Cardiovascular System: Study Notes

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Cardiovascular System

HEART

• The heart is a four-chambered structure composed predominantly of cardiac muscle.
• Heart is a mesodermally derived organ located in mediastinum.
• Heart wall is formed of three layers: Endocardium, Myocardium and Pericardium.
• The heart supports two different circulations: the pulmonary circulation and the systemic circulation.
• It is protected by double-layered pericardium.
• The pericardial (space between pericardial membranes) is filled with pericardial fluid. It reduces friction betweenthe heart walls, and surrounding tissues. It also protects the heart from mechanical shocks.
• Each side of the heart consists of an atrium and a ventricle.
• The atria are separated by an inter-atrial septum and the ventricles are separated by interventricular septum.
• The left side of the heart contains more muscle than the right side because the systemic circulation has a much higher resistance and pressure.
• The atria are separated from the ventricles by the atrioventricular valves (tricuspid on the right, mitral [bicuspid] on the left).

The ventricles are separated from the vasculature by the semilunar valves (pulmonary on the right, aortic on the left).

Conduction System of Heart

• Human heart is myogenic, i.e. normal activities of heart are auto regulated by nodal tissues. Nodal tissues are specialized cardiac musculature present in heart wall.
• It consists of

1. Sino-atrial node (SAN) in the right upper corner of the right atrium
2. Atrio-ventricular node (AVN) in the lower left corner of the right atrium close to the atrio-ventricular septum.

• The signal is delayed at AV nodes to allow for the ventricles to fill completely before they contract.

Cardiac Cycle

• Cardiac cycle is defined as the complete cycle of events in the heart from the beginning of one heart beat to the beginning of the next.
• Each heartbeat is composed of two phases, known as systole and diastole.
• During systole, ventricular contraction and closure of the AV valves occurs and blood is pumped out of the ventricles.
• During diastole, the heart is relaxed, the semilunar valves are closed, and blood from the atria fills the ventricles.
• The cardiac output is the product of heart rate (HR, beats per minute) and stroke volume(SV, volume of blood pumped per beat).

Heart Sounds

• Heart sounds are noises generated by the closing and opening of the heart valves.
• In a healthy individual, there are two normal heart sounds called lub and dub. 
• Lub is the first heart sound (S1). It is associated with the closure of the tricuspid and bicuspidvalves at the beginning of ventricular systole.
• The second heart sound dub (S2) is associated with the closure of the semilunar valvesat the beginning of ventricular diastole

Blood Vessels

Arteries

• Function: Carry blood from the heart to organs (distributing vessels).
• Blood Type: Oxygenated.
• Wall Thickness: Thick-walled.
• Lumen: Narrow.
• Layers: 3 layers, with a thicker Tunica media.
• Pressure: High (120/80 mmHg).
• Valves: No valves (due to high pressure).
• Pulsatile Flow: Yes.
• Collapse: Do not collapse when empty.
• Muscle Action: Elastic and contractile due to thick Tunica media.

Veins

• Function: Return blood from organs to the heart (collecting vessels).
• Blood Type: Deoxygenated.
• Wall Thickness: Thin-walled.
• Lumen: Broad.
• Layers: 3 layers, with a thicker Tunica externa.
• Pressure: Low (0 mmHg).
• Valves: Valves present to prevent backflow.
• Pulsatile Flow: No.
• Collapse: Collapse when empty.
• Muscle Action: Compressed by skeletal muscles; cannot recoil on their own.

Capillaries

• Function: Connect arteries and veins; primary site of exchange between blood and tissues.
• Blood Type: Mixed blood (oxygenated and deoxygenated).
• Wall Thickness: Thinnest (one cell thick to allow diffusion).
• Lumen: Narrowest.
• Layers: 1 layer (only Tunica intima).
• Pressure: Low (17 mmHg).
• Valves: No valves (no space).
• Pulsatile Flow: Yes.
• Collapse: Collapse when empty.
• Muscle Action: None (very thin walls).

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