HUMAN HEART DEFINITION “The muscular organ responsible for pumping blood to human body through blood vessels by repeated contractions is called human heart.” LOCATION In humans, heart is lies between lungs, in the middle of chest cavity (thorax), underneath the breast bone (sternum). It is usually felt to be on the left side because its apex containing strong muscular left ventricle is tilted toward the left side. MASS AND SIZE In normal adults, mass of heart is 250 to 350 grams and it is equal to the size of a clenched fist. PROTECTION Human heart is protected by Pericardium a sac in which heart is enclosed Pericardial present between heart wall and pericardium. It reduces friction between fluid pericardium and heart walls during heart contraction. LAYERS OF HEART The walls of human heart consist of 3 layers Epicardium outer layer of heart Myocardium middle layer of heart(major portion) Endocardium inner layer of heart STRUCTURE Heart Chambers Atria Right artium Valves Ventricles Left atrium Right ventricle Cardic muscles Atrio ventricular valve Left ventricle Right atrio ventricular valve/tricuspid valve Left atrio ventricular valve/ bicuspid valve Semi lunar valve Pulmonary valve Aortic valve Flow sheet showing the structure of human heart Chambers Human heart, like the heart of birds and other mammals, also consists of four chambers. Two atria Two ventricles Atria “The upper half of heart consists of two thin walled receiving chambers called atria”. The two atria are separated by interatrial septum to keep oxygenated and deoxygenated blood separated. Right Atrium receives deoxygenated blood from body via main veins, superior and inferior vena cava. Left Atrium receives oxygenated blood by pulmonary veins from lungs. Ventricles “The lower half of heart consists of two larger, thick walled distributing chambers called ventricles.” The two ventricles are separated by interventricular septum to keep oxygenated and deoxygenated blood separated. Left Ventricle pushes oxygenated blood to aorta and then whole body except lungs. Right Ventricle pushes deoxygenated blood only to lungs. Left ventricle is the largest and strongest heart chamber. Its walls are thicker (about half an inch thick) and are three times more muscular than right ventricle because it pumps blood with enough force to all body parts providing an evidence that the structure of different parts of heart is adapted to its function. Valves “The thin-walled fibrous flaps of tissue preventing backward flow of blood is called valves.” There are two types of valves. Atrioventricular Valves Right Atrioventricular /Tricuspid Valve Left atrioventricular/Bicuspid valve Semilunar Valve Pulmonary Valve containing three flaps and guarding the opening of right atrium and right ventricle. It prevents backward flow of blood from right ventricle to right atrium containing two flaps and guarding the opening of left atrium and left ventricle. It prevents the backward flow of blood from left ventricle to left atrium present at the base of pulmonary trunk. It prevents backward flow of blood from pulmonary trunk into right ventricle. present at the base of aorta. It prevents the backward flow of blood From Aorta to the left ventricle Aortic Valve Cardiac Muscle The bulk of walls of heart is made of cardiac muscles (the term cardiac means related to heart). They are involuntary in action and are composed of branched, striated cells each with a single nucleus. WORKING OF HEART Human heart works as a double pump. i.e., it receives deoxygenated blood (with less oxygen) from body and pumps it to lungs. At the same time, it receives oxygenated blood form lungs and pumps it to all body. It shows two circuits. Pulmonary Circuit Systemic Circuit Pulmonary Circuit “The pathway through which deoxygenated blood is pushed from heart to lungs and in return oxygenated blood is carried from lungs to heart is called pulmonary circuit.” Right atrium contracts to push the deoxygenated blood to right ventricle through the opening guarded by tricuspid valve. Right ventricle contracts to pass the blood into pulmonary trunk through pulmonary valve which carries blood to lungs. The oxygenated blood from lungs is brought to the left atrium by pulmonary veins. The blood in pulmonary circulation is at lower pressure than the blood in systemic circulation. It gives sufficient time for gaseous exchange to occur in lungs. Right atrium Tricuspid valve Right ventricle pulmonary valve Pulmonary trunk Lungs Systemic Circuit “The pathway by which oxygenated blood is pushed from heart to body tissue and in return, deoxygenated blood is carried from body tissue to heart is called systemic circuit”. The left atrium contracts to pump oxygenated blood to the left ventricle through an opening guarded by bicuspid valve. When the left ventricle contracts, it pumps oxygenated blood to all parts of body except lungs through aorta guarded by aortic valve. The right atrium receives deoxygenated blood from body by inferior and superior vena cava. It contracts to push blood into right ventricle. Both atria are filled with blood simultaneously. They contract together to pump blood into both ventricles. Similarly, both ventricles contract simultaneously to pump blood out of heart. Left atrium valve Bicuspid valve Left ventricle aortic semilunar aorta All body tissues Except lungs HEART BEAT DEFINITION “One complete cardiac cycle consisting of alternate contraction and relaxation of heart chambers is called heart beat.” The relaxation of heart chambers fills them with blood while contraction of chambers pushes the blood out of them. TIME PERIOD / DURATION A complete cardiac cycle takes 0.8 seconds 0. STEPS It consists of following steps. 1. Atrial Systole Immediately after filling, both atria contract and pump blood towards ventricles. This period is called atrial systole and it takes 0.1 seconds. 2. Ventricular Systole Now, both ventricles contract and pump blood towards body and lungs. The period of ventricular contraction is called ventricular systole. When ventricles contract, the tricuspid and bicuspid valve close and ‘lubb’ sound is produced and lasts about 0.3 seconds. 3. Cardiac Diastole When atria and ventricles relax, blood is filled in atria. When ventricles relax, semilunar valves namely aortic and pulmonary valves close, ‘dubb’ sound is produced. It takes 0.4 seconds. This ‘lubb dubb’ can be heard with the help of stethoscope. The ventricular relaxation is called ventricular diastole.
