Cardiovascular System: The Heart

Cardiovascular System: The Heart

• The heart's muscle cells are branched and have a single nucleus, connected by intercalated disks
• Myocardial muscle cells are specialized for efficient pumping action
• The heart has four chambers: right atrium, right ventricle, left atrium, and left ventricle
• The right atrium receives deoxygenated blood from three sources (superior vena cava, inferior vena cava, and coronary sinus)
• From the right atrium, blood flows through the tricuspid valve into the right ventricle
• The right ventricle pumps blood through the pulmonary semilunar valve into the pulmonary trunk
• The left atrium receives oxygenated blood from the lungs (four pulmonary veins)
• From the left atrium, blood flows through the bicuspid (mitral) valve into the left ventricle
• The left ventricle pumps blood through the aortic semilunar valve into the aorta
• Blood flows through the heart in a specific path, ensuring that deoxygenated and oxygenated blood do not mix

Cardiac Muscle Histology

• Cardiac muscle cells are branched and have a single nucleus
• These cells are connected to each other by specialized junctions called intercalated disks
• The intercalated disks enable rapid transmission of electrical signals throughout the heart

Blood Circulation

• Pulmonary circulation: The right side of the heart pumps deoxygenated blood to the lungs for gas exchange, returning oxygenated blood to the left side of the heart
• Systemic circulation: The left side of the heart pumps oxygenated blood to the body's tissues, returning deoxygenated blood to the right side of the heart

Heart Cells

• Cardiac muscle is composed of two types of cells: autorhythmic cells (10%) and contractile cells (90%)
• Autorhythmic cells are self-excitable and generate spontaneous action potentials (pacemaker potentials), initiating and regulating the heartbeat
• Contractile cells respond to these signals and contract to pump blood

Conduction System of the Heart

• The SA node (sinoatrial node) is the heart's pacemaker, firing spontaneously at 90-100 times per minute
• Signals from the SA node spread through the atria, causing them to contract
• The AV node (atrioventricular node) delays the signal to allow the atria to fully contract before the ventricles contract
• Impulses move through the AV bundle (bundle of His) and its branches into Purkinje fibers in the ventricles, triggering ventricular contraction

Action Potential in Cardiac Muscle

• Cardiac muscle action potentials have a unique plateau phase due to calcium ion influx maintaining depolarization.
• This plateau prolongs the refractory period, preventing tetanus and ensuring proper cardiac function

Physiology of Contraction

• The refractory period in cardiac muscle ensures that the heart can properly fill between contractions, preventing tetanus (sustained contraction)

Electrocardiogram (ECG)

• An ECG measures electrical activity spread through the heart
• Waves on an ECG correspond to electrical events in the heart (e.g., atrial depolarization, ventricular depolarization, ventricular repolarization)
• The ECG allows for diagnosis of various cardiac abnormalities

P wave

• Represents atrial depolarization, causing atrial contraction

QRS complex

• Represents ventricular depolarization, causing ventricular contraction

T wave

• Represents ventricular repolarization

Cardiac Cycle

• The cardiac cycle is a complete sequence of contraction and relaxation of the heart
• One cycle typically takes around 0.8 seconds
• Key aspects of the cycle include end-diastolic volume (EDV), end-systolic volume (ESV), and stroke volume (SV)

Ventricular Pressures

• The pressure in the aorta is 120 mmHg during systole
• The pressure in the pulmonary trunk is lower, around 30 mmHg
• Ventricular wall thickness allows the left ventricle to exert more force for systemic circulation

Cardiac Output

• Cardiac output (CO) is the volume of blood pumped per minute from the left ventricle into the aorta
• CO is calculated as heart rate (HR) multiplied by stroke volume (SV)

Influences on Stroke Volume

• Stroke volume can be affected by preload, contractility, and afterload
• Preload is determined by ventricular end-diastolic volume (EDV)
• Contractility is the heart's ability to contract more forcefully at a given preload. Influenced by inotropic agents
• Afterload is the pressure the heart must overcome to pump blood into the arteries

Starling's Law

• Starling's Law states that increasing preload increases stroke volume (stretch leads to more forceful contraction)

Influences on Heart Rate

• Heart rate is regulated by the autonomic nervous system (sympathetic and parasympathetic)

Epinephrine (and norepinephrine)

• Increase contractility and heart rate
• Increase the duration of calcium channels opening

Afterload

• Afterload is the pressure exerted by the blood in the aorta against the closing of the semilunar (aortic) valves
• High afterload can reduce stroke volume