Chapter 11

The Cardiovascular System

  • A closed system composed of the heart and blood vessels.

  • Functions to pump blood and facilitate circulation throughout the body.

  • Transports gases, nutrients, cell waste, hormones, and other essential compounds to and from cells in the body.

Anatomy of the Heart

  • Size and Weight:

    • Size: Approximately the size of a fist; 12 cm (5 in) in length, 8 cm (3.5 in) wide, and 6 cm (2.5 in) thick.

    • Mass: Typically 250-350 g (9-12 oz).

  • Location:

    • Situated in the thorax between the lungs in the inferior mediastinum.

  • Orientation:

    • Apex directed toward the left hip, base oriented toward the right shoulder.

Coverings of the Heart

  • Pericardium:

    • A double-walled sac surrounding the heart.

    • Composed of:

      • Fibrous Pericardium: Loose and superficial.

      • Serous Pericardium: Deep to the fibrous layer, has two continuous layers:

        • Parietal Pericardium: Outer layer fused to fibrous pericardium.

        • Visceral Pericardium (Epicardium): Next to the heart.

    • Serous Fluid: Fills the space between the parietal and visceral layers.

Heart Wall Layers

  1. Epicardium:

    • Outer layer, visceral layer of the serous pericardium, made of connective tissue.

  2. Myocardium:

    • Middle layer, composed of cardiac muscle cells and fibrous skeleton.

  3. Endocardium:

    • Inner layer, made of endothelium.

Chambers of the Heart

  • Divided into Four Chambers:

    • Atria (Right and Left):

      • Superior receiving chambers that assist in filling the ventricles.

      • Blood enters under low pressure from veins.

    • Ventricles (Right and Left):

      • Inferior discharging chambers with thick walls that pump blood during contraction.

    • Septa Separating Chambers:

      • Interatrial septum (separates atria).

      • Interventricular septum (separates ventricles).

Cardiac Skeleton (Fibrous Skeleton)

  • Composed of dense connective tissue that:

    • Forms and anchors heart valves.

    • Anchors myocardium and major vessels.

    • Separates atria from ventricles.

    • Channels electrical energy from top to bottom of the heart.

Chambers and Associated Great Vessels

Pulmonary Circulation

  • Right side of the heart sends oxygen-poor blood to the lungs through the pulmonary trunk, which divides into pulmonary arteries. Oxygen-rich blood then returns via pulmonary veins.

Systemic Circulation

  • Left side of the heart pumps oxygen-rich blood into the aorta, circulating to systemic arteries and body tissues.

  • Left Ventricle: Has thicker walls due to pumping blood to the body.

  • Oxygen-poor blood returns to the right atrium via systemic veins (superior/inferior vena cava).

Heart Valves

  • Ensure one-directional blood flow, preventing backflow.

    • Atrioventricular (AV) Valves: Between atria and ventricles.

      • Bicuspid (Mitral) Valve: Left side.

      • Tricuspid Valve: Right side.

    • Semilunar Valves: Between ventricles and arteries.

      • Pulmonary Semilunar Valve: Right ventricle.

      • Aortic Semilunar Valve: Left ventricle.

    • Functionality:

      • AV valves open during heart relaxation and close during contraction, anchored by chordae tendineae.

      • Semilunar valves operate in opposition to AV valves, opening during ventricular contraction.

Cardiac (Coronary) Circulation

  • Heart chambers do not nourish the myocardium.

  • Coronary Arteries: Branch from the aorta and supply oxygenated blood to heart muscle. They fill during heart relaxation.

    • Left Coronary Artery: Anterior interventricular and circumflex arteries.

    • Right Coronary Artery: Posterior interventricular and marginal arteries.

  • Cardiac Veins: Drain myocardium, with blood returning through the coronary sinus to the right atrium.

Heart Physiology

  • Heart Beats: Pumps approximately 1000 times daily, moving around 6000 L of blood.

Skeletal vs Cardiac Muscle Cells

  • Skeletal Muscle Cells:

    • Long cylindrical shape, multinucleated, no gap junctions, less mitochondria.

    • Capable of independent contraction.

  • Cardiac Muscle Cells:

    • Short, branched, interconnected, single or double central nucleus, extensive mitochondria.

    • Contract in a coordinated manner due to gap junctions (intercalated discs).

Cardiac Muscle Contraction

  • All-or-None Law: Activation spreads through gap junctions.

  • Autorhythmicity: Cardiac cells can generate their contractions independently.

  • Refractory Period: Longer than skeletal muscle to prevent tetanic contractions.

Heart's Conduction System

  • Intrinsic Conduction System: Regulates heartbeat rhythm via special nervous tissue.

  • Components:

    • Sinoatrial (SA) Node: Pacemaker, located in the right atrium.

    • Atrioventricular (AV) Node: Junction of atria and ventricles.

    • AV Bundle (Bundle of His) and Purkinje Fibers: Spread excitation through ventricle walls.

Homeostatic Imbalances in Heart Function

  • Ischemia: Inadequate oxygen supply.

  • Fibrillation: Rapid, uncoordinated contractions.

  • Tachycardia: Heart rate over 100 beats/min.

  • Bradycardia: Heart rate below 60 beats/min.

Mechanical Events: Cardiac Cycle

  • Cardiac Cycle: One heartbeat involving contraction (systole) and relaxation (diastole).

  • Average heart rate: ~75 beats/min; cardiac cycle duration: ~0.8 seconds.

  • Key Phases:**

    • Atrial Diastole: Heart relaxed; AV valves open, blood flows to ventricles.

    • Atrial Systole: Atria contract to fill ventricles.

    • Isovolumetric Contraction: Ventricles contract; AV valves close; pressure rises.

    • Ventricular Systole: Blood ejected through semilunar valves.

    • Isovolumetric Relaxation: Ventricles relax; semilunar valves close.

Heart Sounds

  • Lub: Closing of AV valves.

  • Dup: Closing of semilunar valves after systole.

Cardiac Output (CO)

  • Definition: Amount of blood pumped by each ventricle in one minute.

  • Stroke Volume (SV): Blood volume per contraction.

  • Calculation: CO = HR × SV (e.g., CO = 75 bpm × 70 mL/beat = 5250 mL/min).

Regulation of Stroke Volume

  • Starling’s Law: Stretching of cardiac muscle affects contraction strength; preload influences stroke volume.

  • Factors Influencing Preload: Venous return and length of ventricular relaxation.

  • Afterload: Pressure required to eject blood from ventricles.

Regulation of Heart Rate

  • Influenced by:

    • Neural Factors: ANS adjustments (sympathetic and parasympathetic).

    • Hormones/Ions: Epinephrine, thyroid hormones, calcium, sodium, potassium.

    • Physical Factors: Includes age, body temperature, exercise, and body weight.

Blood Vessels - The Vascular System

  • Blood vessels create a closed vascular system transporting blood to tissues and back to the heart.

Types of Blood Vessels

  1. Arteries and Arterioles: Carry blood away from the heart.

  2. Capillary Beds: Exchange site for nutrients, gases, and wastes.

  3. Venules and Veins: Return blood to the heart, with veins often containing valves to prevent backflow.

Microscopic Anatomy of Blood Vessels

  • Three Layers (Tunics):

    • Tunica Intima: Endothelium for friction reduction.

    • Tunica Media: Smooth muscle and elastic tissue (thicker in arteries).

    • Tunica Externa: Protective outer covering, mostly connective tissue.

Differences in Structure

  • Arteries: Thicker, more elastic walls than veins to handle higher pressure.

  • Veins: Thinner walls, larger lumen, designed for low pressure; skeletal muscle aids blood return.

Capillaries

  • Structure: One cell layer thick for efficient exchange.

  • Types: Continuous, fenestrated, and sinusoidal capillaries.

Capillary Beds

  • Networks that facilitate microcirculation; consist of vascular shunts and true capillaries.

Associated Great Vessels

  • Major Arteries: Aorta (largest), pulmonary trunk, left/right pulmonary arteries.

  • Major Veins: Superior/inferior vena cavae, pulmonary veins.

Aorta Region and Branches

  • Arteries: Ascending aorta, aortic arch, thoracic and abdominal aorta.

  • Major Branches from Aorta: Coronary arteries, brachiocephalic trunk, common carotids, subclavians, intercostal arteries, celiac trunk, mesenteric arteries.

Major Veins of Systemic Circulation

  • Draining into Superior Vena Cava: Cephalic, basilic, brachial, and subclavian veins.

  • Draining into Inferior Vena Cava: Great saphenous veins, common iliac veins.

Hepatic Portal System

  • Collects blood from digestive organs, spleen, pancreas, and sends to liver via hepatic portal vein for regulation.

Physiology of Circulation

Vital Signs

  • Include measurements of arterial pulse, blood pressure, respiratory rate, and body temperature.

  • Arterial Pulse: Monitored at superficial arteries.

  • Blood Pressure (BP): Measured using sphygmomanometer; influenced by CO and peripheral resistance (PR).

Factors Affecting Blood Pressure

  • Neural factors (ANS), renal factors, temperature, chemicals, and body position.

Variations in Blood Pressure

  • Normal Range: 110–140 mm Hg systolic and 70–80 mm Hg diastolic.

  • Hypotension: Systolic below 100 mm Hg; often indicates illness.

  • Hypertension: Systolic above 140 mm Hg; chronic cases can be dangerous.

Capillary Exchange

  • Mechanisms for material transport include direct diffusion, intercellular clefts, and vesicular transport.

  • Influence of hydrostatic and osmotic pressures drives fluid movement across capillary membranes.

Developmental Aspects and Aging

  • Aging can lead to issues such as weakened venous valves, varicose veins, atherosclerosis, and hypertension.

  • Homeostatic Imbalances: Conditions like angina pectoris, myocardial infarction, heart murmurs, and congestive heart failure (CHF) can arise from cardiovascular complications.