Cardiovascular System: Structure and Function of the Heart

Cardiovascular System

Introduction

The cardiovascular system is essential for transporting nutrients, gases, and waste products throughout the body. It consists of the heart, which pumps blood, and blood vessels, which carry blood to all tissues. This system supports cellular metabolism and maintains homeostasis.

• The heart pumps blood throughout the body.

• Blood vessels (arteries, veins, capillaries) move blood to and from tissues.

• Blood delivers nutrients and oxygen, removes waste products.

• Oxygen is picked up in the lungs and delivered to tissues; carbon dioxide is collected from tissues and expelled via the lungs.

The Heart

Location and Position

The heart is located in the mediastinum, within the thoracic cavity, superior to the diaphragm and posterior to the sternum. It is not exactly centered; about two-thirds of the heart lies to the left of the midline. The heart is approximately the size of a closed fist.

• Base: Located superiorly and to the right.

• Apex: Located inferiorly and to the left, pointing towards the left.

The Pericardium and Heart Wall

The heart is enclosed in a protective sac called the pericardium, which has three layers:

• Fibrous pericardium: Outermost layer, tough and protective.

• Parietal pericardium: Lines the inside of the fibrous pericardium.

• Visceral pericardium (epicardium): Adheres directly to the heart wall.

• Between the parietal and visceral layers is the pericardial cavity, filled with serous fluid to reduce friction.

The heart wall itself also has three layers:

• Epicardium: Outermost layer, same as visceral pericardium; contains blood vessels and fat.

• Myocardium: Middle layer, composed of cardiac muscle tissue; responsible for contraction and pumping blood.

• Endocardium: Innermost layer, thin epithelium lining the chambers and valves; continuous with the endothelium of blood vessels.

Chambers and Great Vessels

Heart Chambers

The heart has four chambers:

• Right Atrium (RA): Receives deoxygenated blood from the body via the superior vena cava (SVC), inferior vena cava (IVC), and coronary sinus.

• Right Ventricle (RV): Receives blood from the RA and pumps it to the lungs via the pulmonary trunk/artery.

• Left Atrium (LA): Receives oxygenated blood from the lungs via four pulmonary veins.

• Left Ventricle (LV): Receives blood from the LA and pumps it to the body via the aorta.

Septum

• Interatrial septum: Wall between the atria; contains the fossa ovalis, a remnant of fetal circulation.

• Interventricular septum: Wall between the ventricles.

Myocardial Thickness

• Myocardium is thinnest in the atria, thicker in the ventricles.

• LV myocardium is thickest, as it must pump blood to the entire body.

Valves of the Heart

Function and Types

Heart valves ensure unidirectional blood flow and prevent backflow.

• There are four main valves:

  • 2 Atrioventricular (AV) valves: Between atria and ventricles.

  • 2 Semilunar valves: Between ventricles and major arteries.

Atrioventricular Valves

• Right AV (tricuspid) valve: Between RA and RV.

• Left AV (bicuspid/mitral) valve: Between LA and LV.

• Prevent backflow into atria when ventricles contract.

• Chordae tendineae and papillary muscles anchor the valves and prevent inversion.

Semilunar Valves

• Pulmonary semilunar valve: Between RV and pulmonary trunk.

• Aortic semilunar valve: Between LV and aorta.

• Prevent backflow from arteries into ventricles.

• Shaped like pockets; fill with blood to close.

Blood Flow Through the Heart

Pathway of Blood

Blood flows through the heart in a specific sequence to ensure oxygenation and circulation.

1. Deoxygenated blood enters RA from SVC, IVC, and coronary sinus.

2. Passes through tricuspid valve to RV.

3. RV pumps blood through pulmonary semilunar valve into pulmonary trunk/arteries.

4. Blood travels to lungs, picks up O2, releases CO2.

5. Oxygenated blood returns to LA via pulmonary veins.

6. Passes through bicuspid/mitral valve to LV.

7. LV pumps blood through aortic semilunar valve into aorta.

8. Blood is distributed to the body.

Cardiac Cycle

Definition and Phases

The cardiac cycle includes all events in one heartbeat, including contraction (systole) and relaxation (diastole) of the atria and ventricles, and the opening/closing of valves.

• Diastole: Relaxation phase; chambers fill with blood.

• Systole: Contraction phase; chambers eject blood.

Phases of the Cardiac Cycle

1. Relaxation Period: All chambers are in diastole; AV and semilunar valves are closed; blood fills atria, pressure increases.

2. Atrial Systole: Atria contract, pushing remaining blood into ventricles; AV valves open, semilunar valves closed.

3. Ventricular Systole: Ventricles contract, AV valves close (first heart sound: lubb), semilunar valves open, blood ejected into arteries; as blood falls back, semilunar valves close (second heart sound: dupp).

Table: Heart Valves and Locations

Valve	Location	Function
Tricuspid (Right AV)	Between RA and RV	Prevents backflow into RA
Bicuspid/Mitral (Left AV)	Between LA and LV	Prevents backflow into LA
Pulmonary Semilunar	Between RV and pulmonary trunk	Prevents backflow into RV
Aortic Semilunar	Between LV and aorta	Prevents backflow into LV

Additional info:

• Cardiac muscle is striated and branched, allowing for coordinated contraction.

• The heart's conduction system (not detailed in these slides) includes the SA node, AV node, bundle of His, bundle branches, and Purkinje fibers, which regulate heartbeat.