Heart

The heart is an organ in the chest that pumps blood through the body. It consists of four chambers - the right atrium, right ventricle, left atrium, and left ventricle. The heart is composed of a unique type of muscle tissue found only in the heart called cardiac muscle.

The flow of blood through the heart begins with blood entering the right atrium where it is pumped down into the right ventricle. The right ventricle pumps the blood to the lungs where it picks up fresh oxygen gas. Blood from the lungs travels back to the heart into the left atrium. The left atrium pumps it down to the largest and strongest chamber, the left ventricle. The left ventricle pumps the blood to the rest of the body, where it will eventually return to the heart through the right ventricle.

The Cardiovascular System
The heart is the main component of the cardiovascular system but the system also includes other components such as the blood which carries material around the body and the blood vessels that aid the transport of blood around the body. This complex network of veins and arteries is responsible for providing the bodies tissues with the essential nutrients that it needs. The primary role of the heart in this system is to pump the blood around the body with enough pressure to ensure that it reaches the extremities and the vital organs get enough blood. The heart is also responsible for regulating the blood volume and pressure through endocrine functions. The blood itself carries important nutrients to the organs and removes waste products such as carbon dioxide but it also acts as a communication channel to aid the nervous system.

Anatomy of the Heart
The heart is situated in the thoracic cavity and therefore it is protected by the ribcage. It sits just above the diaphragm which separates it from the abdomen and is approximately the size of a human fist. There is a difference in size between males and females with a male heart weighing approximately 350 grams and a female heart weighing 250 grams in relation to body size. The heart is surrounded by the pericardium which is a membranous sac filled with pericardial fluid. The pericardium consists of two layers- the external fibrous and the internal serous. The external layer is an extremely strong membrane and from it the main blood vessels of the heart protrude near the top of the sac. The inner layer is in contact with the pericardial fluid and it is the movement of this fluid that maintains the hearts movements.

The wall of the heart can be divided into three main laters such as the outer epicardium which is made up of connective tissue, the middle layer called the myocardium and the endothelium which is a layer of inner epithelial cells.

Chambers of the Heart
The heart is made up of four separate chambers. The top two chambers are called the atria and the bottom two chambers are the ventricles. The atria receive blood from the circulatory system and the ventricles take the blood that enters the atria and push it back out around the body. In terms of function, the heart is split into the left and right sections. The chambers are separated by the septum so that blood on the left hand side does not mix with blood on the right hand side. This is important in terms of circulation because blood that enters the right hand side of the heart is deoxygenated and needs to be sent to the lungs in order for gas exchange to occur. Once the blood has been oxygenated, the blood flows into the left hand side of the heart and is pumped to the rest of the body. If the two sides mixed, the oxygen levels wouldn't be high enough and the heart would not be efficient in supplying the body with enough nutrients.

The muscular wall of the ventricles is much thicker than that of the atria because the ventricles have to exert enough pressure to pump the blood to the rest of the body. It is also much thicker on the left hand side of the heart than the right because the right only needs to pump to the lungs whereas the left side of the heart must reach everything else.

Blood Vessels of the Heart
The heart has a number of different blood vessels in order to transport blood to and from different areas of the body. The venae cavae are two large veins that carry the deoxygenated blood back to the heart. The blood then passes through the right atrium and right ventricle and out through the pulmonary arteries to to the lungs. The pulmonary veins then carry oxygenated blood back to the heart and this is the only time that deoxygenated blood is present in a vein. The blood flows into the left atrium and the left ventricle before exiting through the aorta which is the major artery that supplies the rest of the body with oxygenated blood. The heart itself is supplied by the coronary arteries and these run right through the heart muscle.

Valves of the Heart
It is important that blood flows in the correct direction otherwise there would be too much pressure on the heart and it would not function efficiently. Therefore, the heart has four main valves that control the direction of flow. The atrioventricular valves separate the atrium and ventricle on each side so that the blood flows from the atrium into the ventricle and not the other way around. The valves will open and close in respoonse to changes in pressure at heart beats. When there is more pressure in the atrium, the valve will open and the blood can flow into the ventricle but as pressure builds in the ventricle, the valve will close to prevent reverse flow. These two valves differ depending on the side of the heart that they reside. On the left hand side, the atrioventricular valve has two cusps and is commonly referred to as the bicuspid valve or mitral valve. The valve in the right side of the heart has three cusps and is known as the tricuspid valve.

There are also semilunar valves present in the heart. The aortic valve can be found between the ventricle and the aorta whilst the pulmonary valve is located between the right ventricle and the pulmonary trunk. It is the opening and closing of these four valves that give the heart beat sound.

Circuits
The cardiovascular system is separated into two main circuits- the systemic circuit which supplies the major organs with blood and the pulmonary circuit which takes blood to the lungs to be oxygenated. The flow of blood through either of these circuits is called parallel flow due to the pattern of flow it takes. The blood does not flow from one organ into the next organ because by the time it reached the last organ, there would not be a sufficient amount of nutrients left and there would be more waste products in the blood. Therefore, the system runs in parallel. The arteries branch off at the organs and each organ gets its own blood flow directly from the heart. This system also offers the body another advantage and that is independent regulation. As each organ has its own supply, that supply can be regulated depending on the demands of the body.

The Cardiac Cycle
The cardiac cycle is the mechanism by which the heart contracts and relaxes in order to pump blood around the body. The cycle consists of one heart beat and is completed in a number of phases. One cycle includes both contraction and relaxation and is therefore divided into two main stages- systole and diastole. Systole is a period of contraction whilst diastole is a period of relaxation.


 * 1) Phase 1 - During the later stages of diastole (relaxation), blood  returns to the heart and enters the atria and passes through into the ventricles. The pressure is high enough to allow the passage of blood through the valves and into the ventricles by a process called venous return. The ventricles fill with blood and the valves close. At the end of diastole, the atria contract to force blood into the ventricle and they then relax. This is known as ventricular filling.
 * 2) Phase 2 - Systole (contraction) begins with the ventricles contracting which increases their internal pressure. At this point the atrioventricular valves are closed and so are the semilunar valves. Once the pressure is high enough, the semilunar valves are forced open and the blood can leave the heart.
 * 3) Phase 3 - This phase includes the ejection of the blood into the aorta and pulmonary arteries. This decreases the pressure in the ventricles and the valves start to close to prevent back flow. The closing of the semilunar valves once more marks the start of diastole.
 * 4) Phase 4 - At this point, the myocardium of the heart is relaxed. There is still some blood in the ventricles but not enough to force the valves to open. As soon as the ventricular pressure has dropped low enough, the atrioventricular valves will open and allow the passage of blood from the atria to the ventricles once more.

These phases are not undertaken in equal amounts. The heart remains for the most part in diastole and will only enter systole for 0.3 seconds.

Contraction of the Heart
In order for the heart to successfully complete the cardiac cycle, it must contract and relax. As the heart is split into the right and left side, the contractions must be synchronous and this is carried out by a conduction system.

The cardiac muscle classes as myogenic because it is stimulated by triggers within the muscle itself rather than external signals. In order to efficiently contract and relax, the heart regulates its own heart beat in a process called autorhythmicity and there are two types of cells involved. The first are the pacemaker cells which are responsible for initiation of action potentials and the second are the conduction fibres which will transmit the action potential.

Pacemaker cells
The pacemaker cells initiate action potentials and are found in two main regions of the heart- the sinoatrial node and the atrioventricular node. The sinoatrial node can be found in the right atrial wall at the point that it joins the vena cava and the atrioventricular node is present at the tricuspid valve. The rates in which action potentials are generated differ between the two nodes and the sinoatrial node is faster at depolarisation.

Conduction fibres
The fibres are designed for efficient transduction of the signal through the myocardium and they therefore are responsible for triggering muscle contractions. They have a larger diameter and are therefore quicker in their conductance of a signal than other cardiac muscle fibres.

Once the signal has passed through the conduction fibres, they first spread through the atria which causes the atria to contract. The next wave then hits the ventricles which will contract after the atria have contracted. Gap junctions are important in the conduction of the signal and these can be found as intercalculated disks within the heart.

Initiation of a Heartbeat
The action potential starts in the sinoatrial node and it travels down the inter-nodal pathways, which are regions of conduction fibres found in the atria, to the atrioventricular node.This node then transmits the action potential slightly slower than the sinoatrial node and therefore the action potential is delayed by 0.1 seconds which is referred to as the atrioventricular node delay. The action potential then travels down the bundle of His which is a bundle of fibres located in the inter-ventricular septum. It then travels to the bundle branches which split left and right in the heart and the impulse further travels through the purkinje fibres which are present in the ventricles.

This process creates a wave of excitation throughout the heart and causes the muscle to contract.

Myocardial Ischemia
In order to function, the heart requires a large amount of oxygen and if there is a lack of oxygen to the tissue, the individual will experience myocardial ischemia. This causes chest pain known as angina pectoris. The ischemia can be caused by spasms in the arteries or an increase in the rate of activity, but in some cases it is more prolonged because the arteries are blocked by atherosclerotic plaque.

Chronic myocardial ischemia leads to a heart attack, also known as a myocardial infarction. If this occurs, the cardiac tissue does not get any oxygen and becomes irreversibly damaged. The cells die and scar tissue forms on the organ. Blockages are usually the main cause of a heart attack and can sometimes be removed by inflating a small balloon in the artery to make it wider.