The Cardiovascular System

Overview of the Cardiovascular System

The cardiovascular system is essential for transporting blood, nutrients, gases, and wastes throughout the body. It consists of the heart and two main circuits: the systemic and pulmonary circuits.

• Heart: A muscular organ that pumps blood through the circulatory system.

• Systemic Circuit: Carries oxygenated blood from the heart to the body and returns deoxygenated blood back to the heart.

• Pulmonary Circuit: Transports deoxygenated blood from the heart to the lungs and returns oxygenated blood to the heart.

Key Concept: Changes in blood volume within the heart chambers induce changes in pressure, which drive the flow of blood through the circulatory system.

Unidirectional Flow and Heart Valves

Blood flows in one direction through the heart, ensured by the opening and closing of heart valves.

• Valves: Structures that prevent backflow and ensure unidirectional movement of blood.

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

Example: The atrioventricular (AV) valves prevent blood from flowing back into the atria when the ventricles contract.

Cardiac Cycle and Stroke Volume

The cardiac cycle refers to the sequence of events in one heartbeat, including the filling and emptying of the ventricles.

• End Diastolic Volume (EDV): The volume of blood in the ventricle at the end of filling (diastole).

• End Systolic Volume (ESV): The volume of blood remaining in the ventricle after contraction (systole).

• Stroke Volume (SV): The amount of blood ejected from the ventricle with each heartbeat.

Formula:

Cardiac Output (CO): The volume of blood expelled from the heart per minute.

Formula:

where HR is heart rate (beats per minute).

Cardiac Muscle Structure and Function

Contractile Cells and Intercalated Disks

Cardiac muscle is composed of contractile cells that are specialized for continuous rhythmic contraction.

• Intercalated Disks: Specialized connections between cardiac muscle cells containing gap junctions and desmosomes.

• Gap Junctions: Allow for the direct transmission of electrical impulses and force between cells, enabling coordinated contraction.

Action Potentials in Cardiac Muscle

Cardiac contractile cells generate long-lasting action potentials, which keep the cell depolarized for an extended period.

• Long Refractory Period: Prevents summation of force and tetanus, ensuring rhythmic contractions.

• Comparison: Cardiac muscle contraction shares features with both skeletal and smooth muscle contraction, such as striations (like skeletal muscle) and involuntary control (like smooth muscle).

Additional info: The plateau phase of the cardiac action potential is due to sustained calcium influx, which prolongs depolarization.

Autorhythmic Cells and Cardiac Conduction

Autorhythmic Cells and Pacemaker Activity

Autorhythmic cells are specialized cardiac cells that generate spontaneous action potentials, setting the rhythm and pace of the heart.

• Locations: Found in the sinoatrial (SA) node, atrioventricular (AV) node, and Purkinje fibers.

• SA Node: The fastest autorhythmic cells, normally setting the pace for the entire heart (the natural pacemaker).

Electrocardiogram (ECG)

An ECG is a diagnostic tool that records the electrical activity of the heart and helps detect pathologies affecting heart rate, rhythm, conduction blocks, or coordination.

• Applications: Used to diagnose arrhythmias, conduction abnormalities, and myocardial infarction.

Regulation of Cardiac Output

Autonomic Nervous System Control

The autonomic nervous system (ANS) regulates heart rate and stroke volume through its sympathetic and parasympathetic branches.

• Heart Rate (HR): Controlled by antagonistic effects of the sympathetic (increases HR) and parasympathetic (decreases HR) branches.

• Mechanism: The ANS alters the membrane potential and rate of depolarization in autorhythmic cells.

• Stroke Volume (SV): Regulated by intrinsic properties of contractile cells and modulated by the sympathetic branch (increases contractility).

Summary Table: Key Components of Cardiac Function