Heart
- Cone-shaped hollow muscular organ. The size of one's heart is approximately the size
of his/her fist. - Weight in males is 300-350 grams and female is 250-300 grams.
- Lies in the thoracic cavity between T5-T8 in the mediastinal space (In rabbit, heart lies in the left part of the thoracic cavity between arms.
- The heart is myogenic which means the cardiac muscles in the heart contract by a pulse produced by a myocyte (the cardiac muscle)
Layers of the Heart
1. Pericardium:
Heart is covered by a tough, 2 layered fibro-serous sac called pericardium. The outer layer is non distensible fibrous pericardium and inner layer is thin serous pericardium which further consists of outer parietal layer (attached to fibrous pericardium) and inner visceral layer (adhered to the heart and is called epicardium).
2. Myocardium:
Middle, thickest layer [BPKIHS 2012, I0M]. highly vascular, composed of cardiac muscle fibres joined together by intercalated disc. The connective tissue in myocardium acts as cardiac skeleton.
3. Endocardium:
Innermost layer, consists of endothelium of squamous cells restüng on thin basement membrane of loose connective tissue.
External structure of Heart:
The human heart is 4 chambered (two atria and 2 ventricles) and is divided by a septum into two halves - right and left
(a) Auricles (Atria):
Two in number and demarcated externally from ventricles by irregular groove called coronary sulcus and from each other by inter-atrial sulcus
Right auricle is larger than left auricle .
Pulmonary vein enters inside left auricle.
Two venacava enters inside the right auricle of human wnile 3 venacava (2 superior-right & left pre-cavals and 1 inferior venacava or post-caval)
(b) Ventricles:
Two in number and demarcated externally from each other by an oblique groove called interventricular sulcus which contains coronary blood vessels.
Left ventricle is larger than right ventricle.
NOTE:
Sinus venosus and conus arteriosus: Sinus venosus and conus/truncus/bulbus arteriosus are accessory chambers in the heart of lower vertebrates (fishes and amphibians).
In rabbit, sinus venosus is formed in the embryo but later it becomes a part of wall of right auricle.
In frog, sinus venosus spreads upon most of the dorsal side of heart and conus arteriosus lies obliquely upon the ventral surface of right atrium
Internal Structure of Heart
Auricles:
Thin walled, act as reservoirs for blood entering the heart
Right auricle is bigger than left auricle and both are separated by a myomembranous partition called interatrial or interauricular septum.
During embryonic stage, in place of this septum, septum primum and septum secondum are present having a gap called foramen ovalis between them. A flap called Eustachian flap is present from the opening of inferior venacava upto foramen ovalis, which prevents the entry of blood in the fetal heart to lungs because in fetal life, lungs are not functional and purification of blood is done by placenta.
At the time of birth, there is closure of foramen ovalis but there remains depression on both side. Depression on the right is atrium is called fossa ovalis and on the left is called fossa lunata
Inner surface of auricles is smooth. A network of muscular ridges called musculi pectinati or trabeculi pectinati present internally in the region of the auricular appendages and give comb like appearance.
Right auricle receives deoxygenated blood via superior and inferior vencava and left auricle receives oxygenated blood via pulmonary veins.
Ventricles:
Right and left ventricles are separated by an interventricular septum which is obliquely curved towards right, so that the left ventricle is larger than right one but the cavity of left ventricle is relatively smaller because the myocardium of left ventricle is 3 times thicker than right ventricle.
Walls of the ventricles are internally raised into a number of thick, muscular, columnar projections called columnae carnae or trabecular carnae; and a few large muscular elevations called papillary muscles or musculli papillares which are 3 in right ventricle and 2 in left ventricle.
Papillary muscles act as anchors for chordae tendinae.
Chordae tendinae [MOE 2010]: Strong, inelastic thread like tendons present in the mammalian heart but absent in frog. One end is attached to the cusps of atrioventricular valves and the other end to the papillary muscles of the ventricles. These muscles contract during ventricular systole and pull the valves downwards. They hold the valves in place.
Chordate tendinae and the papillary muscle prevent the backflow of the blood from the valve. The caroticosystemic aorta crosses over the pulmonary aorta. At the place of crossing the pulmonary aorta and the caroticosystemic aorta are connected by muscular band known as ligamentum arteriosum which during embryonic stages was represented as a single artery known as ductus arteriosum or duct of botalli.
Valves:
The valves of mammalian heart are tendinous cords
a. Atrio-ventricular valves:
Auricles open into the ventricles through auriculo-ventricular apertures or AV apertures guarded by one way Atrioventricular valves or parachute valves and are located dorsally or posteriorly. There are 2 types of AV valves in mammals:
1. Right AV valve or Tricuspid valve: Between the right auricle and right ventricle and consists of 3 membranous flaps or cusps
2. Left A.V. valve or Bicuspid or Mitral valve: Between the left auricle and left ventricle and consists of 2 flaps or cusps. The bicuspid valve resembles mitre or topi of bishop, hence, also called Mitral valve.
- These valves allow the passage of blood from auricles into ventricles but prevent backflow.
- In frog, the AV valves are semilunar type and not of cuspid type. A single row of AV valves due to single ventricle is present.
- Narrowing of valve is called stenosis
b. Semilunar valves:
The aortic valve is present between left ventricle and aorta and Pulmonary valve is present between right ventricle and pulmonary artery. [BPKIHS]
Other valves:
(c) Eustachian valve are present at the opening of inferior vena cava (post caval) in the right auricle in rabbit but in human, the vestige of eustachian valve is present over the opening of post caval vein and allows the passage of blood in right auricle.
Haversian valve is present in human over the opening of precaval vein and allows the passage of blood in right auricle but absent in rabbit.
Thebesian or coronary valve is present over the opening of coronary sinus in right auricle in mammals and allows the passage of blood in right auricle.
Sinus of valsalva: When the semilunar valves open towards aorta, a gap remains between the flaps and the wall of aorta which is called sinus of valsalva. When ventricles relax, blood is filled in this sinus and is called Drooping of blood.
Corpora Arantii: These are the thick nodules present on the edges of the flaps of semilunar valves which prevent the reverting of these valves into the ventricles.
Blood is supplied to the Heart by coronary artery and venous drainage is by coronary sinus directly into the right atrium.
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