Cardiovascular System - The Heart

Cardiovascular System - The Heart

• The heart is a cone-shaped organ, roughly the size of a clenched fist, located in the mediastinum of the chest cavity.
• It pumps blood throughout the body's blood vessels.
• The heart is tilted diagonally, with two-thirds of its mass positioned to the left of the chest's midline.
• The pointed end of the heart is called the apex, extending forward, downward, and to the left, between the 5th and 6th ribs roughly at the midclavicular line.
• The broad superior part is called the base, located superiorly, posteriorly, and to the right.
• The heart's base is not mobile, fixed by its connections to major blood vessels.
• The apex moves during ventricular contractions, striking the left chest wall near the 5th intercostal space.

Structure and Function of the Pericardium

• The heart is enclosed within a protective sac called the pericardium.
• Fibrous pericardium is a tough, fibrous outer layer preventing excessive stretching of the heart.
• Serous pericardium is a thinner inner lining with two layers:
  • Parietal layer: lines the inner surface of the fibrous pericardium.
  • Visceral layer: covers the outer surface of the heart.
• The space between the parietal and visceral layers is filled with serous pericardial fluid, reducing friction as the heart moves.

Chambers and Valves of the Heart

• The heart has four chambers: two atria (upper) and two ventricles (lower).
• The atria collect blood returning to the heart.
• The ventricles pump blood out of the heart.
• A muscular septum (wall) divides the heart into left and right halves.
• The right and left atria are separated by the interatrial septum.
• The right and left ventricles are separated by the interventricular septum.
• Atrioventricular (AV) valves (tricuspid and mitral valves) regulate blood flow between atria and ventricles.
• Semilunar valves (pulmonary and aortic valves) control flow from the heart to the pulmonary trunk (to the lungs) and the aorta (to the body), respectively.

Heart Valves

• The valves in the heart ensure one-way blood flow through the heart.
• Right and Left Atrioventricular valves (tricuspid and mitral valves) prevent backflow from the ventricles into the atria during ventricular contraction.
• Semilunar valves (pulmonary and aortic valves) prevent blood from flowing back into the ventricles from the pulmonary artery and aorta, respectively, during ventricular relaxation.

Cardiac Muscle Tissue

• Cardiac muscle cells (cardiomyocytes) are striated, branched, and have a single central nucleus.
• They connect to each other at intercalated discs, facilitating rapid transmission of electrical signals.
• Cardiac muscle has a unique autorhythmic property, meaning specialized cells in the conduction system spontaneously generate action potentials which drive the heart's rhythmic contractions.

Cardiac Conduction System

• The Sinoatrial (SA) node, located in the right atrium, is the heart's natural pacemaker, initiating rhythmic contractions.
• Action potentials spread from the SA node through the atria and to the Atrioventricular (AV) node.
• The AV node slows the impulse conduction before it enters the ventricles, allowing the atria to fully empty into the ventricles.
• Action potentials then spread through the bundle branches and Purkinje fibers, causing the ventricles to contract.

Cardiac Cycle

• The cardiac cycle is the sequence of events in one complete heartbeat, including contraction and relaxation of the atria and ventricles.
• Systole refers to the period of ventricular contraction, and diastole refers to the period of ventricular relaxation.
• Various phases of the cycle include the ventricular filling, isovolumetric contraction, ventricular ejection, and isovolumetric relaxation.

Regulation of the Heart

• Intrinsic regulation involves the heart's inherent properties, such as the Frank-Starling mechanism, which adjusts stroke volume to the amount of blood returning to the heart.
• Extrinsic regulation occurs due to neural control (sympathetic and parasympathetic) and hormonal influences, notably epinephrine and norepinephrine.

Electrocardiogram (ECG)

• An ECG measures the electrical activity of the heart.
• Characteristic waves (P, QRS, T) on an ECG correspond to different phases of the cardiac cycle.

Systemic, Pulmonary, and Coronary Circulation

• Systemic circulation involves the left side of the heart, delivering oxygenated blood to the body and returning deoxygenated blood to the right side of the heart.
• Pulmonary circulation involves the right side of the heart, carrying deoxygenated blood to the lungs for oxygenation and returning oxygenated blood to the left side of the heart.
• Coronary circulation provides blood supply to the heart muscle itself, branching off the aorta.

Heart Disorders

• Coronary artery disease involves narrowing or blockage of coronary arteries, reducing blood supply to the heart muscle, leading to angina pectoris or myocardial infarction.
• Arrhythmias are abnormal heart rhythms.
• Congenital heart defects are structural problems present at birth.