Human Heart

Heart is situated in the thoracic cavity in between two lungs, slightly tilted to the left it has size of clenched fist.

It is protected by a double walled membrane bag pericardium enclosing pericardial fluid outer- parietal pericardium and inner visceral pericardium.

The heart is four chambered two chamber are relatively smaller called atria and two large lower chambers called ventricle. A thin muscular wall called inter atrial septum separates left and right atrium. A thick walled inter ventricular septum separates left and right ventricles. Atrium and ventricles are separated by thick fibrous tissue called atrio ventricular septum.

opening of left and right ventricles into pulmonary artery and aorta respectively are provided with semilunar valves.

The atrioventricular opening between left atrium and left ventricle is guarded by bicuspid valves (it has two flaps). The atrioventricular opening between right atrium and right ventricle is guarded by tricuspid valve (it has three flaps).

Super vena cava carry blood from upper region. Inferior vena cava carry blood from lower body parts. Coronary sinus carry blood from heart itself. Entire heart is made up of cardiac muscles. The wall of ventricle is much thicker than that of atria. A specialised cardiac musculature called nodal tissue is also distributed in heart.

A patch of this tissue is present in the right upper corner of right atrium called Sino atrial node (SAN). Another patch of this tissue is seen at the lower left corner of right atrium close to atrio ventricular septum called Atrio ventricular Node (AVnode).

A bundle of nodal fiber, atrio ventricular bundle continues from AV node which pass through atrio ventricular septa to emerge on the top of inner ventricular septum and immediately divides into right and left bundle. The branches give rise to many through ventricular musculature of respective sides called as purkinje fiber. Fibers along with left and right bundle are called as bundle of HIS.

The nodal tissue has the ability to generate the action potentials without any external stimuli i.e., it is auto excitable. However, the number of action potentials generated in a minute vary at different parts of nodal system.

SAN can generate maximum number of action potentials i.e., 70-75 times per minute and is responsible for initiation and maintaining the rhythmic contractile activity of heart. Therefore, it is called Pacemaker.

 

Circulation of Heart

1)     Pulmonary Circulation- Deoxygenated blood from the right ventricle is pumped out through the right ventricle is pumped out through pulmonary arteries to lungs where it gets oxygenated. Oxygenated blood is poured back to heart.

2)     Systematic circulation- Oxygenated blood from the left atrium is poured into left ventricle from left ventricle largest artery arises which distributes oxygenated blood into different parts.

 

 

 

Portal system

1)     Hepatic portal system- it consists of hepatic portal vein that collects the blood through capillaries from digestive organ like intestine. It does not take the blood directly to heart but breaks up into capillary system in liver and deposit sugar and other nutrients.

2)     Hypophysical portal system- Portal system also exist between hypothalamus and anterior part of pituitary which transports hormone to pituitary.

Systole- it is referred to contraction of cardiac chamber.

Diastole- it is referred to expansion of cardiac chamber.

Joint diastole- relaxed state of both atria and ventricle.

Sequential changes occurring inside cardiac chamber during one complete cardiac cycle

1)     Atrial Filling-

·        Filling of right atrium with deoxygenated blood from great veins and left atrium with oxygenated blood from pulmonary veins.

·        As the pressure increases in the atria the bicuspid and tricuspid valves open and blood flows into respective relaxed ventricle.

·        Pressure of blood is very low in ventricle so the semilunar valves of arteries remain closed and blood does not flow out of ventricles.

2)     Atrial Systole-

·        Two atria contracts forcing most of the blood to ventricles

·        Closing of great veins root by compression occurs.

·        Bicuspid and tricuspid valves are open (lasts for about 0.1 seconds)

3)     Atrial Diastole-

·        Sharp closing of AV valves to stop back flow of blood to ventricles. This produces first heart beat sound “lubb”.

·        Rise of ventricular pressure but it is still lower than the pressure in great arteries and hence semilunar valves are still closed.

·        Ventricles contract as closed chamber and the pressure increases more than the pressure inside aorta forcing the opening of valves.

4)     Joint Diastole-

·        Ventricle relax and pressure falls below to that in aorta.

·        Closing of semilunar valves in pulmonary artery and aorta produces the second heart beat sound “dub”.

·        This prevents back flow of blood into ventricles.

·        As the low ventricular pressure is still greater than the atrial pressure AV Valves remains closed.

·        Continued ventricular diastole decreases the pressure tremendously and now both atria and ventricle are in joint diastole.

One Complete Systole and Diastole takes 0.8 second and forms a cardiac cycle begins with atrial systole. Human heart pumps 6500 liters blood per day continuously.

 

Heart Rate- (Number of Times heart beat’s per minute) Smaller animals have higher heart rate because of high metabolic activity. They need more oxygen to be pumped and more oxygen to tissue. This is the reason why small animals have higher rate than large animals.

Size of animal is inversely proportional to its heart rate.

Trachycardia- it refers to the abnormal increase in heartbeat. It may be due to internal factors like emotion, stress, anxiety, anger, excitement etc.

Bradycardia- it refers to the abnormal decrease in heartbeat. Athlete who have a high heart rate may suffer from bradycardia.

Arteries Pulse- It is the wave of distension followed by constriction experienced in arteries as a result of ventricular systole or diastole. As ventricles contracts blood is pumped out into arteries with force. It causes distension of the elastic wall of arteries and is felt as a pulse when a figure is placed on artery near the wrist.

Blood Pressure- it is the pressure exerted by the blood against the wall of arteries as the arteries already contains blood, pressure in them increases due to sudden flow of blood during ventricular systole and falls slightly during ventricular diastole.

Systolic Pressure- it is the pressure exerted by arteries due to systole of ventricles (120 mmHg).

Diastolic Pressure- it is the pressure exerted by arteries due to diastolic activity in ventricles (90mmHg).

 

Note-

·        value of blood pressure changes with age, sex, and health of person.

·        Sphygmomanometer is the instrument used to measure the pressure of blood.

·        Normal Blood pressure of person is 120/90 mmHg.

·        Blood pressure can increase due to arteriosclerosis where due to hardening of arteries their lumen become narrower and so blood pressure increases.

 

Electrocardiograph (ECG)

ECG is graphic representation of the electric current produced by the excitation of cardiac muscles. the instrument used to record the changes is an electrocardiograph.

1)     P- wave- it is a amall upward wave that indicates the depolarisation of atria (atrial Contraction) it is caused by activation of SA node.

2)     QRS Wave- it begins after a fraction of second of P wave. It represents the ventricular depolarisation (ventricular contraction). It is caused by the impulse of the contraction from AV node through the bundle of HIS and Purkinje Fiber and the contraction of the ventricular muscles. Thus, this wave is due to spread of electric impulse through ventricles.

3)     T Wave- it is dome shaped which indicates ventricular repolarisation (ventricular relaxation).