The Heart

The heart is a hollow muscular pumping organ. It is somewhat conical in shape. It is about the size of a man’s fist. It lies between the lungs in the thoracic cavity. The heart is enclosed in a thin tough transparent membrane, the epicardium. There is a fluid between the heart and the pericardium. The fluid reduces the friction between the pericardium and the heart. The pericardium is inextensible. The wall of the heart is composed of epicardium, myocardium and endocardium. The thick midlayer (myocardium) consists of cardiac muscle tissue that provides the contractile force behind blood flow through the heart. The smooth inner lining (endocardium) is made brachiocephalic of epithelial and trunk connective tissues. Internally the heart is divided by a vertical partition into two halves, the right and left. The vertical partition is called septum. Each half is again divided by a horizontal partition into an upper thin atrium (Platria) and a lower thick walled ventricle. Thus the heart consists of four chambers, the right and left atria and the right and left ventricles. The atrium of each side communicates with the corresponding ventricle through an opening, the atrio-ventricular opening.

  

                                                                                          

Veins: The two great veins the superior and inferior vena cavae open into the right atrium. The orifice of the superior vena cava is directed obliquely and has no valves. Two, right and left pulmonary veins, open into the left atrium and their orifices are not provided with valves.

Valves: The heart has four sets of membranous flaps that serve as valves. The coordinated opening and closing of the valves permit blood to move in one direction. Atrio-ventricular valves are inlet valves. Tricuspid valve is present in the opening between right atrium and right ventricle. It is made up of three flaps of connective tissues. Bicuspid valve is present in the opening between left atrium and left ventricle. It consists of two flaps. The fore ends of the flaps are attached to the wall of the ventricle by fibrous cord, the cordae tendinae, which prevents the over extension of flaps into the auricles during the ventricular contraction. Semilunar valves are outlet valves. One semilunar valve is located at the passageway leading from the left ventricle to the aorta and another is located between the right ventricle and the pulmonary artery.

human heart dissected

Valves 

Circulation of blood in the Heart: atria receive blood and the ventricles distribute it the blood flows into the right atrium through the vena cavae.

Superior Vena Cava: 
It brings the deoxygenated blood from the upper region of the body.

Inferior Vena Cava: it brings the deoxygenated blood from

the lower region of the body. From right atrium blood flows to the right ventricle, then to the lungs by means of pulmonary arteries for oxygenation. The oxygenated blood returns to left atrium by means of pulmonary veins. Then the blood flows to the left ventricle. From left ventricle the blood is pumped into aorta which carries blood to all parts of the body. 

Circulation of Blood in Heart

Cardiac Cycle: The cardiac cycle is a sequence of events during one heartbeat.

Atria Relaxation: Blood enters the right atrium from the body through the

vena cavae. Blood enters left atrium from the lungs through the pulmonary

veins. Both atria fill up and the blood flow into them ceases.

The blood circulation may be summarized as follows:

—>Veins (conduct blood from organs)—*right atrium—> right ventricle—*pulmonary arteries> capillaries in the lungs—>pulmonary veins—>left atrium—*Left ventricle —>aorta—> arteries (conduct blood to organs)—> capillaries.

Atria Contraction: Blood is pushed through the atrio-ventricular valves into
the still relaxed ventricles. The ventricles become full and pressure forces the valves to close preventing a back flow of blood into the atria.

Ventricle Relaxation: Blood is received from the contracting atria.

Ventricle Contraction: Blood pushed against the semi lunar valves at the
 base of the aorta and pulmonary arteries. Right and left ventricles have equal volume and both the ventricles pump almost simultaneously, so that equal amount of blood enter and leave the heart. 

Ventricle Relaxation: Blood is received from the contracting atria.

Ventricle Contraction: Blood pushed against the semi lunar valves at the
 base of the aorta and pulmonary arteries. Right and left ventricles have equal volume and both the ventricles pump almost simultaneously, so that equal amount of blood enter and leave the heart.

One heart beat-a cardiac cycle of relaxation-diastole and contraction systole takes about 0.8 second. The heart muscle rests. 0.1 to 0.3 second between the beats. Each heart beat is accompanied by two heart sounds, lub and dub. The lub dub heart sounds are not caused by the contraction of the heart, but by sudden closing of the valves. The heart beats about 75 times per minute (at rest) pumping out about 5 liters of blood. The heart beat varies with age, sex, size and state of health.