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Cardiovascular System - Coggle Diagram
Cardiovascular System
The Heart
Anatomy of the heart
Size, location and orientation
- Size of a human fist, weighing less than a pound.
- Located in the thoracic cavity
- Snugly enclosed within the inferior mediastinum (medial section of the thoracic cavity).
- The heart is flanked on each side by the lungs.
- Its pointed apex is directed towards the left hip and rests on the diaphragm, approximately at the level of the fifth intercostal space.
- Its broad posterosuperior aspect or base, from which the great vessels of the body emerge, points towards the right shoulder and lies beneath the second rib.
Coverings and Walls of the Heart
- The heart is enclosed by a sac called the pericardium that is made up of three layers: an outer fibrous layer and an inner serous membrane pair.
- The loosely fitting superficial part of this sac is referred to as the fibrous pericardium. This fibrous layer helps protect the heart and anchors it's to surrounding structures, such as the diaphragm and sternum.
- Deep to the fibrous pericardium is the slippery two-layered serous pericardium.
- The parietal layer of the serous pericardium or parietal pericardium, lines the interior of the fibrous pericardium.
- The visceral layer or the visceral pericardium, also called the epicardium is part of the heart wall. The visceral pericardium is the inner most layer of the pericardium and the outermost layer of the heart wall.
- Lubricating serous fluid is produced by the serous pericardial membranes and collects in the pericardial cavity between these serous layers. This fluid allows the heart to beat easily in a relatively frictionless environment as the serous pericardial layers slide smoothly across each other.
Heart Walls:
- The heart walls are composed of three layers: the outer epicardium (the visceral pericardium), the myocardium, and the innermost endocardium.
- The myocardium consists of thick bundles of cardiac muscles twisted and whorled into ringlike arrangements. It is the layer that actually contracts. Myocardial cells are linked together by intercalated discs, which contain both desmosomes and gap junctions. The gap junctions at the intercalated discs allow ions to flow from cell to cell, carrying a wave of excitement across the heart. The myocardium is reinforced internally by a network of dense fibrous connective tissue called the "skeleton of the heart".
- The endocardium is a thin, glistening sheet of endothelium that lines the heart chambers. It is continuous with the lining of the blood vessels leaving and entering the heart.
Chambers and Associated Great Vessels:
- The heart has four hollow cavities, or chambers- two atria and two ventricles. Each of these chambers is lined with endocardium, which helps blood flow smoothly through the heart.
- The superior atria are primarily receiving chambers. They assist with filling the ventricles. Blood flows into the atria under low pressure from the veins of the body and then continues on to fill the ventricles.
- The inferior, thick-walled ventricles are the discharging chambers, or actual pumps of the heart. When they contract, blood is propelled out of the heart and into circulation. The right ventricle forms most of the heart's anterior surface; the left ventricle forms its apex.
- The septum that divides the heart longitudinally is referred to as the interatrial septum where it divides the atria and the interventricular septum where it divides the ventricles.
- The heart functions as a double pump, with arteries carrying blood away from and veins carrying blood towards the heart.
- The right side works as the pulmonary circuit pump. It receives oxygen-poor blood from the veins of the body through the large superior vena cava and inferior vena cava and pumps it out through the pulmonary trunk.
The pulmonary trunk splits into the right and left pulmonary arteries, which carry blood to the lungs, where oxygen is picked up and carbon dioxide is unloaded. Oxygen-rich blood drains from the lungs and is returned to the left side of the heart through the four pulmonary veins. This circuit, from the right ventricle ( the pump) to the lungs and back to the left atrium ( receiving chamber) is called the pulmonary circulation.
- Oxygen- rich blood returned to the left atrium flows into the left ventricle and is pumped into the aorta, from which the systemic arteries branch to supply essentially all blood tissues.
- After oxygen is delivered to tissues, oxygen- poor blood circulates from the tissues back to the right artium via the systemic veins, which finally empty their cargo into either the superior or inferior vena cava.
- This second circuit from the left ventricle through the body tissues and back to the right atrium is called the systemic circulation. Because the left ventricle pumps blood over the much longer systemic pathway through the body, its walls are substantially thicker then those of the right ventricle.
Heart Valves:
- The heart is equipped with four valves, which allow blood to flow in only one direction through the heart chamber- from the atria through the ventricles and out the great arteries leaving the heart.
- The atrioventricular (AV) valves are located between the atria and ventricles on each side. These valves prevent backflow into the atria when the ventricles contract.
- The left AV valve- the bicuspid value also called mitral valve- consists of two flaps or cusps of endocardium.
- The right AV valve, the tricuspid valve, has three cusps. Tiny white cords, the chordae tendineare (like heart strings) - anchor the cusps to the walls of the ventricles. When the heart is released and blood is passively filling its chambers, the AV valve cusps hang limply into the ventricles.
- As the ventricles contract, they press on the blood in their chambers and the pressure inside the ventricles begins to rise. This forces the AV valves to cusps upward, closing the valves. At this point, the chordae tendineae tighten and anchor the cusps in a closed position.
- The second set of valves, the semilunar valves, guards the bases of the two large arteries leaving the ventricular chambers. Thus, they are known as the pulmonary semilunar valve and aortic semilunar valve.
- Each semilunar valve has three cusps that fit tightly together when the valves are closed. When the ventricles are contracting and forcing blood out of the heart, the cusps are forced open and flattened against the walls of the arteries by the tremendous force of rushing blood. Then when the ventricles relax, the blood begins to flow backwards toward the heart and the cusps fill with blood like a parachute filling with air, closing the valves.
- Each set of valves operate at a different time. The AV valves are open during heart relaxation and closed when the ventricles are contracting. The semilunar valves are closed during heart relaxation and are forced open when the ventricles contract.
Cardiac Circulation
- The blood contained in the heart does not nourish the myocardium.
- The functional blood supply that oxygenates and nourishes the myocardium is provided by the right and left coronary arteries. The coronary arteries branch from the base of the aorta and encircle the heart in the coronary sulcus (atrioventricular groove) at the junction of the atria and ventricles.
- The coronary arteries and their major branches (the anterior interventricular artery and circumflex artery on the left and the posterior interventricular artery and marginal artery on the right) are compressed (flow is inhibited, not stopped completely) when the ventricles are contracting and fill when the heart is relaxed. The myocardium is drained by several cardiac veins, which empty into an enlarged vessel on the posterior of the heart called the coronary sinus. The coronary sinus in turn, empties into the right atrium.
Physiology of the heart
Intrinsic conduction system of the heart setting the basic rhythm
- Two systems act to regulate heart activity. One of these involves the nerves of the autonomic nervous system which acts like brakes and gas pedals to decrease or increase the heart rate, depending on which division is activated. The second system is the intrinsic conduction system or nodal system, that is built into the heart tissue and sets its basic rhythm like a drummer set the beat for a rock band playing a song.
- The intrinsic conduction system is composed of a special tissue found nowhere else in the body; it is much like a cross between muscle and nervous tissue. This system causes heart muscle deploarisation in only one direction- from atria to the ventricles. Enforces a heart rate of 75 beats per minute.
- One of the most important parts of the intrinsic conduction system is a crescent-shaped node of tissue called the sinoatrial (SA) node, located in the right atrium . The sinoatrial node serves as the heart's pacemaker.
- Other components include the atrioventricular (AV) node at the junction of the atria and ventricles, the atrioventricular (AV) bundle and the right and left bundle branches located in the interventricular septum and finally the purkinje fibers which spread within the myocardium of the ventricle walls.
- The SA node has the fastest rate of depolarisation in the whole system, it starts each heartbeat and sets the pace for the whole heart.
- From the SA node, the impulse spreads through the atrial walls to the AV node and then the atria contract. At the AV node, the impulse is delayed briefly to give the atria time to finish contracting. It then passes rapidly through the AV bundle, the bundle branches and the purkinje fibers, resulting in a "wringing " contraction of the ventricles that begins at the heart apex and moves toward the atria. This contraction effectively ejects blood superiorly into the large arteries leaving the heart.
- Tachycardia is a rapid heart rate ( over 100 beats per minute ). Bradycardia is heart rate that is substantially slower than normal (less than 60 beats per minute )
Cardiac Cycle and Heart Sounds
- In a healthy heart, the atria contract simultaneously. Then, as they start to relax, the ventricles begin to contract.
- Systole means heart contraction.
- Diastole means heart relaxation.
- The cardiac cycle refers to the event of one complete heartbeat , during which both atria and ventricles contract and then relax .
- Average heart rate is approximately 75 beats per minute. Cardiac cycle length is normally 0.8 seconds.
We consider the cardiac cycle in terms of events occurring in five periods:
1. Atrial diastole ( ventricular filling ) :
- Heart completely relaxed . Pressure in the heart is low, the AV valves are open and the blood is flowing passively through the atria into the ventricles. The semi-lunar valves are closed.
2. Atrial Systole:
- The ventricles remain in diastole as the atria contract, forcing blood into the ventricles to complete ventricular filling.
3. Isovolumetric contraction:
- Atrial Systole ends and ventricular systole begins. The initial rise in intraventricular pressure closes the AV valves, preventing backflow of blood into the atria. For a moment, the ventricles are completely closed chambers.
4. Ventricular Systole (ejection phase):
- The ventricles continue to contract, causing the intraventricular pressure to surpass the pressure in the major arteries leaving the heart. This causes the semilunar valves to open and blood to be ejected from the ventricles. During this phase the atria are again relaxed and filling with blood.
5. Isovolumetric Relaxation:
- As ventricular diastole begins, the pressure in the ventricles falls below that in the major arteries and the semilunar valves close to prevent backflow into the ventricles. For another moment, the ventricles are completely closed chambers and intraventricular pressure continues to decrease. Meanwhile, the atria has been in diastole, filling with blood. When ventricular pressure falls below atrial pressure, the AV valves open and the cycle repeats.
- When using a stethoscope, you can hear two distinct sounds during each cardiac cycle. These heart sounds are often described by the two syllables "lub" and "dup". The first heart sound (lub) is caused by the closing of the AV valves. The second heart sound (dup) occurs when the semilunar valves close at the end of ventricular systole.
Cardiac Output
- Cardiac output (CO) is the amount of blood pumped out by each side of the heart (actually each ventricle) in 1 minute. It is the product of the heart rate (HR) and the stroke volume (SV). Stroke volume is the volume of blood pumped out by a ventricle with each heartbeat. In general, stroke volume increases as the force of ventricular contraction increases.
- Regulation of stroke volume:
- A healthy heart pumps out about 60 percent of the blood present in its ventricles. It is approximately 70ml with each heartbeat. Three factors influence stroke volume; preload, contractractility and afterload.
According to the Frank-Starling law of the heart the critical factor controlling stroke volume is how much the cardiac muscle cells are stretched by the filling of the chambers just before they contract. This is called preload. The more the muscle cell are stretched, the stronger the contraction, and the higher the stroke volume. Two things influence preload; the amount of venous return (blood returning to the heart) and the length of time the ventricles are relaxed and filling (determined by heart rate). Anything that increases the volume or speed of venous return also increases stroke volume and force of contraction.
- Contractility is the ability of cardiac muscle to generate tension. Contractility is influenced by factors other than stretch, including the amount of calcium ions present, sympathetic nervous stimulation and hormones such as epinephrine and thyroxine. As contractility increases, stoke volume increases.
- Afterload is the amount of pressure the ventricles must overcome to eject blood.
- Afterload is determined by the blood pressure in the pulmonary trunk and aorta pressing back on the semilunar valves.
- Factors Modifiyong Basic Heart Rate:
- In healthy people, stroke volume tends to be relatively constant. However when blood volume drops suddenly or when the heart has been seriously weakened stroke volume declines, and cardiac output is maintained by faster heartbeat. Although heart contraction does not depend on the nervous, its rate can be changed temporarily by the autonomic nerves. Some factors that has an influence on heart rate.
- Neural (ANS) control. During times of physical or emotional stress, the nerves of the sympathetic division of the autonomic nervous system more strongly stimulate the SA and AV nodes and the cardiac muscle itself. As a result, the heart beats more rapidly. Parasympathetic nerves, primarily vagus nerve fibers, slow and steady the heart, giving it more time to rest during noncrisis times.
- Hormones and ions. Various hormones and ions can have a dramatic effect on heart activity. Both epinephrine, which mimics sympathetic nerve and is released in response to sympathetic nerve stimulation and thyroxine, a thyroid hormone, increase heart rate and contractility. A reduced level of ionic calcium in the blood depresses the heartbeat, whereas an excessive level of blood calcium ions causes such prolonged contractions that the heart may stop entirely. Either excess of lack of needed ions such as sodium and potassium also modifies heart activity. A deficit of potassium ions in the blood, for example, causes the heart to beat feebly and abnormal heart rhythms appear.
- Physical factors: A number of physical factors, including age, gender, exercise and body temperature influences heart rate.
Blood Vessels
Blood circulates inside the blood vessels which forms a closed transport system called the vascular system. Like a system of roads the vascular system has its freeways, second roads and alleys. As the heart beats, it propels blood into the large arteries leaving the heart. As large arteries branch, blood moves into successively smaller and smaller arteries and then into the arterioles which feed the capillaries in the tissue. Clusters of capillaries called capillary beds are drained by venules and finally empty into the great veins (venae cavae) entering the heart.
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A closed system of the heart and blood vessels:
- The heart pumps blood
- Blood Vessels allow blood to circulate to all parts of the body
Function of the cardiovascular system:
- Transport oxygen, nutrients, cell wastes, hormones to and from cells, regulation of body temperature.