This pacemaking and conducting system structures are composed of myocardiai cells although

ed arborising fibers of Purkmje It should be clearfy borne in mind That these structures, viz. tha SAN, AVN, bundle of his and Purkinje's fibers, together from a system, whose function is to create impulse (pace making) and convey the impulse (conduction of impulse} lo every part of the heart. This pacemaking and conducting system structures are composed of myocardiai cells although their functions have become highly specialized, viz. pace making and conducting the impulse These speuahzed cells, thus, do not have sufficient contractility The muscle of the heart (including the above mentioned pace making and conducting system) are called myocardium. Pericardium The entire heart is enclosed by a sac called pentardium In between the two layers of pencardium. a cavity called pencardial cavity exists which contains a very small quantity of fluid If there is an acute enlargement of heart the pericardium may oppose the enlargement However, if the cardiac enlargement is chronic. the pencardium also grows and therefore does not oppose the expansion of hear! The heart valves AV valves Each orifice between atnum and the corresponding ventricle, contains alriovernncular (AV) valves Thus, There are two AV valves, right and the left. These valves function in such a way that blood can tow from atrium to ventricle but not from ventncle to atnum .Therefore, these valves are open during ventncular diastole bul are closed during ventncufer systole The right AV valve consists of three cusps or leaflets (fig 5 1 2) whereas the left consist of two cusps and hence they are known as tricuspid and bicuspid (mitral) valves respectively During ventncular systole, Ihe free margins of the cusps meet each other and seal off the orifice so efficiently that no blood can escape from the ventricle into the corresponding atrium /I the ventricular side of the cusps, strong chord like structures, catted chordae tendmeae (fig 5.1 3) are attached These chords arise from muscular structures (looking lika muscle bellies) tailed 'papillary muscles', which in tum. arise from the vanrncutir myocardium That is. these papillary muscles and chordae tendmeae run through Ihe cavities of the ventricle and get attached with the undersurlace of the AV valves. Fig 5 1 2 The cusps of the different waives When venlncilar systole (contraction of tha ventncle) occurs. the papillary muscles also contract and thereby rhey exert a pull on the cusps of the AV valves This pull ensures that there is no prolapse of the valve due to high pressure (during ihe ventncular systole)L into the atrium The chordae lendmeae and the papillary muscles may therefore be viewed as different members of a single system, aimed to prevent the prolapse of Ihe valves as mentioned above For studymg the mechanical properties of heart muscle, and the effects of drugs on them. The erpenmental physiologist or pharmacologist usually chooses isolated papillary muscles of a mammalian heart Semilunar valves.Aortic and pulmonary valves are known as temilunar valves They are present in The beginning or aorta and pulmonary artery respectively. Both the aortic and the pulmonary valves contain three cusps These valves allow one way traffic of blood from the right ventricle to the pulmonary artery, or from the lefl ventricle to the aortaj but not in the reverse direction',. Ventricular muscles As stated already, the left ventricular musculature is quite heavy, the right ventricular one is comparatively thin. The muscles of Ihe venlncles take their origin from the atnoxentncular fibrous rings (annull fibrosi) of the heart (fig 5 1 2) There are four Fibrous rings (annull fibrosi) m the heart, situated at Ihe amoventricular orifices as wall as in the regions of pulmonary and aortic valves (Tig.5 1.2) Fig 5 13. Chordae landmae Because of these nngs, the musculature of the atria and those of the ventricles are competely separaled from each other. The only communication between the atria and the venlncles being The atnoventncular bundle of His This issue needs to be carefully understood Although, we do not say nowadays that anatomically hear! is a syncylium (vide infra), from the physiological point of view, tha heart muscle slill can be regaided at father, a sort of syncytium However the atrinventricular rings cornplately cut off tha continuity of the atrial and ventricular musculature Ultimilaly, tharafore. tha two atria togethei may be viewed at if acting as a single cell and the two ventricles acting as another single cell. The ventricular muscles, at already stated, take their origin from the AV rings [Vanous bundles of muscles, have been described by some authors, but many authors consider these to be artifacts and will not be discussed here any morel ) is more important to understand the effects of contraction of ventricular muscles. When the left ventricle contracts and the muscles are allowed lo shorten (isotmmc contraction), the main directions in which tha left ventricle shortens are, (i) base to apex. thal is the base moves towards the apex and the vertical diameter is reduced, and (ii) side to side, that is, the distance between the inlervanlncular septum and The lataral watt rjf the left ventricle is reduced As a result, the contracting left venlncle greatly squtezes on its content (The blood) The blood cannot escape into the left atrium (owing To the firm closure of the mitral valve) and so the blood forces open The aortic valve and escapes into Ihe aorta The right ventricle mainly shortens m ils transverse diameter There is another factor, called, left ventricular aid. This mechanism is produced by the bulging of the intervent