UNIT 2 - Cardiovascular System

Anatomy of the Heart

External Anatomy

  • The heart is located in the inferior mediastinum, above the diaphragm and behind the sternum.
  • Its apex points down, forward, and to the left, while the base points up, backward, and to the right.
  • Most of the heart is situated in the left hemithorax.
  • The heart's position changes with ventilation and its own beating.
  • Key external features include the superior vena cava, base of the heart, right border, right lung, inferior surface, diaphragm, arch of the aorta, pulmonary trunk, left lung, left border, and apex of the heart.

Layers of the Heart

  • Endocardium: The innermost layer, composed of endothelium.
  • Myocardium: The middle layer, consisting of striated cardiac muscle.
  • Pericardium: The double outer layer.
    • Fibrous Pericardium: A rigid and closed outer layer.
    • Serous Pericardium: A double-layered membrane composed of the visceral and parietal layers.

Cardiac Chambers

  • Atria: Upper chambers with appendages.
  • Ventricles: Lower chambers.
  • Sulci: External grooves marking the limits of the heart chambers (coronary and interventricular).
  • Septa: Internal separations (interventricular and interatrial).
Right Atrium
  • Receives blood from:
    • Superior vena cava.
    • Inferior vena cava.
    • Coronary sinus.
  • Tricuspid valve regulates blood flow to the right ventricle.
Right Ventricle
  • Pumps blood to the pulmonary trunk.
  • The pulmonary valve regulates blood flow to the pulmonary trunk.
Left Atrium
  • Receives blood from the four pulmonary veins.
  • The bicuspid or mitral valve regulates blood flow to the left ventricle.
Left Ventricle
  • Has a thicker and stronger myocardium.
  • Pumps blood to the aorta, regulated by the aortic valve.
  • Circular section
Atrio-Ventricular Valves
  • Bicuspid Valve:
    • Open: Chordae tendineae are slack, and papillary muscles are relaxed.
    • Closed: Chordae tendineae are taut, and papillary muscles are contracted.
Semilunar Valves
  • Pulmonary valve
  • Aortic valve

Coronary Circulation

  • Coronary veins converge in the coronary sinus, located on the posterior coronary sulcus.
  • The coronary sinus gathers venous blood from the heart walls and empties it into the right atrium.

Anatomy of Blood Vessels

Main Vessels: Aorta

  • The aorta is the main artery of the human body.
  • It receives blood from the left ventricle and maintains the high pressure generated by the ventricle.
  • Four parts of the aorta:
    • Ascending aorta
    • Aortic arch
    • Thoracic aorta
    • Abdominal aorta
  • Each part gives rise to distribution arteries that carry blood to different parts of the body.
Ascending Aorta
  • Gives rise to the coronary arteries.
Aortic Arch
  • Brachiocephalic trunk:
    • Right subclavian artery
    • Right common carotid artery
  • Left common carotid artery
  • Left subclavian artery
Thoracic Aorta
  • Visceral branches: Supply the inner organs of the cavity.
  • Parietal branches: Supply the outer muscles of the cavity, such as the intercostals.
Abdominal Aorta (from cranial to caudal)
  1. Celiac Trunk: Supplies the liver, spleen, and stomach.
  2. Superior Mesenteric: Supplies the small intestine and the right colon.
  3. Renal: Left and right arteries to the kidneys.
  4. Inferior Mesenteric: Supplies the colon and rectum.
  • End of the Aorta: Divides into the iliac arteries, which supply the inferior limbs.

Main Vessels: Venae Cavae

Superior Vena Cava
  • Gathers venous blood from the head, neck, upper limbs, and thoracic cavity, and empties it into the right atrium.
    • Jugular Veins: Collect blood from the head.
    • Subclavian Veins: Collect blood from the arms.
    • Brachiocephalic Veins: Formed by the convergence of the jugular and subclavian veins, merging into the superior vena cava.
    • Thoracic Cavity: Veins from each organ and muscle converge into the azygos vein, which runs close to the vertebral column and merges with the superior vena cava.
Inferior Vena Cava
  • Gathers venous blood from the abdominal cavity and lower limbs, and empties it into the right atrium.
    • Abdominal Cavity: Veins from each organ (renal, hepatic, etc.) converge directly into the vena cava.
    • Lower Limbs: Iliac veins gather blood from each leg and merge at the caudal part of the inferior vena cava.

Functions of the Heart

  • The heart is an organ made of cardiac muscle, responsible for pumping and maintaining blood flow through the vascular system.
  • It can contract rhythmically, generating an electrical signal on its own (myogenic origin).

Types of Cells

Myocardial Contractile Cells
  • Joined by gap junctions at the intercalated disks, allowing fast transmission of electrical signals.
  • Types:
    • Myocardial contractile atrial cells.
    • Myocardial contractile ventricular cells.
Myocardial Autorhythmic Cells (1%)
  • Structurally different, lacking sarcomeres for contracting.
  • Types:
    • Pacemaker Cells: Rhythmically produce pacemaker action potentials (PAs), located in the sinoatrial (SA) and atrioventricular (AV) nodes.
    • Conduction Cells: Include internodal pathways, bundle branches, and Purkinje fibers.
  • Pacemaker cells are responsible for the myogenic electric activity of the heart.
  • The heart functions without innervation.

Electrical Events

Pacemaker Action Potential

  • The heart generates its own impulses to trigger myocardial cell contraction via action potentials from pacemaker cells.
  • Action potentials reach contractile cells through gap junctions.
  • Characteristics:
    • Unstable resting membrane potential.
    • Automatic reaching of the threshold.
    • Role of calcium in depolarization.
  • If channels are voltage-gated channels that open at −60mV-60mV, allowing simultaneous sodium (in) and potassium (out) current.
  • Key voltages and ion movements:
    • −60mV-60 mV: Na+Na^+ in, K+K^+ out.
    • −40mV-40 mV: Ca2+Ca^{2+} entries.
    • K+K^+ out.
  • The autonomic nervous system modulates the firing rate of pacemaker cells.
    • Sympathetic Activity (Noradrenaline): Increases heart rate.
    • Parasympathetic Activity (Acetylcholine): Decreases heart rate.
Sympathetic Activity (Noradrenaline)
  • Increases heart rate by binding to receptors on pacemaker cells, increasing Na+Na^+ and Ca2+Ca^{2+} influx, and speeding up depolarization.
Parasympathetic Activity (Acetylcholine)
  • Decreases heart rate by binding to receptors on pacemaker cells, increasing K+K^+ efflux, decreasing Ca2+Ca^{2+} influx, and slowing down depolarization.

Myocardial Contractile Cells Action Potential

  • Contractile cells need a stimulus to contract, just like skeletal muscle fibers, receiving it from pacemaker cells via gap junctions.
  • Contractile myocardial cells trigger their own action potential before contraction.
  • Phases:
    • Phase 4: Resting Membrane Potential: Stable at −90mV-90 mV (only permeable to K+K^+).
    • Phase 0: Depolarization: Voltage-gated Na+Na^+ channels open, sodium enters, depolarizing the cell up to +20mV+20 mV, then close.
    • Phase 1: Repolarization I: Voltage-gated K+K^+ channels open, potassium goes out (short phase).
    • Phase 2: Plateau: Potassium permeability diminishes, and Ca2+Ca^{2+} channels open. The balance of ions sustains voltage and increases action potential duration.
    • Phase 3: Fast Repolarization II: Calcium channels close, potassium channels remain open, and potassium flows out of the cell quickly, repolarizing the membrane.
  • The plateau phase prevents tetanus.
  • Plateau increases action potential duration, allowing muscle relaxation and membrane repolarization to occur almost simultaneously, preventing a second action potential until the muscle has relaxed.

Generation and Conduction of the Electrical Impulse

  1. Depolarization of the SA node.
  2. Fast spreading toward the left atrium and the AV node.
  3. Depolarization of the atrial myocardium.
  4. Pacemaker depolarization spreads from the AV node through the bundle branches of the septum.
  5. Pacemaker depolarization reaches the apex, and ventricle myocardial depolarization begins (bottom to top).
  • Total electrical activity is driven by the SA node.
    • In a healthy heart, the SA node fires at 80-100 action potentials per minute, faster than the heart rate at rest, modulated by the parasympathetic system.
    • In case of SA node failure, the AV node and bundle branches can generate the electrical signal at lower rates (40 action potentials per minute) without autonomic regulation.
    • Receptors for autonomic neurotransmitters (acetylcholine and noradrenaline) are only present at the SA node.

Mechanical Events

Cardiac Cycle

  • Cardiac cycle: Electrical and mechanical events that lead to pressure and volume changes, filling and pumping blood, in and from the heart.
    • Diastole: Cardiac muscle relaxes, allowing chamber filling.
    • Systole: Cardiac muscle contracts, pumping blood out.
  • Phases are not simultaneous in atria and ventricles; separate atrial and ventricular cycles.
Atrial Cycle
  • Diastole: Very long to ensure blood inflow.
  • Systole: Shorter, helps finish filling ventricles.
Ventricular Cycle
  • Diastole:
    • Isovolumic relaxation (pressure decrease).
    • Ventricular filling: fast/slow/active.
  • Systole:
    • Isovolumic contraction (pressure increase).
    • Ventricular ejection.
  • AV valves open during ventricular filling.
  • AV valves close during isovolumic contraction.
  • Semilunar valves open during ventricular ejection.
  • Semilunar valves close during isovolumic relaxation.

Cardiac Parameters

  • Stroke Volume (Vs): Total volume of blood ejected by the heart in a single systole (80 mL).
  • Ventricular End Diastolic Volume (VEDV): Total volume of blood inside the ventricle at the end of diastole (120 mL).
  • Ejection Fraction: Percentage of VEDV ejected in a single systole: (Vs/VEDV)∗100(Vs/VEDV) * 100.
  • Systolic Residual Volume (SRV): Volume of blood remaining inside the ventricle after systole: (VEDV−Vs)(VEDV - Vs). Around 40 mL.
  • Heart Rate (HR): Number of contractions or beatings in one minute. At rest, between 55-100 bpm, regulated by autonomic activity.
  • Cardiac Output (Q): Total volume of blood ejected from the heart in one minute. Main parameter to assess cardiac function. In a resting adult, the average is 5 liters/min. Calculated as stroke volume times heart rate: Q=Vs∗HRQ = Vs * HR.

Cardiac Function Regulation

  • The heart adjusts the volume of blood pumped depending on tissue needs, regulating cardiac output (Q).
  • Q varies with age, posture, and exercise.
  • Normally, the heart pumps as much blood as it receives through the venae cavae, so cardiac output and venous return are equal.
  • Frank-Starling's Law: Ventricle's force of contraction is proportional to the amount of blood at the end of diastole (preload).
Distribution of Cardiac Output
  • Blood is distributed to each tissue based on its needs:
    • Brain: 14%
    • Heart: 4%
    • Digestive system: 27%
    • Kidneys: 20%
    • Skeletal muscles: 21%
    • Skin: 5%
    • Bones and other tissues: 9%

Regulation of Vascular Function

Classification

  • The heart pumps blood to a closed and double circuit:
    • Systemic or general circulation.
    • Pulmonary circulation.
Anatomical Classification of Blood Vessels
  • Arteries: Carry blood from the heart to other organs.
  • Arterioles: Small-diameter arteries inside tissues.
  • Capillaries: Smaller vessels that allow gas and nutrient exchange with cells.
  • Venules: First portion after capillaries.
  • Veins: Venules merge to form bigger veins, carrying blood back to the heart.
Functional Classification of Blood Vessels
  • Arteries: Elastic to maintain pressure generated by the ventricle.
  • Arterioles: No elastic tissue; regulate diameter to allow or prevent blood flow into different tissues.
  • Capillaries: Very thin membrane (endothelium only) to facilitate exchange.
  • Venules and Veins: Blood reservoirs; very low pressures return blood to the heart.

Arterial Circulation

  • During ventricular contraction, ventricles contract and semilunar valves open. Aorta and arteries expand and store pressure in elastic walls.
  • During ventricular relaxation, the semilunar valve shuts, preventing flow back into the ventricle. Elastic recoil of arteries sends blood forward into the circulatory system.
  • When the aortic valve opens, the pressure in the aorta is 80 mmHg. Ejection of blood increases aortic pressure up to 120 mmHg. These correspond to diastolic and systolic pressures.
Blood flow and Pressure
  • Blood flows due to the pressure gradient generated by the left ventricle and maintained by the arteries.
  • The biggest pressure drop happens at the capillaries, leaving the venous part with almost no pressure gradient.
Tonic Control of Arterioles Diameter
  • Mediated by sympathetic neurons releasing norepinephrine onto alpha receptors of the blood vessels to maintain tonic activity.
  • Increased signal rate leads to blood vessel constriction, while a decreased signal rate leads to blood vessel dilation.

Venous Return

  • The venous portion drives blood back to the heart and acts as a blood reservoir (60-70% of total blood volume).
  • Vein walls have high compliance, altering their shape to store blood.
  • Blood pressure at the venous portion is lower (average gradient of 10 mmHg), depending mostly on distension and volume.
Mechanisms Facilitating Venous Return
  • Venous Valves: Present in main veins of the lower body to prevent backflow.
  • Skeletal Muscle Pump: Contraction of skeletal muscles compresses veins, forcing blood up.
  • Respiratory Pump: Compression by respiratory muscles at the thoracic cavity.
  • Cardiac Suction: From the right atrium.

Capillaries

  • The exchange portion includes precapillaries (arterioles ending), capillaries, and postcapillary venules.
  • Blood can be directed straight to the venous portion through arteriovenous bypasses or distributed slowly over capillary beds, regulated by precapillary sphincters (smooth muscle).
Blood Flow Regulation in Capillaries
  • If blood goes directly to the venous portion (less exchange), precapillary sphincters contract, closing access to capillary beds, forming the arteriovenous bypass.
  • If blood goes to capillary beds (more exchange), precapillary sphincters relax, allowing blood spreading and decreasing blood speed.

Capillary Exchange

Pressures Regulating Liquid Exchange
  • Hydrostatic Pressure: Fluid pressure against the vessel wall.
    • Hydrostatic pressure inside the capillary (Pc): Higher at physiological values.
    • Hydrostatic pressure of the interstitial fluid (PIF).
  • Oncotic Pressure: Pressure due to protein concentration (Donnan effect).
    • Oncotic pressure at the capillary (∏\prodc): Higher at physiological values.
    • Oncotic pressure of the interstitial fluid (∏\prodIF): Almost no protein content at physiological conditions.
Starling Forces
  • Hydrostatic and oncotic pressures act within the capillary and in the IF, determining the direction and amount of fluid movement.
Fluid Movement
  • Filtration forces favoring fluid passage from capillary to interstitial fluid: Capillary hydrostatic pressure and interstitial fluid oncotic pressure.
  • Absorption forces favoring fluid passage from interstitial fluid to capillary: Interstitial fluid hydrostatic pressure and capillary oncotic pressure.
  • Net Pressure: PN=kf[(Pc+∏IF)–(PIF+∏c)]PN = kf [(Pc + \prod IF) – (PIF + \prod c)]
    • If PN>0PN > 0, FILTRATION predominates.
    • If PN<0PN < 0, ABSORPTION predominates.
  • KfKf: Membrane filtration coefficient.
  • Homeostasis: Filtration = Absorption.
Edema
  • Fluid excess in body tissues, resulting in a loss of balance between filtration and absorption.
  • Causes:
    • Increase in capillary hydrostatic pressure (high blood pressure, heart failure).
    • Decrease in plasma protein concentration (severe malnutrition, liver failure).
    • Increase in interstitial protein content (inflammation, inadequate drainage of lymph).

Lymphatic System

Composition
  • Lymph: Liquid formed at the interstitial space of capillaries, gathered and transported by lymphatic vessels, and returned to the bloodstream.
  • Composition: Water, plasmatic protein, lipids, white cells, and cellular debris.
Anatomy of the Lymphatic System
  • Lymph vessels (capillaries) are blind-ended and lie close to all blood capillaries.
  • Their endothelium is thinner than blood capillaries, walls are anchored to connective tissue by filaments, creating openings between cells for larger molecules.
Lymph Nodes
  • Lymph is filtered through lymph nodes arranged in groups:
    • Cervical
    • Submandibular
    • Supraclavicular
    • Axillary
    • Inguinal
    • Femoral
  • Isolated nodes and diffuse lymphatic tissue also exist (Peyer’s patch, tonsils, GALT).
Lymph Node Structure
  • Nodes are masses of B and T lymphocytes within a capsule, performing defense (antigen presentation) and leukocyte maturation.
    • Medulla: B cells and macrophages
    • Cortex: T cells
    • Trabeculae
Lymphatic Circulation
  • Lymph enters the node through four afferent vessels, is filtered toward sinusoids, and exits through a single efferent duct. Harmful particles are removed.
  • Lymph flow depends on vessel contraction and valves. It is aided by skeletal muscle contraction.
Lymph Ducts
  • Lymph vessels progressively increase in size until forming ducts, emptying into venous circulation at the junction of subclavian and jugular veins.
    • The upper right half of the body is drained by the right lymphatic duct.
    • The upper left half and the inferior part are drained by the thoracic duct.
    • Abdominal lymph accumulates at the cisterna chyli.

Autonomic Regulation of Cardiovascular Function

  • Involves the CNS (medulla-pons) and the ANS (sympathetic and parasympathetic).
  • The brain cortex, limbic system, and hypothalamus can activate or inhibit the centers of the medulla.

Nervous Regulation: Reflexes

  • Involves receptors, control centers (medulla-pons), and effectors.
    • Receptors:
      • Mecanoreceptors
      • Baroreceptors
      • Chemoreceptors
    • Effectors:
      • Vessels (arterioles and veins)
      • Heart (pacemaker cells and ventricular myocardium)
Reflex Responses to Receptors
  • Mecanoreceptors: Respond to distension of the right atrial walls (Starling reflex, maintaining venous return and cardiac output balance).
  • Baroreceptors: Respond to changes in blood pressure, adapting arteriole resistance and heart rate (located in the aortic arch).
  • Chemoreceptors: Respond to changes in oxygen partial pressure, carbon dioxide, and pH in arterial blood (located in the aortic arch).
Baroreceptor Reflex
  • Increases in blood pressure stimulate baroreceptors, which inhibit the vasomotor center, decreasing sympathetic activity and increasing parasympathetic activity.
Chemoreceptor Reflex
  • Chemoreceptors in the medulla oblongata, carotid body, and aortic body detect changes in blood gases and pH and trigger adjustments to maintain homeostasis.
  • An increase in blood pressure stimulates baroreceptors, inhibiting the vasomotor center, and decreasing sympathetic activity while increasing parasympathetic activity. This results in vasodilation, less contraction force, and decreased heart rate, ultimately lowering blood pressure.