10.1 Heart Anatomy and Physiology

The heart, a muscular organ in the chest, powers our circulatory system. It's divided into four chambers and has valves that ensure blood flows in one direction. The heart's structure and function are crucial for understanding the cardiovascular system.

The cardiac cycle is the sequence of events in a single heartbeat. It involves the contraction and relaxation of heart chambers, regulated by electrical signals. This process keeps blood flowing through our body, delivering oxygen and nutrients to tissues.

Heart Anatomy and Function

Anatomical Structures and Locations

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• The heart is a muscular organ located in the thoracic cavity between the lungs that pumps blood through the circulatory system
• The pericardium is a fibrous sac that surrounds and protects the heart, consisting of an outer fibrous layer and an inner serous layer
• The heart wall is composed of three layers: the epicardium (outer layer), myocardium (middle muscular layer), and endocardium (inner layer)
• The heart is divided into four chambers: right atrium, right ventricle, left atrium, and left ventricle
• The interatrial septum separates the right and left atria, while the interventricular septum separates the right and left ventricles

Heart Valves and Coronary Arteries

• Heart valves ensure unidirectional blood flow through the heart
  • The tricuspid valve is located between the right atrium and right ventricle
  • The pulmonary semilunar valve is located between the right ventricle and pulmonary trunk
  • The mitral (bicuspid) valve is located between the left atrium and left ventricle
  • The aortic semilunar valve is located between the left ventricle and aorta
• The right coronary artery and left coronary artery, which branches into the left anterior descending and circumflex arteries, supply the myocardium with oxygenated blood
  • Coronary arteries originate from the base of the aorta, just above the aortic valve
  • Coronary arteries run along the surface of the heart, providing blood supply to the cardiac muscle tissue

Cardiac Cycle Physiology

Phases of the Cardiac Cycle

• The cardiac cycle refers to the sequence of events that occur during one complete heartbeat, consisting of systole (contraction) and diastole (relaxation) of the atria and ventricles
• The cardiac cycle is divided into five phases: atrial systole, isovolumetric contraction, ventricular ejection, isovolumetric relaxation, and ventricular filling
  • During atrial systole, the atria contract, pumping blood into the ventricles while the AV valves (tricuspid and mitral) are open and the semilunar valves (pulmonary and aortic) are closed
  • During isovolumetric contraction, the ventricles begin to contract, the AV valves close, and ventricular pressure rises without a change in volume
  • During ventricular ejection, the semilunar valves open when ventricular pressure exceeds the pressure in the aorta and pulmonary trunk, allowing blood to be ejected from the ventricles
  • During isovolumetric relaxation, the ventricles relax, the semilunar valves close, and ventricular pressure drops without a change in volume
  • During ventricular filling, the AV valves open when atrial pressure exceeds ventricular pressure, allowing blood to flow from the atria into the ventricles

Regulation of the Cardiac Cycle

• The cardiac cycle is regulated by intrinsic autorhythmic cells (pacemaker cells) and extrinsic factors
  • Intrinsic regulation involves the SA node, AV node, and Purkinje fibers, which generate and conduct electrical impulses
  • Extrinsic regulation involves the autonomic nervous system (sympathetic and parasympathetic) and hormones (epinephrine and norepinephrine) that modulate heart rate and contractility

Electrical Conduction System of the Heart

Components of the Electrical Conduction System

• The electrical conduction system of the heart consists of specialized cardiac muscle cells that initiate and coordinate heart contractions
• The sinoatrial (SA) node, located in the right atrium, is the primary pacemaker of the heart and initiates the heartbeat by spontaneously depolarizing
• The atrioventricular (AV) node, located in the interatrial septum, receives the electrical impulse from the SA node and delays it before transmitting it to the bundle of His
• The bundle of His, located in the interventricular septum, divides into the left and right bundle branches, which rapidly conduct the electrical impulse to the respective ventricles
• The Purkinje fibers, located in the ventricular walls, are the terminal branches of the conduction system and rapidly distribute the electrical impulse throughout the ventricles

Electrocardiogram (ECG)

• The electrocardiogram (ECG) is a graphical representation of the electrical activity of the heart
  • The P wave represents atrial depolarization
  • The QRS complex represents ventricular depolarization
  • The T wave represents ventricular repolarization
• The ECG provides valuable information about heart rate, rhythm, and conduction abnormalities (arrhythmias)

Atria vs Ventricles: Blood Pumping Roles

Atrial Function

• The atria are the receiving chambers of the heart, collecting blood from the veins and pumping it into the ventricles
  • The right atrium receives deoxygenated blood from the systemic circulation via the superior and inferior vena cava
  • The left atrium receives oxygenated blood from the pulmonary circulation via the pulmonary veins
• The atria have thinner walls and generate lower pressures compared to the ventricles, as they pump blood over a shorter distance into the ventricles
• The atria contract first, filling the ventricles with blood before ventricular contraction

Ventricular Function

• The ventricles are the discharging chambers of the heart, pumping blood out to the lungs and the body
  • The right ventricle pumps deoxygenated blood to the lungs via the pulmonary trunk and pulmonary arteries
  • The left ventricle pumps oxygenated blood to the systemic circulation via the aorta
• The ventricles have thicker walls, particularly the left ventricle, and generate higher pressures to pump blood over longer distances to the lungs and the body
• The ventricles contract after the atria, ejecting blood out of the heart and into the pulmonary and systemic circulations