Cardiovascular System: Structure, Function, and Cardiac Cycle

The heart wall itself is a multi-layered organ:

• Epicardium: The outermost layer, consisting of loose areolar connective and adipose tissue.
• Myocardium: The thickest layer of the heart, composed of cardiac muscle cells (cardiomyocytes) and the cardiac skeleton.
• Endocardium: The innermost layer, made of simple squamous endothelial tissue.

Histology of the Cardiomyocyte

The functional unit of the heart is the cardiomyocyte. These specialized muscle cells are designed for endurance and synchronized activity. Unlike skeletal muscle, cardiomyocytes typically have a single, centrally located nucleus and are relatively short and wide. They possess branched myofibrils and an abundance of mitochondria to support continuous aerobic metabolism, allowing them to resist fatigue.

A defining feature of cardiac tissue is the presence of intercalated discs. These are areas where cell membranes are fused together, allowing the entire tissue to function as a single, coordinated unit known as a syncytium. This structural arrangement, combined with autorhythmic properties, ensures that the heart can beat rhythmically and independently.

Chambers and the Interventricular Septum

The heart is divided into four distinct chambers: two upper atria and two lower ventricles.

The Atria (Receiving Chambers)

The atria are thin-walled (approximately $1\text{ mm}$) chambers designed to receive blood.

• The right atrium receives deoxygenated blood from the body via the superior and inferior vena cava. Crucially, the right atrial wall contains the heart's natural pacemakers: the SA-node and AV-node.
• The left atrium receives oxygenated blood returning from the lungs via the pulmonary veins.

The Ventricles (Pumping Chambers)

The ventricles are thick-walled, muscular chambers responsible for forceful ejection. They are separated by the interventricular septum, a muscular wall that prevents the mixing of oxygenated and deoxygenated blood.

There is a significant physiological difference between the two ventricles due to the different pressures they must overcome:

• Right Ventricle: Has a wall thickness of approximately $3\text{ mm}$. It pumps deoxygenated blood to the lungs at relatively low pressure.
• Left Ventricle: Has a much thicker wall, approximately $1\text{ cm}$. Because it must pump oxygenated blood to the entire body, it generates much higher force, reaching pressures such as $120\text{ mmHg}$.

Exam Tip: Remember the "3x rule" for the left ventricle. It contains approximately 3x more cardiomyocytes and 3x more crossbridges than the right ventricle, allowing it to generate 3x more force.

Valves and Unidirectional Flow

To prevent the backflow of blood and ensure one-directional flow, the heart utilizes two types of valves:

1. Atrioventricular (AV) Valves: Located between the atria and the ventricles.
   • Tricuspid Valve: Located on the right side.
   • Bicuspid (Mitral) Valve: Located on the left side.
2. Semilunar (SL) Valves: Located between the ventricles and the great vessels.
   • Pulmonary Semilunar Valve: Between the right ventricle and the pulmonary artery.
   • Aortic Semilunar Valve: Between the left ventricle and the aorta.

The Cardiac Cycle and Pressure Dynamics

The cardiac cycle describes the sequence of electrical and mechanical events during one heartbeat. This cycle is driven by pressure gradients, where blood always moves from areas of high pressure to low pressure.

The cycle can be broken down into several key stages:

• Atrial and Late Ventricular Diastole: Ventricular pressure is lower than atrial pressure; AV valves are open, and blood flows passively into the ventricles.
• Atrial Systole: The atria contract, increasing atrial pressure and forcing the remaining blood into the ventricles, reaching the End Diastolic Volume (EDV).
• Early Ventricular Systole: The ventricles begin to contract. As ventricular pressure rises above atrial pressure, the AV valves close.
• Late Ventricular Systole: Ventricular pressure exceeds the pressure in the great vessels (e.g., $120\text{ mmHg}$ in the left ventricle), forcing the semilunar valves open and ejecting blood. The volume remaining in the heart after this is the End Systolic Volume (ESV).
• Ventricular Diastole: The ventricles relax, pressure drops, and the semilunar valves close to prevent backflow.

Clinical Note: Hypertension is defined as blood pressure greater than $120\text{ mmHg}$. Chronic high pressure can lead to serious complications like stroke or hemorrhage.