Chapter 20 – The Heart

Overview of the Circulatory System

The circulatory system is essential for transporting substances throughout the body, maintaining homeostasis, and supporting cellular function. It consists of the heart, blood vessels, and blood.

• Purpose: To deliver oxygen and nutrients to tissues, remove waste products, and transport hormones and immune cells.

• Main Components: Heart, blood vessels (arteries, veins, capillaries), and blood.

• Transported Substances: Oxygen, carbon dioxide, nutrients, hormones, metabolic wastes, and immune cells.

• Closed System: The circulatory system is described as "closed" because blood is contained within vessels and does not leave them under normal conditions.

Blood Pressure and Flow

Blood pressure is the force exerted by circulating blood on the walls of blood vessels. It is necessary to create a pressure gradient that drives blood through the closed circulatory system.

• Hydrostatic Pressure: Generated by the contraction of the heart, pushing blood through vessels.

• Pressure Gradient: Required to move blood from areas of higher pressure to lower pressure.

• Equation: , where is the pressure difference and is resistance.

Location, Size, and Shape of the Heart

The heart is a muscular organ located in the mediastinum of the thoracic cavity, slightly left of the midline.

• Location: Between the lungs, behind the sternum, above the diaphragm.

• Size and Shape: About the size of a closed fist; cone-shaped with a pointed apex and broad base.

• Necessity: The heart acts as a pump to maintain blood flow throughout the circulatory system.

Pulmonary and Systemic Circuits

The heart pumps blood through two main circuits: pulmonary (to the lungs) and systemic (to the rest of the body).

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

• Systemic Circuit: Left side of the heart pumps oxygenated blood to the body tissues.

• Anatomical Differences: The left ventricle has a thicker myocardium to generate higher pressure for systemic circulation.

Structure and Function of the Pericardium

The pericardium is a double-walled sac that surrounds and protects the heart.

• Layers: Fibrous pericardium (outer), serous pericardium (inner, with parietal and visceral layers).

• Function: Protects the heart, anchors it in the chest, and reduces friction during contractions.

Layers of the Heart Wall

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

• Epicardium: Outer layer; provides protection and contains blood vessels.

• Myocardium: Middle, muscular layer; responsible for contraction and pumping blood.

• Endocardium: Inner layer; lines the chambers and valves, reducing friction.

Cardiac Muscle and Specialized Structures

The myocardium is composed of cardiac muscle cells, which are unique in structure and function.

• Cardiac Muscle: Striated, involuntary muscle with intercalated discs.

• Intercalated Discs: Specialized junctions connecting cardiac cells, allowing synchronized contraction.

• Gap Junctions: Channels within intercalated discs that permit direct electrical communication between cells.

• Function: Ensures coordinated heartbeats and efficient pumping.

Muscle Bundles in Atria and Ventricles

The atria and ventricles have different muscle thicknesses due to their distinct roles in circulation.

• Atria: Thinner walls; pump blood into ventricles.

• Ventricles: Thicker walls, especially the left ventricle; pump blood into pulmonary and systemic circuits.

• Myocardium Thickness: Thickest in the left ventricle to generate high pressure for systemic circulation.

Structure and Function of Atria and Ventricles

The heart has four chambers: two atria and two ventricles, each with specific structural and functional characteristics.

• Atria: Upper chambers; receive blood from veins.

• Ventricles: Lower chambers; pump blood into arteries.

• External Appearance: Atria are smaller and less muscular; ventricles are larger and more muscular.

• Functional Differences: Atria act as receiving chambers; ventricles as pumping chambers.

Definitions: Vein and Artery

Veins and arteries are the main types of blood vessels, differing in structure and function.

• Vein: Vessel that carries blood toward the heart.

• Artery: Vessel that carries blood away from the heart.

Major Blood Vessels Entering and Leaving the Heart

Several major vessels transport blood to and from the heart chambers.

Atrioventricular (AV) Valves

AV valves separate the atria from the ventricles and prevent backflow of blood.

• Tricuspid Valve: Located between right atrium and right ventricle; has three cusps.

• Bicuspid (Mitral) Valve: Located between left atrium and left ventricle; has two cusps.

• Function: Ensure one-way blood flow from atria to ventricles.

Semilunar Valves

Semilunar valves prevent backflow from arteries into the ventricles.

• Aortic Valve: Between left ventricle and aorta.

• Pulmonary Valve: Between right ventricle and pulmonary trunk.

• Function: Allow blood to exit ventricles and prevent return during relaxation.

Circulation Patterns

Blood flows through the heart in a specific sequence, involving pulmonary and systemic circuits.

• Circulation Through the Heart: Right atrium → right ventricle → pulmonary arteries → lungs → pulmonary veins → left atrium → left ventricle → aorta → body.

• Pulmonary Circulation: Right ventricle → pulmonary arteries → lungs → pulmonary veins → left atrium.

• Systemic Circulation: Left ventricle → aorta → body tissues → veins → right atrium.

Coronary Circulation

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

• Coronary Arteries: Right and left coronary arteries branch from the base of the aorta.

• Major Branches: Right coronary artery (marginal, posterior interventricular branches); left coronary artery (anterior interventricular, circumflex branches).

• Function: Supply oxygen and nutrients to heart muscle.

Anastomosis in Coronary Circulation

Anastomosis refers to the connection between blood vessels, providing alternate routes for blood flow.

• Anastomosis: Direct connection between arteries or veins.

• Importance: Ensures continued blood supply if one route is blocked.

• Coronary Anastomoses: Found in the heart to protect against ischemia.

Coronary Veins and Drainage

Coronary veins collect deoxygenated blood from the myocardium and drain it into the right atrium.

• Coronary Sinus: Main vessel that drains most coronary veins.

• Drainage Site: Empties into the right atrium.