Chapter 18: The Heart – Structure, Function, and Circulation

The Heart as Part of the Cardiovascular System

Components and Functions

The heart is a central organ in the cardiovascular system, responsible for pumping blood throughout the body. The cardiovascular system consists of three main components:

• Heart: Acts as a pump to circulate blood.

• Blood vessels: Serve as delivery routes for blood.

• Blood: Serves as the delivery medium for oxygen, nutrients, carbon dioxide, and wastes.

The heart ensures that oxygen and nutrients reach cells, while carbon dioxide and wastes are removed.

Location and Structure of the Heart

Position in the Mediastinum

The heart is located in the mediastinum, a central compartment in the thoracic cavity. It lies between the lungs, behind the sternum, and above the diaphragm. The apex of the heart points downward and to the left, while the base anchors the heart and faces toward the right shoulder.

• Size: About the size of a fist (~300 g).

• Point of maximum impulse: Between the 5th and 6th ribs at the midclavicular line.

Surrounding Structures

The heart is surrounded by the pericardium, a double-layered sac that encloses the heart and part of the great vessels.

• Fibrous pericardium: Dense irregular connective tissue that protects, anchors, and prevents overstretching.

• Serous pericardium: Simple squamous epithelium with two layers: parietal (lines inside fibrous pericardium) and visceral (lines heart, also called epicardium). Produces serous fluid to prevent friction.

Layers of the Heart Wall

Structural Layers

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

• Epicardium: External layer, simple squamous epithelium.

• Myocardium: Middle contractile layer, composed of cardiac muscle cells, making up 95% of the heart wall.

• Endocardium: Inner layer, simple squamous epithelium, lines chambers and valves, continuous with endothelium of blood vessels, minimizes friction.

Heart Chambers and Basic Anatomy

Chamber Structure

The heart has four chambers:

• Atria: Superior chambers, divided by the interatrial septum. They are receiving chambers with lower pressure and thinner myocardium.

• Ventricles: Inferior chambers, divided by the interventricular septum. They act as pumps with higher pressure and thicker myocardium.

Key anatomical features include:

• Fossa ovalis: Remnant of the foramen ovale, a fetal shunt.

• Papillary muscles and chordae tendineae: Attach to valves and prevent backflow during ventricular contraction.

• Sulci: Grooves marking chamber separation (atrioventricular, anterior interventricular, posterior interventricular).

Circuits of the Heart

Pulmonary and Systemic Circuits

The heart functions as two pumps:

• Pulmonary circuit: Right heart → lungs → left heart. Delivers O2-poor blood to lungs for gas exchange; returns O2-rich blood to left heart.

• Systemic circuit: Left heart → body → right heart. Delivers O2-rich blood to body; returns O2-poor blood to right heart.

Both ventricles pump the same amount of blood, but the left ventricle has a thicker wall due to higher systemic pressure.

Fibrous Skeleton of the Heart

Structure and Function

The fibrous skeleton is dense connective tissue running through cardiac muscle. It supports heart valves and great vessels, serves as a point of attachment for cardiac muscle, and electrically insulates atria from ventricles, ensuring proper timing of contractions.

Pathway of Blood Through the Heart

Sequential Flow

Blood follows a specific pathway through the heart and body:

1. SVC and IVC → Right Atrium (RA)

2. Tricuspid valve → Right Ventricle (RV)

3. Pulmonic valve → Pulmonary Trunk

4. Left & Right Pulmonary arteries → Lungs (gas exchange)

5. Left & Right Pulmonary veins → Left Atrium (LA)

6. Mitral valve → Left Ventricle (LV)

7. Aortic valve → Aorta

8. Body capillaries (gas exchange)

Coronary Circulation

Blood Supply to the Heart

Coronary arteries branch from the aorta and supply blood to cardiac muscle tissue, which has high energy requirements and can only produce ATP aerobically. Blockage leads to myocardial infarction (heart attack), resulting in muscle death and replacement by scar tissue. Anastomoses provide multiple pathways for blood flow.

Heart Valves

Types and Functions

Heart valves ensure unidirectional blood flow by opening and closing in response to pressure changes:

• Atrioventricular (AV) valves: Tricuspid (RA to RV), Mitral (LA to LV).

• Semilunar valves: Pulmonic (RV to pulmonary trunk), Aortic (LV to aorta).

Blood flows from atria → ventricles → vessels.

Valve Functioning

Valve operation depends on pressure differences:

• Atrial pressure > ventricular pressure: AV valves open.

• Ventricular pressure > atrial pressure: AV valves close.

• Ventricular pressure > aortic pressure: Aortic valve opens.

• Aortic pressure > ventricular pressure: Aortic valve closes.

Summary Table: Heart Valves and Their Functions

Valve	Location	Function
Tricuspid (AV)	RA to RV	Prevents backflow into RA
Mitral (AV)	LA to LV	Prevents backflow into LA
Pulmonic (Semilunar)	RV to Pulmonary Trunk	Prevents backflow into RV
Aortic (Semilunar)	LV to Aorta	Prevents backflow into LV

Key Terms and Concepts

• Myocardial infarction: Death of cardiac muscle due to lack of oxygen.

• Anastomosis: Connection between two or more arteries.

• Chordae tendineae: Tendinous cords preventing valve prolapse.

• Papillary muscles: Muscles anchoring chordae tendineae.

Relevant Equations

Cardiac Output

Cardiac output is the volume of blood pumped by the heart per minute:

$\text{Cardiac Output} = \text{Stroke Volume} \times \text{Heart Rate}$

Blood Pressure

Blood pressure is determined by cardiac output and resistance:

$\text{Blood Pressure} = \text{Cardiac Output} \times \text{Total Peripheral Resistance}$

Conclusion

The heart is a complex organ with specialized structures and functions that ensure efficient circulation of blood throughout the body. Understanding its anatomy, circuits, and valve mechanisms is essential for comprehending cardiovascular physiology.