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D4: The Heart - Coggle Diagram
D4: The Heart
Cardiac conduction
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Sinoatrial node (SA): specialised cardiac muscle cells found where the superior vena cava joins the right atrium
Signals from the sinoatrial node that cause contraction cannot pass directly from atria to ventricles.
Instead the impulse spreads along the atria to the atrioventricular node (AV), from where it spreads to the ventricle.
In the interventricular walls there are specialised fibres grouped to form a bundle called Bundle of His.
This bundle splits into two branches that go to the left and right ventricles, leading to other specialised cells called Purkinje fibres.
These fibres carry the impulse at a very high speed. Conducting fibres ensure coordinated contraction of the entire ventricle wall. The ventricular contraction is called the systole. Blood then flows out of the heart as the semilunar (pulmonary and aortic) valves open and give access to the aorta and pulmonary artery.
Sound: The closing of the atrioventricular valves at the beginning of the ventricular contraction (systole) produces the first sound, the lub. Immediately afterwards, the closing of the semilunar valves just after the ventricular systole and beginning of the diastole causes the second sound, the dub sound.
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Cardiac muscle cells
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Intercalated discs
These discs represent the attachment site between adjacent cells. Intercalated discs contain adhering junctions between cells that hold cells together and gap junctions, which allows communication between cells.
Gap junctions
Densely packed protein channels that permit intercellular passage of ions and small molecules. Electrical activation of the heart requires cell-cell transfer of current via gap junctions.
These cells also have a long refractory period, during which the cells cannot contract for a second time. This ensures that the contraction or systole of the heart is separated by a resting period or diastole.
Action potentials: The mechanism by which the cardiac muscle contracts is similar to that of the striated skeletal muscle.
The membrane action potential activated myosin-ATPase ---increase in Ca ions round the myofilaments. This leads to the sliding of the thick and thin filaments.
The plateau of the action potential in cardiac muscle prevents the heart from contracting before time.
Pacemaker cells have their own intrinsic rhythms of activity; therefore, as one action potential is completed, immediately another action potential is generated in the membrane, even in the absence of nerve impulse. This is why its rhythm is considered myogenic
coronary heart disease: decreased blood flow and oxygen to the heart muscle caused by narrowed heart arteries. It is also called cardiac ischemia or ischemic heart disease.
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As life expectancy continues to improve, high-fat diets, cigarette smoking, and sedentary lifestyles become more common. This increases the chances of mortality caused by atherosclerosis and CHD
There have been many studies on the classical risk factors for CHD, such as smoking, alcohol consumption, blood pressure, obesity and cholesterol, which affect all ethnic groups alike. The genetic predisposition to CHD in some ethnic groups could be explained by insulin resistance syndrome and associated chances of diabetes
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Artificial pacemakers
the heart beating too slowly (bradycardia) or too quickly (tachycardia), or beating irregularly (arrhythmia)
This pacemaker consists of a battery-powered generator and wires with electrodes to connect to the heart muscle. If the pacemaker senses that the heartbeat is above a certain rate, it will stop sending signals to the heart. The pacemaker can also sense when the heartbeat slows down too much. It will automatically turn back on and start pacing the heart again.
Electrocardiogram: checks the electrical activity of the heart. Electrodes that detect electrical changes on the skin are placed on the patient’s body
The R-R interval between successive beats allows the calculation of the heart rate. The P-wave represents the SA node electrical activity and contraction of the atria. The QRS complex corresponds to the excitation of the ventricles. The T-wave is the relaxation of the ventricles at the end of the contraction.
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