Cardiovascular System: Heart Chambers and Valves

• The cardiovascular system consists of the heart, blood vessels and blood.

Heart Chambers

• The heart has four chambers: the left atrium, the right atrium, the left ventricle, and the right ventricle.
• The left and right atria are the superior chambers that receive blood and send it to the ventricles.
• The left and right ventricles are the inferior chambers that pump blood away.
• The left side of the heart has oxygenated blood, while the right side has deoxygenated blood

Heart Valves

• Heart valves prevent backflow to ensure one-way blood flow.
• Atrioventricular (AV) valves are located between an atrium and a ventricle on each side of the heart
• Semilunar valves are between a ventricle and an arterial trunk
• The right AV valve is also known as the tricuspid valve.
• The left AV valve is also known as the bicuspid or mitral valve.

System Circulations

• The cardiovascular system has two major circulations: pulmonary circulation, systemic circulation.
• Pulmonary circulation transports blood from the right side of the heart to the alveoli of the lungs for gas exchange and back to the left side.
• Systemic circulation transports blood from the left side of the heart to the systemic cells of the body for nutrient and gas exchange, and back to the right side of the heart.

Pulmonary Circulation Blood Flow

• Deoxygenated blood enters the right atrium from the venae cavae and coronary sinus, then passes through the right AV valve into the right ventricle.
• Blood passes through the pulmonary semilunar valve, entering the pulmonary trunk, and goes through the pulmonary arteries to both lungs.
• The blood enters pulmonary capillaries of both lungs for gas exchange.
• Oxygenated blood enters the right and left pulmonary veins and returns to the left atrium of the heart.

Systemic Circulation Blood Flow

• Oxygenated blood enters the left atrium
• Passes through the left AV valve (bicuspid or mitral valve)
• Enters the left ventricle
• Then passes through aortic semilunar valve
• Enters the aorta
• It proceeds to be distributed by the systemic arteries
• The enters systemic capillaries for nutrient and gas exchange
• Deoxygenated blood drains into the superior vena cava (SVC), inferior vena cava (IVC), and coronary sinus, and then finally enters the right atrium

Pericardium

• The pericardium surrounds the heart and has three layers: the fibrous pericardium, the parietal layer of serous pericardium, and the visceral layer of serous pericardium (epicardium).
• The pericardial cavity is located between the parietal and visceral layers of the serous pericardium and contains serous fluid.

Heart Wall Layers

• The heart wall varies in thickness, ventricles (pumping chambers) have thicker walls than atria.
• The left ventricle has a thicker wall than the right ventricle because the left ventricle must generate high pressure to force blood through systemic circulation, while the right only pumps to nearby lungs.
• There are three layers of the heart wall: epicardium, myocardium and endocardium.

Cardiac Muscle Metabolism and Properties

• Cardiac muscle has a high demand for energy, has an extensive blood supply, numerous mitochondria, and myoglobin and creatine kinase.
• Cardiac muscle can use different types of fuel molecules like fatty acids, glucose, lactic acid, amino acids, and ketone bodies.
• Cardiac muscle mostly relies on aerobic metabolism and is susceptible to failure when oxygen is low.
• Interference with blood flow to heart muscle can cause cell death.

Fibrous Skeleton

• The fibrous skeleton is made of dense irregular connective tissue.
• Provides structural support at the boundary of atria and ventricles.
• It forms fibrous rings to anchor valves, provides framework for attachment of cardiac muscle, and acts as an electrical insulator to prevent ventricles from contracting at the same time as atria.

Coronary Circulation

• Coronary circulation delivers blood to the heart wall.
• Coronary arteries transport oxygenated blood to the heart wall.
• Coronary veins transport deoxygenated blood away from the heart wall toward the right atrium.

Heart's Conduction System

• The conduction system initiates and conducts electrical events to ensure proper timing of contractions.
• It has specialized cardiac muscle cells that have action potentials but do not contract.
• Its activity is influenced by the autonomic nervous system.

Cardiac Center and Innervation

• The cardiac center is located in the medulla oblongata.
• It contains cardioacceleratory and cardioinhibitory centers, and sends signals via sympathetic and parasympathetic pathways.
• It receives signals from baroreceptors and chemoreceptors in the cardiovascular system.
• The cardiac center modifies cardiac activity but does not initiate it, however, it influences rate and force of heart's contractions.

Divisions of the Autonomic Nervous System

• Parasympathetic innervation decreases heart rate.
• Sympathetic innervation increases heart rate and force of contraction.

Contraction of the Heart

• Action potential must first be initiated in the SA node
• Is propagated throughout the atria and the conduction system
• The action potential is propagated across the sarcolemma of cardiac muscle cells, resulting in muscle contraction.
• Thin filaments slide past thick filaments and sarcomeres shorten within cardiac muscle cells.

SA Node Activity

• Resting membrane potential(RMP) of the SA node = -60mV.
• Slow voltage-gated Na+ channels open to cause inflow of Na+, which changes the membrane potential from -60mV to -40mV.
• This causes Fast voltage-gated Ca2+ channels open to cause calcium (Ca2+) inflow
• Causes membrane potential to change from -40 mV to just above 0 mV.
• Causes voltage-gated Ca2+ channels to close and Voltage-gated K+ channels to open.
• K+ outflow then allows membrane potential returns to RMP -60 mV and K+ channels close

Action Potential Through Cardiac Conduction System

• Generated at the sinoatrial (SA) node and spreads via gap junctions between cardiac muscle cells throughout the atria to atrioventricular (AV) node.
• There is a delay at the AV node before it passes to the AV bundle within the interventricular septum.
• The AV bundle conducts the action potential to the left and right bundle branches and then to the Purkinje fibers.
• The actin potential then is spread via gap junctions between cardiac muscle dells throughout the ventricles

Cardiac Muscle Cell at Rest

• At rest, a cardiac muscle cell has a resting membrane potential (RMP) of -90 mV, with all three voltage channels closed.
• Three phases: depolarization, plateau, and repolarization.
• Fast voltage-gated Na+ channels open, allowing Na+ to rapidly enter the cell, reversing the polarity from negative to positive (-90 mV to +30 mV).
• Then these channels then close voltage-gated, K+ channels open, causing K+ outflow of cardiac muscle cells.
• Slow voltage-gated, Ca2+ channels open, causing Ca2+ to enter the cell, with no electrical change and the depolarized state maintained.
• Voltage-gated, and Ca2+ channels then close
• Voltage-gated K+ channels remain open, and K+ moves out of the cell and is reversed from positive to negative (+30 mV to –90 mV).

ECG Reading

• Skin electrodes detect electrical signals of cardiac muscle cells.
• It is a Common diagnostic tool known as an electrocardiogram (ECG/EKG).

Waves and Segments of a Normal ECG

• The P wave reflects electrical changes of atrial depolarization originating in the SA node.
• The QRS complex refers to ​​Electrical changes associated with ventricular depolarization, and the Atria also simultaneously repolarizing.
• The T wave refers to Electrical change associated with ventricular repolarization.
• P-Q segment is associated with atrial cells’ plateau (atria are contracting).
• S-T segment is associated with the ventricular plateau (ventricles are contracting).

Intervals of a Normal ECG

• P-R interval is time from beginning of the P wave to beginning of the QRS deflection- which is from atrial depolarization to beginning of ventricular depolarization
• Time needed to transmit action potential through entire conduction system
• Q-T interval is time from the beginning of the QRS to the end of the T wave and reflects the time of ventricular action potentials as well as length that depends upon heart rate

Cardiac Cycle

• Encompasses all events in the heart from the start of one beat to the start of the next
• Includes both systole (contraction) and diastole (relaxation)
• Contraction increases pressure; relaxation decreases it
• Blood moves down its pressure gradient (high to low) where all
• Valves ensure that flow is forward (closure prevents backflow)

Ventricular Activity

• Is the most important driving force to raise ventricular pressure
• AV valves are pushed closed
• Semilunar valves pushed open and blood ejected to artery
• Ventricular relaxation lowers ventricular pressure where semilunar valves close and have No pressure from below keeping them open, and AV valves are open where there is No pressure pushing them closed

Cardiac Cycle Phases

• Includes all events in the heart from the start of one beat to the start of the next and there are 5 phases:
• Atrial Contraction and Ventricular filling - atria contract, ventricles relax, ventricle pressure is less than atria and arterial trunk. All valves open except atrial and ventricular trunk
• Isometric Contraction - Atria relax, Ventricles contract, valves closed, and has EDV of 130mL
• Ventricle Ejection - Atria relax, Ventricles contract, valves closed, and as SV of 70mL
• Isometric Relaxation
• Atrial Relaxation and Ventricular Filling

Cardiac Output

• The amount of blood pumped by a single ventricle in one minute and is measured in liters per minute
• Measure of the effectiveness of the cardiovascular system
• It increases in healthy individuals during exercise
• Is determined by heart rate (beats per minute) and stroke volume (amount of blood ejected per beat)
• calculated using formula: HR × SV = CO and is typically 75 beats/min × 70 ml/beat = 5,250 ml/min = 5.25 L/min

Stroke Volume

• Amount of blood ejected in one beat from one ventricle and in influenced by:

  • Venous return: volume of blood returned to the heart where there Is a direct relation to SV which Determines amount of ventricular blood prior to contraction
    • preload- pressure stretching heart wall before shortening
  • Inotropic agents:
  • Afterload is

• Volume determines preload, where pressure stretches heart wall before shortening

• The Frank-Starling law (Starling's Law) states When EDV increases, the greater stretch of heart wall results in more optimal overlap of thick and thin filaments where the heart then contracts more forcefully when filled with more blood, so SV increases The venous return may also be increased by increased venous pressure or increased time to fill where venous pressure increases during exercise as muscles squeeze veins and ​​the time available to fill increases with slower heart rate such as Athletes

Cardiac Output

• Increases with heart rate and inotropy and decreases as afterload increases