Heart

INTRODUCTON

The heart is about the size of a first and is located in the thoracic cavity just behind the sternum and in beteen the lungs. The heart is situated beteen the two lungs and behind the sternum in the thorax. It is surrounded by a tough sac, the pericadium, the outer part of which consists of inelastic white fibrous tissue. The inner part is made up of two membranes. The inner membrane is attached to the heart and the outer one is attached to the fibrous tissue.

Pericardial fluid is secreted between them nd reduces the friction between the heart wall and surrounding tissues when the heart is beting. The inelastic nature of the pericrdium as whole prevents the heart from being overstretched or over filled with blood. There are four chambers in human heart, two upper thin walled atria (single arium) and two loer thick alled ventricles. The walls of atria are thin because they only hve to pump blood to ventricles.

The right side of heart is completely seperated from the left. The right side deals with deoxygenated blood and the left side with oxygenated blood. Human heart is a hollow muscular contractile organ, the centre of the circulatory system. It provides the propulsive force for circulating the blood throughout the vascular system. Heart’s wall possess three layers : the outer epicardium (a serous layer), the middle myocardium (composed of cardic muscles) and inner endocardium ( a layer that lines the four chambers of the heart and covers the vlves).

The heart is enclosed in fibrous sac, the pericardium. Cardia muscles hs a longer refractile period than skeletal muscle and consequently does not fatigue. The right atrium receives deoxygenated blood from the general circulaton of the body hile the left atrium receives oxygented blood from the lungs. The muscular wall of the left ventricle is at least three times as thick as that of right ventricles. This difference is due to fact that the right ventricle only hs to pump blood to the lungs, which are very near the heart in the thorax, whereas the ventricle pumps blood all round the body.Therefore, the blood entering the aorta from the left ventricle is at a much higher blood pressure than the blood entering the pulmonary artery.

The circulation from the lungs is called the pulmonary circulation and the circultion round the body is called the systematic circulation. As the atria contract they force blood into the ventricles and rings of muscle which surround the venae cavae and pulmonary veins their point of entry into the atria contrct and close off the veins. This prevents blood returning into veins. The left atrium is seperated from the left ventricle by a bicuspid (two-flapped) valve, while a tricuspid valve seperates the right atrium from right ventricle Jointly, these are known as the artioventricular valves. Attached to the ventricle side of the flaps are fibrous cords hich in turn attach to conical-shped papillary muscles which are extensions of the inner wall of the ventricle. The artioventricular valves are pushed open hen the flaps of each valve press tightly closed so preventing return of blood to the atria . At the same time the papillary muscles contract so tightening the fibrous cords. This prevents the valves from being turned inside out. Semilunar valves are found at the points where the pulmonry artery and aort leave the heart. These prevent blood from getting back into the ventricles. Just beyond the aortic valve are the opening of two coronary arteries. These are the only blood vessels which supply oxygenated blood to the walls of the heart.

STRUCTURE OF CARDIAC MUSCLE

The walls of the heart are composed of cardiac muscle fibres, connective tissue and tiny blood vessels. Each muscles posses one or two nuclei and many large mitochondria. Each fibre is mde up of many myofibrills. These contain actin and myosin filaments which bring about contraction in the striped appearance of the muscle. They account for the striped appearance of the muscle. Fibres. The dark bands known as intercalated discs, are cell surfce membrnes seperating individual muscle cells.The structure of the membrnes is modified to allow ions to diffuse rapidly across them. This allows rapid spred of excitation (action potentials) through the muscle. When one cell become excited, the action potential spreads quickly to all the others, so that the whole mass of fibres behves as one unit.The fibres branch and cross-connect with each other to form a complex net-like arrangement.Cardiac muscle contracts more sloly than skeletal muscle and does not ftigue as easily. No neurons are present in the all of the heart.

THE CARDIAC CYCLE

The cardiac cycle refers to the sequence of events which takes place during the completion of one heartbeat.It involves repeted contraction and relaxation of the heart muscle. Contraction is called systole and relaxation is called diastole. It occurs as follows:

Atrial diastole During the time when the atria and the ventricles are both relaxed, blood returning to the heart under low pressure in the veins enters the two atria. Oxygenated blood enters the left atrium and deoxygenated blood enters the right atrium. At first the bicuspid valve and tricuspid valve are closed but, as the atria fill with blood, pressure in them rises. Eventually it becomes greater than that in the relaxed ventricles and the valves are pushed open.

Atrial systole- When atrial diastole ends, the two atria contract simultaneously. This is termed atrial systole nd results in blood being pumped into the ventricles.

Ventricular systole-Almost immediately the ventricles contract.This is ventricle systole.When this occurs pressure in the ventricle rises nd closes the atrioventricular valves, preventing blood from returning to the atria.The pressure forces open the seminular valvesof the aorta and pulmonary rtery nd blood enters these vessels.The closing of the atrioventriculr vlves during ventriculr systole produces the first heart sound, described as ‘lub’.

Ventricular diastole-Ventricular systole ends and is followed by ventricular diastole.The high pressure developed in the aorta and pulmonary artery tends to force some blood back towards the ventricles and this closes the seminulr valves of the aorta and pulmonary artery.Hence backflow of blood is prevented.The closing of the valves causes the heart sound ‘dub’.

During a complete heartbeat, blood flows through the heart because of sequence of coordinated contraction known as the cardiac cycle. In one cardiac cycle, both atria simultaneously relax and contract, followed by the relaxation and contrction of both ventricles.It is during systole that the atria and ventricles pummp blood.If the heart is beating at the rate of 75 times per minute, each cycle takes about 0.8 seconds to complete.Diastole and systole are generally of equal durtion, each lasting about 0.4 second.

CAUSES OF RHYTHMIC HEART BEAT

The heart acts as a pump. It expands and contracts regulry. The contraction is called systole and relaxation is known as distole. Both the auricles contract simultaneously which is called auricular systole. The relaxation of both the auricles is called similarly auricular diastole.In the same way the contraction and relaxtion of ventricles are called ventricular systole and ventricular distole respectively. A complete heartbeat is formed by one systole and one diastole. One heart beat takes 0.8 sec in a normal man. So in a normal man there will be 72 times per minute

The rhythmic contraction and expansion of heart is controlled by a special kind of nervous tissue which lie in bundles.One is sinus or sinu-auriculr node(SA node) and called pace-maker. The other is auriculo-ventricular node(AV-node).Both are situated in right auricle.Fibres from sinus node extend in both the auricles and bunch of fibres extends in the inter-ventricular septum where the bundle divides into two. This bunch is called Bundle of His. The two branches extend further in the ventricular wall. There each branch forms a netork of fibres clled Purkinje system. Due to the ventricles contract almost simultaneously.

The heart beat begins from sinus node hich leads to contraction of right auricle and then left auricle.This stimulates the auriculo-ventricular node and the sensation go to the ventricular wall through Purkinje system. This results in the contraction of the ventricles. Thus in mmmals an alternate contraction and expansion of the auricles and ventricles take place. As the hert can control its own activity is called myogenic heart.(like frog)

HEART SOUNDS

As heart valves snap during each heartbeat, distinctive sounds can be detected. These sounds can be heard easily with a simple instrument called Stethoscope. The heart sounds are typically characterized as a series of lub-dup sounds. The lub represents the sound produced hen the two AV vlves close during ventricular contraction and the dup represents the sound produced when the seminulr vlves close during ventriculr relaxation. If any of these vlves do not close tightly, some blood will be forced through them producing sishing sound called a murmur.A heart murmur results from small deformities of a valve. Severe murmurs may substntially reducde the efficient flow of blood through the hert and hinder a person’s ability to sustain normal activity.

Coordination of heartbeat

The heart contains a system of specialized muscles cells called the cardiac conduction system that initiates and conducts electical impulses. These impulses stimulate the heart muscle to contrct. The cardiac conduction system consists of four specilized structures : the sinoatrial (SA) node, the artioventricular (AV) nod, the trioventriculr bundle nd the purkinje fibres.The sinoatrial (SA) node is a small mass of specialized muscle tissue in the upper wall of the right atrium. The SA node automatically initiates an impulse, which travels across the atria, stimulating atria muscle cells to contract. This node is often called the pacemaker because it sets the ‘pace’ of each heartbeat.As the impulse spreads along the atria, it stimulates a second mass of specialized muscle cells called atrioventriculr (AV) node t the bse of right atrium close to the septum. The V node connects wwith the atrioventricular bundle(AV bundle), hich has a right branch and a left branch.Each branch conducts the impulse down to the tip(apex) of the heart and forms additional branches called Purkinje fibres. These fibres conduct impulses to the myocardium of the ventricles, stimulating their contraction.The passage of an electical impulse through the the cardiac conduction system tkes about 0.2 second.This sequence of events not only stimultes the myocardium but also ensures coordinated contraction of the atria and ventricles in two ways

(a) First it ensures that atria contract before the ventricles do. This occurs because each impulse is momentrily delayed t the AV node, giving both atria time to empty their contents into the ventricles before the ventricles contrct.

(b) Second , coordination allows the lower portion of the ventricles to contrcat before the upper portion does. This ensurs that blood blood is pumped in the direction of the pulmonary trunk and aorta.

Regulation of Heart Activity

The cardiac conduction system establishes a heart rate of about 75 beats per minute. This is sufficient to meet the body’s needs while at rest, but is not sufficient to meet the expnded needs of our tissues during periods of physical activity or strss. During these periods, the heart contracts faster, pumps blood faster, and moves blood more rapidly through the body. This helps mntain homeostastis more waste products such as CO₂ and lactic acid. Cardiac output is the volume of blood pumped by each ventricle in 1 minute. It is determined by twofactors : hert rate and stroke volume. The stroke volume is the volume of blood ejected by a ventricle at each contraction. For an adult at rest, the heart rate is about 75 beats per minute and stroke volume is about 70 millimetres(ml) per contraction. Using these values, the crdiac output wwould be 5250 ml per minute or 5.25 lites per minute.

Nervous Control

The control centre for the ANS is located in a specific area of brain, the medull oblongata. When this area is stimulted, specific nerves of the ANS send stimulatory impulse to S and AV nodes s well as to the myocardium of the ventricles. As a result, there is increase in heart rte and the heart muscle contracts more forcefully.The control centre for the ANS also receives nerve signals from parts of the brain associated with the emotions. This is one reason hy emotions such as nger, fer nd excitement causes elevated heart activity.

Hormonal Control

The hormone epinephrine is released into the blood by the adrenal glands above each kidney. Hen epinephrine reches the heart, it stimultes the SA node, causing an increse in the hert rate nd the force of contrction.drenline is secreted by the medulla of the adrenal glnds and noradrenaline is also secreted in smaller amounts. Both stimulate the heart, although adrenline is more effective. Cardiac output nd blood pressure are increased by incresing rate.

Blood pressure

As blood is ejected from the ventricles during systole, it exerts a force knon as blood pressure against the wall of a vessel. When a volume of blood passes through an artery, the pressure exerted by the blood causes the artery to bulge. Because of the elastic quality of the artery wall, the bulge is not permanent and the artery will recoil to its normal size. In arteries close to the body surfce, the bulge and recoil can be felt by placing one’s finger on the skin directly over an artery, which is known s pulse.Each pulse corresponds to one heartbeat and consequently, the pulse rate(pulses per minute) is equl to the heart rate. There are several sites on the body where arteries are close to the surface and the pulse can be detected. The systolic arterial blood pressure rises during activity or exictment and falls during sleep. In a normal adult the blood pressure indictes systolic pressure of 120 and diastolic of 80. Blood pressure is measured by a device called sphygmomanometer.