The Cardiovascular System: Structure and Function

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The Cardiovascular System

Overview and Functions

The cardiovascular system is a closed system composed of the heart and blood vessels. Its primary function is to transport oxygen, nutrients, cell wastes, and hormones to and from the cells of the body. The heart acts as a pump, while blood vessels provide the route for blood circulation throughout the body.

Transport: Delivers essential substances to tissues and removes metabolic wastes.
Regulation: Maintains homeostasis, including temperature and pH balance.
Protection: Circulates immune cells and clotting factors.

Anatomy of the Heart

Location and Orientation

The heart is approximately the size of a human fist and weighs less than a pound. It is located in the thoracic cavity, between the lungs in the inferior mediastinum. The apex points toward the left hip and rests on the diaphragm, while the base points toward the right shoulder.

Location of the heart within the thorax Surface anatomy of the heart in the thorax Transverse section showing mediastinum and heart

Heart Wall and Coverings

The heart wall consists of three layers, each with distinct functions and structures. The heart is also enclosed by a double-walled sac called the pericardium, which protects and anchors the heart.

Fibrous Pericardium: Tough, dense connective tissue that anchors the heart and prevents overexpansion.
Serous Pericardium: Composed of a parietal layer (lining the fibrous pericardium) and a visceral layer (epicardium) that covers the heart surface.

Heart wall and pericardial layers

Epicardium: Outermost layer; also known as the visceral pericardium.
Myocardium: Middle layer composed of cardiac muscle; responsible for contraction.
Endocardium: Inner layer; a thin endothelium lining the heart chambers.

Heart wall layers: epicardium, myocardium, endocardium

Clinical Note: Pericarditis

Pericarditis: Inflammation of the pericardium, often caused by infection. Can lead to fluid accumulation (cardiac tamponade), restricting heart movement and reducing cardiac output.

Chambers and Associated Great Vessels

Heart Chambers

The heart contains four chambers: two atria (superior, receiving chambers) and two ventricles (inferior, discharging chambers). The atria receive blood under low pressure from veins, while the ventricles have thick walls and pump blood into circulation.

Anterior view of heart showing major vessels Frontal section showing interior chambers and valves

Septa of the Heart

Interatrial septum: Separates the right and left atria.
Interventricular septum: Separates the right and left ventricles.

Muscular interventricular septum

Double Pump Function

Pulmonary Circuit: Right side pumps deoxygenated blood to the lungs via the pulmonary trunk and arteries; oxygenated blood returns via pulmonary veins to the left atrium.
Systemic Circuit: Left side pumps oxygenated blood to the body via the aorta; deoxygenated blood returns to the right atrium via the superior and inferior vena cava.

Diagram of pulmonary and systemic circulation

Heart Valves

Types and Functions

Heart valves ensure unidirectional blood flow and prevent backflow. There are two main types:

Atrioventricular (AV) Valves: Between atria and ventricles (right: tricuspid; left: bicuspid/mitral).
Semilunar Valves: Between ventricles and major arteries (pulmonary and aortic semilunar valves).

Heart valves and their locations

Valve Operation

AV Valves: Anchored by chordae tendineae; open during relaxation, closed during contraction.
Semilunar Valves: Closed during relaxation, open during ventricular contraction.
Valves open and close in response to pressure changes within the heart.

Operation of the AV valves: opening phase Operation of the AV valves: filling phase Operation of the AV valves: atrial contraction Operation of the AV valves: ventricular contraction Operation of the AV valves: closure phase Operation of the AV valves: chordae tendineae tightening

Heart Wall: Fibrous Skeleton and Contraction Pattern

Fibrous Skeleton

The fibrous skeleton of the heart provides structural support and anchors cardiac muscle fibers. Cardiac muscle cells are arranged in spiral bundles, allowing efficient contraction patterns.

Atrial contraction: Compresses chamber walls to move blood into ventricles.
Ventricular contraction: Begins at the apex and compresses superiorly, similar to wringing a mop.

Fibrous skeleton and valves of the heart Contraction pattern of cardiac muscle bundles

Summary Table: Heart Chambers and Valves

Chamber	Receives Blood From	Pumps Blood To	Valve
Right Atrium	Superior/Inferior Vena Cava, Coronary Sinus	Right Ventricle	Tricuspid (Right AV)
Right Ventricle	Right Atrium	Pulmonary Trunk	Pulmonary Semilunar
Left Atrium	Pulmonary Veins	Left Ventricle	Bicuspid (Mitral, Left AV)
Left Ventricle	Left Atrium	Aorta	Aortic Semilunar

Key Terms and Concepts

Pericardium: Double-walled sac enclosing the heart.
Epicardium: Outermost layer of the heart wall.
Myocardium: Muscular, contractile middle layer of the heart wall.
Endocardium: Thin, endothelial inner lining of the heart chambers.
Chordae Tendineae: Tendinous cords anchoring AV valve cusps to ventricular walls.
Fibrous Skeleton: Dense connective tissue framework supporting the heart.

Additional info: The heart's double pump function is essential for separating pulmonary and systemic circulation, ensuring efficient oxygenation of blood and delivery to tissues. The fibrous skeleton also acts as an electrical insulator, preventing direct transmission of electrical impulses between atria and ventricles.