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The total blood volume of an individual varies by only 100-200 ml. This volume is efficiently maintained by the capillary shift, kidney function and autonomic nervous control. The autonomic nervous system also controls the capillary beds and vascular tone. This action will also adjust rapidly to changes in blood pressure, hence it is not as important in the long run as it is in the short term.
Hypervolemia is an increase in blood volume while hypovolemia is a decrease in blood volume. When blood volume changes the body manages either of these events in such a way as to re-establish a normal volume of five liters.
Potentially, there is much more vascular space than the available five liters of blood could fill. This space is maintained by the action of the sympathetic nerves which keep the vessels in a moderate state of contraction and keep some capillary beds partially closed. |
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right atrium . The atrium contracts forcing blood through the tricuspid valve, into the right ventricle . Once the right ventricle fills, it then contracts, forcing the tricuspid valve to close and the pulmonary valve to open. This permits the blood to enter the pulmonary artery and travel to the lungs. As the right ventricle relaxes, the pulmonary valve closes to prevent backflow.
In the lungs , carbon dioxide is released and oxygen is picked up by the hemoglobin in the red blood cells. This oxygenated blood travels via the four pulmonary veins into the left atrium which then contracts, forcing blood past the mitral valve into the left ventricle . The left ventricle then contracts, closing the mitral valve and opening the aortic valve forcing the blood to enter the aorta and continue into the general circulation. |
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| The fibrous skeleton of the heart is made up of dense connective tissue arranged in annulus rings. It serves as the area for myocardial tissue origination and attachment, as well as for providing valvular support. |
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There are two sets of heart valves: the atrioventricular valves between the atria and ventricles, and the semilunar valves between the ventricles and aorta or pulmonary artery. Both the atrioventricular and semilunar valves are made of dense connective tissue covered with endocardium and have an inner framework of fibrous tissue.
There are two atrioventricular valves: the tricuspid valve and the mitral valve. During diastole, the pointed ends of the atrioventricular valves open into their respective ventricle and blood flows freely from the atria through the valves and into the ventricle. The valves are anchored by the papillary muscles which are conical muscular projections attached to the myocardium and connected to the valves by the chordae tendineae. |
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| The tricuspid valve is attached to the right atrioventricular annulus. It separates the right atrium from the right ventricle.The atrial surface of the valve, which coapts or overlaps during systole, is smooth. The opposite or ventricular side, is roughened by the attachment of the chordae tendineae. |
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| The mitral valve is attached to the left atrioventricular annulus. It has two unequal triangular leaflets which separate the left atrium and ventricle. The left ventricle is separated by the aortic leaflet of the mitral valve into an inflow tract posteriorly and an outflow tract anteriorly. |
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| Pulmonary & Aortic Semilunar Valves |
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Definition
The semilunar valves constitute the second set of heart valves and are named for each of the three crescent-shaped cusps that make up each valve. Each of the two large arteries receiving blood from the ventricles has a semilunar valve at its proximal end that prevents the backflow of blood into the ventricle during diastole. The valves are supported by an annulus ring.
As the ventricles contract the semilunar valves are forced to open and the cusps are forced against the artery wall. At the end of systole blood from the arteries fills the cusps and forces them toward the ventricles. The free edges of the cusps meet or coapt to form a barrier preventing the backflow of blood into the ventricles.
The right and left coronary arteries arise just above the cusps of aortic semilunar valve. A dilation of the aorta occurs adjacent to each cusp each forming the Sinuses of Valsalva. |
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| Valve Dysfunction - Stenosis and Regurgitation |
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Valvular dysfunction often appears as one of two major types, either a stenosis or a regurgitation. A stenosis describes the inability of the valve to open to its normal position.
Regurgitation describes the inability of the valve to effectively shut and impede the backward flow of blood. The regurgitation of blood can cause a jet lesion on the wall of the chamber opposite the regurgitation. |
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| Deep Sinospiral Muscle Layer |
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Definition
| The middle muscle is the deep sinospiral muscle group which originates from the mitral annulus and encircles both ventricles at the base of the heart |
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| Deep Bulbospiral Muscle Layer |
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The deepest muscle on the left is the deep bulbospiral muscle group which originates from the septal portion of the mitral valve annulus. The deep bulbospiral muscle layer divides the left ventricular wall, wraps around the aortic outlet in a sphincter like form and re-attaches to the septal portion of the mitral valve annulus.
The deep bulbospiral and sinospiral fascicles interdigitate in the muscular IV septum. Upon ventricular contraction, the orientation of the muscle bands causes shortening in the diameter of the heart. The spiral muscles, upon contraction, also shorten the heart from apex to base . |
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The autonomic nervous system (ANS) is broken down into the sympathetic and parasympathetic divisions. The sympathetic division activates the body to cope with stressor situations, i.e. the "fight or flight" response. The parasympathetic division oversees digestion, elimination and glandular functions, i.e. the resting and digesting response.
Heart Function is controlled via the Autonomic Nervous System (ANS). Two different sets of efferent autonomic nerves, the parasympathetic (Vagus) and the sympathetic (sympathetic chain), act to regulate heart function. Stimulation of parasympathetic nerves will decrease all activities of the heart. Stimulation of the sympathetic innervation has the opposite effect and it is a "readiness regulator" in times of stress.
Acetylcholine and norepinephrine are the major neurotransmitters released by the ANS. Acetylcholine binds to cholinergic receptors while norepinephrine binds to adrenergic receptors. There are two main classes of adrenergic receptors, alpha and beta, which are further divided into alpha-1, alpha-2, beta-1 and beta-2.
An agonist is a drug with an affinity towards a specific receptor site and also has the ability to produce an effect. An antagonist is a drug that also has an affinity towards a specific receptor site, but does not have the ability to produce a physiological effect. Instead, it blocks the action and effect of an agonist. |
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The coronary arteries arise just distal to the aortic valve in the Sinuses of Valsalva and traverse the heart's outer surface. Contrary to other arteries, they fill with oxygenated blood from the hemodynamic pressure wave that closes the aortic valve during diastole.
The right coronary artery, supplying the right side of the heart, contains only one major branch, while the left coronary artery contains two branches, the left anterior descending branch (LAD) and the circumflex branch. Smaller branches from the major branches penetrate the myocardium. After cellular exchange is completed in the capillaries, venous blood is picked up by venules and veins which ultimately empty into the coronary sinus and the blood then enters the right atrium. About 4 - 5% of the total circulating blood flows through the coronary arteries. |
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| Coronary Artery Pathology |
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The occlusion of a coronary artery is often associated with atherosclerosis. A partial or total blockage prevents the flow of arterial blood beyond the site of the stenosis, causing ischemia, angina and/or ischemic necrosis of the myocardium which is referred to as a myocardial infarction (MI). Less often, coronary vessels may be occluded by fat globules, tumor emboli or clot emboli from other parts of the body, though this usually occurs at sites of pre-existing stenosis. The spasm of a coronary artery can also occur producing the same results.
Atherosclerosis is a disease caused by faulty or incomplete lipid metabolism. The body fails to metabolize fats normally, thus cholesterol, phospholipid and neutral fat are deposited on the walls of arteries. Over time the body builds fibrous tissue around these fatty deposits (called plaques) which can become calcified into "bony-hard" plates in later stages. This process can occur in any artery.
These plaques and/or plates can cause acute occlusion three ways: 1) the fatty deposit may break through the inner wall of the vessel, thus occluding it, 2) the fatty deposit may dislodge and occlude a smaller vessel distally, and 3) fatty deposits may erode the arterial wall resulting in a hematoma which totally or partially occludes the lumen of the artery. |
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| Electrical Activity of the Heart: Membrane Resting Potential |
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Definition
Cardiac conduction system cells have a membrane resting potential such that the outer surface of the resting cell membrane carries a positive electrical charge and the inner surface of the membrane carries a negative electrical charge. During the resting stage, the cardiac conduction system cells are polarized. The normal resting potential of the cardiac conduction system is approximately -85 to -95 millivolts (mV).
The changes in electrical potential that occur result from the movement of sodium and potassium ions across the sarcolemma of the cell. This movement of charged ions allows for the changes in electrical potential necessary to propagate the electrical charge to the next excitable cell. The movement of the sodium and potassium ions is mediated by the sodium-potassium pump. |
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| Action Potentials - Excitability |
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| The cells in the cardiac conduction are excitable meaning that they have the ability to respond to an electrochemical stimulus. Electrochemical stimulation makes the cell membrane permeable to the flow of sodium, potassium and calcium ions. The action potential for ventricular muscle is 105mV, meaning that the membrane potential must change from approximately -90mV to approximately +20mV, a difference of about 110mV. |
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| Phases of Cardiac Electrical Activity |
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Definition
Phase 0 is the phase of a stable resting potential, when the cells are polarized and in an excitable state awaiting a stimulus which will cause depolarization. When a stimulus above the threshold potential strikes the cell the cell begins to depolarize. Sodium ions rush into the cell causing the electrochemical difference potential between the inside and outside of the cell to race toward zero.
During Phase 1, the depolarization phase, the electrochemical voltage change is so rapid that the voltage overshoots the zero potential and tops out around +20mV. Phase 1 is a very short phase where the potential difference comes to rest near 0mV.
Phase 2, referred to as the plateau phase, is where the transmembrane action potential is maintained near 0mV by the infusion of calcium ions. The cell is in a depolarized state and restoration of the resting membrane potential is beginning to take place. At the end of this phase, the cell begins to repolarize.
Phase 3 is known as the rapid repolarization phase. The cell restores itself to the original polarized state of -90mV. |
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On the posterior part of the right atrial wall, adjacent to the junction of the superior vena cava and the right atrium, is a specialized conduction system cell area called the sinoatrial node (SA node). It is horseshoe shaped and located just beneath the epicardium.
Although the depolarization of the SA node is the first step of the cardiac cycle, it does not produce enough energy to be recorded by the electrocardiograph.
The depolarization wave spreads down these specialized cardiac conduction system fibers over the atria by way of internodal and interatrial pathways known as Bachmann's bundle. These specialized fibers transmit impulses six times faster than do ordinary cell-to-cell interconnections. These pathways trigger the contraction of other cells in the atrium more rapidly than if the depolarization wave followed the cell-to-cell interconnections.
The SA node sets the heart rate at 72 beats per minute rather than the 60 beats per minute intrinsic to the atria alone or the 20-40 beats per minute intrinsic to the ventricals alone |
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| The atrioventricular node, or AV node, is made up of another cluster of specialized cardiac conduction system cells. The AV node forms a pathway for impulse conduction that bridges between the atria and ventricles. Depolarization of the AV node is relatively slow due to the intrinsic characteristics of its cells. This causes a delay in the transmission of the depolarization wave to the ventricles. The transmission of the wave through the AV node is relatively weak, thus is considered silent on the electrocardiogram |
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| Bundle of His - Right and Left Bundle Branches |
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Definition
The depolarization wave leaves the AV node and enters a compact tract of cardiac conduction system fibers known as the bundle of His. The bundle of His proceeds down the interventricular septum to the inferior border of the membranous septum where it bifurcates forming the right and left bundle branches
. The right bundle branch spreads the electrical impulse to the right ventricle. The left bundle branch spreads the electrical impulse to the interventricular septum and left ventricle. The left bundle branch divides into two branches. The first branch, the anterior superior fascicle, conducts the impulse to the anterior and superior segments of the left ventricle. The second branch, the posterior inferior fascicle, conducts the impulse to the posterior and inferior segments of the heart. |
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The depolarization wave is finally conducted to the Purkinje Fibers at the ends of the bundle branches. These specialized cardiac conduction system fibers form a rapid conduction network within the myocardium and are responsible for propagating the depolarization wave to all cardiac muscle cells. The QRS complex of the electrocardiogram represents the ventricular depolarization of contraction.
Following ventricular contraction and the QRS complex is a brief period of low electrical activity. On the electrocardiogram this appears as the S-T segment. Ventricular repolarization is represented by the T-wave on the electrocardiogram. The T-wave deflection is also in the same direction of the largest deflection of the QRS complex because the electrical forces of repolarization are transmitted from the epicardial surface to the endocardial surfaces in the same direction as depolarization. |
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