Chapter 18: The Heart – Anatomy, Physiology, and Circulation

Cardiovascular System

Overview of the Cardiovascular System

The cardiovascular system is a closed system consisting of the heart and blood vessels. Its primary function is to deliver oxygen and nutrients to tissues and remove carbon dioxide and other waste products.

• Heart: Pumps blood throughout the body.

• Blood vessels: Allow blood to circulate to all parts of the body.

• Main function: Transport essential substances and remove metabolic wastes.

The Heart

Location and Structure

The heart is a muscular organ located in the thorax between the lungs, with its pointed apex directed toward the left hip. It is approximately the size of a human fist.

• Location: Thoracic cavity, between the lungs.

• Apex: Points toward the left hip.

• Size: About the size of a closed fist.

Chambers of the Heart

The heart consists of four chambers, divided into two atria and two ventricles. The right and left sides act as separate pumps.

• Atria (Receiving chambers):

  • Right atrium

  • Left atrium

• Ventricles (Discharging chambers):

  • Right ventricle

  • Left ventricle

Pathway of Blood Through the Heart

Pulmonary and Systemic Circuits

The heart pumps equal volumes of blood through two main circuits: the pulmonary circuit and the systemic circuit.

• Pulmonary circuit: Short, low-pressure circulation that carries blood from the right ventricle to the lungs for gas exchange and returns oxygenated blood to the left atrium.

• Systemic circuit: High-resistance, long pathway that carries oxygen-rich blood from the left ventricle to all body tissues and returns deoxygenated blood to the right atrium.

• Ventricular anatomy: The structure of the ventricles reflects the differences in pressure and resistance between the two circuits.

Diagram: Pulmonary and Systemic Circuits

Key features:

• Pulmonary arteries carry deoxygenated blood from the right ventricle to the lungs.

• Pulmonary veins return oxygenated blood to the left atrium.

• Aorta distributes oxygenated blood from the left ventricle to the body.

• Venae cavae return deoxygenated blood to the right atrium.

Heart Valves

Types and Functions

Heart valves ensure unidirectional blood flow through the heart chambers and prevent backflow.

• Atrioventricular (AV) valves: Located between atria and ventricles.

  • Bicuspid (mitral) valve: Left side

  • Tricuspid valve: Right side

• Semilunar valves: Located between ventricles and major arteries.

  • Pulmonary semilunar valve: Right ventricle to pulmonary artery

  • Aortic semilunar valve: Left ventricle to aorta

Chordae tendineae: Tendinous cords that anchor AV valves to papillary muscles, preventing valve prolapse during ventricular contraction.

Cardiac Muscle Tissue

Microscopic Anatomy

Cardiac muscle cells are striated, short, branched, and interconnected. They are specialized for continuous rhythmic contraction.

• Intercalated discs: Specialized junctions between cells containing desmosomes (prevent separation during contraction) and gap junctions (allow ion passage for electrical coupling).

• Endomysium: Connective tissue matrix connecting muscle cells to the fibrous skeleton.

• Mitochondria: Numerous and large, occupying 25-35% of cell volume, supporting aerobic metabolism.

Comparison: Skeletal vs. Cardiac Muscle

Feature	Skeletal Muscle	Cardiac Muscle
Cell shape	Striated, long, cylindrical, multinucleate	Striated, short, branched, one or two nuclei
Intercellular junctions	None	Intercalated discs (desmosomes, gap junctions)
Contraction as a unit	No, motor units must be stimulated individually	Yes, functional syncytium via gap junctions
T tubules	Abundant, narrow	Fewer, wider
Sarcoplasmic reticulum	Elaborate, terminal cisterns	Less elaborate, no terminal cisterns
Source of Ca2+ for contraction	Sarcoplasmic reticulum only	Sarcoplasmic reticulum and extracellular fluid
Pacemaker cells	No	Yes
Types of metabolism	Aerobic and anaerobic	Aerobic only (more mitochondria)

Summary

• Day 1: Anatomy of the heart, valves, blood circulation, and cardiac muscle tissue

• Day 2: Cardiac conduction

• Day 3: Cardiac cycle, cardiac output, and effects of sympathetic stimulation on the heart

Additional info: These notes are based on textbook-style slides and images, with academic context added for clarity and completeness.