Chapter 13: Cardiac Function Notes

Overview of the Cardiovascular System

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

• Heart: A muscular pump responsible for moving blood through the vessels; it regulates blood pressure and volume with sensory and endocrine functions.

• Blood Vessels: Tubular conduits for transport of blood that also function as regulators of blood distribution and pressure.

• Blood: Connective tissue fluid carrying nutrients and waste, essential for cell communication and hormone transport.

• This system is closed, functioning internally to control molecule delivery and removal to meet body demands.

• The system is highly coordinated with the nervous and endocrine systems.

Basic Terminology

• Artery: Blood vessels carrying blood away from the heart.

• Vein: Blood vessels carrying blood to the heart.

• Atrium: Chamber of the heart receiving blood.

• Ventricle: Chamber of the heart ejecting blood.

Anatomy of the Heart

• The heart resembles an upside-down cone pointing to the left.

• Apex: Points downward and to the left, fused to the diaphragm by the pericardium (a double-layered serous membrane).

• Chambers: Four chambers - two atria and two ventricles. The left ventricle has a thicker wall than the right due to pumping blood over greater distances.

Valves of the Heart

1. Atrioventricular Valves (AV Valves): Separate atria from ventricles, permitting blood flow when atrial pressure exceeds ventricular pressure.

   • Bicuspid/Mitral Valve: On the left side (two flaps).

   • Tricuspid Valve: On the right side (three flaps).

2. Semilunar Valves: Separate ventricles from arteries, opening when ventricular pressure exceeds arterial pressure.

   • Aortic Semilunar Valve: Between left ventricle & aorta.

   • Pulmonary Semilunar Valve: Between right ventricle & pulmonary trunk.

Layers of the Heart

• Myocardium: Thickest layer composed of cardiac muscle, substrate for contraction.

• Endocardium: Inner lining made of simple squamous epithelium, continuous with the endothelium of blood vessels.

• Epicardium/Visceral Pericardium: Forms the top layer and is also epithelial.

• Pericardium: Sac surrounding the heart providing lubrication to prevent friction with other organs.

Anatomy of Blood Vessels

• Types: Arteries, veins, and capillaries.

• Veins: Carry blood towards the heart, larger lumen, contain valves to prevent backflow.

• Arteries: Carry blood away from the heart, thicker muscle layer allowing for vasoconstriction and vasodilation.

• Capillaries: Smallest vessels, crucial for nutrient and gas exchange; composed of thin endothelial layers lacking smooth muscle.

Blood Composition

• Blood Components: Plasma (55%), buffy coat (containing platelets and leukocytes), and erythrocytes (red blood cells).

• Erythrocytes: Biconcave, flexible shape for effective travel through capillaries; carry oxygen due to hemoglobin.

Circulatory System Anatomy

• Two Circuits:

  1. Pulmonary Circuit: Oxygen-poor blood sent to lungs, oxygen-rich blood returned to heart.

  2. Systemic Circuit: Oxygen-rich blood distributed to body, oxygen-poor blood returned to heart.

• Terms: Oxygen-rich (blood flowing away from heart) vs. oxygen-poor (blood returning to heart). Blood is always red, veins appear blue due to optical illusions, not actual blood color.

Cardiac Cycle

• Defines all heart events in a heartbeat, occurs in stages:

  1. Ventricular Filling & Atrial Contraction: AV valves open, blood fills ventricles, atria contract to complete filling.

  2. Isovolumetric Contraction: Ventricles contract, pressure builds, valves remain closed.

  3. Ventricular Ejection: Blood is ejected when semilunar valves open.

  4. Isovolumetric Relaxation: Semilunar valves close, ventricles relax, pressure drops.

• Aortic, Atrial, and Ventricular Pressures

  • Atrial Pressure: Increases during atrial contraction, aiding in ventricular filling. It is lower than ventricular pressure when the AV valves open.

  • Ventricular Pressure: Rises during ventricular contraction to exceed aortic pressure for blood ejection. It decreases during diastole for atrial filling.

  • Aortic Pressure: Peaks during ventricular ejection. It gradually decreases as blood moves through the systemic circulation, aided by the elastic recoil of the aorta, which helps maintain blood flow.

Electrical Activity of the Heart

• Myogenic Contraction: Heart generates its own contractions through specialized myocytes (autorhythmic cells).

• Pacemaker Cells: Located in the SA node, generate action potentials guiding heart rhythm.

• Conduction System: Signal travels from the SA node to the AV node, then along the bundle of His, Purkinje fibers activate ventricles from apex upward, ensuring effective ejection of blood.

Electrocardiograms (ECGs)

• ECGs: Measure electrical activity of the heart, reflect mechanical activity indirectly. Readings consist of:

  • P-Wave: Atrial depolarization.

  • QRS Complex: Ventricular depolarization.

  • T-Wave: Ventricular repolarization.

Cardiac Output and Regulation

• Cardiac Output (CO): Amount of blood pumped by a ventricle in one minute, influenced by heart rate and stroke volume.

• Formulas: CO = HR x SV.

  • Ex: Healthy Adult

    • CO = (72 bpm) x (0.07 l/b) = 5.0 l/m

• Factors Affecting CO:

  • Extrinsic regulation: neural and hormonal control.

  • Increased sympathetic activity raises CO; parasympathetic decreases.

• Factors Affecting Stroke Volume (SV):

  1. End Diastolic Volume (Preload): Increased volume leads to increased SV (Starling's Law).

  2. Contractility: Strength of contraction, increased by sympathetic activity and epinephrine.

  3. Afterload: Pressure against which the heart must work; increased afterload decreases SV.

Sounds of the Heartbeat

• The heartbeat sound ("lubb-DUPP") comes from turbulent blood flow during valve closure, not from the opening/closing of valves.

Summary of Factors Affecting Stroke Volume and Cardiac Output

1. Increased venous return leads to increased end-diastolic volume and increased stroke volume.

2. Increased sympathetic activity boosts contractility and stroke volume.

3. Decreased afterload leads to increased stroke volume.