Cardiovascular System: Structure and Function Study Guide

Overview of the Cardiovascular System

Introduction

The cardiovascular system is essential for transporting nutrients, gases, hormones, and waste products throughout the body. It consists of a network of blood vessels, a central pump (the heart), and valves that regulate fluid flow. In mammals, the system is organized into pulmonary and systemic circuits, each serving distinct roles in oxygenating blood and distributing it to tissues.

• Cardiovascular System: Composed of pipes (vessels), valves, and a pump (heart).

• Pulmonary Circuit: Carries deoxygenated blood from the right side of the heart to the lungs for oxygenation.

• Systemic Circuit: Distributes oxygenated blood from the left side of the heart to the rest of the body.

• Capillary Beds: Sites of exchange between blood and tissues.

Heart Anatomy

Position, Size, and Shape

The heart is located in the mediastinum of the thoracic cavity, slightly to the left of the midline, and anchored to the diaphragm. Its superior end (base) is where major blood vessels attach, while the inferior end (apex) points downward and to the left.

• Base: Superior portion where vessels attach.

• Apex: Inferior, pointed end.

• Orientation: The heart is tilted, with the right side more anterior.

Serous Membranes and the Pericardium

The heart is enclosed by the pericardium, a double-layered serous membrane that reduces friction and anchors the heart within the thoracic cavity.

• Visceral Pericardium (Epicardium): Inner layer covering the heart surface, contains adipose tissue.

• Parietal Pericardium: Outer layer, forms the pericardial sac.

• Pericardial Cavity: Space between layers, filled with lubricating fluid.

• Function: Prevents excessive expansion, reduces friction, anchors heart.

• Pathology: Pericarditis is inflammation of the pericardium, causing pain and friction rub.

Heart Wall Structure

The heart wall consists of three layers, each with distinct functions and tissue types.

• Epicardium: Outer layer, contains fat and blood vessels.

• Myocardium: Middle, muscular layer responsible for contraction; thickness varies between chambers.

• Endocardium: Inner endothelial layer lining the chambers and valves.

Fibrous Skeleton

The fibrous skeleton is a framework of connective tissue that provides structural support, electrical insulation, and attachment points for cardiac muscle and valves.

• Functions: Supports valves, separates atria and ventricles electrically, anchors muscle fibers.

• Location: Concentrated around valve rings and interventricular septum.

Chambers of the Heart

The heart contains four chambers: two atria and two ventricles, each with specific roles in blood circulation.

• Atria: Thin-walled chambers that receive blood returning to the heart.

• Ventricles: Thick-walled chambers that pump blood out of the heart; left ventricle wall is thicker due to higher pressure required for systemic circulation.

• Auricles: Ear-like extensions of atria, contain endocrine cells that release atrial natriuretic peptide (ANP) to lower blood pressure.

Valves of the Heart

Valves ensure unidirectional blood flow through the heart and prevent backflow.

• Atrioventricular (AV) Valves: Separate atria from ventricles; right AV (tricuspid), left AV (bicuspid/mitral).

• Semilunar Valves: Located at the exits of ventricles; pulmonary valve (right), aortic valve (left).

• Chordae Tendineae: Tendinous cords that anchor AV valves to papillary muscles, preventing inversion during ventricular contraction.

Valve Function and Pressure Dynamics

• Valves open and close in response to pressure differences between chambers.

• Example equation for pressure-driven valve opening: $P_{atrium} > P_{ventricle}$: AV valve opens $P_{ventricle} > P_{artery}$: Semilunar valve opens

• Systemic blood pressure in the aorta typically ranges from 80 mmHg (diastole) to 120 mmHg (systole).

Coronary Circulation

The heart muscle (myocardium) requires its own blood supply, provided by the coronary arteries and veins.

• Coronary Arteries: Branch from the aorta, supply oxygenated blood to the heart wall.

• Coronary Veins: Drain deoxygenated blood from the myocardium into the coronary sinus, which empties into the right atrium.

• Blood Flow: Most coronary blood flow occurs during diastole, as contraction compresses arteries.

• Anastomoses: Alternative routes for blood flow, important for protection against blockage.

Pathology: Myocardial Infarction (MI)

• Definition: Death of heart muscle due to lack of blood supply (ischemia).

• Cause: Blockage of coronary arteries, often by atherosclerotic plaque.

• Symptoms: Chest pain, tissue death within minutes.

• Difference between Ischemia and Hypoxia: Ischemia is lack of blood supply; hypoxia is lack of oxygen.

Summary Table: Major Heart Structures and Functions

Structure	Location	Function
Atria	Upper chambers	Receive blood returning to heart
Ventricles	Lower chambers	Pump blood out of heart
AV Valves	Between atria and ventricles	Prevent backflow into atria
Semilunar Valves	At exits of ventricles	Prevent backflow into ventricles
Coronary Arteries	Surface of heart	Supply myocardium with oxygen
Coronary Veins	Surface of heart	Drain deoxygenated blood from myocardium

Key Terms and Definitions

• Epicardium: Outer layer of heart wall.

• Myocardium: Muscular middle layer.

• Endocardium: Inner endothelial layer.

• Pericardium: Double-layered sac enclosing the heart.

• Ischemia: Reduced blood supply to tissue.

• Hypoxia: Reduced oxygen supply to tissue.

• Atrial Natriuretic Peptide (ANP): Hormone released by atria to lower blood pressure.

Additional info:

• Cardiac output depends on heart rate (beats per minute) and stroke volume (volume of blood pumped per beat): $\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}$

• Fatty acids are a major energy source for cardiac muscle.

• Heart muscle fibers are arranged in spiral/helical bands for efficient contraction.