Cardiovascular System: Structure, Function, and Circulation

Cardiovascular System Overview

Introduction

The cardiovascular system consists of three interrelated components: blood, the heart, and blood vessels. Its primary functions are to deliver vital substances (such as oxygen, nutrients, and hormones) to tissues and remove wastes (carbon dioxide and cellular byproducts).

• Blood: Transports gases, nutrients, hormones, and waste products.

• Heart: Muscular organ that pumps blood throughout the body.

• Blood Vessels: Network of tubes (arteries, veins, capillaries) that carry blood to and from all body regions.

Structural System of the Heart

Location and Orientation

The heart is located near the anterior chest wall, behind the sternum, and lies in the mediastinum between the pleural cavities.

• Base: Superior end of the heart where great vessels connect.

• Apex: Inferior, pointed tip of the heart.

Pericardial Sac and Pericardium

• Pericardial Sac: Double-walled membrane that prevents heart displacement, acts as a physical barrier against infection, and contains pain receptors.

• Pericardium: Serous membrane surrounding the heart and lining the pericardial cavity.

  • Parietal pericardium: Lines inner surface of pericardial sac.

  • Visceral pericardium (epicardium): Covers the heart, considered the outer wall layer.

Heart Wall Layers

• Epicardium: Outer, smooth layer that minimizes friction (same as visceral pericardium).

• Myocardium: Thickest, muscular layer providing contractile force for blood flow.

• Endocardium: Internal lining, continuous with blood vessels.

Chambers and Septa

• Four chambers: Right and left atria (collect blood returning to heart), right and left ventricles (pump blood from heart).

• Interatrial septum: Separates the two atria.

• Interventricular septum: Separates the two ventricles, prevents blood mixing.

Valves of the Heart

• Atrioventricular (AV) Valves:

  • Right AV (tricuspid) valve: Separates right atrium from right ventricle.

  • Left AV (mitral/bicuspid) valve: Separates left atrium from left ventricle.

  • Open during ventricular relaxation (diastole), close during contraction (systole).

• Semilunar Valves:

  • Pulmonary semilunar valve: Between right ventricle and pulmonary arteries.

  • Aortic semilunar valve: Between left ventricle and aorta.

  • Open during systole, close during diastole.

Blood Flow Through the Heart

Blood flows through the heart in a specific sequence, ensuring oxygenation and distribution to tissues.

1. Deoxygenated blood enters the right atrium via the superior and inferior vena cava.

2. Passes through the tricuspid valve into the right ventricle.

3. Right ventricle pumps blood through the pulmonary valve into pulmonary arteries to the lungs.

4. Oxygenated blood returns from lungs via pulmonary veins to the left atrium.

5. Passes through the mitral valve into the left ventricle.

6. Left ventricle pumps blood through the aortic valve into the aorta and systemic circulation.

Differences Between Ventricles

• Right ventricle: Thinner wall, lower pressure, pumps blood short distance to lungs.

• Left ventricle: Very thick wall, produces 4-6 times more pressure, propels blood to entire systemic circuit.

Coronary Circulation

Coronary circulation supplies blood to cardiac muscle tissue.

• Right coronary artery: Supplies right atrium and parts of both ventricles.

• Left coronary artery: Supplies left atrium, left ventricle, and interventricular septum.

Conduction System of the Heart

Heartbeat Coordination

• Contraction of atria and ventricles occurs in a coordinated manner.

• Two types of cardiac muscle cells:

  • Contractile cells: 99% of cardiac cells, produce contractions that propel blood.

  • Conducting system cells: Initiate and distribute electrical impulses.

Cardiac Conduction System

• Allows for automaticity (autorhythmicity): cardiac muscles contract without neural or hormonal input.

• Nodal cells (SA and AV nodes) establish rate of contraction.

• Conducting cells distribute stimuli to myocardium:

  • AV bundle (bundle of His)

  • Right and left bundle branches

  • Purkinje fibers

Pacemaker Cells

• Nodal cells reach threshold and fire first, establishing heart rate.

• SA node (cardiac pacemaker): Embedded in posterior wall of right atrium, initiates contraction at 70-80 beats/min.

• If SA node fails, AV node takes over at 40-60 beats/min.

Path of an Impulse (Action Potential)

1. Impulse starts from SA node, travels through atria (atrial contraction).

2. Impulse reaches AV node, delays to ensure atria contract before ventricles.

3. Impulse travels to AV bundle, bundle branches, and Purkinje fibers (ventricular contraction).

Myocardial Electrical Activity

• Cardiac muscle cells generate electrical activity (action potentials) via movement of ions (Na+, K+, Ca2+, Cl-).

• Depolarization: Electrical activation, inside of cell becomes less negatively charged.

  • P wave: Atrial depolarization

  • QRS wave: Ventricular depolarization

• Repolarization: Deactivation, return to resting membrane potential.

Functional Aspects of Cardiac Activity

Heart Dynamics

• Stroke volume (SV): Volume of blood ejected by a ventricle in one beat.

• Cardiac output (CO): Volume of blood ejected from left ventricle in one minute.

Formula for Cardiac Output

Cardiac output is calculated as:

$CO = SV \times HR$

• SV = volume ejected during ventricular systole (average 70 mL)

• HR = heart rate (beats per minute, normal is 60-100)

Autonomic Effects on Heart Rate

• Parasympathetic pathway releases acetylcholine (ACh): slows heart rate.

• Sympathetic pathway releases norepinephrine (NE): increases heart rate.

Blood Vessels

Vascular Pathway of Blood Flow

• Arteries: Carry blood away from heart to organs, branch into arterioles.

• Arterioles: Small vessels, regulate blood flow into capillaries.

• Capillaries: Smallest vessels, site of chemical and gaseous exchange.

• Venules: Transport blood from capillaries to veins.

• Veins: Return blood to atria of heart.

Structure of Vessel Walls

• Tunica intima: Epithelial innermost layer, composed of squamous epithelial cells, continuous with endocardium.

• Tunica media: Middle layer with smooth muscle, elastic tissue, and collagen; regulates vasomotor tone.

• Tunica externa: Outer layer of loose connective tissue, anchors vessel to surrounding tissue.

Arteries and Arterioles

• Arteries: Large vessels supplying distinct body regions.

• Arterioles: Branch from arteries, regulate blood flow into capillaries.

• All arteries have accompanying veins.

Veins and Venules

• Venules receive blood from capillaries, empty into veins.

• Veins have thinner walls than arteries, expand easier (less muscle), and contain valves to prevent backflow.

Comparison of Artery and Vein

Feature	Artery	Vein
Wall Thickness	Thick (more muscle)	Thin (less muscle)
Valves	Absent	Present
Direction of Blood Flow	Away from heart	Toward heart
Pressure	High	Low

Capillaries

• Thin walls permit efficient exchange between blood and interstitial fluid.

• Permeable to ions, nutrients, wastes, gases, and water.

Maintaining Adequate Blood Flow

• Blood flow equals cardiac output (CO).

• Increased CO leads to increased flow through capillaries; decreased CO leads to reduced flow.

• Blood flow also influenced by pressure and resistance:

  • Increased pressure increases flow.

  • Increased peripheral resistance decreases flow (due to vascular resistance, viscosity, turbulence).

Blood Pressure in the Systemic Circuit

• Blood pressure changes as it proceeds through systemic circuit.

• Highest in aorta (120 mmHg), lowest in venae cavae (2 mmHg).

• Recorded as systolic/diastolic (120/80 mmHg).

• Pulse: Rhythmic alternating changes in pressure with each heartbeat.

Capillary Pressures

• Pressure drops from 35 to 18 mmHg along capillary length.

• Cause filtration of water and solutes out of bloodstream into tissues.

• Most materials reabsorbed into capillaries; remainder picked up by lymphatic vessels.

Venous Pressure

• Pressure at start of venous system is 18 mmHg (about one-fifth of arterial system).

• Large veins provide low resistance, ensuring increased flow despite low pressure.

Homeostatic Regulation of Perfusion

Tissue Perfusion

• Affected by cardiac output, peripheral resistance, and blood pressure.

• Goal: Ensure blood flow changes occur at the right time, in the right location, without drastically altering pressure and flow to vital organs.

• Accomplished through autoregulation, neural processes, and endocrine processes.

Pattern of Circulation

Pulmonary and Systemic Circuits

• Pulmonary circuit: Right side of heart pumps deoxygenated blood to lungs for oxygenation.

• Systemic circuit: Left side of heart pumps oxygenated blood to all body tissues.

• These circuits are serially connected; output of one becomes input of the other.

Summary Table: Blood Flow Sequence

Chamber/Valve	Blood Type	Destination
Right Atrium	Deoxygenated	Right Ventricle
Right Ventricle	Deoxygenated	Pulmonary Arteries (Lungs)
Left Atrium	Oxygenated	Left Ventricle
Left Ventricle	Oxygenated	Aorta (Systemic Tissues)

Additional info:

• These notes are based on college-level Anatomy & Physiology content, suitable for ANAT 1005 or similar courses.

• Images referenced in the original file illustrate anatomical structures and blood flow pathways.