HEART (THE PUMPING ORGAN)
All vertebrates possess a muscular chambered heart. Depending upon the different types of circulation, heart is of following three types:
FOUR CHAMBERED: Posses 2 atria and 2 ventricles; oxygenated and deoxygenated blood do not get mixed and pumped separately; double circulation. e.g., birds and mammals.
THREE CHAMBERED: Posses 2 atria and 1 ventricle; left atrium receives oxygenated and right atrium receives deoxygenated blood, which gets mixed in ventricle, incomplete double circulation. e.g., amphibians and reptiles(except crocodile).
TWO CHAMBERED: Posses 1 atrium and 1 ventricle; heart always receives deoxygenated blood which passes through it for once; single circulation. e.g., fishes
HUMAN HEART
Human heart is a muscular organ of mesodermal origin, situated between the lungs of thorocic cavity. An average adult human heart is about 12 cm. Its weight ranges from 280-340 gm in males and 230-280 gm (average 250 g) in females. The heart is surrounded by a protective covering called pericardium (two-layered) which is filled with pericardial fluid.
The heart is divided into 4 chambers in human beings; these chambers are:
Auricles or Atrium There are auricles or atrium, left and right separated by inter-auricular septum and are superior in position. SA node (Sinoatrial Node) is situated in the upper wall of right atrium.
Ventricles The two interior chambers of the heart are the right and left ventricle separated by inter-ventricular septum.
Note: The walls of ventricle are thicker than that of atria because ventricles have to pump blood into various organs, while atria receives blood.
PARTS OF THE HUMAN HEART AND THEIR FUNCTIONS
LEFT ATRIUM– It receives oxygenated blood from the lungs via pulmonary vein.
LEFT VENTRICLE– It pumps blood to all parts of the body except the lungs via aorta.
RIGHT ATRIUM– It receives deoxygenated blood coming from the body via vena cava.
RIGHT VENTRICLE– It pumps blood to the lungs for oxygenation.
VALVES– It prevents backflow of blood. There are 4 types of valves i.e., two atrioventricular (AV) valve and two semilunar (SL) valve.
- Atrioventricular (AV) valves are present between the atria and the ventricles. It mainly prevents the back flow from the ventricles into the atrium during systole. It includes mitral valve and tricuspid valve.
– Mitral valve or bicuspid valve allows the blood to flow from left atrium into left ventricle.
– Tricuspid valve present between the right atrium and right ventricle and stops the back flow of blood between them.
- Semilunar valves are present in the arteries leaving the heart.
AORTA– It carries oxygenated blood.
SEPTUM– It prevents mixing of oxygenated and deoxygenated blood.
PULMONARY ARTERY– It carries deoxygenated blood to the lungs.
PULMONARY VEIN– It carries oxygenated blood from lungs to the heart.
SUPERIOR VENA CAVA– It returns deoxygenated blood from head and arms to the heart.
INFERIOR VENA CAVA– It returns deoxygenated blood from lower limbs and organs to the heart.
WORKING OF HUMAN HEART
The working mechanism of heart includes contraction (systole) and relaxation (diastole) of the atria and ventricles. The atria and ventricles contract alternately and constitutes a heart beat. SA node spreads waves of contraction across the walls of the atria which further reaches to the AV node (pacesetter).
AV node further stimulates the ventricles to contract simultaneously by transmitting the impulse . The ventricles force blood through arteries and hence exert great pressure on the blood.
PUMPING ACTION OF HEART
The human heart functions to pump oxygenated (left portion) and deoxygenated blood (right portion) to various body organs.
DOUBLE CIRCULATION
It is the passage of blood occurring twice in the heart through separate pathways for completing one cycle. It consists of pulmonary and systemic circulation. Double circulation is generally found in fishes, amphibians, birds, reptiles, and man where arterivenous heart (means when it receives both venous or deoxygenated and arterial or oxygenated blood) is present.
The advantage of double circulation is that the blood can be sent to the lungs to pick up oxygen and then be returned to the heart to be pumped again before travelling around the body.
HEART BEAT AND ITS REGULATION
Heart beat is the rhythmic contraction (systole) and relaxation (diastole) of the cardiac muscles. The heart of the healthy person beats 72 times per minute, while for a hard core physical worker it is 180 times per minute. Heart beat is regulated by the ability of the heart to contract spontaneously at a regular rate.
Heart sounds (beatings) can be heard by stethoscope. The first heart sound is ‘lub’ created by the closure of the atriventricular valves, at the beginning of the ventricular systole and opening of semilunar valves. The second heart sound is ‘dub’ created at the end of ventricular systole when the semilunar valve closes and the AV valve opens.
ELECTROCARDIOGRAPH (ECG)
ECG is the graphical record of electrical currents produced by the excitation of the cardiac muscles. Any deviation from normal ECG represents abnormal heart functioning.
BLOOD VESSELS
The intricate network of hollow tubes that transport blood throughout the body are called blood vessels. Blood vessels (except capillaries) are made up of three layers. i.e., tunica externa, tunica media and tunica interna.
There are three types of blood vessels:
1- Arteries- These are elastic vessels that transport blood away from the heart to various organs. Through the arteries only pure blood flows, but through the pulmonary arteries impure blood flows.
2- Veins- These are elastic vessels that transport blood to the heart from various organs of the body. Veins carry only impure blood (doxygenated blood), but pulmonary vein carry only pure (oxygenated) blood.
3- Capillaries- These are small vessels that transport blood from the arteries to the veins and are located within the tissues of the body. Through the thin walls of the capillaries, exchange of wastes, nutrients, carbon di-oxide and oxygen takes place.
BLOOD PRESSURE
Blood pressure is produced by the blood (present in blood vessels) against the walls of the blood vessels by the contraction of left ventricle. Blood pressure is recorded in millimeters of mercury, similar to barometric pressure with the help of an instrument called sphygmomanometer. The blood pressure is high in the arteries, gradually drops in the arterioles and capillaries, and become very low in the veins. The average blood pressure is expressed as 120/80mmHg.
When the left ventricle contracts (pushing the blood into the aorta), the pressure produced is known as systolic blood pressure (120mmHg). When the complete diastole occurs and the heart is resting, the pressure within the vessels is called diastolic blood pressure (80mmHg).
POINTS TO REMEMBER
- Everyday our heart valves open and close normally 100,000 times.
- The blood circulatory system was invented by William Harvey.
- The thickest portion of the human heart is the wall of left ventricle.
- The size of human heart is just one fist.
- The SA node is responsible for initiating and maintaining the rhythmic contractions of heart. Therefore it is also called pacemaker.
- A woman’s heart typically beats faster than a man’s.
- When the body is at rest, blood takes only 6 sec to go from heart to lungs and back, only 8 sec for it to go to the brain and back and only 16 sec for it to reach the toes and travel all the way back to the heart.
- Cocaine affects the heart’s electrical activity and causes spasm of the arteries, which can lead to heart attack or stroke.
- Left atrium pump 75% of the blood to left ventricle without any effort, therefore, a person with defect in left atrium or mitral valves is able to survive normally.