Cardiovascular System - The Heart and Circulation

Anatomy of the Cardiovascular System

  • The cardiovascular system comprises the heart and blood vessels.

  • Heart Function: It acts as a muscular organ situated in the mediastinum between the two lungs. It pumps blood throughout the circulatory system, delivering nutrients and oxygen to tissues while removing carbon dioxide and waste products.

  • Blood Vessel Function: These vessels circulate blood throughout the body.

Structure of the Heart

  • The heart consists of four chambers: two atria and two ventricles.

  • The musculature of the ventricles is significantly thicker than that of the atria. The force of contraction depends directly on these muscles.

Chambers and Valves

  • Right Side of the Heart (Deoxygenated Blood):

    • Right Atrium: A thin-walled, low-pressure chamber. It contains the Sinoatrial (SA) node (the pacemaker) and the Atrioventricular (AV) node.

    • Venous Return: It receives deoxygenated blood via the Superior Vena Cava (from head, neck, and upper limbs) and the Inferior Vena Cava (from lower body parts).

    • Right Ventricle: A thick-walled chamber that receives blood from the atrium through the Tricuspid Valve. It pumps blood to the lungs through the Pulmonary Artery (via the pulmonary valve).

  • Left Side of the Heart (Oxygenated Blood):

    • Left Atrium: A thin-walled, low-pressure chamber that receives oxygenated blood from the lungs via Pulmonary Veins. This is a physiological exception where an artery carries venous blood and a vein carries arterial blood.

    • Left Ventricle: Features a very thick wall to pump arterial blood to different body parts through the Systemic Aorta. It receives blood from the left atrium through the Mitral (Bicuspid) Valve.

Septa of the Heart

  • Interatrial Septum: A fibrous septum separating the right and left atria.

  • Interventricular Septum: Separates the right and left ventricles. The upper portion is membranous, while the lower portion is muscular.

Layers of the Heart Wall

  1. Pericardium: The outer covering. It consists of an outer Parietal Pericardium and an inner Visceral Pericardium. The Pericardial Cavity (pericardial space) between them contains a thin film of fluid.

  2. Myocardium: The middle layer composed of cardiac muscle fibers (myocytes). It forms the bulk of the heart and is responsible for its pumping action. These fibers are involuntary. Myocardium contains three fiber types:

    • Contractile unit fibers.

    • Pacemaker fibers.

    • Conductive system fibers.

  3. Endocardium: The innermost layer. It is a thin, smooth, glistening membrane formed by a single layer of endothelial cells. It is continuous with the endothelium of blood vessels.

Heart Valves

  • Atrioventricular (AV) Valves: Located between the atria and ventricles. They open only toward the ventricles to prevent backflow into the atria.

    • Bicuspid (Mitral) Valve: Located on the left side; has 22 cusps.

    • Tricuspid Valve: Located on the right side; has 33 cusps.

    • Anatomy of AV Valves: Attached to an atrioventricular ring. The cusps are connected to Papillary Muscles by Chordae Tendineae. Papillary muscles arise from the inner surface of the ventricles and ensure the closure of cusps during ventricular contraction.

  • Semilunar Valves: Located at the openings of blood vessels arising from ventricles. They are made of 33 flaps and have a half-moon shape.

    • Aortic Valve: At the opening of the systemic aorta.

    • Pulmonary Valve: At the opening of the pulmonary artery.

Actions of the Heart

  • Chronotropic Action: Refers to heart rate (frequency).

    • Tachycardia: Increase in heart rate.

    • Bradycardia: Decrease in heart rate.

  • Inotropic Action: Refers to the force of contraction.

    • Positive Inotropic: Increase in force.

    • Negative Inotropic: Decrease in force.

  • Dromotropic Action: Refers to the velocity of impulse conduction.

    • Positive Dromotropic: Increase in conduction velocity.

    • Negative Dromotropic: Decrease in conduction velocity.

  • Bathmotropic Action: Refers to the excitability of cardiac muscle.

    • Positive Bathmotropic: Increase in excitability.

    • Negative Bathmotropic: Decrease in excitability.

Blood Vessel Anatomy and Physiology

The Arterial System

  • Comprises the aorta, arteries, and arterioles. Walls consist of three layers:

    1. Tunica Adventitia: Outer connective tissue layer (continuation of parietal pericardium).

    2. Tunica Media: Middle smooth muscle layer.

    3. Tunica Intima: Inner endothelial layer (continuation of endocardium).

  • Elastic Laminae: The external elastic lamina sits between the adventitia and media; the internal elastic lamina sits between the media and intima.

  • Vessel Dimensions and Characteristics:

    • Aorta: Maximum diameter of about 25 mm25\,mm.

    • Arteries: Gradually decrease to 4 mm4\,mm at the ends.

    • Arterioles: Diameter ranges from 30 μ30\,\mu to 10 μ10\,\mu. Known as Resistant Vessels because they offer peripheral resistance to blood flow.

  • Capillaries: Small, thin-walled vessels; diameter of 55 to 8 μ8\,\mu. They are functionally critical for material exchange between blood and tissues.

The Venous System

  • Includes venules, veins, and venae cavae.

  • Venules: Connect from capillaries; diameter of about 20 μ20\,\mu. Known as Capacitance Vessels because they hold large quantities of blood.

  • Veins: Diameter of approximately 5 mm5\,mm.

  • Venae Cavae: Diameter of about 30 mm30\,mm.

  • Venous Walls: Comprised of less elastic tissue and more smooth muscle compared to arteries.

Complications in Blood Vessels

  • Arteries: Susceptible to Arteriosclerosis (hardening and loss of elasticity) and Atherosclerosis (narrowing of the lumen due to cholesterol deposition).

  • Arterioles: Increased smooth muscle tone leads to Hypertension.

  • Capillaries: Increased permeability can lead to Shock or Edema.

  • Veins: Inflammation leads to Thrombosis (clot formation). A dislodged clot is a Thrombus, which can cause an Embolism, obstructing flow to vital organs like the brain, heart, or lungs.

Divisions of Circulation

Systemic (Greater) Circulation

  • Blood travels from the left ventricle through the arterial system to tissues, then returns to the right atrium via the venous system. It supplies oxygenated blood to the body.

Pulmonary (Lesser) Circulation

  • Blood is pumped from the right ventricle to the lungs via the pulmonary artery. Oxygenated blood returns to the left atrium through pulmonary veins.

Coronary (Cardiac) Circulation

  • Supplies the myocardium. Myocardial cells cannot receive nutrients effectively through diffusion from the heart chambers.

  • Coronary Arteries: Branches of the ascending aorta.

    • Left Coronary Artery: Includes the Anterior Interventricular branch (Left Anterior Descending - LAD) and Circumflex branch.

    • Right Coronary Artery: Includes the Posterior Interventricular branch and Marginal branch.

  • Dynamics: Blood flow in coronary arteries is minimal during contraction (as they are squeezed shut) but high during relaxation when aortic pressure propels blood into them.

  • Coronary Veins: Deoxygenated blood drains into the Coronary Sinus located in the coronary sulcus. Tributaries include the Great, Middle, Small, and Anterior cardiac veins.

Histology of Cardiac Muscle

  • Fibers are striated (actin and myosin in sarcomeres), branched, and shorter than skeletal muscle.

  • Contains centrally located nuclei; occasionally multinucleated.

  • Intercalated Discs: Irregular transverse thickenings of the sarcolemma connecting fibers. They contain Desmosomes (holding fibers together) and Gap Junctions (allowing action potential conduction, enabling the myocardium to contract as a single coordinated unit).

  • Mitochondria: Larger and more numerous (25%25\% of cytosolic space) than in skeletal muscle.

  • Sarcoplasmic Reticulum (SR): Smaller than in skeletal muscle, leading to a smaller intracellular reserve of Ca2+Ca^{2+}.

Cardiac Conduction System

  • The heart contains autorhythmic (self-excitable) fibers that generate action potentials without outside stimulus. They act as the pacemaker and the conduction system.

  • Sequence of Excitation:

    1. Sinoatrial (SA) Node: The natural pacemaker located in the right atrium; initiates action potentials about every 0.6 sec0.6\,sec (uncorrected rate of 100100 times per minute). At rest, Acetylcholine from the Parasympathetic ANS slows this to 7575 action potentials per minute (0.8 sec0.8\,sec intervals).

    2. Atrioventricular (AV) Node: Located in the interatrial septum. The signal slows here to allow atria to empty into ventricles.

    3. Atrioventricular (AV) Bundle (Bundle of His): The only electrical connection between atria and ventricles.

    4. Right and Left Bundle Branches: Extend through the interventricular septum.

    5. Purkinje Fibers: Large-diameter fibers that rapidly conduct the impulse from the apex upward to the ventricular myocardium.

Action Potential in Contractile Fibers

  • Depolarization (Phase 0): Opening of voltage-gated fast Na+Na^+ channels; Na+Na^+ flows in.

  • Plateau (Phase 2): Sustained depolarization lasting 0.25 sec0.25\,sec. Caused by opening of voltage-gated slow Ca2+Ca^{2+} channels (influx of Ca2+Ca^{2+}) and partial opening of K+K^+ channels (efflux of K+K^+).

  • Repolarization (Phase 3): Recovery of the resting membrane potential (−90 mV-90\,mV) as Ca2+Ca^{2+} channels close and additional voltage-gated K+K^+ channels open (outflow of K+K^+).

  • Refractory Period (Phase 4): The time interval during which a second contraction cannot be triggered. In cardiac muscle, this lasts longer than the contraction itself to ensure relaxation is underway before another beat can start.

Electrocardiogram (ECG/EKG)

  • Definition: A recording of electrical signals produced by all heart muscle fibers during each heartbeat.

  • Principal Waves and Complexes:

    • P wave: Small upward deflection; represents atrial depolarization (spreading from SA node through atria). Duration: 0.1 sec0.1\,sec. Amplitude: 0.10.1 to 0.12 mV0.12\,mV.

    • QRS complex: Rapid ventricular depolarization. Duration: 0.080.08 to 0.10 sec0.10\,sec. Includes QQ (small negative), RR (tall positive, 1 mV1\,mV), and SS (small negative).

    • T wave: Dome-shaped upward deflection; indicates ventricular repolarization (0.2 sec0.2\,sec, 0.3 mV0.3\,mV). Occurs as ventricles relax.

    • U wave: Occasionally seen; attributed to repolarization of papillary muscles.

  • Intervals and Segments:

    • P-R Interval: From the start of PP wave to the start of QQ wave; conduction time from SA node to ventricles.

    • S-T Segment: End of SS wave to start of TT wave. Represents the plateau phase of ventricular depolarization. Elevation indicates acute Myocardial Infarction (MI); depression indicates insufficient oxygen.

    • Q-T Interval: Start of QQ wave to end of TT wave. Indicates total ventricular electrical activity (depolarization and repolarization).

    • R-R Interval: Time between two consecutive RR waves; used to calculate heart rate and duration of one cardiac cycle (0.8 sec0.8\,sec normally).

  • Clinical Abnormalities:

    • Larger PP wave: Atrium enlargement.

    • Enlarged QQ wave: Myocardial infarction.

    • Enlarged RR wave: Ventricle enlargement.

    • Flat TT wave: Insufficient oxygen (Coronary artery disease).

    • Elevated TT wave: Hyperkalemia.

The Cardiac Cycle

  • A single cardiac cycle lasts 0.8 sec0.8\,sec at a heart rate of 75 beats/min75\,beats/min.

  • Atrial Events:

    • Atrial Systole (0.1 sec0.1\,sec): Atria contract, pushing the final 25 mL25\,mL (10%10\%) of blood into ventricles. This is also called the "last rapid filling phase" or "presystole."

    • Atrial Diastole (0.7 sec0.7\,sec): Period of atrial filling.

  • Ventricular Events:

    • Ventricular Systole (0.3 sec0.3\,sec):

      • Isometric Contraction (0.05 sec0.05\,sec): All valves are closed; pressure rises while volume remains constant. Produces the First Heart Sound (S1).

      • Ejection Period (0.22 sec0.22\,sec): Semilunar valves open. Divided into Rapid Ejection (0.13 sec0.13\,sec) and Slow Ejection (0.09 sec0.09\,sec).

    • Ventricular Diastole (0.5 sec0.5\,sec):

      • Protodiastole (0.04 sec0.04\,sec): End of systole and start of diastole.

      • Isometric Relaxation (0.08 sec0.08\,sec): SL valves close (producing the Second Heart Sound (S2)). All four valves are closed; ventricular pressure drops without volume change.

      • Rapid Filling (0.11 sec0.11\,sec): AV valves open; 70%70\% of ventricular filling occurs. Produces Third Heart Sound (S3).

      • Slow Filling (0.19 sec0.19\,sec): Also called diastasis; accounts for 20%20\% of filling.

      • Last Rapid Filling (0.11 sec0.11\,sec): Caused by atrial systole; accounts for final 10%10\% of filling.

Cardiac Volumes and Fractions

  • End-Diastolic Volume (EDV): Total volume in the ventricle at the end of relaxation (130130 to 150 mL150\,mL).

  • End-Systolic Volume (ESV): Volume remaining in each ventricle after contraction (60 mL60\,mL).

  • Stroke Volume (SV): Volume ejected per beat (SV=EDV−ESVSV = EDV - ESV). At rest, this is approximately 70 mL70\,mL.

  • Ejection Fraction: The portion of EDV ejected. Normal: 60%60\% to 65%65\%.

  • Cardiac Output (CO): Total blood volume ejected by a ventricle per minute (approx. 5 liters5\,liters in adults).

Heart Sounds

  • Auscultation: Listening to body sounds via a stethoscope.

  • First Sound (S1; "Lubb"): Louder/longer; caused by AV valve closure.

  • Second Sound (S2; "Dupp"): Shorter/softer; caused by SL valve closure.

  • Third Sound (S3): Caused by blood turbulence during rapid ventricular filling.

  • Fourth Sound (S4): Caused by blood turbulence during atrial systole.

  • Murmur: Abnormal clicking или rushing sound heard between or masking normal sounds.

Regulation of Stroke Volume

  1. Preload: Degree of stretch on the heart before contraction (proportional to EDV). EDV is determined by the duration of ventricular diastole and venous return.

  2. Contractility: Forcefulness of contraction of individual muscle fibers.

  3. Afterload: The pressure that must be exceeded to eject blood (aortic pressure approx. 80 mmHg80\,mmHg; pulmonary pressure approx. 20 mmHg20\,mmHg).

Blood Pressure (BP)

  • Definition: The lateral hydrostatic pressure exerted by blood on vessel walls. It is determined by cardiac output, blood volume, and vascular resistance.

  • Components:

    • Systolic BP: Maximum pressure in arteries during heart contraction (Normal: 120 mmHg120\,mmHg; range 110110-140 mmHg140\,mmHg).

    • Diastolic BP: Minimum pressure in arteries during heart relaxation (Normal: 80 mmHg80\,mmHg; range 6060-90 mmHg90\,mmHg).

    • Pulse Pressure: Difference between systolic and diastolic pressure (Normal: 40 mmHg40\,mmHg).

    • Mean Arterial Pressure (MAP): Average arterial pressure (MAP=diastolic pressure+13pulse pressureMAP = \text{diastolic pressure} + \frac{1}{3} \text{pulse pressure}).

Regulation Mechanisms of BP

  • Cardiovascular (CV) Center: Located in the medulla oblongata. Controls heart rate and vessel diameter via the vasomotor center (vasoconstrictor and vasodilator centers).

  • Neural Regulation:

    • Baroreceptors: Located in the carotid sinus and aortic wall; respond to stretch/pressure changes. Increased pressure triggers decreased sympathetic and increased parasympathetic stimulation to lower BP.

    • Chemoreceptors: Monitor O2O_2, CO2CO_2, and H+H^+ levels. Hypoxia, acidosis, or hypercapnia triggers sympathetic stimulation to increase BP.

  • Renal Mechanism:

    1. ECF volume regulation.

    2. Renin-Angiotensin-Aldosterone (RAA) System: Renin is secreted by juxtaglomerular cells in response to low blood volume. It leads to the production of Angiotensin II (a potent vasoconstrictor) and results in Aldosterone secretion, which increases salt and water reabsorption.

  • Hormonal Regulation:

    • Epinephrine/Norepinephrine: Increase CO and cause vasoconstriction.

    • Antidiuretic Hormone (ADH/Vasopressin): Released from the posterior pituitary during dehydration; causes vasoconstriction and water reabsorption.

    • Atrial Natriuretic Peptide (ANP): Released by the atria; lowers BP by causing vasodilation and promoting salt/water loss in urine.

  • Autoregulation: Ability of tissues (especially brain, cardiac, and skeletal) to adjust blood flow locally based on metabolic demand via physical changes (warming = dilation) or chemicals (K+K^+, H+H^+, lactate, nitric oxide).

Disorders

  • Hypertension: Persistently elevated BP. Stage 1 (140140-159 mmHg159\,mmHg systolic); Stage 2 (>160\,mmHg systolic).

  • Angina Pectoris: Chest pain due to ischemia (insufficient blood supply).

  • Myocardial Infarction: Death of cardiac muscle due to prolonged severe ischemia.

  • Heart Failure: Inability to pump enough blood. Congestive Heart Failure (CHF) involves chronic congestion in the lungs or peripheral circulation.

  • Arrhythmia: Irregular heart rhythm; includes bradycardia (<60\,bpm) and tachycardia (>100\,bpm).