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Starting from the 4 chambers of Heart (Structure of Heart) and Properties of Cardiac Muscles to the Conduction system (Action potential) and Phases of Cardiac cycles, each and every aspect of Cardiovascular System is explained in detail. Electrocardiography and related terminologies are also explained thoroughly.
Typology: Study notes
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Cardiovascular system is made up of heart & blood vessels i.e. arteries, veins, capillaries. These blood vessels distribute blood throughout the body. The capillaries are the thinnest and exchange of O2 and CO2 at tissue levels occurs in the capillaries. The heart is located in the chest between the lungs behind the sternum and above the diaphragm. At the 3rd^ week of Gestation, the heart starts its work. So the heart is the first organ developed in human body.
The heart consists of two sides i.e. Left side and Right side. Similarly, there are four chambers in the heart: Right Atrium, Right Ventricle, Left Atrium and Left Ventricle.
Right Atrium has a Sinoatrial Node between the superior and inferior Vena Cava. The SA Node is responsible for initiating impulse. Receives deoxygenated blood from superior vena cava and inferior vena cava. Drains blood to Right Ventricle through AV (Artioventricular) valve /Tricuspid valve.
From Right Ventricle blood goes to Pulmonary Artery to lungs to pulmonary veins and then to Left Atrium. All Arteries contain oxygenated blood expect Pulmonary arteries.
Receives oxygenated blood from Pulmonary Veins. Drains blood to Left Ventricle through Mitral Valve/Bicuspid Valve.
Through left Ventricle, oxygenated blood comes out through Aorta/Systemic Artery.
There are 2 septa in the heart that do the work of separation. The septum between the two Atria is called Interatrial Septum. The Septum between the two Ventricles is called Intraventricular Septum.
There are four Valves in the heart. Tricuspid and Bicuspid Valves are responsible between the two chambers. Semilunar Valves: Pulmonary Valve (Right Side) Aortic Valve (Left Side)
Right Left Atrium Atrium Right Left Ventricle Ventricle Right Left Atrium Atrium Right Left Ventricle Ventricle Right Left Atrium Atrium Right Left Ventricle Ventricle
Functional Syncytium: Cardiac Muscles are interconnected by Intercalated discs. These intercalated discs transmit signals between the fibers. Between these intercalated discs are passages called Gap Junction. These gap junctions help in the free transmission of ions from one place to another. The parts in intercalated disc other than the gap junction are called Desosomes. Due to functional syncytium, when stimulus is given to one fiber, all fibers contract/relax. There are two types of syncytium: Atrial Syncytium Ventrical Syncytium. Auto excitability: Excitability is the response of any living tissue due to stimulus. Auto excitability in cardiac muscles produces own impulses and respond. Action Potential: Action Potential is the first response to stimulus. Action potential in cardiac muscles is different from action potential in skeletal and smooth muscles. The resting membrane potential of skeletal muscles is -90mV, and the rmp of smooth muscles is -50 to -95 mV. In cardiac muscles, the rmp is -95mV. After a stimulus is generated across the membrane, Initial Depolarization begins. During Initial Depolarization sodium ions from extracellular fluid begins to enter the membrane through sodium channels. This causes increase in positive sign inside the membrane. Initial depolarization reaches upto +20mV. The second step is Initial Repolarization. During initial repolarization, efflux of K ions takes place. Due to this efflux, the membrane starts to regain its negative charge. The third step in cardiac muscles is Final Depolarization. Final depolarization is also called Plateau Phase. Plateau phase/sustained depolarization takes place due to slow influx of calcium ions. The concentration of negativity and positivity inside and outside of the membrane is equal preventing repolarization. The significance of plateau phase is to prevent another impulse or to prevent multiple contractions. The final step of action potential in cardiac muscles is Final Repolarization. During final repolarization, calcium channels close and potassium channels open causing the efflux of potassium ions. This efflux restores the negativity inside the membrane bringing it back to the rmp.
Conductivity: Transfer of cardiac impulse from pace-maker (SA node) to all cardiac muscle fibers is called conductivity. The main components of the cardiac conduction system are the SA node, AV node,
Cardiac contraction is the end result of action potentials that are initiated at the pace-maker cells in the SA node. Due to functional syncytium, the atria contract simultaneously but the conduction of impulse from the atria to ventricle is done through a conducting system. From SA node the impulse is transmitted to AV node and then to Bundle of His.
and pathological reasons. Bradycardia can be physiological as in athletes or during sleep. The pathological reasons of bradycardia can be hypothyroidism, hypothermia, polycythemia. Polycythemia (also known as polycythaemia or polyglobulia) is a disease state in which the hematocrit (the volume percentage of red blood cells in the blood) is elevated. Cardiac Conduction:
The impulse is generated from the SA node. The SA node generates 70-80b/min and it has the fastest rate of discharge of impulse than other components. The AV node and AV bundle generates 40-60b/min while the Right and Left Bundle Branches and the Purkinje Fibers generate 35-40b/min. Speed of Impulse: Speed is inversely proportional to time. The faster an object move, the lesser time it takes to cover the distance. The SA node generates the impulse at 0.0s. The impulse travels from the SA node to the AV node in 0.03s. In the AV node the impulse rests for 0.09s. So from the SA node to the exit of the AV node the impulse takes total 0.12s (0.03+0.09). The AV bundle is right below the AV node; impulse rests here for 0.04s (total 0.12+0.04= 0.16s ). From the AV bundle, the impulse travels to the Purkinje fibers which take 0.06-0.08s. So the total time from the SA node to the purkinje fibers is 0.16+0.06/0.08= 0.22/0.24s.
From the SA node impulses also travel to the Left Atrium, this takes 0.1s but this travelling is not a part of the conducting system. Conduction pathway includes conduction of impulse from the atria to the ventricles. The lowest speed of the impulse is in the AV node i.e. 0.05m/s because the impulse rests here for the longest time. The delay that occurs in the AV node is called the AV nodal delay. This is the 0.09s delay in the conducting pathway when the impulse reaches from the SA node to the AV node and rests in the AV node and delays transmission of impulse to the ventricles. The cause of this delay is that the AV nodal fibers are arranged in such a way that there are lesser gap junctions between them, causing impulse to take more time in jumping from one fiber to another. The advantage of this delay is that it allows the atria to fully contract, emptying whole blood into the ventricles before ventricular contraction. The highest speed of impulse is in the Right & left Bundle Branches i.e. 4m/s. This is because the impulse travels a large surface area in a very short time. The cause of this highest speed is due to the presence of more gap junctions. Due to these numerous gap junctions, the impulse jumps quickly from one fiber to another. The advantage of this high speed is that it creates effective pressure in the ventricular chambers which enables both the ventricles to contract simultaneously. Sometimes, due to some functional abnormality, the SA node fails to generate impulses. In the absence of the SA node, impulses are generated by the AV node and AV bundle. In case the AV node fails to generate an impulse too, right and left bundle branches and Purkinje fibers take over. SA node generates impulse at the fastest rate. But sometimes other components generate impulses faster than the SA node. These components that generate impulses faster than the pacemaker are called ectopic pacemakers. There are two causes of the activation of ectopic pacemakers; Blockage in the transmission of impulse in the SA node. Generation of abnormal foci (cells). Effect of Autonomic Nervous System on Conducting System: ANS consists of Sympathetic Nervous System and Parasympathetic Nervous System. Sympathetic nervous system does the work of excitement while parasympathetic nervous system relaxes. Function Effect of Sympathetic Nervous System Effect of Parasympathetic Nervous System SA nodal discharge rate Increases Decreases Speed of transmission of impulse Increases Decreases Time taken in transmission of impulse Decreases Increases AV nodal delay Decreases Increases
ECG Paper: ECG paper is made up of horizontal and vertical lines that are at a regular interval of 1mm. Every 5th horizontal and vertical line is thickened. 1mm of thin line is called a small square and 5mm of thickened line is called a large square. 1mm on the x axis represents 0.4s of time whereas 1mm on the y axis represents 1mV of amplitude. ECG Leads: ECG leads are electrodes that are attached on adjacent tissues of the heart on the skin to record ECG. Placement of ECG leads follows the Eunthovin’s Triangle. It is an imaginary triangle guiding the placing of leads on the skin. It places the leads between the right arm, left arm and the left foot making a triangle in which the heart rests in the centre. Right arm Left arm Left foot The number of ECG leads is 12 which include both bipolar & unipolar leads. The bipolar leads have both negative and positive poles whereas the unipolar leads have only one active lead.
3 Bipolar limb leads; Lead I Lead II Lead III 3 unipolar limb leads; avR avL avF 6 unipolar chest leads; C C C C C C 12 leads 9 unipolar leads 3 limb leads 6 chest leads 3 bipolar leads 3 limb leads
0.08s. The normal amplitude of Q wave is 0.1mV, of R wave is 1mV (comprises of 2 large squares) and of S wave is 0.4mV. Abnormality of QRS complex: “Tall QRS complex” is due to ventricular hypotrophy whereas “Small QRS complex” is due to AV bundles blockage. T Wave: T wave is produced due to the ventricular repolarization. Its normal time is 0.15s and normal amplitude is 0.3mV. It is produced due to the efflux of K ions so if any abnormality comes it will be due to alteration in K ions transport. Abnormality of T waves: Hyperkalemia is an abnormality of T waves when efflux of K ions increases. “Tall or tented T waves” is due to hyperkalemia whereas “Short or absent T waves” is due to hypokalemia. “Inverted T waves” is due to myocardial infarction. U wave: U wave is produced due to slow repolarization of papillary muscles. It is not a significant wave of ECG.
Intervals comprise of two or move waves. PR Interval: Interval starts from the beginning of P wave to the beginning of Q or R wave and depolarize the atria. Its time duration is 0.16 to 0.20s. If the time duration prolongs, it is because of first degree heart blockage (minor blockage of AV bundles or AV node). QRS Interval: This interval starts from the beginning of Q wave to the end of S wave. Its time duration is 0.06 to 0.08s. QT Interval: It starts from the beginning of Q wave to the end of T wave. Arterial repolarization, ventricular repolarization along with ventricular depolarization is done here. Its time duration is 0.4s. ST Interval: It starts at the end of S waves to the end of T waves. Ventricular repolarization is done here. Its time duration is 0.32s.
Isoelectric line when there is no action potential or no activity in the ECG is called a segment. PQ Segment: Isoelectric line between the end of P wave and the beginning of Q wave. It shows the AV nodal delay. Its time duration is 0.09s. ST Segment: Isoelectric line between the end of S wave and the beginning of T wave. It shows the plateau phase.
“ST Segment elevation” occurs due to myocardial infarction whereas “ST Segment depression” occurs due to angina (chest pain or discomfort when heart muscles don’t get enough oxygen rich blood).
