STRUCTURE OF HEART
EXTERNAL STRUCTURE :-
Heart is situated at the ventral side of mediastinal space of thoracic cavity in between the lungs. Left lung has cardiac notch, heart is 5×3.5 inches in size, weight 300 gm. It is enclosed in coelomic epithelium. Its triangular superior-broad portion is tilted slightly towards right (dorsal) side. Its lower narrow portion is tilted towards left side. Covering of heart is called pericardium which consists of two layers :
(i) Outer : Parietal pericardium in which two sub layers are present :
(a) Outer : Fibrous connective tissue layer
(b) Inner : Simple squamous epithelium (Serous membrane)
(ii) Inner : Visceral pericardium or epicardium made up of simple squamous epithelium (Serous membrane).
The narrow space in between these two membranes is called pericardial cavity.
A serous fluid is present in this cavity it is called pericardial fluid. It is secreted by the pericardium. Pericardial cavity is a true coelom (as it lies between two layers of mesoderm).
Functions of pericardial fluid :
(1) It prevents the heart from external jerks.
(2) It provides moisture to heart. It prevents the two membranes from collapsing.
(3) It prevents the heart from the bad effect of friction at the time of contraction.
The heart of man is four chambered 2 auricles and 2 ventricles. It is pinkish in colour and conical in shape.
The broad upper part of heart is called auricular part or base and lower conical part is called ventricular part (its tip is called apex).
In between the auricles and ventricles, a clear groove is present, which is known as coronary sulcus. This groove is more towards auricles, by the effect of this the auricular surface is smaller than ventricles.
(a) Auricles- Auricular part of heart is smaller and of dark colour. Its walls are thin. It is divided into right and left auricles by fissure called interauricular sulcus, which is shifted slightly towards left. Therefore out of these two, right auricular surface is bigger than left auricle.
Each auricle forms an bulbous structure called auricular appendages. It covers a small part of ventricle of its side.
(b) Ventricles- Ventricular part is broad, muscular and of light colour. Ventricles have thicker walls than auricles.
The grooves which divide the two ventricles are termed as Interventricular groove or sulcus. It is oblique or tilted toward Right. It does not reach till the tip or apex of the heart, So the right ventricle is smaller than the left ventricle.
Left ventricle is more muscular and thick walled then right because it has to pump blood into those arteries which take blood throughout the body while right ventricle has to pump blood only to the lungs.
left part of the heart ( i.e. left auricle and left ventricle) contain the blood which is to be pumped into the systemic circulation, therefore it is called systemic heart. The main purpose of such a circulation is to transport oxygen, as well as nutrients to the body tissues, and to remove carbon dioxide and other harmful nitrogenous waste from them
Pulmonary heart –
Right part of the heart ( i.e. right auricle and right ventricle) contain the blood which is to be pumped in pulmonary circulation for oxygenation, therefore it is called pulmonary heart. The pulmonary circulation is responsible for regular oxygenation of the impure deoxygenated blood which is received by the right auricle.
Blood supply of heart (Coronary circulation)
The oxygenated blood is supplied to the heart musculature for its consumption with the help of two coronary arteries, left and right. These arteries arise from the common origin at arch of aorta. The left and right coronary arteries then further subdivides into a number of branches carrying blood to different regions of heart. The impure blood from heart walls return back via coronary veins which drain into the coronary sinus. The coronary sinus opens in the right atrium.
Ischemic heart diseases
If the lumen of any of the coronary artery gets narrowed due to obstruction or cholestrol deposition, the cardiac tissues enter a condition of more demand and less supply whenever the person performs exertion. Under such hypoxic conditions a pain might arise in heart muscles ,this condition is called Angina Pectoris. This condition is reversible when the demand supply ratio is restabilised. (i.e. when the person stops exertion and rests). A Coronary Artery Bypass Grafting (CABG) may be required to provide additional channel of blood supply in such cases .
In Coronary Artery Bypass Grafting, a part of internal mammary artery or a segment of patients own saphenous vein is used as the By pass channel .
Myocardial Infarction (M.I.) –This is cellular death of cardiac tissue due to anoxia .
When the blood supply to the heart completely stops due to complete block of a coronary artery, under reduced oxygen condition the heart tries to re-establish the blood supply by working even harder, thus aggravating the situation even further. Due to this reason the cardiac tissue starts dying by necrosis and myocardial infarction sets in, this is an irreversible condition. It is also called as HEART ATTACK in common language .
A blockage of left anterior descending artery (LAD) can be most fatal for the heart . (widows artery)
Internal Structure of Heart The internal structure of heart is discussed under following headings .
1. Cut section of heart : The wall of heart is made of three layers from outside inwards :-
i) Epicardium – outermost layer, mesodermal in origin. Made of simple squamous epithelium.
ii) Myocardium – middle layer, thickest , mesodermal. Made of cardiac muscles which are striated but involuntary .
iii) Endocardium – innermost layer, endodermal in origin. Made of simple squamous epithelium
Interauricular septum– it is a partition between the left and right auricles. It is shifted slightly towards left, so the Right auricle is slightly bigger than left. An oval depression (Fossa Ovalis) is present on its Posterior part.
It is remainant offoramen ovalepresent in foetal stage which closes at birth. In foetal circulation the lungs are non functional and by-passed so the blood directly reaches the left atrium from right. Atrium through foramen ovale.
Interventricular septum– it is a partition dividing the right and left Ventricles. It is shifted towards right. So the left Ventricle is bigger than right .
Auriculo ventricular septum – it separates the two auricles from the two ventricles. It is shifted upwards towards Auricles. Therefore auricles are smaller than ventricles .
3} Chambers The mammalian heart has four chambers .
Right Auricle or Atrium - Inlets :- it receives one S.V.C. , one I.V.C. and one opening of coronary sinus in man. (It receives two S.V.C. (rt. & lt.) in case of rabbit )SVC = superior vena cava = anterior vena cava = pre caval ; IVC= inferior vena cava = posterior vena cava = post cavals. The SVC & IVC bring impure blood from the upper and lower body parts respectively. The Coronary sinus receives impure blood from the rt. & lt.
Coronary veins and drains it in the right auricle Outlets :- this impure blood drains through the right AV foramen into the right ventricle
Right Ventricle - Inlets :- receives impure blood through right AV foramen from right auricle Outlets:- drains the impure blood into pulmonary artery through which it reaches lungs for oxygenation.
Left Auricle - Inlets :- receives oxygenated blood from lungs via pulmonary vein Outlets:- this pure blood is drained into left ventricle through left AV foramen .
Left Ventricle - Inlets :- receives pure blood through left AV foramen from left auricle Outlets:- drains pure blood into the Aorta from where it is supplied to systemic organs.
4} Walls Auricles :- The inner wall surface here presents a series of transverse muscular ridges called musculi pectinati. They run forwards and downwards towards AV foramen, giving appearance of the teeth of a comb (combed muscles).
Ventricles :- The inner wall is rough due to presence of muscular ridges trabeculae carneae or columnae carneae. These continue as papillary muscles , whose one end is attached to the ventricular wall and the other end connected to the cusps of AV valves by chordae tendineae. These chordae tendineae are collagenous and inelastic chords one end of which is inserted in the papillary muscles and other end is connected to the flaps of AV valves. These are meant for preventing the pushing of flaps into atrium during ventricular contraction.
5} Valves Rt. Atrium :- All its inlets are guarded with valves to prevent backflow of the blood. The SVC opening is said to be guarded by Haversian valve. The IVC which opens below this has its opening guarded by a valve called Eustachian valve(during embryonic life the valve guides the inferior vena caval blood to the left auricle through foramen ovale) The opening of coronary sinus in rt.
Atrium is guarded by Thebesian valve.
Left. Atrium :- At its inlet is pulmonary vein (four veins in man and two in rabbit), these have no guarding valve.
AV foramen :- The right AV foramen has a unidirectional valve called tricuspid valve (made of three flaps or cusps) which allows entry of Blood from Rt. Atrium to Rt ventricle and prevents its backflow.
The unidirectional valve present on left AV foramen is made of two cusps only ,hence called bicuspid valve. (also called as the Mitral valve).
Rt. Ventricle :- Its outlet is in the pulmonary artery . it is guarded by a pulmonary semilunar valve Lt. Ventricle :- Its outlet is in the systemic aorta. This opening is guarded by an aortic semilunar valve. Both these semilunar valves are made of three cusps each and are unidirectional in nature.
From the two ventricles the pulmonary artery & systemic aorta arise out in the form of arches (called as pulmonary and systemic arches). These arches cross each other ,and at the point of crossing they are attached by ligamentum arteriosum. Ligamentum arteriosum is the remainant of ductus arteriosus. ductus arteriosus is a small channel connecting the lumen of the two arches which gets closed at the time of birth.
Congenital Heart Disease
These are the diseases present since birth due to defects in the development of heart where there is incomplete separation of oxygenated and deoxygenated parts of blood due to which there is mixing of pure and impure blood. Some common types are :-
1) Patent foramen ovale :- The foramen ovale fails to close after birth as a result of which there is an atrial septal defect, through which the impure blood in Rt atrium reaches the left atrium and mixes with the pure blood present there.
2) Patent ductus arteriosus :- The ductus arteriosus fails to close after birth due to which the impure blood in the pulmonary arch mixes with the pure blood in the aortic arch.
3) Ventricular septal defect :- The inter ventricular septum is incomplete thus allowing mixing of the blood in the two ventricles.
Mixing of impure deoxygenated blood in the pure oxygenated blood in any of the above cases leads to decrease in the quantity of oxygen supply to the body. Thus, symptoms of cyanosis (bluish discolouration of skin) develop.
Such babies are called Blue babies. These childrens get exhausted early due to inefficient oxygen supply .
Conduction Pathway The conducting system of Heart
It is made of myocardium that is specialised in for initiation and conduction of the cardiac impulse. Its fibres are finer than other myocardial fibres, these are completely cross striated and posess special nerve like properties (= self excitatory neuromuscular pathway).
The conducting system has the following parts :-
S.A. Node (Pacemaker)
Inter nodal pathway
Bundle of His
Purkinje fibres (Rt & Lt)
Rate of conduction is fastest in bundle of His and slowest in AV node
1) Sinoatrial node (SA node). It is known as the “pacemaker” of the heart. It is present in right upper corner of the right atrium. It generates impulses at the rate of about 72 per minute. And initiates heart beat. Discovered by Keith and Fack.
2) Internodal pathway : it is the network of neuromuscular pathway that connects the SA node to the AV node.
3) Atrioventricular node (AV Node). It is smaller than SA node and is situated in the lower left corner of the right atrium close to the atrioventricular septum. It is capable of generating impulse at rate of about 40/min. Discovered by Kent.
4) Bundle of His (AV Bundle). it is the connection between the atrial and ventricular musculature. It begins at the AV node and then divides into left and right branches as it descends down towards ventricles.
The left branch of the AV bundle descends on the left side of the interventricular septum and is distributed to the left ventricle after dividing into Purkinje fibres.
5) The Purkinje fibres. These are distributed through the endocardium of the ventricles and propagate the impulse in the entire ventricle musculature. (18-25 per min.)
Why SA node is called the pacemaker of the heart?
Although impulse is produced by the entire neuromuscular pathway, the frequency of impulse generation is maximum in case of SA node in comparison to other parts of pathway. Hence it guides the rhythm of heart beat and is called the pacemaker of the heart . The AV node on the other hand just conducts the impulse forwards.
WORKING OF HEART
Rhythmic contraction and relaxation of heart is called heart beat. Actually, contraction and relaxation occur separately in atria and ventricles. However, ventricular movements are quite prominent and forceful. Therefore, heart beat is synonym with ventricular or apex beat. The rate of heart beat in an adult male is on the average 72 per minute. It is higher in women, children and infants and lower in aged persons. It increases temporarily with activity and disease. In animals heart beat is connected with size. In mammals, smaller animals have higher heart beat.
Adult human - 72 per min.
Rabbit - 210 per min.
Elephant - 28 per min
Foetus - 140-160 per min.
Rat - 400-600 per min.
Sparrow - 500 per min.
New born 120-140 per min
Shrew - 600-800 per min.
Canary bird - 1000 per
Child - 100 per min
Blue whale - 25 per min.
Frog - 64 per min.
Heart beat is entirely controlled by nervous supply in arthopods and some annelids. It is called neurogenic heart beat and the heart is called neurogenic heart. In molluscs and vertebrates heart beat originates from a special muscular tissue. Such a heart beat is called myogenic heart beat and the heart is called myogenic heart. Human heart is myogenic.
Differences between Neurogenic and Myogenic Hearts
Impulse of heart beat comes from outside heart.
The impulse of heart beat develops within the heart.
Impulse is generated by nervous system.
Impulse is generated by a special muscular tissue;
Nerve fibres are spread over the heart to bring about contraction and relaxation
There are special conducting muscle fibres for spreading the impulse.
Heart will stop beating if removed from the body
It will continue to beat for some time, if detached heart is supplied with proper nourishment and favourable conditions.
Each heart beat has two components, systole and diastole. Systole represents contraction while diastole represents relaxation of heart chambers.
Heart beats are listened with the help of an instrument called stethoscope (invented by Laennec).
Regulation of Heart Beat Centre for heart beat Regulation is located in medulla oblongata. (Brain stem)
(i) Nervous Control – The " Cardiac-centre"(neural centre) which regulates heart-beat is found in Medulla-oblongata of the brain it can moderate the cardiac function through ANS. This cardiac-centre has two units
(i) Cardio- accelerator centre.
(ii) Cardio-inhibitory centre.
From the cardio-acceleratory centre, a pair of sympathetic nerves go into the S.A. node. This centre increases the rate of heart-beat.
While the cardio-inhibitory centre sends impulses to the S.A. node through cardiac branch of Vagus-nerve.This center reduces the rate of heart-beat.
(b) Hormonal Control
(1) From the sympathetic nerve-fibres a hormone sympathins or nor-adrenaline is secreted which increases the heart-beat.
(2) From the parasympathetic nerve-fibres hormone Acetyl-choline is secreted which decrease the heart beat.
(3) The Adrenaline hormone and Thyroxine hormone secreted by Adrenal medulla and thyroid respectively also increase heart beat.
Hormonal Control -
Autonomic Nervous System -
Tachycardia. It is the condition where heart rate exceeds 90 per minute for an average adult.
Common causes of tachycardia :-
i) Temperature. Rate of heart beat increases. Fever causes tachycardia because increased body temperature increases the rate of metabolism of the sinus node, which in turn directly increases its excitability and rhythm.
ii) Stimulation by sympathetic nerves. Stimulation of the sympathetic nerves releases the hormone norepinephrine at the sympathetic nerve endings. Therefore this leads to increase in the heart rate.
iii) Weak condition of the heart. Weakening of the myocardium usually increases the heart rate because the weakened heart does not pump blood into the arterial tree to a normal extent ,and this causes sympathetic reflexes to increase heart rate.
iv) Shock/loss of blood. When a patient looses blood and passes into a state of shock or semi shock, reflex stimulation of heart occurs which increases the frequency of heart beat to compensate for less delivery.
v) Exercise. Physical exertion cause an increased consumption of oxygen by tissues. In order to meet the increased demand the heart has to work faster .
vi) Sinus tachycardia. Increased frequency of impulse discharges from the SA node will in turn increase the heart rate.
Bradycardia.It is the condition where the heart rate falls below 60 per minute in an average adult.
Common causes of bradycardia :-
(i) Temperature: Fall in body temperature leads to fall in the rate of SA node metabolism, which in turn reduces its Excitability and rhythm.
(ii) Stimulation by parasympathetic Vagus: Parasympathetic stimulation of acetylcholine secreted by vagus has an Inhibitory effect on the SA node. (opposite phenomenon of sympathetic stimulation occurs here).
(iii) Stronger condition of the heart: The athlete’s heart is considered stronger than that of a normal person. This allows it to pump greater stroke volume output per heart beat. When the athlete is at rest, this excessive quantity of pumped blood causes a negative feed back response resulting in bradycardia.
(iv) Rest: When at rest or sleeping, the oxygen demand of body is lesser this gives a negative feedback resulting in falling heart rate.
(v) Sinus bradycardia: Reduced frequency of impulse discharge from SA node will reduce the heart rate.
The ratio of heart rate to respiratory rate in an average adult under normal circumstances is 4:1