Human Heart : Unique Pumping Muscle of Human Body

Position of Heart within Human Body

Human Heart:

The human heart has a mass of between 250 and 350 grams and is about the size of a fist. It is located anterior to the vertebral column and posterior to the sternum. It is enclosed in a double-walled sac called the pericardium. The superficial part of this sac is called the fibrous pericardium. This sac protects the heart, anchors its surrounding structures, and prevents overfilling of the heart with blood.

Heart Structure (Exterior)

Heart Structure (Exterior):

The heart is composed of 3 layers of tissue

1) Pericardium

2) Myocardium

3) Endocardium

Pericardium:

The pericardium is made up of two(2) sacs. The outer sac consists of fibrous sac is continuous with the tunica adventitia of the great blood vessels above and is adherent to the diaphragm below. Its inelastic, fibrous nature prevents over distension of the heart. The outer layer of the serous membrane, the parietal pericardium, lines the fibrous sac. The inner layer, the visceral pericardium, or epicardium, which is continuous with the parietal pericardium, is adherent to the heart muscle.

A similar arrangement of a double membrane forming a closed space is seen also with pleura, the membrane enclosing the lungs. The serous membrane consists of flattened epithelial cells. It secrete serous fluid into the space between the visceral and parietal and visceral pericardium is only a potential space. In heart the two layers are close association, with only the thin film of serous fluid between them.

Myocardium:

This composed of specialized cardiac muscle found in heart. It is not under voluntary control but like skeletal muscle cross stripes are soon on microscopic examination.

Each fiber(Cell) has a nucleus and one or more branches. The ends of the cells and their branches are in very close contact with the ends and branches of adjacent cells. This arrangement gives cardiac muscle the appearance of being a sheet of muscle rather than a  very large number of individual cells.

Endocardium:

This lines the chambers and valves of the heart. It is a thin smooth glistening membrane that permits smooth flow of blood inside the heart. It consists of flattened epithelial cells, and it is continuous with the endothelium lining the blood vessels.

Interior Structure of Heart

INTERIOR STRUCTURE OF HEART:

 The heart is divided into a right and left side by the septum, a partition consisting of myocardium covered by endocardium. After birth blood cannot cross the septum from one side to the other.Each side is divided by an atrioventricular valve into an upper chamber,the atrium, and a lower chamber, the ventricle.The atrioventricular valves are formed by double folds of endocardium strengthened by a little fibrous tissue.The right atrioventricular valve (Tricuspid valve) has 3 flaps or cusps and the left atrioventricualr valve (mitral valve) has 2 cusps.Flow of the blood in the heart is one way; blood enters the heart via the atria and passes into the ventricles below.

The valves between atria and ventricles open and close passively according to changes in pressure in the chambers.They open when the pressure in the atria is greater than that in the ventricles.During ventricular systole(Contraction) the pressure in the ventricles rises above that in the atria and the valves snap shut ,preventing backward flow of blood.The valves are preventing backward flow of blood.The valve are prevented from opening upwards into the atria by tendinous chords called chordae tendineae, which extent from the inferior surface of the cusps to little projections of myocardium into the ventricles, covered with endothelium, called papillary muscles . 

THE CARDIAC CYCLE:

The function of the heart is to maintain a constant circulation of blood throughout the body.The heart acts as a pump and its action consists of a series of events known as the cardiac cycle. During each heartbeat, or cardiac cycle,the heart contracts and then relaxes.The period of contraction is called systole and that of relaxation, diastole.

Stages of the cardiac cycle:

Stages of the cardiac cycle:

The normal number of  cardiac cycles per minute ranges from 60 to 80. Taking 74 as an example each cycles lasts about 0.8 of a second and consists of:

1) Atrial systole- contraction of the atria.

2) Ventricular systole- Contarction of the ventricles.

3)Complete cardiac diastole- relaxation of the atria and ventricles

The superior vena cava and the inferior vena cava transport deoxygenated blood into the right atrium at the same time as the 4 pulmonary veins bring oxygenated blood into the left atrium.The atrioventricular valves are open and blood flows passively through to the ventricles.The SA node triggers a wave of contraction that spreads over the myocardium of both atria,emptying the atria and completing ventricular filling.When the electrical impulse reaches the AV node it is slowed down,delaying atrioventricular transmission.

This delay means that the mechanical result of the atrial stimulation,atrial contarction,lags behind the electrical activity by a fraction of a second.This allows the atria to finish emptying into the ventricles before the ventricles begin to contract.After this brief delay,the AV node triggers its own electrical impulse, which quickly spreads to the ventricular muscle via AV bundle,The bundle branches and Purkinje fibres.

This result in a wave of  contraction which sweeps upwards from the apex of the heart and across the walls of both ventricles pumping the blood into the pulmonary artery and the aorta(Ventricular systole 0.3 sec).The high pressure generated during ventricular contraction is greater than that in the aorta and forces the atrioventricular valves to close preventing backflow of blood into the atria.

                                

After contraction of the ventricles there is complete cardiac diastole, a period of 0.4 secs,When atria and ventricles are relaxed.During this time the myocardium recovers in preparation for the next heartbeat,and the atria refill in preparation for the next cycle.

FLOW OF BLOOD THROUGH THE HEART

                                                                                        3D Animation Showing Blood Flow within Heart.

FLOW OF BLOOD THROUGH THE HEART:

The two largest veins of the body,the superior and inferior vena cava, empty their contents into the right atrium.The blood passes via the right atrioventricular valve into the right ventricle,and from there it is pumped into the pulmonary artery or trunk (The only artery in the body which carries deoxygenated blood). The opening  of the pulmonary artery is guarded by the pulmonary valve, formed by three semilunar cusps.The valve prevents the backflow of blood into the backflow of blood into the right ventricle when the ventricular muscle relaxes. After leaving the heart the pulmonary artery divides into left nad right pulmonary arteries, which carry the venous blood to the lungs where exchange of gases takes place; carbon dioxide is excreted and oxygen is absorbed. 

Heart Lung Tissue Circulation of Human Body

Two pulmonary veins from each lung carry oxygenated blood back to the left atrium.Blood then passes through the left atrioventricular valve into the  left ventricle, and from there it is pumped into the aorta,the first artery of the general circulation.The opening of the aorta is guarded by the aortic valve, formed by three semilunar cusps.

                                                

From this sequence of events it can be seen that the blood passes from the right to the left side of the heart via the lungs or pulmonary circulation . However, it should be noted that both atria contract at the same time and this is followed by the simultaneous contraction of both ventricles.

                                                

The muscle layer of the walls of the atria is thinner than that of the ventricles.This is consistent with the amount of work they do. The atria usually assisted by gavity,propel the blood to the lungs and round the whole body. The pulmonary trunk leaves the heart from the upper part of the right ventricle,and the aorta leaves from the upper part of the left ventricle.

SA Node and AV Node of Human Heart

SA NODE(SINOATRIAL NODE) AND AV NODE (ATRIOVENTRICULAR NODE):

The heart has an intrinsic system whereby the cardiac muscle is automatically stimulated to contract without the need of external stimulation.The property is called autorhythmicity.However,the intrinsic system can be stimulated or depressed by nerve impulses initiated in the brain and by circulating chemicals, including hormones.

                                                                      
                                                                    3D Animation showing SA and AV Node of Heart.

                                                                            

Small group of specialised neuromuscular cells in the myocardium initiate and conduct impuses,causing coordinated and synchronised contraction of the heart muscle.

Sinoatreal node(SA node):

This small mass of specialised cells lies in the wall of the right atrium near the opening of the superior vena cava.The SA node is pacemaker of the heart because it normally initiates impulses more rapidly than other groups of neuromuscular cells.Firing of the SA node causes atrial contraction.

Atrioventricular node(AV node):

This small mass of neuro muscular tissue is situated in the wall of the atrial septum near the atrioventricular valves.Normally,the AV node conducts impulses that arrive via the atria and that originated from the SA node.There is a delay here; the electrical signal takes 0.1 of a second to pass through into the ventricles.This allows the atria to finish contracting before the ventricles start.

Atrioventricular bundle(Bundle of His):

This is a mass of specialised fibres that originate from the AV node. The AV bundle crosses the fibrous ring that separates atria and ventricles then,at the upper end of the ventricular septum,it divides into right and left bundle branches.Within the ventricular myocardium the branches bundle, bundle branches and purkinje fibres convey electrical impulses from the AV node to the apex of the myocardium where the wave of ventricular contraction begins, then sweeps upwards and outwards pumping blood into the pulmonary artery and aorta.

Blood Pressure Measuring Device

BLOOD PRESSURE:

This is the force or pressure that the blood exerts on the walls of blood vessels.Keeping blood pressure within normal limits is very important .If it is high blood vessel can be damaged,causing clots or bleeding from sites of blood vessel rupture.If it is falls to low ,then blood flow through tissue beds may be inadequate.This is particularly dangerous for such essential organs as the heart brain or kidney.

                                                         

Blood pressure varies according to the time of day, the posture,gender and age of the individual.During bed rest at night the blood pressure tends to be lower.It increases with age and is usually higher in women than in men. Atrial blood pressure is measured with a sphygmomanometer and is usually expressed with the systolic pressure written above the diastolic pressure:

                                                                   

                                                                     120                            16

                                                            BP =-------- mm Hg  or  BP = ----- kPa.

                                                                       80                             11  

                                                    

                                                                             Cardiac       Peripheral

                                                  Blood pressure = Output    *   resistance    

Electric Charges Curve of P,Q,R,S and T

Electric Charges in the Heart:

As the body fluid and tissues are good conductors of electricity, the electrical activity within the heart can be detected by attaching electrodes to the surface of the body. The Pattern of electrical activity may be displayed on an oscilloscope screen or traced on paper. The apparatus used is an electrocardiograph and the tracing is an electrocardiogram (ECG).

Electrocardiograph machine and ElectrocardiogramTracing

The normal ECG tracing show five waves which by convention, have been named P,Q,R,S and T. The P wave arises when the impulse from the SA node sweeps over the atria. The QRS complex represents the very rapid spread of the impulse from the AV node through the AV bundle and the Purkinje fibers and the electrical activity of the ventricular muscle.

The T wave represents the relaxation of the ventricular muscle. The ECG described above originates from the SA node and is known as Sinus Rhythm. The rate of sinus rhythm is 60 to 100 beats per minute. A faster heart rate is called tachycardia and a slower heart rate bradycardia.

Human Heart Post