Electrical Activity & Autorhythmicity
Autorhythmic Cells: 1% of heart, initiate APs |
Contractile Cells: 99%, mechanical pumping |
Pacemaker Activity:- |
1. Funny channels: (Na⁺ in) and K⁺ channels close. |
2. T-type Ca²⁺ channels open |
3. Threshold: L-type Ca²⁺ channels open |
4. Repolarization: K⁺ channels open |
Note that: : Long refractory period coincides with plateau (Prevents summation/tetanus) |
Contractile Cell Action Potential
Type of Cell: Contractile (99% of cardiac cells) |
Resting potential: –90 mV |
1. Depolarization: Fast Na⁺ channels open |
2. Initial Repolarization: Transient K⁺ channels open |
3. Plateau: L-type Ca²⁺ channels open, reduced K⁺ efflux |
4. Repolarization: Regular K⁺ channels open |
Return to Rest: Leaky K⁺ channels restore resting potential |
Pacemaker vs. Cardiac cell
Pacemaker: gradual depolarization, no true resting potential, Ca²⁺-dependent spike |
Contractile: stable resting potential, Na⁺-dependent spike, plateau from L-type Ca²⁺ |
Conduction Pathway
1. SA nodes |
2. AV nodes |
3. Bundle of his |
4. Right/left bundle branches |
5. Purkinje fibers |
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Structure of the Heart & bloodflow
Anatomy of the Heart
Heart Wall Layers: |
Endocardium: Inner layer, lines chambers |
Myocardium: Cardiac muscle layer, responsible for contraction |
Epicardium: Outer layer, also part of pericardium |
Pericardium: Protective sac around the heart |
Valve Composition: |
Connective tissue: Mostly collagen, provides structural support |
Endothelium: Inner lining of heart and blood vessels |
Heart Structure & Blood Flow
4 Heart Valves: Tricuspid, Pulmonary, Mitral, Aortic Flow Sequence: Right atrium → Right ventricle → Lungs → Left atrium → Left ventricle → Body Oxygenation: Pulmonary arteries = O₂-poor, Pulmonary veins = O₂-rich |
Cardiac Cycle & Heart Sounds Phases:
S1 (“lub”) = AV valves close |
S2 (“dub”) = Aortic/pulmonary valves close |
Murmur: Stenotic or insufficient valves → turbulence |
Phases: Mid-to-late diastole → Ventricular systole → Early diastole |
Heart Valve Disorders
Stenotic = valve doesn’t open completely |
Insufficient = valve doesn’t close completely |
Causes murmurs (turbulent flow) |
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Blood Pressure
Pulse Pressure = Systolic – Diastolic |
Mean Arterial Pressure (MAP) = Diastolic + ⅓(Pulse Pressure) |
Measured by: Sphygmomanometer (Korotkoff sounds) |
Blood pressure during dynamic exercise
Systolic increases |
Diastolic ~same |
MAP increases progressively |
Blood Flow
Flow influenced by: Radius (power of 4 effect), Length, Viscosity |
Pressure gradient = Flow × Resistance |
Cardiac Output
CO = Heart Rate × Stroke Volume |
Stroke Volume = EDV – ESV |
Regulation: 2 Types |
Type 1: Intrinsic: Frank–Starling Law = Increased venous return increases ventricular filling (increased EDV), which results in a larger stroke volume due to the length–tension relationship. |
Extrinsic: Sympathetic stimulation → ↑ , contractility → ↑ SV |
MAP Regulation
MAP = C.O x TPR |
Contributors: Stroke vol.,Heart rate, Blood Vol., Blood Viscosity, Arteriolar Radius, Sympathetic/Parasympathetic activity |
ECG Components
P wave: Atrial depolarization |
PR segment: AV node delay |
QRS complex: Ventricular depolarization (atrial repolarization hidden) |
T wave: Ventricular repolarization |
TP interval: Ventricles relaxing/filling |
ST segment: Ventricles contracting/emptying |
ECG Abnormalities
Rate: Tachycardia |
Rhythm: Extrasystole, ventricular fibrillation |
Conduction: Complete heart block |
Myopathies: Myocardial infarction |
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Capillary exchange
Lipid-soluble substances: pass through endothelial cells |
Small water-soluble substances: pass through pores |
Exchangeable proteins: moved via vesicular transport |
Plasma proteins: generally cannot cross capillary wall |
Net Filtration Pressure (NFP)
NFP = Capillary Hydrostatic Pressure − Blood Colloid Osmotic Pressure |
Affects direction of fluid movement (filtration vs. reabsorption) |
Positive NFP = fluid pushed out (filtration) |
Negative NFP = fluid pulled in (reabsorption) |
Baroreceptor Reflex
Stimulus: ↑ or ↓ blood pressure |
Sensors: Carotid sinus and aortic arch |
2 effects can occur: |
↑ BP =↑ afferent firing = ↓ HR, ↓ contractility & vasodilation |
↓ BP = ↓ afferent firing =↑ HR, ↑ contractility & vasoconstriction |
Veins & Venous Return
Valves prevent backflow |
Factors that facilitate return: |
Sympathetic stimulation, Skeletal muscle pump, Respiratory activity, Increased blood volume. |
Exercise Physiology: Cardiovascular Response
Systole and diastole both decrease, but diastole decreases more |
Systolic & MAP increase; diastolic remains about the same |
Cardiac output shifts to muscles, heart, skin |
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