How the heart works

Electrical Conduction System

• The heart is composed primarily of muscle tissue. A network of nerve fibers coordinates the contraction and relaxation of the cardiac muscle tissue to obtain an efficient, wave-like pumping action of the heart.

The sinus node (1) is located high in the heart's right upper chamber (right atrium) and it sends out an electrical impulse - about 60 times a minute when you are at rest.

The impulse quickly spreads throughout the muscular atrial wall (2). This causes the atrium to contract, pushing the blood from the upper chambers into the lower chambers.

The impulse then travels to the lower chambers via the AV node (3). This is a cord-like structure that is usually the only electrical connection between the heart's upper and lower chambers. The AV node delays the conduction of the impulse so the lower chambers have time to fill with blood.

The impulse then travels very quickly throughout the lower chambers. This causes the muscular walls of the lower chambers (ventricles) (4) to contract pushing the blood out of the right side of the heart to the lungs and the left side of the heart to the body.

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Systole

• The contraction of the cardiac muscle tissue in the ventricles is called systole. When the ventricles contract, they force the blood from their chambers into the arteries leaving the heart. The left ventricle empties into the aorta and the right ventricle into the pulmonary artery. The increased pressure due to the contraction of the ventricles is called systolic pressure.

Diastole

• The relaxation of the cardiac muscle tissue in the ventricles is called diastole. When the ventricles relax, they make room to accept the blood from the atria. The decreased pressure due to the relaxation of the ventricles is called diastolic pressure.

P = Atria ContractingQRS = Ventricles ContractingT = Ventricles relaxing

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How is the cardiac cycle controlled?

Medulla oblongata and the heart…

• The hearts control center is in the medulla oblongata

• It is a part of the parasympathetic and sympathetic nervous systems

• Parasympathetic fibers attach to the SA and AV nodes in the heart and innervate the these areas

• SA node controls the heart rate

Parasympathetic nerves and the heart…

• Carry continuous impulses to the SA and AV nodes, changing heart rate– An increase in impulses slows heart rate– A decrease in impulses increases heart rate

• Acetycholine is the “break” of the parasympathetic nerves, decreasing the heart rate

• When absent, heart rate increases

Sympathetic nerves and the heart…

• Also attach to the SA and AV nodes, as well as other areas of the heart

• Secrete norepherine in response to impulses, which increases heart rate and force of heart muscle (myocardial) contractions

Medulla oblongata and the heart…

• Baroreceptors or pressure receptors sense changes in pressure of the heart..

• This message gets sent to the medulla oblongata, and it then regulates the heart pressure– High pressure gets reduced, low pressure

gets increased

Other factors that affect heart rate• Impulses from the cerebellum- specifically the

hypothalamus– Fight or flight response

• Fainting (decrease heart rate)• Anxiety attack (increased heart rate)

• Temperature– Rising temp = rise in heart rate– Lower temp = decrease in heart rate

• Ions – Too much potassium (K+) hyperkalemia

• Decrease rate and contractions = arrhythmia

– Too much calcium (Ca+2) hypercalcemia• Increases heart action, prolonged contractions

How is the cardiac cycle measured?

EKG

• An electrocardiogram (EKG) diagnostic test is often performed to evaluate a patient's electrical heart activity. It provides cardiologists with a blueprint of the propagation of the electrical signal within different regions of the heart.

ECG Strip

• The ECG strip shows the electrical activity of the heart. Electrical signs cause the heart to contract or pump. Each signal begins in the atria. The signal then immediately moves down the ventricles. The heart muscle relaxes for an instant before the next electrical signal begins.