulatory circuits
    • Right atrium receives blood from the systemic circuit
    • Left atrium receives blood from the pulmonary circuit
    
  This picture shows not only the basic structure of the inside of the heart but also blood flow through the heart.
  

• Blood passing through the right atrium enters the right ventricle, which are the inferior chambers
• 
  Pulmonary Trunk is one of the two large arteries. It carries the right ventricle into the pulmonary circuit
• 
  Aorta the second large artery. Conducts blood from the left ventricle into the systemic circuit
  

• Pulmonary circuit- the heart pumps blood to the lungs, then back to the heart. From this pathway, blood picks up oxygen and drops off carbon dioxide in the lungs.
  • Put simply, blood is oxygenated in the lungs.
  • One pair of chambers, the right atrium and right ventricle, is directly connected to the pulmonary circuit. In this circuit, deoxygenated blood from the body enters the heart through the rightatrium. That chamber then pushes the deoxygenated blood on to the right ventricle, which then pumps that blood to the lungs. After the blood has passed through the lungs, it is oxygenated, full of oxygen.
• Systemic circuit- the longer of the two, the heart pumps oxygenated blood through the rest of your body. As the blood passes through organ after organ, tissue after tissue, blood drops off oxygen, and picks up carbon dioxide.
  • the heart must pump the oxygenated blood to your organs, tissues, and bones. The blood from the lungs reenters the heart through the left atrium. The left atrium them pushes blood into the left ventricle, which in turn pumps the oxygenated blood throughout your body. Notice that blood always enters the heart through an atrium, and always leaves the heart through a ventricle.

Right atriventricular valve	Between right atrium and right ventricle	Prevents back flow of blood into right atrium when ventricles contract
Pulmonary semilunar valve	Between right ventricle and pulmonary trunk	Prevents back flow of blood into right ventricle upon ventricle relaxation
Left atriventricular valve	Between left atrium and left ventricle	Prevents back flow into left atrium upon ventricle contraction
Aortic semilunar valve	Between left ventricle and ascending aorta	Prevents back flow of blood into left ventricle upon ventricle relaxation

Fibrous Skeleton

How does the heart beat?

The atria and ventricles work together, alternately contracting and relaxing to pump blood through your heart. The electrical system of your heart is the power source that makes this possible.

The heart's electrical system

Yourheartbeatistriggeredbyelectricalimpulsesthattraveldownaspecialpathwaythroughyourheart:

1. SAnode(sinoatrialnode)–knownastheheart’snaturalpacemaker
   Theimpulsestartsinasmallbundleofspecializedcellslocatedintherightatrium,calledtheSAnode.Theelectricalactivityspreadsthroughthewallsoftheatriaandcausesthemtocontract.Thisforces bloodintotheventricles.
   TheSAnodesetstherateandrhythmofyourheartbeat.NormalheartrhythmisoftencallednormalsinusrhythmbecausetheSA(sinus)nodefiresregularly.
2. AVnode(atrioventricularnode)
   TheAVnodeisaclusterofcellsinthecenteroftheheartbetweentheatriaandventricles,andactslikeagatethatslowstheelectricalsignalbeforeitenterstheventricles.Thisdelaygivestheatriatimeto contractbeforetheventriclesdo.
3. His-PurkinjeNetwork
   Thispathwayoffiberssendstheimpulsetothemuscularwallsoftheventriclesandcausesthemtocontract.Thisforcesbloodoutofthehearttothelungsandbody.

TheSAnodefiresanotherimpulseandthecyclebeginsagain.

Atrest,anormalheartbeatsaround50to99timesaminute.Exercise,emotions,feverandsomemedicationscancauseyourhearttobeatfaster,sometimestowellover100beatsperminute.How fast does the normal heart beat?Howfasttheheartbeatsdependsonthebody'sneedforoxygen-richblood.Atrest,theSAnodecausesyourhearttobeatabout50to100timeseachminute.Duringactivityorexcitement,yourbodyneedsmoreoxygen-richblood;theheartraterisestowellover100beatsperminute.Medicationsandsomemedicalconditionsmayaffecthowfastyourheart-rateisatrestandwithexercise.How do you know how fast your heart is beating?Youcantellhowfastyourheartisbeating(yourheartrate)byfeelingyourpulse.Yourheart-rateistheamountoftimesyourheartbeatsinoneminute.Youwillneedawatchwithasecondhand.
Placeyourindexandmiddlefingerofyourhandontheinnerwristoftheotherarm,justbelowthebaseofthethumb.Youshouldfeelatappingorpulsingagainstyourfingers.Countthenumberoftapsyoufeelin10seconds.Multiplythatnumberby6tofindoutyourheart-rateforoneminute:Pulsein10secondsx6=____beatsperminute(yourheart-rate)Whenfeelingyourpulse,youcanalsotellifyourheartrhythmisregularornot.