Cardiovascular System Terms 1-60

Fundamental Components of the Cardiovascular System

The cardiovascular system is a complex network designed for the transport of blood throughout the body. At its core are arteries, which are defined as the vessels that carry blood away from the heart. These branch into arterioles, which are small arterial branches that transport blood specifically into a capillary network. Veins serve the opposite function, acting as the blood vessels that carry blood from a capillary bed back toward the heart. Venules are categorized as small, thin-walled veins that receive blood directly from the capillaries. Situated between the arterioles and venules are the capillaries; these are small blood vessels with thin walls that permit the critical exchange of nutrients, gases, and other substances between the blood and the surrounding tissues.

Structural Layers and Anatomy of the Heart

The heart is a multi-layered organ situated within the mediastinum, which is the central tissue mass that divides the thoracic cavity into two pleural cavities. The heart is surrounded by the pericardium, a fibrous sac. Specifically, the fibrous pericardium is a tough outer layer that prevents the chambers of the heart from overfilling with blood. The epicardium, which is also known as the visceral pericardium, constitutes the outer layer of the heart. Directly beneath this is the myocardium, the middle and muscular layer of the heart composed of specialized cardiac muscle. The endocardium serves as the inner surface lining of the heart. Clinical conditions related to these layers include pericarditis, an inflammation of the pericardium, and endocarditis, which is the inflammation of the endocardium. Furthermore, cardiac tamponade refers to the compression of the heart resulting from the accumulation of fluid within the pericardial space.

External features of the heart include the base, which forms the superior border. The auricle is an externally visible flap formed by the collapse of the outer wall of a relaxed atrium. The surface of the heart is marked by sulci, which are grooves or furrows (singular: sulcus). Specifically, the anterior ventricular sulcus is a shallow depression on the anterior surface of the heart that marks the boundary between the left and right ventricles. Another notable structure is the ligamentum arteriosum, a fibrous strand found in adults that is the remnant of the ductus arteriosus from the fetal stage.

Cardiac Cycles and Rhythmic Regulation

The heart operates through a series of rhythmic contractions and relaxations. Systole is defined as a period of contraction within a chamber of the heart, while diastole refers to a period of relaxation. The intrinsic ability of the heart to set its own rhythm without the need for input from the nervous system is known as Autorhymicity. Heart rate is measured in beats per minute, and specific terms describe clinical variations: bradycardia is an abnormally low heart rate, while tachycardia is an abnormally high heart rate. Stroke volume is defined as the specific amount of blood pumped during a single heart beat.

Heart Chambers and Valvular Functions

The heart is divided into four distinct chambers. The right atrium is the superior receiving chamber on the right side that receives deoxygenated blood. Below it is the right ventricle, an inferiorly located discharging chamber on the right side containing deoxygenated blood. On the left side, the left atrium is the superior receiving chamber containing oxygenated blood, and the left ventricle is the inferior discharging chamber that also contains oxygenated blood.

To ensure unidirectional blood flow, the heart utilizes several valves. The tricuspid valve, which has three cusps or flaps, is located between the right atria and the right ventricle. The bicuspid valve, also known as the mitral valve, is the left atrioventricular valve and possesses two cusps. The pulmonary semilunar valve is situated between the right ventricle and the pulmonary trunk; its function is to prevent the backflow of blood from the pulmonary arteries back into the right ventricle. Similarly, the aortic semilunar valve is located between the left ventricle and the aorta, where it prevents blood from flowing backward from the aorta into the left ventricle.

Systematic and Pulmonary Circulatory Circuits

Blood is transported through two primary circuits. The pulmonary circuit carries blood from the right side of the heart to the lungs and returns it to the left atrium. Key vessels in this circuit include the pulmonary trunk, a major vessel originating from the right ventricle, and the pulmonary arteries, which are branches of the pulmonary trunk that carry blood from the right atrium to the lungs. The pulmonary veins then carry oxygenated blood from the lungs back to the left atrium.

The systemic circuit carries blood from the left side of the heart to the various body tissues and returns it to the right side of the heart. The aorta is the large, elastic artery that carries blood away from the left ventricle and into this circuit. Returns to the heart are handled by the vena cavae: the superior vena cava carries blood to the right atrium from parts of the body superior to the heart, while the inferior vena cava carries blood from parts of the body inferior to the heart.

Histology and Comparative Anatomy of Blood Vessels

Blood vessel walls are composed of three distinct layers. The tunica intima, also called the tunica interna, is the innermost layer including an endothelial lining and an underlying layer of connective tissue with elastic fibers. In arteries, the outer margin of this layer contains a thick layer of elastic fibers known as the internal elastic membrane. The tunica media is the middle layer, consisting of concentric sheets of smooth muscle within a framework of loose connective tissue. The tunica externa, or tunica adventitia, is the outermost connective tissue sheath. In arteries, it contains collagen and scattered elastic fibers. In veins, it is generally thicker than the tunica media and contains networks of elastic fibers and bundles of smooth muscle cells.

There are several key differences between arteries and veins. The lumen of an artery remains round even when empty, whereas a vein would appear flattened or collapsed without blood. Veins generally possess a larger lumen. The tunica intima of an artery often appears rippled due to vessel constriction, while the tunica intima of a vein is usually smooth. Furthermore, the tunica media of arteries is thicker and contains an external elastic membrane. In contrast, the tunica externa of veins is often thicker than its own tunica media and contains smooth muscle cells.

Microcirculation and Capillary Specialization

Capillaries are classified into three types based on their structure. In a continuous capillary, the endothelium forms a complete lining. Fenestrated capillaries contain "windows" or pores that penetrate the endothelial lining, allowing for the rapid exchange of water and solutes as large as small peptides. Sinusoids resemble flattened, irregularly shaped fenestrated capillaries and often have gaps between adjacent endothelial cells. The flow of blood into these capillaries is regulated by a pre-capillary sphincter, which is a ring of smooth muscle cells at the entrance of each capillary. Contraction or relaxation of these cells changes the diameter of the entrance to control blood flow.

Regulatory Mechanisms and Reflexes

The diameter of blood vessels is regulated by the vasomotor center through two primary processes. Vasoconstriction is the decrease in the diameter of arterioles caused by the contraction of smooth muscles in the tunica media, which increases peripheral resistance. This can occur due to local factors, hormones, or stimulation of the vasomotor center. Vasodilation is the increase in the diameter of arterioles due to the relaxation of smooth muscles in the tunica media, which decreases peripheral resistance. This can be triggered by local factors, hormones, or decreased stimulation of the vasomotor center. Physiological reflexes also maintain homeostasis: baroreceptor reflexes respond to changes in blood pressure, while chemoreceptor reflexes respond to changes in carbon dioxide, oxygen, or $PH$ levels in the blood.

Venous Dynamics and Clinical Pathologies

The majority of blood volume is located within the systemic venous system. Venous return refers to the flow of blood from the periphery back to the right atrium. To facilitate this, veins contain valves that permit blood flow in one direction only. Venoconstriction is a mechanism that reduces the diameter of the veins and the volume of blood they contain. This process helps maintain the volume within the arterial system at near-normal levels even during significant blood loss.

When the venous system fails, clinical conditions can arise. Varicose veins occur when the walls of veins near the valves weaken or stretch, causing the valves to malfunction. This leads to blood pooling and the veins becoming grossly distended and distorted. Hemorrhoids are a specific type of varicose vein, characterized by swollen and inflamed veins in the rectum and anus that cause discomfort and bleeding.