GAP Ch19 Cardiovascular System
The cardiovascular system, also known as the circulatory system, is essential for sustaining life by facilitating the circulation of blood throughout the body. This complex system involves not only the heart, which acts as the primary pump, but also a vast network of blood vessels that transport oxygenated and deoxygenated blood to and from various tissues and organs. The efficient functioning of the cardiovascular system is crucial for maintaining homeostasis, delivering nutrients, regulating temperature, and removing waste products from cellular metabolism.
Heart Location and Structure
Location: The heart resides in the thoracic cavity, positioned above the diaphragm and situated medially between the lungs in the mediastinum. This strategic placement allows it to effectively pump blood to both the lungs and the rest of the body.
Pericardium: The heart is encased in a tough protective membrane known as the pericardium, which reduces friction as the heart beats. This structure comprises two layers:
Fibrous Pericardium: Provides overall protection to the heart and helps maintain its position within the thoracic cavity.
Serous Pericardium: Consists of two layers—the parietal layer that lines the fibrous pericardium and the visceral layer, also known as the epicardium, which adheres directly to the heart's surface.
Heart Chambers: The heart consists of four chambers:
Atria (Upper Chambers):
Right Atrium: Receives deoxygenated blood from the body through the superior and inferior vena cavae.
Left Atrium: Receives oxygenated blood from the lungs via the pulmonary veins.
Ventricles (Lower Chambers):
Right Ventricle: Pumps deoxygenated blood to the lungs for oxygenation through the pulmonary trunk, which branches into right and left pulmonary arteries.
Left Ventricle: Pumps oxygenated blood to the body through the aorta, the body's largest artery.
Heart Shape and Size
Shape: The heart resembles a pinecone, being broad at the top near the atria and tapering towards the apex at the bottom.
Size: Approximately the size of a fist, the heart's dimensions can vary based on factors such as an individual's gender, body size, and level of physical fitness.
Circulatory Pathways
Pulmonary Circuit: This circuit transports blood to and from the lungs. Deoxygenated blood is pumped from the right ventricle to the lungs through the pulmonary trunk, where carbon dioxide is exchanged for oxygen. Oxygenated blood returns to the heart via the pulmonary veins into the left atrium.
Systemic Circuit: Carries oxygenated blood from the left ventricle through the aorta to various body tissues. Once oxygen is delivered, blood returns deoxygenated to the right atrium through the superior and inferior vena cavae. This dual system of circulation ensures that oxygen-rich blood reaches every cell in the body while simultaneously removing carbon dioxide and other metabolic wastes.
Heart Wall Layers
Epicardium: The outermost layer of the heart, which contains blood vessels that supply the heart muscle as well as protective fibrous tissue.
Myocardium: The thick, muscular middle layer composed of cardiac muscle, responsible for the heart's pumping action. This layer is the most substantial, enabling strong contractions.
Endocardium: The innermost layer, which lines the chambers of the heart and covers the valves, creating a smooth surface that promotes efficient blood flow and prevents clotting.
Cardiac Muscle and Functionality
The heart muscle contracts through specialized cells called myocardial cells, which exhibit autorhythmicity, allowing the heart to beat independently of external stimulation.
Intercalated Discs: These structures connect myocardial cells and facilitate synchronized contractions by coordinating electrical impulses across the heart tissue, ensuring that the heart contracts in a coordinated manner.
The left ventricle has a thicker wall compared to the right ventricle, a structural adaptation that enables it to generate higher pressure necessary for systemic circulation.
Heart Valves
Atrioventricular Valves: Ensure one-way blood flow between the atria and ventricles:
Bicuspid (Mitral) Valve: Located between the left atrium and left ventricle.
Tricuspid Valve: Located between the right atrium and right ventricle.
Semilunar Valves: Prevent backflow of blood from the aorta and pulmonary arteries:
Aortic Valve: Opens to allow blood to flow from the left ventricle into the aorta.
Pulmonary Valve: Opens to allow blood to flow from the right ventricle into the pulmonary trunk.
Cardiac Cycle Phases
Systole: The contraction phase of the heart cycle, which includes:
Atrial systole: Atria contract, pushing blood into the ventricles.
Ventricular systole: Ventricles contract, ejecting blood into the aorta and pulmonary trunk.
Diastole: The relaxation phase, allowing the heart chambers to fill with blood, preparing for the next cycle of contractions.
Electrical Conduction System
Sinoatrial (SA) Node: Acts as the natural pacemaker for the heart, generating electrical impulses that initiate the heartbeat.
Atrioventricular (AV) Node: Receives impulses from the SA node and relays them to the ventricles, resulting in a necessary delay for efficient pumping.
Bundle of His and Purkinje Fibers: Conduct electrical impulses through the ventricles to ensure coordinated and effective contraction of the heart muscle.
Coronary Circulation
Coronary Arteries: Branch from the aorta and supply the heart muscle (myocardium) with oxygenated blood necessary for its energy needs.
Coronary Veins: Drain deoxygenated blood from the heart muscle into the coronary sinus, which then returns blood to the right atrium. This dual supply and return system is crucial for sustaining heart function and health.