Circulatory System of Class 11
(i) Shape and position : Reddish, roughly conical, highly muscular, mesodermal hollow organ of the size of one’s first. Its average weight in males is about 300 gm. and in females about 250 gm. It lies behind the sternum in the mediastinum space of thoracic cavity in between the two lungs. The broader base faces upward and backward. The narrower apex is directed downward, forward and slightly towards left, lying between 5th and 6th ribs and rests on the diaphragm.
(ii) Protective covering : Heart is enclosed in a tough, 2 layered fibroserous sac, the 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).
Between the parietal and visceral layers, occurs a narrow potential space, the pericardial cavity which is derived from coelom and is filled with serous pericardial fluid for frictionless movement and protection from shock and mechanical injury.
(iii) Histology : The heart wall consists of connective tissue, blood vessels and cardiac muscle fibres in 3 different layers – Epicardium, Myocardium and Endocardium.
(a) Endocardium : Innermost layer lining the cavity of heart and consisting of endothelium of squamous cells resting on thin basement membrane of loose connective tissue.
(b) Myocardium : Middle, highly vascular layer, composed of cardiac muscle fibres joined together by intercalated disc. The connective tissue in myocardium acts as cardiac skeleton. Endocardium is thickest where the myocarduim is thinnest and vice-versa.
(c) Epicardium : Visceral pericardium, joined to myocardium by connective tissue.
(iv) External structure : Human heart is 4-chambered and is divided by septa into two halves – right and left. Each half has one darker, thin walled auricle in the broader upper region and one lighter, thick- walled ventricle in the narrower lower region.
(a) Auricles (Atria) : Two in no., demarcated externally from ventricles by irregular groove called coronary sulcus and from each other by interatrial sulcus. When atria contract, small curtain like flaps called auricular appendages or appendices project from sides of auricles and overhang the corresponding ventricles.
(b) Ventricles : Two in no. demarcated externally from each other by an oblique groove called interventricular sulcus which contains coronary blood vessels. The right ventricle does not reach apex.
(c) 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.
(v) Internal structure
(a) Auricles : Atria are thin walled. They act as reserviors 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, at the place of this septum, there are present septum primum and septum secondum having a gap (aperture) called foramen ovalis between them. From the opening of inferior vena cava upto foramen ovalis, there is a flap called Eustachian flap which prevents the blood in the foetal heart go to lungs because in foetal life, lungs are not functional purification of blood is done by placenta. At the time of birth, there is closure of foramen ovalis but there remains depression on posterior part of the right surface of interauricular septum in rabbit. In man this depression is present on both the side. because of least regenerative power in human being. The depression towards right atrium is called fossa ovalis and depression towards left atrium is called fossa lunata.
PFO (Patient Foramen Ovalis) or septal defect : In case there is no closure of foramen ovalis, then disease is called PFO. In this condition, there is mixing of blood after birth which gives bluish appearance to the body called as Cyanosis. Such child is called Blue Baby.
The inner surface of auricles is smooth. A network of muscular ridges called musculi pectinati or trabeculi pectinati occurs internally in the region of the auricular appendages and give comb like appearance.
(ii) Ventricles : The right and left ventricles are demarcated by an interventricular septum which is obliquely curved towards right, so that the left ventricle is larger than right one. However, the cavity of left ventricle is relatively smaller and nearly circular because the myocardium of left ventricleis 3 times thicker than right ventricle whose cavity is larger and somewhat crescentic.
The walls of the ventricles are internally raised into a number of thick, muscular, column shaped 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. These muscles act as anchors for chordae tendinae.
Chordae tendinae : Numerous, strong, inelastic thread like tendons present in the mammalian heart but absent in frog. One end of these threads is attached to the cusps of A.V. valves and the other end to the papillary muscles of the ventricles. These muscles contract during ventricular systole and pull the valves downwards, thus, preventing their everting into atria. The chordae tendinae hold the valves in place.
Regurgitation : If there is weeakening of papillary muscles or breaking of chordae tendinae, then AV valves revert into auricles. So, blood goes in opposite direction, it is called regurgitation. Sometimes, there is narrowing of valves. So, there remains gap between the valves which causes regurgitation.
Moderator band : Right ventricle contains a prominent muscular trabeculum called moderator band which extends from the interventricular septum to anterior papillary muscle.
(vi) Major blood vessels associated with heart : The blood vessels that enter or leave the heart are called Great Blood Vessels.
(a) Superior vena cava or precaval : Brings deoxygenated blood from head and upper parts of the body into the right auricle through an opening which is single in human and cat and two in rabbit as there are 2 precavals – right and left in rabbit. In frog, right and left precavals open into sinus venosus.
(b) Inferior vena cava or post caval : Drains deoxygenated blood from middle and lower parts of the body into the right auricle through a single opening which is bordered by a membranous, falciform fold which is a remnant of the foetal valve of Eustachian. In frog, post caval opens into sinus venosus.
(c) Coronary sinus : Returns deoxygenated blood from heart wall into right auricle through a single opening.
(d) Pulmonary vein : Four pulmonary veins, two from each lung, carry oxygenated blood from the lungs and open into the left auricle through four openings. In rabbit, the pulmonary veins open in the left auricle through 2 openings.
(e) Pulmonary aorta/arch : Arises from upper left corner of right ventricle through a single opening and divides into right and left pulmonary arteries which carry deoxygenated blood to the lungs for oxygenation.
(f) Systemic aorta : Arises from upper right corner of left ventricle through a single opening and has 3 regions – ascending aorta, arch of aorta and descending aorta. It distributes oxygenated blood to various body parts except lungs.
Ligamentum arteriosus : During foetal life, because the lungs are non-functional hence blood of pulmonary aorta comes into systemic aorta through a small duct called ductus botalli or ductus arteriosus soon
after birth, deposition of elastin fibre blocks this duct, forming a new structure called ligamentum botalli or ligamentum arteriosus.
PDA (Patient Ductus Arteriosus) : If the ligamentum arteriosus remains open, the condition is called PDA. In this case, there is mixing of blood which leads to blue baby.
Valves : The valves present in the mammalian heart are tendinous cords.
(a) Eustachian valve : Present on the opening of inferior vena cava (post caval) in the right auricle in rabbit, whereas in human, the vestige of eustachian valve is present over the opening of post caval vein. It allows the passage of blood in right auricle.
(b) Haversian valve : Present in human but absent in rabbit. It is present over the opening of precaval vein and allows the passage of blood in right auricle.
(c) Thebesian or coronary valve : Present over the opening of coronary sinus in right auricle in mammals and allows the passage of blood in right auricle.
(d) Atrio-ventricular valves : Auricles open into the respective ventricles through wide passages called auriculo ventricular apertures or A.V. apertures which are guarded by one-way A.V. valves or parachute valves and are located dorsally or posteriorly. There are 2 types of valves in mammals.
(1) Right A.V. valve or Tricuspid valve : Present between right auricle and right ventricle. It consists of 3 membranous flaps or cusps.
(2) Left A.V. valve or Bicuspid or Mitral valve : Present between left auricle and left ventricle. It consists of 2 flaps or cusps. The bicuspid valve resembles mitre or topi of bishop, hence, also called as Mitral valve.
The upper edges of the flaps are attached to the margins of the A.V. apertures while the lower edges project freely into the ventricles. The free edges of these flaps are connected by chordae tendinae to he papillary muscles of the ventricles. These valves allow the passage of blood from auricles into ventricles but prevent backflow.
In frog, the A.V. valves are semilunar type and not of cuspid type. There is single row of A.V. valves due to single ventricle.
(e) Semilunar valves : At the base of pulmonary arch and systemic aorta, three membranous, pocket-shaped flaps called semilunar valves are present which are set in a ring with their cavities directed away from the ventricles. They allow the passage of blood from ventricles to respective blood vessels, but prevent the return of blood.
Sinus of valsalva : When the semilunar valves open towards aorta, there remains gap between the flaps and the wall of aorta. This gap is called sinus of valsalva. When ventricles relax, blood is filled in this sinus and is called Drooping of blood.
Corpora Arantii : Thick nodules present on the edges of the flaps of semilunar valves which prevent the reverting of these valves into the ventricles.
The mechanism which results in sealing off of a leaking or severed blood vessel to stop bleeding is called hemostatic mechanism. A leakage in a blood vessel inside the body is plugged or sealed by blood platelets which adhere together to pile up into a sticky plug called thrombus. This process is called agglutination of the platelets. Sometimes, a thrombus is so large as to block the bloodstream in the concerned vessel or capillary. This may prove fatal if it happens in such organs as brain, lungs, heart, etc.
Bleeding in tissue spaces or at body surface due to damage to a blood vessel is stopped by a different process called blood coagulation or clotting.
As described by Howell, this process is a complex series of sequential changes which can be grouped into three major steps as follows:
(i) First step: As a blood vessel or certain capillaries are torn at the site of an injury and bleeding starts, the platelets adhere to the damaged tissues and release a phospholipid, called platelet factor-3. Traumatized tissues at the injury also relase a lipoprotein factor called thromboplastin. These two factors combine with calcium ions (Ca++) and certain proteins of the blood plasma to form an enzyme named prothrombinase.
(ii) Second step: In presence of Ca++, the prothrombinase inactivates an anticoagulant of blood called heparin or antiprothrombin. Heparin normally keeps the blood in ‘sol’ condition for its easy flow in the blood vessels. When heparin is inactivated, the prothrombinase easily catalyzes breakdown of an inactive plasma protein, prothrombin, into an active protein called thrombin and some small peptide chains.
(iii) Third step: Lastly, the thrombin, acting as an enzyme, first brings about depolymerization of a soluble plasma protein, the fibrinogen, into its monomers. Later, it stimulates repolymerization of these monomers into long, insoluble, fibre-like polymers of a different protein called fibrin. The thin, long and solid fibres or fibrin form a dense network upon the wound. Blood corpuscles become entrapped in this network. Thus, a red clot format at the wound in about 2 to 8 minutes after injury. It stops further bleeding. Soonafter, the clot starts contracting and a pale yellow fluid, called serum, starts oozing out from it.
This serum is blood plasma minus the corpuscles and fibrinogen.
The prothrombin and fibrinogen of blood plasma are formed in liver with the help of vitamin K. Hence, deficiency, not only of these proteins, but also of vitamin K causes haemophilia in which blood clotting
takes so much time that the patient generally dies due to excessive bleeding. This is a genetic disease.