L 16

Cardiovascular System Lecture Notes (L16)

• The cardiovascular system is a complex system of organs responsible for transporting blood throughout the body.
• The heart is a hollow muscular organ, roughly the size of the owner's fist.
• The heart is located in the thoracic cavity, within the mediastinum.
• It lies obliquely, positioned more towards the left side of the body.
• The base of the heart is situated above, and the apex is positioned below.
• The heart is positioned between the 1/3 and 2/3 of the body's long axis, from ribs 2 to 5.
• Anteriorly, the heart is protected by the sternum and ribs.
• Posteriorly, structures such as the esophagus, trachea, and descending aorta are situated behind the heart.
• Inferiorly, the central tendon of the diaphragm is located below the heart.
• Superiorly, large blood vessels like the great vessels are above the heart
• Laterally, the heart is flanked by the lungs.

Heart Structure

• The heart has three tissue layers: pericardium, myocardium, and endocardium.
• The pericardium is a fibroserous sac, consisting of a fibrous sac (open) and a serous sac (closed).
• The fibrous sac is continuous with the tunica adventitia of the great vessels and adheres to the diaphragm, preventing overdistention.
• The pericardium is made of two layers - parietal (outer) and visceral (inner, or epicardium).
• The fluid-filled space between the layers is called the pericardial cavity and contains serous fluid.
• Serous fluid allows for smooth movement between the heart and surrounding structures.

Heart Chambers and Valves

• The heart is divided into four chambers: two atria and two ventricles.
• The atria are the upper chambers.
• The ventricles are the lower chambers.
• The heart has atrioventricular (AV) valves and semilunar valves.
• AV valves are formed by double folds of endocardium strengthened by fibrous tissue which are the mitral and tricuspid valves.
• The AV valves allow blood to flow from the atria to the ventricles but prevent backflow.
• Semilunar valves (pulmonary and aortic valves) regulate blood flow out of the heart into arteries.

Cardiac Muscle (Myocardium)

• The myocardium is specialized cardiac muscle found only in the heart.
• Cardiac muscle cells are branched and contain a single nucleus.
• Cardiac muscle cells are connected by intercalated discs, enabling coordinated contraction.
• The myocardium is thickest at the heart's apex and thins towards the base.
• Its thickness is greatest in the left ventricle.

The Conduction System of the Heart

• Specialized cells within the heart form the intrinsic conduction system that automatically stimulates cardiac muscle contraction rhythmically without external impulses.
• The system consists of the sinoatrial (SA) node, atrioventricular (AV) node, AV bundle (bundle of His), right and left bundle branches, and Purkinje fibers.

Nerve Supply

• The vagus nerves (parasympathetic) supply the SA and AV nodes and atrial muscle.
• Parasympathetic stimulation slows the heart rate and decreases the force of contraction.
• Sympathetic nerves supply the SA and AV nodes and the myocardium of the atria and ventricles.
• Sympathetic stimulation increases the heart rate and enhances the force of contraction.

Circulatory Pathways

• Blood vessels are functionally divided into two circuits: the pulmonary circuit and the systemic circuit.
• The pulmonary circuit carries blood between the heart and lungs for gas exchange (oxygenation).
• The systemic circuit pumps oxygenated blood to the rest of the body from the heart.

Venous Drainage

• Blood returns to the heart via veins.
• The coronary sinus is a major vessel responsible for collecting deoxygenated blood from the heart wall.