Cardiovascular System: The Heart – Structure, Function, and Circulation

Cardiovascular System: Heart

Overview of the Cardiovascular System

The cardiovascular system is responsible for transporting blood, nutrients, gases, and wastes throughout the body. The heart acts as a central pump, circulating blood through two main circuits: the pulmonary and systemic circuits.

• Blood: Composed of plasma (liquid component) and formed elements (cells such as erythrocytes, leukocytes, and platelets).

• Hematopoiesis: The process of blood cell formation, primarily occurring in the bone marrow.

Structure of the Heart

The heart is a muscular organ located in the mediastinum of the thoracic cavity. It is divided into four chambers and is covered by protective membranes.

• Location: Lies in the mediastinum, between the lungs, protected by the sternum and ribs.

• Double Pump: The right side pumps blood to the lungs (pulmonary circuit), and the left side pumps blood to the rest of the body (systemic circuit).

• Chambers: Two atria (receiving chambers) and two ventricles (pumping chambers).

Heart Wall Layers

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

• Epicardium (Visceral Pericardium): The outermost layer, providing protection.

• Myocardium: The thick middle layer composed of cardiac muscle, responsible for contraction.

• Endocardium: The innermost layer, lining the heart chambers and valves.

Pericardium and Pericardial Cavity

The heart is enclosed in a double-layered sac called the pericardium, which protects and anchors the heart.

• Fibrous Pericardium: Tough outer layer that prevents overexpansion.

• Serous Pericardium: Composed of parietal and visceral layers, with the pericardial cavity containing lubricating fluid.

• Clinical Terms: Pericardiocentesis (draining fluid), Pericarditis (inflammation), Hemopericardium (blood in cavity), Cardiac Tamponade (compression due to fluid).

Cardiac Muscle and Intercalated Discs

Cardiac muscle cells are connected by specialized junctions that facilitate coordinated contraction.

• Intercalated Discs: Complex junctions allowing electrical and mechanical connection between cells.

• Fascia Adherens: Binds adjacent cells and transmits contractile force.

• Gap Junctions: Allow rapid transmission of electrical signals.

• Endomysium: Loose connective tissue containing blood vessels and nerves.

Heart Chambers and Landmarks

The heart contains four chambers, each with unique features and functions.

• Right Atrium: Receives oxygen-poor blood from the body via the superior and inferior vena cava and coronary sinus.

• Right Ventricle: Pumps blood to the lungs via the pulmonary trunk and pulmonary valve.

• Left Atrium: Receives oxygen-rich blood from the lungs via four pulmonary veins.

• Left Ventricle: Pumps blood to the body via the aorta and aortic valve; has a thicker wall than the right ventricle.

• Landmarks: Coronary sulcus, anterior and posterior interventricular sulci, fossa ovalis (remnant of fetal foramen ovale).

Heart Valves

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

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

• Semilunar Valves: At the junction of ventricles and great arteries; pulmonary and aortic valves.

• Valve Structure: Composed of endocardium and connective tissue core, anchored by the cardiac skeleton.

Cardiac Skeleton

The cardiac skeleton is a dense connective tissue structure that supports the heart valves and provides electrical insulation.

• Functions: Anchors valve cusps, prevents over-dilation, provides insertion points for cardiac muscle, and insulates electrical impulses.

Blood Flow Through the Heart

Blood flows through the heart in a specific sequence, passing through all four chambers and valves.

1. Oxygen-poor blood enters the right atrium from the body.

2. Passes through the tricuspid valve into the right ventricle.

3. Pumped through the pulmonary valve into the pulmonary trunk and lungs.

4. Oxygen-rich blood returns to the left atrium via pulmonary veins.

5. Passes through the mitral valve into the left ventricle.

6. Pumped through the aortic valve into the aorta and systemic circulation.

Heart Sounds

Heart sounds are produced by the closing of valves during the cardiac cycle.

• First Sound ("lub"): Closing of AV valves.

• Second Sound ("dub"): Closing of semilunar valves.

• Locations: Heard at specific intercostal spaces on the chest.

Cardiac Cycle

The cardiac cycle consists of alternating contraction and relaxation phases.

• Systole: Contraction of a heart chamber.

• Diastole: Relaxation and expansion of a heart chamber.

• Normal Heart Rate: Approximately 80 beats per minute at rest.

• Blood Pressure: Measures left ventricular systolic over diastolic pressure.

Conduction System of the Heart

The heart's intrinsic conduction system coordinates rhythmic contractions through specialized cardiac muscle cells.

• Sinoatrial (SA) Node: Pacemaker located in the right atrium; initiates impulses.

• Atrioventricular (AV) Node: Receives impulses from SA node; delays transmission by 0.1 seconds.

• AV Bundle (Bundle of His): Conducts impulses from AV node to bundle branches.

• Bundle Branches: Right and left branches run through the interventricular septum.

• Purkinje Fibers: Distribute impulses to ventricular walls, causing contraction from apex upward.

Cardiac Control Centers

Heart rate is regulated by external neural controls.

• Parasympathetic: Vagus nerve decreases heart rate.

• Sympathetic: Cervical and thoracic ganglia increase heart rate.

Fetal Circulation

Fetal circulation includes unique structures that bypass the nonfunctional fetal lungs.

• Umbilical Arteries: Carry deoxygenated blood to the placenta.

• Umbilical Vein: Returns oxygenated blood from the placenta.

• Ductus Venosus: Bypasses the liver, delivering blood to the inferior vena cava.

• Foramen Ovale: Shunts blood from right to left atrium, bypassing pulmonary circuit.

• Ductus Arteriosus: Connects pulmonary trunk to aorta, further bypassing lungs.

• Postnatal Changes: Foramen ovale becomes fossa ovalis; ductus arteriosus becomes ligamentum arteriosum.

Table: Comparison of Heart Chambers

Chamber	Main Function	Key Features
Right Atrium	Receives deoxygenated blood from body	Superior/inferior vena cava, coronary sinus, fossa ovalis
Right Ventricle	Pumps blood to lungs	Tricuspid valve, pulmonary valve, trabeculae carneae
Left Atrium	Receives oxygenated blood from lungs	Pulmonary veins, mitral valve
Left Ventricle	Pumps blood to body	Aortic valve, thickest wall, papillary muscles

Key Equations

• Blood Pressure:

  • $\text{Blood Pressure} = \frac{\text{Systolic Pressure}}{\text{Diastolic Pressure}}$

• Heart Rate:

  • $\text{Heart Rate} = \frac{60}{\text{R-R Interval (seconds)}}$

Example: Patent Foramen Ovale (PFO)

A congenital defect where the foramen ovale fails to close after birth, allowing abnormal blood flow between the atria. This can lead to complications such as stroke or hypoxemia.

Additional info: Expanded explanations and table added for clarity and completeness.