The Cardiovascular System

Overview of the Cardiovascular System

The cardiovascular system is responsible for the transport of blood, nutrients, gases, and wastes throughout the body. It consists of the heart and an extensive network of blood vessels (arteries, veins, and capillaries). The heart acts as a muscular pump, maintaining the flow of blood through these vessels to ensure that tissues receive oxygen and nutrients and that waste products are removed efficiently.

• Heart: Muscular organ that pumps blood through the vessels.

• Blood Vessels: Tubular structures (arteries, veins, capillaries) that transport blood to and from all body tissues.

Pulmonary and Systemic Circuits

Major Divisions of Circulation

The cardiovascular system is divided into two main circuits: the pulmonary circuit and the systemic circuit. Each circuit serves a distinct function in the transport of blood.

• Pulmonary Circuit: Carries deoxygenated blood from the right side of the heart to the lungs for gas exchange, then returns oxygenated blood to the left side of the heart.

• Systemic Circuit: Delivers oxygenated blood from the left side of the heart to the rest of the body and returns deoxygenated blood to the right side of the heart.

Pulmonary Circuit

The pulmonary circuit is supplied by the right side of the heart. It transports blood to the lungs, where carbon dioxide is exchanged for oxygen. Arteries in this circuit carry blood away from the heart, while veins return blood to the heart.

• Right ventricle pumps deoxygenated blood into the pulmonary trunk and arteries.

• Blood flows through pulmonary capillaries in the lungs for gas exchange.

• Oxygenated blood returns to the left atrium via pulmonary veins.

Systemic Circuit

The systemic circuit is supplied by the left side of the heart. It delivers oxygen-rich blood to all body tissues and returns oxygen-poor blood to the right side of the heart.

• Left ventricle pumps blood into the aorta, the largest artery.

• Blood travels through systemic arteries, arterioles, and capillaries to supply tissues.

• Deoxygenated blood returns via systemic veins to the right atrium (through the superior and inferior vena cava).

Heart Anatomy

Position, Size, and Shape of the Heart

The heart is located in the thoracic cavity within the mediastinum, slightly tilted to the left. The average adult heart weighs about 300 grams. The base is the broad superior portion, while the apex is the pointed inferior end.

The Pericardium

The heart is enclosed in a double-walled sac called the pericardium. This structure protects the heart, anchors it within the thorax, and prevents overfilling.

• Parietal pericardium: Outer fibrous layer.

• Visceral pericardium (epicardium): Inner serous layer that covers the heart surface.

• Pericardial cavity: Space between the layers, filled with serous fluid to reduce friction.

The Heart Wall

The heart wall consists of three layers, each with a specific function:

• Epicardium: Outer layer, also known as the visceral pericardium.

• Myocardium: Middle, muscular layer responsible for contraction.

• Endocardium: Inner endothelial lining of the heart chambers.

Chambers of the Heart

Internal Features and Septa

The heart has four chambers: two superior atria and two inferior ventricles. The interatrial septum separates the atria, while the interventricular septum separates the ventricles. The fossa ovalis is a remnant of the fetal foramen ovale.

Atria

The atria are thin-walled chambers that receive blood returning to the heart. Each atrium has an auricle to increase its volume. The right atrium receives blood from three major veins: the superior vena cava, inferior vena cava, and coronary sinus.

Ventricles

The ventricles are thick-walled chambers that pump blood into the arteries. The right ventricle forms most of the anterior surface, while the left ventricle dominates the posterior-inferior surface. Key internal features include trabeculae carneae, papillary muscles, and chordae tendineae.

• Right ventricle: Pumps blood into the pulmonary trunk (pulmonary circuit).

• Left ventricle: Pumps blood into the aorta (systemic circuit).

Heart Valves

Types and Functions of Heart Valves

The heart contains four valves that ensure unidirectional blood flow. Each valve consists of thin flaps called cusps.

• Atrioventricular (AV) valves: Between atria and ventricles (tricuspid and mitral/bicuspid valves).

• Semilunar valves: Between ventricles and major arteries (pulmonary and aortic valves).

Atrioventricular (AV) Valves

The right AV valve (tricuspid) and left AV valve (mitral or bicuspid) prevent backflow from the ventricles into the atria. Each cusp is anchored to papillary muscles by chordae tendineae, which tense during ventricular contraction to prevent valve prolapse.

Semilunar Valves

The pulmonary semilunar valve and aortic semilunar valve are located between the ventricles and the great arteries. Each has three cusps and prevents backflow into the ventricles after contraction.

Blood Flow Through the Heart

Pathway of Blood

Blood flows through the heart in a specific sequence, ensuring separation of oxygenated and deoxygenated blood. Blood from the body enters the right atrium, passes to the right ventricle, and is pumped to the lungs. Oxygenated blood returns to the left atrium, moves to the left ventricle, and is pumped to the body.

• Right atrium → right ventricle → pulmonary trunk/arteries → lungs

• Lungs → pulmonary veins → left atrium → left ventricle → aorta → body

Note: Blood in the left and right chambers does not mix under normal conditions.

Coronary Circulation

Arterial Supply

The heart has its own blood supply, known as coronary circulation. The left coronary artery supplies the left atrium and ventricle, while the right coronary artery supplies the right atrium and ventricle. This circulation is essential for sustaining the heart's workload.

Venous Drainage

After passing through the capillaries, blood drains into the coronary veins, which empty into the coronary sinus and then into the right atrium.

Clinical Considerations

Homeostatic Imbalances

Blockage of coronary arteries can lead to serious conditions:

• Angina pectoris: Chest pain due to reduced blood flow to the heart muscle.

• Myocardial infarction (heart attack): Death of heart muscle tissue due to prolonged lack of blood supply.

Atherosclerosis

Atherosclerosis is a condition where arteries become stiff and narrowed due to the buildup of plaques composed of lipids, connective tissue, and smooth muscle. This can block blood flow and is a major risk factor for heart disease. Treatments include bypass surgery, angioplasty, or stent placement.