Cardiovascular System: Anatomy and Circulation

• The cardiovascular (CV) system includes the heart and blood vessels, while the circulatory system encompasses the CV system and the lymphatic system.
• CV system functions include transporting oxygen, nutrients, hormones, and removing waste products like carbon dioxide.
• Key organs of the CV system are the heart, blood vessels (arteries, veins, capillaries), and blood.

Systemic and Pulmonary Circulations

• Systemic circulation carries oxygenated blood from the left ventricle to the body and returns deoxygenated blood to the right atrium.
• Pulmonary circulation carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium.
• Systemic circulation involves higher pressure and longer distances compared to pulmonary circulation.

Heart Anatomy

• The heart is located in the thorax, within the mediastinum.
• The heart is enclosed in a double-layered sac called the pericardium.
• The heart wall consists of three layers: the epicardium (outer), myocardium (middle, cardiac muscle), and endocardium (inner).
• The heart has four chambers: right atrium, right ventricle, left atrium, and left ventricle.
• Atria receive blood, and ventricles pump blood out of the heart.

Blood Flow Through the Heart

• Deoxygenated blood flows from the body into the right atrium, then to the right ventricle, and out to the lungs via the pulmonary artery.
• Oxygenated blood flows from the lungs into the left atrium, then to the left ventricle, and out to the body via the aorta.
• Atrioventricular (AV) valves (tricuspid on the right, bicuspid/mitral on the left) prevent backflow from ventricles into atria.
• Semilunar valves (pulmonary and aortic) prevent backflow from arteries into ventricles.

Cardiac Muscle (Myocardium)

• Cardiac muscle is striated and involuntary.
• Intercalated discs connect cardiac muscle cells, containing gap junctions for rapid electrical communication.
• Gap junctions allow coordinated contraction of the heart muscle.

Ventricle Wall Thickness

• The left ventricle is thicker than the right ventricle.
• The left ventricle pumps blood to the entire body against higher resistance
• The right ventricle pumps blood only to the lungs.

Pacemaker Cells

• Pacemaker cells are specialized cardiac muscle cells that spontaneously depolarize, initiating heart contractions.
• Key pacemaker cells are located in the sinoatrial (SA) node and the atrioventricular (AV) node.

Cardiac Muscle Contraction Physiology

• Ventricular contraction cell action potential phases:
  • Phase 0 (depolarization): Rapid Na+ influx.
  • Phase 1 (initial repolarization): Na+ channels close, K+ channels open.
  • Phase 2 (plateau): Ca2+ influx balances K+ efflux.
  • Phase 3 (repolarization): Ca2+ channels close, K+ efflux increases.
  • Phase 4 (resting membrane potential).
• Calcium ions play a crucial role in muscle contraction by binding to troponin, which moves tropomyosin and exposes myosin-binding sites on actin.
• Calcium channels (L-type) are essential for the plateau phase and calcium-induced calcium release.

Pacemaker Cell Action Potential

• Pacemaker cell action potential does not have a stable resting membrane potential.
• It gradually depolarizes due to "funny" Na+ channels and decreased K+ permeability.
• Threshold is reached, voltage-gated Ca2+ channels open, causing rapid depolarization.

Cardiac Conduction System

• The cardiac conduction system includes the SA node, AV node, Bundle of His, left and right bundle branches, and Purkinje fibers.
• The SA node initiates the electrical signal, which spreads through the atria, to the AV node, then rapidly through the ventricles.
• Gap junctions facilitate rapid and coordinated spread of electrical signals.

SA Node vs. AV Node

• The SA node is the primary pacemaker of the heart due to its faster intrinsic firing rate.
• The AV node delays the signal slightly, allowing atrial contraction to complete before ventricular contraction.

EKG/ECG

• P wave: Atrial depolarization.
• QRS complex: Ventricular depolarization (atrial repolarization is hidden).
• T wave: Ventricular repolarization.
• PR interval: Time from start of atrial depolarization to start of ventricular depolarization.
• ST segment: Time between ventricular depolarization and repolarization.
• Abnormalities detected on an EKG/ECG include:
  • Tachycardia (fast heart rate).
  • Bradycardia (slow heart rate).
  • Arrhythmias (irregular heartbeats).

Cardiac Cycle

• Cardiac cycle: One complete heartbeat, including systole (contraction) and diastole (relaxation).
• Systole: Contraction phase, during which blood is ejected from the atria or ventricles.
• Diastole: Relaxation phase, during which the atria or ventricles fill with blood.
• Valves open and close passively based on pressure gradients.

Atrial and Ventricular Events

• Atrial systole occurs late in ventricular diastole, pushing remaining blood into the ventricles.
• Ventricular systole begins with isovolumetric contraction, followed by ejection of blood.
• Ventricular diastole begins with isovolumetric relaxation, followed by rapid filling and then slow filling as the atria fill.
• Normal heart sounds (S1 and S2) correspond to valve closures: S1 (AV valves), S2 (semilunar valves).
• Heart murmurs indicate abnormal blood flow, often due to valve defects.

EDV, ESV, SV, and EF

• EDV (End Diastolic Volume): Volume of blood in the ventricle at the end of diastole.
• ESV (End Systolic Volume): Volume of blood remaining in the ventricle at the end of systole.
• SV (Stroke Volume): Volume of blood ejected per beat (SV = EDV - ESV).
• EF (Ejection Fraction): Percentage of EDV ejected per beat (EF = SV/EDV).

Wiggers Diagram

• Wiggers Diagram: Correlates EKG waveforms, heart sounds, and pressure/volume changes during the cardiac cycle.

Ventricular Pressure and Volume Changes

• Right ventricular pressure is lower than left ventricular pressure.
• Volume changes are similar but occur at lower pressures in the right ventricle.

Cardiac Output

• Cardiac Output (CO): Volume of blood pumped by each ventricle per minute.
• Heart Rate (HR): Number of heartbeats per minute.
• Stroke Volume (SV): Volume of blood ejected per beat.

Factors Altering Heart Rate, Contractility, SV, and CO

• Chemical signals (e.g., hormones, drugs)
• Electrolyte imbalances
• Autonomic nervous system activity
• Factors Altering Heart Rate:
  • Epinephrine increases heart rate.
  • Potassium decreases heart rate.
• Factors Altering Contractility:
  • Calcium increases contractility.
  • Digitalis increases contractility.
• Factors Altering Stroke Volume:
  • Afterload increases stroke volume.
• Factors Altering Cardiac Output :
  • Positive chronotropic agents increase heart rate, increasing CO.

Frank-Starling Law

• Frank-Starling Law: Stroke volume increases with increased EDV (preload).
• Increased venous return leads to increased EDV and stronger contraction.

Venous Return

• Venous return: The rate of blood flow back to the heart.
• Factors altering venous return:
  • Blood volume.
  • Venoconstriction.
  • Skeletal muscle pump.
  • Respiratory pump.

Preload and Afterload

• Preload: The degree of stretch on the heart before it contracts (related to EDV).
• Afterload: The resistance against which the heart must pump blood.
• Factors that alter preload:
  • Venous return.
  • Blood volume.
• Factors that alter afterload:
  • Arterial blood pressure.
  • Vascular resistance.

Calculating Cardiac Output

• Cardiac Output (CO) = Stroke Volume (SV) x Heart Rate (HR)
• Factors affecting CO:
  • Changes in Blood Volume.
  • Physical Activity
  • Emotional State

Autonomic Nervous System Effects

• Sympathetic nervous system increases heart rate and contractility via norepinephrine.
• Parasympathetic nervous system decreases heart rate via acetylcholine (primarily affecting the SA node).