Structure and Function of Human heart

Dr. K. Rama Rao
Department of Zoology
Govt, Degree College
TEKKALI; Srikakulam Dt: A.P.

1. Dr. K. Rama Rao Department of Zoology Phone: [phone] Structure and Function of heart Dr. K. Rama Rao Department of Zoology Phone: [phone]

2. The  heart is located on the left side of the chest, beneath the breastbone. The heart is composed of smooth muscle. It has four chambers which contract in a specific order, allowing the human heart to pump blood from the body to the lungs and back again with high efficiency.

3. External Structure: Human heart is four chambered, consisting of two atria and two ventricles . ( i ) Grooves ( Sulci ): The left and right atria are separated externally by a groove . The atria are demarcated externally from the ventricles by an oblique groove called atrioventricular sulcus . These have coronary arteries, through which the heart receives blood.

4. (ii) Atria (sing, atrium): The left atrium is smaller than the right atrium. The right atrium is a roughly quadran­gular chamber. The superior vena cava, inferior vena cava and coronary sinus open into the right atrium .

5. (a) The superior vena cava carries blood from the body’s upper region. (b) The inferior vena cava is larger than the superior and carries blood from the lower body’s region. (c) The coronary sinus carries the majority of blood from the heart itself. The coronary veins open into the coronary sinus. (d) The left atrium receives oxygenated blood from the lungs through two pairs of pulmonary veins.

6. (iii) Ventricles: There are present left and right ventricles with thick walls. The wall of the right ventricle is thinner than that of the left ventricle. The left ventricle is longer and narrower than the right ventricle.

7. (iv) Pulmonary Trunk and Aorta: The pulmonary trunk arises from the right ventricle. It divides into left and right pulmonary arteries that carry deoxygenated blood to the lungs. The aorta arises from the left ventricle.

8. It is divisible into the ascending aorta, arch of aorta and descending aorta . The right and left coronary arteries arise from the ascending aorta.

9. Internal Structure: The internal structure of the heart can be better studied by dissect­ing it from the ventral side. ( i ) Atria: The two thin walled atria are separated from each other by the interatrial septum. The right atrium receives the openings of superior vena cava, inferior vena cava and coronary sinus. The opening of inferior vena cava is guarded by Eustachian valve.

10. The left artrium receives four openings of pulmonary veins.

11. (ii) Bicuspid and Tricuspid Valves: The artrioventricular opening between the left atrium and the left ventricle is guarded by the bicuspid valve, also called mitral valve (having two flaps). The right atrio -ventricular opening is guarded by the tricuspid valve, as it has three flaps.

12. (iii) Ventricles: Attached to the flaps of the bicuspid and tricuspid valves are special fibrous cords, the chordae tendineae , which are joined to the other ends with the special muscles of the ventricular wall, the papillary muscles. The chordae tendineae prevent the bicuspid and tricuspid valves from collapsing back into the atria during powerful ventricular contractions.

13. (iv) Semilunar valves: As stated in the external structure, the pulmonary trunk and aorta arise from the right and left ventricles respectively. At the base of the pulmonary trunk and aorta are located three half-moon shaped pockets known as pulmonary semilunar valves and aortic semilunar valves respectively. These valves allow the free and forward flow of blood, but prevent any backward flow.

14. The valves of the heart are: (a) Bicuspid valve, (b) Tricuspid valve, (c) Aortic semilunar valves and (d) Pulmonary semilunar valves.

15. The heart collects blood through both the atria and then distributes it through the ventricles. The action of heart includes contractions and relaxations of the atria and ven­tricles. A contraction of the heart is called a systole and its relaxation a diastole. The atria and ventricles contract alternately.

16. The contraction of heart (systole) and the relaxation of heart (diastole) constitute the heart beat. The contraction of atria is initiated and activated by the sinoatrial node (SA Node— pace maker) which spreads waves of contraction across the walls of the atria via muscle fibres at regular intervals.

17. When the wave of contraction originating from the sinoatrial node reaches the atrio­ventricular node (AV Node— pace setter), the latter is stimulated and excitatory impulses are rapidly transmitted from it to all parts of the ventricles via bundle of His and Purkinje’s fibres .

18. These impulses stimulate the ventricles to contract simultaneously. The ventricles force bloohd trough long system of arteries and hence must exert great pressure on the blood.

19. Heart Beat: What is heart beat? Heart beat is the rhythmic contraction and relaxation of the heart. Each heart beat includes one systole (contraction phase) and one diastole (relaxation phase) of the heart to distribute and receive blood to and from the body. The heart of a healthy person beats 72 times per minute (average).

20. Beating is an inherent capacity of the heart. The heart of a resting human being pumps about 5 litres of blood per minute. This means that a quantity of blood equal to the total amount contained in the body passes through the heart each minute. During exercise both the number of beats per minute and the amount of blood pumped per beat are greatly increased.

21. Types: The heart beat is of two types: Neurogenic and Myogenic . The neurogenic heart beat is initiated by a nerve impulse coming from a nerve ganglion. annelids and most arthropods. The myogenic heart beat is initiated by a patch of modified heart muscle itself. It is found in hearts of molluscs and vertebrates including human beings.

22. Origin of heart beat: The mammalian heart is myogenic , it is regulated by the nerves. The heart beat originates from the sinoatrial node (SA Node)— pace maker, which lies in the wall of the right atrium near the opening of the superior vena cava. The SA node is a mass of neuromuscular tissue.

23. Sometimes, the SA-node may become damaged or defective. So the heart does not function properly. This can be remedied by the surgical grafting of an artificial pace maker in the chest of the patient. The artificial pace maker stimulates the heart at regular intervals to maintain its beat.

24. Conduction of heart beat: Another mass of neuromuscular tissue, the atrio -ventricular node (AV node) is situated in the wall of the right atrium. The AV node picks up the wave of contraction propagated by SA node. A mass of specialized fibres , the bundle of His, originates from the AV node. The bundle of His divides into two branches, one going to each ventricle.

25. Within the mycocardium of the ventricles the branches of bundle of His divide into a net work of fine fibres called the Purkinje fibres . The bundle of His and the Purkinje fibres convey impulse of contraction from the AV node to the myocardium of the ventricles. Regulation of heart beat (= Regulation of Cardiac Activity).

26. The rate of heart beat is regulated by two mechanisms: Neural Regulation: “ The cardiac centre is formed of cardio-inhibitor and cardio-accelerator parts. (ii) Hormonal Regulation: Adrenaline (epinephrine) and noradrenaline ( norepineph­rine ) hormones are secreted by the medulla of the adrenal glands. Noradrenaline accelerates the heart beat under normal conditions

27. Thyroxine hormone secreted by thyroid gland increases oxidative metabolism of the body cells. This requires more oxygen and thus indirectly increases heart beat.

28. Heart Rate: Human heart beats about 72 times per minute in an adult person at rest. This is called heart rate of that person. The heart rate increases during exercise, fever, fear and anger.

29. Because smaller animals have the higher metabolic rate, their heart rate is higher than larger animals. An elephant has normal heart rate of about 25 per minute whereas mouse has a normal heart rate of several hundred per minute.

30. Cardiac Output (Heart Output): The amount of blood pumped by heart per minute is called sraidiae output or heart output. Heart of a normal person beats 72 times per minute and pumps out about 70 mL of blood per beat. Thus the cardiac output is 72 x 70 or 5040 mL per minute i.e., about 5 litres per minute which is equivalent to the total body blood volume (about 5.5 litres ).

31. Pulse: Pulse is the rhythmic contraction and relaxation in the aorta and its main arteries. Thus pulse is a wave of increase which passes through arteries as the left ventricle pumps its blood into the aorta. Pulse is a regular jerk of an artery. Therefore, it is also called arterial pulse.

32. The pulse rate are as follows: The pulse rate in children is more rapid than in adults. (ii) The pulse rate is more rapid in the female than in the male. (iii) When the person is standing up the pulse rate is more rapid than when he/she is lying down.

33. (iv) When any strong emotion is experienced the pulse rate is increased, for example, anger, excitement, fear, etc. (v) Any exercise increases the rate of the pulse.