Lect 26 Structure of the Heart and the Conduction System
Structure of the Heart
The circulatory system consists of the heart (pump), blood vessels (passageways), and blood (transport medium). The heart ensures the delivery of oxygen and nutrients to tissues and removal of carbon dioxide and waste products.
Heart is about the size of your fist, weighing 250-350 grams, and located slightly to the left in the thoracic cavity within the mediastinum. The exact location is between the lungs, slightly angled, with its apex pointing towards the left hip.
The heart has 4 chambers: upper atria and lower ventricles, separated by a septum. The septum prevents mixing of oxygenated and deoxygenated blood.
Atria receive blood from veins, while ventricles pump blood away through arteries. The atria have thinner walls compared to ventricles, reflecting their lower pressure pumping requirements.
Tissue Layers of the Heart
Pericardium (outer layer):- Fibrous pericardium (outer): A tough, dense connective tissue layer that protects the heart and anchors it to the surrounding structures.
Serous membrane (inner): parietal and visceral (epicardium) with serous fluid in between. The serous pericardium consists of two layers: the parietal layer (lines the inner surface of the fibrous pericardium) and the visceral layer (also known as the epicardium), which adheres directly to the heart. The serous fluid reduces friction during heartbeats.
Myocardium (middle layer):- Cardiac muscle tissue and fibrous tissue skeleton. This layer is responsible for the heart's pumping action. The fibrous skeleton provides structural support and electrical insulation, crucial for coordinated contractions.
Endocardium (inner layer):- Lining of the heart lumen, composed of endothelium (simple squamous epithelium). The smooth surface of the endocardium minimizes friction as blood flows through the heart.
Vessels of the Heart
Arteries carry blood away from the heart. Veins carry blood to the heart. Capillaries connect arteries and veins, facilitating exchange of substances.
Great vessels include superior and inferior vena cava, pulmonary arteries, pulmonary veins (4), and aorta. These vessels are directly connected to the heart and are responsible for transporting large volumes of blood.
Pathway of blood:- Systemic: aorta → arteries → arterioles → capillaries → venules → veins → vena cava. This circuit delivers oxygenated blood to body tissues and returns deoxygenated blood to the heart.
Pulmonary: pulmonary artery → lungs → pulmonary veins. This circuit oxygenates blood in the lungs and returns it to the heart.
Chambers of the Heart
Right Atrium: Receives deoxygenated blood from the superior and inferior vena cava (draining systemic circulation) and the coronary sinus (draining the heart muscle itself).
Left Atrium: Receives oxygenated blood from the pulmonary veins (returning blood from the lungs).
Right Ventricle: Pumps deoxygenated blood to the lungs via the pulmonary artery. It has thinner walls compared to the left ventricle.
Left Ventricle: Pumps oxygenated blood to the body via the aorta; it is thicker than the right ventricle because it needs to generate higher pressure to pump blood throughout the systemic circulation.
Valves of the Heart
Atrioventricular (AV) valves: Prevent backflow between atria and ventricles.- Tricuspid: Between right atria and ventricle (3 cusps). Ensures unidirectional blood flow from the right atrium to the right ventricle.
Bicuspid (Mitral): Between left atria and ventricle (2 cusps). Ensures unidirectional blood flow from the left atrium to the left ventricle.
Chordae tendineae and papillary muscles keep valves closed during ventricular ejection. These structures prevent the AV valves from inverting into the atria during ventricular contraction.
Semilunar valves: Prevent backflow from pulmonary trunk and aorta.- Pulmonary: Between right ventricle and pulmonary trunk. Prevents backflow of blood from the pulmonary artery into the right ventricle.
Aortic: Between left ventricle and aorta. Prevents backflow of blood from the aorta into the left ventricle.
Heart Functions
Right side pumps deoxygenated blood to the lungs (pulmonary circuit). Receives blood from the body and sends it to the lungs for oxygenation.
Left side pumps oxygenated blood to the body (systemic circuit). Receives blood from the lungs and sends it to the rest of the body.
Gas exchange:- Lungs: O2 diffuses into blood, CO2 diffuses out. Oxygen moves from the air into the blood, while carbon dioxide moves from the blood into the air.
Systemic capillaries: O2 diffuses into tissues, CO2 diffuses into blood. Oxygen moves from the blood into the tissues, while carbon dioxide moves from the tissues into the blood.
Maintains blood pressure and secretes atrial natriuretic factor (ANF) to lower blood pressure. ANF promotes sodium and water excretion by the kidneys, reducing blood volume and pressure.
Coronary Circulation
Right and left coronary arteries branch off from the ascending aorta to supply the heart muscle with oxygen and nutrients. Blockage of these arteries can lead to myocardial infarction (heart attack).
Coronary veins (great, small, middle cardiac veins) drain into the coronary sinus, which drains into the right atrium. This system ensures deoxygenated blood from the heart muscle returns to the general circulation.
Cardiac Muscle Histology
Branched cells with a single nucleus, shorter and wider than skeletal muscle fibers, and contain more myoglobin and mitochondria. These features support the high energy demands of cardiac muscle.
Intercalated discs join adjacent cells through desmosomes and gap junctions. Desmosomes provide physical connection, while gap junctions allow for rapid electrical communication.
Cardiac Muscle Electrophysiology
Autorhythmic cells initiate and conduct electrical signals. They do not require external stimulation to generate action potentials.
Contractile cells (99%) perform the mechanical work of pumping. These cells contract in response to electrical signals from autorhythmic cells.
Types of Gated Channels: Voltage-gated Na^+, Ca^{2+} and ligand-gated voltage-gated K^+ channels. These channels facilitate ion movement across the cell membrane, essential for action potential generation and propagation.
Resting Membrane Potential
Established by ion gradients (Na^+, Ca^{2+}, K^+) and semi-permeable cell membrane through ion channels. The resting membrane potential is crucial for maintaining cell excitability.
Voltage-gated channels open at specific voltages. This allows for controlled ion flow and action potential generation.
The Action Potential
Stages of the Action Potential:1. Slow Leaking of Na^+.
Voltage-gated Ca^{2+} channel opens when threshold is reached;
Ca^{2+} ions enter the cell causing depolarization to +20 mV.
Voltage-Gated Ca^{2+} channels close, and K^+ channels open.
K^+ ions leave the cell causing repolarization
Na^+/K^+ pump restores resting membrane potential.
Cardiac Conduction System
Involves three populations of pacemaker cells:- Sinoatrial (SA) node: Fastest intrinsic rate of 60 action potentials/minute; located in the upper right atrium. Often referred to as the heart's natural pacemaker.
Atrioventricular (AV) node: Slower rate of 40 action potentials/minute; located posterior and medial to the tricuspid valve. Delays the impulse allowing atria to contract before ventricles.
Purkinje fiber system: Slowest group; depolarizes about 20 times per minute; includes AV bundle, bundle branches, and Purkinje fibers. Rapidly transmit the action potential throughout the ventricles.
Conduction Pathway:1. SA node generates action potential; spreads to atrial cells then to AV node (0.03 seconds).
AV node delays conduction (0.13 seconds) allowing atria to contract before ventricles. This delay is essential for proper cardiac function.
Action potential is conducted to bundle branches, then Purkinje fibers.
Ventricles depolarize (0.06 seconds); entire action potential duration is about 0.22 seconds. This allows for coordinated ventricular contraction.
SA node is the normal pacemaker, establishing sinus rhythm. The fastest depolarizing pacemaker sets the heart rate. If the SA node fails, other pacemaker cells can take over, but at a slower rate.
Contractile Cells Action Potentials
Rapid Depolarization Phase - Voltage-gated Na^+ channels open; influx of Na^+. This rapid influx causes a sharp increase in membrane potential.
Initial Repolarization Phase - Na^+ channels inactive; small outflow of K^+. This brief repolarization is due to the inactivation of Na^+ channels and a transient outflow of K^+.
Plateau Phase - Depolarization sustained at 0 mV; slow opening of Ca^{2+} channels, inflow of Ca^{2+}; outflow of K^+. The influx of Ca^{2+} balances the outflow of K^+, maintaining the plateau.
Repolarization Phase - Na^+ and Ca^{2+} channels close; more K^+ channels open, outflow of K^+. The closure of Ca^{2+} channels and increased K^+ outflow cause rapid repolarization.
Cardiac APs are slower than skeletal muscle APs, allowing for longer relaxation and filling time. This prevents tetanus and ensures efficient blood filling.