2402 Ch 19 McKinley

Chapter 19: Cardiovascular System: Heart

Overview of the Cardiovascular System

• Definition: The cardiovascular system consists of the heart and blood vessels. It is essential for transporting blood throughout the body, which includes delivering oxygen (O2) and nutrients, while removing carbon dioxide (CO2) and waste products.

Functions of the Cardiovascular System

• Transport: The primary function is to circulate blood, delivering essential substances to tissues and organs.

• Perfusion: This is the delivery of blood per time per gram of tissue (measured in mL/min/g). Adequate perfusion is crucial for maintaining cellular health and requires the heart to pump continuously through open, healthy blood vessels.

Components of the Cardiovascular System

• Blood Vessels: Comprised of arteries, veins, and capillaries, they act as the "soft pipes" of the system.

• The Heart: A hollow organ with four chambers (two atria and two ventricles). It functions as the center of the cardiovascular system.

Circulation Types

• Pulmonary Circulation: Involves the movement of blood from the right side of the heart to the lungs and back to the left side of the heart.

• Systemic Circulation: Transports blood from the left side of the heart throughout the body and returns it to the right side of the heart.

Heart Anatomy

• External Anatomy: The heart has distinct anterior and posterior views, which reveal its chambers and external structures.

• Wall Structure:

  • Composed of three layers: epicardium (outer), myocardium (middle layer that is thicker in ventricles), and endocardium (inner).

  • The left ventricle has the thickest wall because it pumps blood to the entire body.

Heart Valves

• Function: Ensure one-way blood flow through chambers. Types include:

  • Atrioventricular Valves: Prevent backflow into the atria (includes tricuspid and bicuspid/mitral valves).

  • Semilunar Valves: Prevent backflow into the ventricles (includes pulmonary and aortic semilunar valves).

• Mechanism: Valves open and close based on pressure differences during heart contractions.

Clinical Views

• Heart Murmurs: Indicate conditions like valvular insufficiency (leaky valves) or valvular stenosis (narrowed valves).

• Fibrous Skeleton: Composed of dense connective tissue, providing structural support, anchoring valves, and acting as an electrical insulator to prevent simultaneous atrial and ventricular contractions.

Coronary Vessels

• Coronary Circulation: Supplies blood to the heart muscle through coronary arteries and veins. It is vital due to the high metabolic demands of the cardiac muscle.

• Coronary Heart Disease: Associated with plaque buildup in coronary arteries, possibly leading to angina or myocardial infarction (heart attack).

Microscopic Structure of Cardiac Muscle

• Cardiac Muscle Cells: Characterized by short, branched fibers with one or two nuclei. They include intercalated discs containing desmosomes (for mechanical connection) and gap junctions (for electrical connection).

• Metabolism: High energy demands with reliance on aerobic metabolism, utilizing various fuel sources (fatty acids, glucose, etc.).

Heart Activity Control

• Conduction System: Controls heart rhythm via specialized cardiac muscle cells that generate action potentials. Key components include the SA node (primary pacemaker) and AV node (backup pacemaker).

• Nervous System Influence: The autonomic nervous system modulates cardiac activity, affecting heart rate and force of contraction.

The Cardiac Cycle

• Definition: Encompasses all events from one heartbeat to the start of the next, including systole (contraction) and diastole (relaxation).

• Stages:

  • Atrial and Ventricular Filling: Blood fills ventricles after atrial contraction.

  • Isovolumic Contraction: Ventricles contract with closed valves.

  • Ventricular Ejection: Blood is ejected into arteries as ventricular pressure exceeds arterial pressure.

  • Isovolumic Relaxation: Ventricles relax and pressure drops, causing closure of semilunar valves.

• Importance of Ventricular Balance: Equal blood volumes must be pumped by both sides of the heart to prevent edema.

Cardiac Output and its Regulation

• Cardiac Output (CO): The volume of blood pumped by a ventricle in a minute (CO = heart rate x stroke volume).

• Factors Influencing Heart Rate and Stroke Volume:

  • Chronotropic Agents: Affect how fast the heart beats.

  • Preload and Afterload: Determinants of stroke volume; preload relates to the volume of blood returning to the heart.

  • Inotropic Agents: Influence the contractility of cardiac muscle.

  • Autonomic Reflexes: Help adjust heart rate based on physiological demand.

Development of the Heart

• Begins in the embryo's third week with the formation of two heart tubes, which fuse into a single primitive heart tube that begins beating by day 22.

• Significant structures formed during development include the foramen ovale, which directs blood shunting in the embryonic stage until birth.