Lecture 18: Cardiovascular System Anatomy

Cardiovascular System Overview

• Heart anatomy, histology, and conduction pathways are key elements
• Knowledge of these elements is crucial for understanding the objectives

Objectives

• Compare pulmonary and systemic circuits to understand blood flow
• Explain the structure and function of the pericardium
• Name the three layers of heart walls - Epicardium, Myocardium, Endocardium
• Identify important anatomical features and the structure of the heart
• Describe the role that heart valves play in maintaining unidirectional blood flow
• Explain the heart's pacemaker and internal electrical conduction system
• Describe the metabolic characteristics of muscle cells

Pericardium

• The pericardium is a double-walled sac enclosing the heart for protection
• The outermost layer is the fibrous layer, which is predominantly tough
• The parietal layer lines the fibrous pericardium, providing additional support
• The visceral layer, also known as the epicardium, is the outermost layer of the heart itself
• The pericardial cavity, or pericardial sac, exists between the visceral and parietal layers
• This cavity contains pericardial fluid which minimises friction during heartbeats
• Pericarditis is the inflammation of the pericardium
• Pericarditis results in increased friction with heartbeats

Heart Wall Layers

• The epicardium is a visceral pericardium and contains adipose tissue
• The epicardium surrounds the external myocardial surface
• The myocardium is composed of cardiac muscle responsible for contraction
• Its thickness is proportional to the workload required of the heart chambers
• The myocardium features bundles of muscle spiraling around the heart
• The endocardium is the smooth inner lining of the heart
• The endocardium surrounds the heart's internal myocardial surface and is endothelial

Surface Anatomy

• The surface anatomy looks at both the anterior and posterior views to understand the positioning of key structures

Chambers of the Heart

• The atria receive blood and send it to the ventricles
• Auricles act as reservoirs for the atria
• Ventricles pump blood away from the heart
• The right ventricle pumps blood into the pulmonary trunk via arteries
• The left ventricle pumps blood into the aorta
• The interventricular septum separates the ventricles, preventing mixing of blood

Valves of the Heart

• Valves ensure one-way blood flow through the heart
• Valves prevent blood from flowing backwards
• The tricuspid valve is located between the right atria and right ventricle chambers
• The bicuspid (mitral) valve is positioned between the left atria and left ventricle chambers
• The pulmonary semilunar valve is at the base of the pulmonary trunk
• The aortic semilunar valve is at the base of the aorta

Circulation Pathways

• Pulmonary circulation carries blood to the lungs for gas exchange
• Systemic circulation distributes blood to every organ and system in the body, including the heart and lungs

Cardiac Muscle Characteristics

• Cardiac muscle is comprised of cardiomyocytes
• Cardiac muscle is striated, short, thick, and branching
• Intercalated discs connect cardiomyocytes
  • Mechanical junctions enable strength within the muscle tissue
  • Electrical junctions facilitate cell-cell ion flow, synchronising contractions

Metabolism in Cardiac Muscle

• Cardiac muscle relies almost entirely on aerobic respiration
• The muscle utilizes myoglobin and glycogen stores for energy
• Adaptable to different organic fuels, ensuring continuous function
• Large amount of mitochondria (25% of each cell) supports high energy demands
• Cardiac muscle is highly vulnerable to oxygen deficiency
• Cardiac muscle performs minimal anaerobic fermentation
• As a result of metabolism, exhibits fatigue resistance

Conduction in Cardiac Tissue

• Conduction relies on cardiomyocytes and heart muscle cells
• Large T tubules in cells allow for calcium from extracellular fluid
• Electrical junctions within intercalated disks facilitate synchronous contractions
  • Gap junctions relay of ion flow from one cell to another, which leads to coordinated heart beats and synchronous contractions

Conduction Pathway in the Heart

• The conduction pathway utilizes internal pacemaker cells and nerve-like conduction pathways
• The conduction pathway is electrical in nature
• The sinoatrial (SA) node functions as the pacemaker of the heart
• Signals spread from the SA node towards the atrioventricular (AV) node, which are then perpetuated by cardiomyocytes via gap junctions
• The atrioventricular (AV) node then transfers the signal through the right and left branches
• From the AV bundle, signals spread through the Purkinje fibers
• Purkinje Fibers transfer signals upwards at the apex (bottom point) of the heart
• The conduction pathway is perpetuated by cardiomyocytes, and occurs via gap junctions

Electrocardiogram (ECG/EKG)

• The P wave represents atrial depolarization
• The PR segment indicates nodal delay between the SA and AV nodes
• The QRS complex shows ventricles depolarizing and atria repolarizing
• The T wave signifies the repolarization of the ventricles
• The TP segment is evident between heart beats, indicating rest before next cycle initiation