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Cardiovascular System in Health - Exam Preparation Notes

The cardiovascular system, also known as the circulatory system, comprises the heart and blood vessels, functioning as a closed system where the heart pumps blood through a network of vessels, delivering oxygen and nutrients to cells while removing waste products like carbon dioxide. Understanding its structure and organization is crucial for grasping how it operates effectively within the human body.

The heart is a fist-sized, cone-shaped organ located in the thorax between the lungs, specifically in the inferior mediastinum. It lies obliquely from the second rib to the fifth intercostal space and rests on the diaphragm's superior surface. The heart's apex is directed towards the left hip, coming into contact with the chest wall between the fifth and sixth ribs, while the flat base points towards the right shoulder.

The heart's wall consists of three layers, each with distinct functionalities: the epicardium, which is the outer layer made of connective tissue; the myocardium, the middle layer composed primarily of cardiac muscle responsible for pumping action; and the endocardium, the innermost layer made of endothelium that lines the heart chambers.

The heart has four chambers: two atria (superior) and two ventricles (inferior). The right atrium receives blood from the body via the superior vena cava (from above the diaphragm) and the inferior vena cava (from below the diaphragm), alongside blood from the coronary sinus. The left atrium receives blood from the lungs through pulmonary veins. The ventricles act as pumps, directing blood into circulation. The right ventricle sends blood to the pulmonary trunk for gas exchange in the lungs, while the left ventricle ejects blood into the aorta for systemic circulation. Key valves, including the atrioventricular valves (tricuspid and mitral) and semilunar valves (aortic and pulmonary), regulate blood flow through the heart, ensuring unidirectional flow during contraction and relaxation phases.

Blood flow through the heart follows two circuits: the pulmonary circuit, where blood travels from the right side of the heart to the lungs for gas exchange and back to the left side of the heart; and the systemic circuit, where blood moves from the left side of the heart through body tissues before returning to the right side of the heart. There are differences in the structure of the ventricles; the right ventricle has a thinner wall and pumps at lower pressure suited for short-distance pulmonary circulation, while the left ventricle, with a thicker wall, generates high pressure required for systemic circulation, facing much greater resistance.

The heart has an intrinsic conduction system responsible for initiating and coordinating contractions, mainly featuring the sinoatrial (SA) node, the heart's natural pacemaker located in the right atrium; the atrioventricular (AV) node at the junction of the atria and ventricles; and the AV bundle (Bundle of His) that conducts impulses into the interventricular septum. The bundle branches and Purkinje fibers spread impulses throughout the ventricular myocardium, leading to coordinated contraction.

The electrical activity of the heart can be analyzed using an electrocardiogram (ECG), which captures the depolarization and repolarization events: the P wave (atrial depolarization), QRS complex (ventricular depolarization), and T wave (ventricular repolarization). Cardiac output (CO) is defined as the volume of blood pumped by each ventricle per minute, calculated using the formula:
CO = HR \times SV
where $HR$ is heart rate (typically 75 beats per minute) and $SV$ is stroke volume (approximately 70 mL per contraction).

Factors that increase heart rate include stimulation by the sympathetic nervous system, hormones like epinephrine, and physical activity, while the parasympathetic nervous system and certain chemical signals can decrease heart rate, thereby playing a vital role in maintaining homeostasis. Blood vessels are categorized into arteries, veins, and capillaries, each with distinct structures; arteries possess thicker walls and elasticity, carrying blood away from the heart under high pressure; veins have thinner walls and larger lumens, containing valves to prevent backflow while transporting blood back to the heart under lower pressure; and capillaries are microscopic vessels with a single layer of cells that allow for the exchange of substances between blood and tissues.

Blood pressure (BP) is the force blood exerts on vessel walls, measured in mmHg, and varies during the cardiac cycle; systolic pressure is the peak pressure during ventricular contraction, while diastolic pressure is the lowest pressure when the ventricles relax. Maintaining BP is crucial for effective circulation and involves neural, renal, and hormonal regulation. Consequently, changes in heart rate, blood volume, and vascular resistance can lead to significant fluctuations in blood pressure, affecting overall cardiovascular health. Aging, significantly impacting the cardiovascular system, results in increased arterial stiffness, reduced cardiac muscle strength, and heightened blood pressure, potentially leading to conditions such as coronary artery disease and congestive heart failure, emphasizing the need for ongoing cardiovascular health management in aging populations.