🐅Animal Physiology Unit 7 – Cardiovascular System: Heart & Circulation

Heart Structure and Function

• Consists of four chambers: two atria (upper chambers) and two ventricles (lower chambers)
• Atria receive blood from the body and lungs while ventricles pump blood out to the lungs and body
  • Right atrium receives deoxygenated blood from the body via the superior and inferior vena cava
  • Left atrium receives oxygenated blood from the lungs via the pulmonary veins
  • Right ventricle pumps deoxygenated blood to the lungs through the pulmonary artery
  • Left ventricle pumps oxygenated blood to the body through the aorta
• Myocardium, the muscular wall of the heart, contracts to generate the pumping action
  • Consists of specialized cardiac muscle cells called cardiomyocytes
  • Cardiomyocytes are connected by intercalated discs, allowing for coordinated contraction
• Heart valves ensure unidirectional blood flow and prevent backflow
  • Atrioventricular valves (tricuspid and mitral) separate the atria from the ventricles
  • Semilunar valves (pulmonary and aortic) control blood flow out of the ventricles
• Coronary arteries supply the heart muscle with oxygenated blood
  • Left and right coronary arteries branch off from the aorta
  • Blockage of coronary arteries can lead to myocardial infarction (heart attack)
• Pericardium, a double-layered sac, surrounds and protects the heart
  • Consists of the outer fibrous pericardium and inner serous pericardium
  • Pericardial fluid between the layers reduces friction during heart contractions

Blood Vessels and Circulation

• Arteries carry blood away from the heart to the body's tissues
  • Thick, elastic walls to withstand high pressure
  • Contain smooth muscle for vasoconstriction and vasodilation to regulate blood flow
• Veins carry blood from the body's tissues back to the heart
  • Thinner walls compared to arteries due to lower blood pressure
  • Contain valves to prevent backflow of blood
  • Skeletal muscle contractions and respiratory movements aid in venous return
• Capillaries are the smallest blood vessels, connecting arterioles to venules
  • Thin walls (one cell layer) to facilitate exchange of nutrients, gases, and waste products
  • Precapillary sphincters regulate blood flow through capillary beds
• Systemic circulation carries oxygenated blood from the left ventricle to the body and returns deoxygenated blood to the right atrium
• Pulmonary circulation carries deoxygenated blood from the right ventricle to the lungs for oxygenation and returns it to the left atrium
• Coronary circulation supplies the heart muscle with oxygenated blood via the coronary arteries
• Hepatic portal circulation carries nutrient-rich blood from the gastrointestinal tract and spleen to the liver for processing before entering the systemic circulation

Cardiac Cycle and Heart Sounds

• Cardiac cycle refers to the sequence of events that occur during one complete heartbeat
  • Consists of systole (contraction) and diastole (relaxation) of the atria and ventricles
  • Atrial systole, ventricular systole, and complete cardiac diastole are the main phases
• Sinoatrial (SA) node, the heart's natural pacemaker, initiates the cardiac cycle
  • Located in the right atrium, generates electrical impulses that spread through the heart
  • Atrioventricular (AV) node, located between the atria and ventricles, delays the impulse
• Electrocardiogram (ECG) records the electrical activity of the heart during the cardiac cycle
  • P wave represents atrial depolarization, QRS complex represents ventricular depolarization, and T wave represents ventricular repolarization
• Heart sounds, lub-dub, are caused by the closing of heart valves during the cardiac cycle
  • First heart sound (S1) occurs at the beginning of ventricular systole due to the closing of AV valves
  • Second heart sound (S2) occurs at the beginning of ventricular diastole due to the closing of semilunar valves
• Cardiac output is the volume of blood pumped by the heart per minute
  • Calculated as stroke volume (volume of blood pumped per beat) multiplied by heart rate
  • Regulated by factors such as venous return, preload, afterload, and contractility

Blood Pressure Regulation

• Blood pressure is the force exerted by blood against the walls of blood vessels
  • Systolic pressure is the maximum pressure during ventricular contraction
  • Diastolic pressure is the minimum pressure during ventricular relaxation
• Baroreceptors, pressure-sensitive receptors in the blood vessels, detect changes in blood pressure
  • Located in the carotid sinus and aortic arch
  • Send signals to the cardiovascular center in the medulla oblongata to regulate blood pressure
• Short-term regulation of blood pressure involves neural and hormonal mechanisms
  • Sympathetic nervous system increases heart rate, contractility, and vasoconstriction to raise blood pressure
  • Parasympathetic nervous system decreases heart rate to lower blood pressure
  • Renin-angiotensin-aldosterone system (RAAS) increases blood volume and vasoconstriction to raise blood pressure
• Long-term regulation of blood pressure involves the kidneys and fluid balance
  • Kidneys adjust sodium and water excretion to maintain blood volume and pressure
  • Natriuretic peptides (ANP and BNP) promote sodium and water excretion to lower blood pressure
• Chemoreceptors detect changes in blood oxygen, carbon dioxide, and pH levels
  • Carotid and aortic bodies send signals to the respiratory center to adjust ventilation
  • Changes in ventilation can affect blood pressure by altering blood pH and CO2 levels

Cardiovascular Adaptations

• Exercise induces cardiovascular adaptations to meet increased metabolic demands
  • Cardiac hypertrophy (enlargement of the heart muscle) increases stroke volume
  • Increased capillary density in skeletal muscles improves oxygen and nutrient delivery
  • Enhanced oxygen extraction by tissues due to increased myoglobin content
• Diving response is a cardiovascular adaptation in aquatic mammals and birds
  • Bradycardia (decreased heart rate) and peripheral vasoconstriction conserve oxygen
  • Blood is shunted to vital organs (brain and heart) while limiting flow to non-essential tissues
• High-altitude adaptations help maintain oxygen delivery in low-oxygen environments
  • Increased red blood cell production (erythropoiesis) to enhance oxygen-carrying capacity
  • Vasodilation of pulmonary blood vessels to improve blood flow and gas exchange in the lungs
• Gravitational effects on the cardiovascular system are evident in upright posture
  • Baroreceptor reflexes and skeletal muscle pump maintain blood pressure and venous return
  • Orthostatic hypotension can occur due to pooling of blood in the lower extremities
• Temperature regulation involves cardiovascular adjustments
  • Vasodilation and increased skin blood flow promote heat loss in warm environments
  • Vasoconstriction and reduced skin blood flow conserve heat in cold environments

Disorders and Diseases

• Atherosclerosis is the buildup of plaque in the arteries, leading to narrowing and hardening
  • Risk factors include high cholesterol, hypertension, smoking, and diabetes
  • Can lead to coronary artery disease, myocardial infarction, and stroke
• Hypertension, or high blood pressure, increases the risk of cardiovascular diseases
  • Primary (essential) hypertension has no identifiable cause and is often related to lifestyle factors
  • Secondary hypertension results from underlying conditions such as kidney disease or endocrine disorders
• Congestive heart failure occurs when the heart fails to pump blood effectively
  • Can be caused by coronary artery disease, hypertension, valvular disorders, or cardiomyopathy
  • Leads to fluid accumulation in the lungs (pulmonary edema) and peripheral tissues (edema)
• Arrhythmias are abnormalities in the heart's rhythm or conduction
  • Tachycardia is an abnormally fast heart rate, while bradycardia is an abnormally slow heart rate
  • Atrial fibrillation is a common arrhythmia characterized by irregular and rapid atrial contractions
• Valvular disorders affect the function of heart valves, leading to regurgitation or stenosis
  • Mitral valve prolapse is a common condition where the mitral valve bulges into the left atrium
  • Aortic stenosis is a narrowing of the aortic valve, obstructing blood flow from the left ventricle

Key Experiments and Discoveries

• William Harvey's discovery of blood circulation (1628)
  • Demonstrated that blood flows in a continuous circuit through the body
  • Laid the foundation for modern understanding of the cardiovascular system
• Otto Frank's law of the heart (1895)
  • Described the relationship between ventricular end-diastolic volume and stroke volume
  • Introduced the concept of the Frank-Starling mechanism, which relates preload to cardiac output
• Willem Einthoven's invention of the electrocardiogram (ECG) (1903)
  • Developed the string galvanometer to record the electrical activity of the heart
  • Established the standard ECG leads and waveforms (P, QRS, T) used in clinical practice
• Corneille Heymans' discovery of baroreceptors and chemoreceptors (1920s-1930s)
  • Demonstrated the role of carotid sinus and aortic arch baroreceptors in blood pressure regulation
  • Identified the function of carotid and aortic body chemoreceptors in respiratory control
• Renin-angiotensin-aldosterone system (RAAS) elucidation (1940s-1950s)
  • Series of experiments by various researchers uncovered the components and functions of the RAAS
  • Revealed the role of the RAAS in blood pressure regulation and fluid balance

Clinical Applications

• Electrocardiography (ECG) is used to diagnose and monitor heart conditions
  • 12-lead ECG provides a comprehensive view of the heart's electrical activity
  • Holter monitors and event recorders allow for continuous or intermittent ECG recording
• Echocardiography uses ultrasound to visualize the heart's structure and function
  • Assesses chamber sizes, wall thickness, valve function, and blood flow
  • Doppler echocardiography measures the velocity and direction of blood flow
• Cardiac catheterization and angiography provide detailed information about the heart and coronary arteries
  • Pressure measurements, oxygen saturation, and blood sampling can be performed
  • Coronary angiography visualizes the coronary arteries and detects blockages or narrowing
• Pacemakers and implantable cardioverter-defibrillators (ICDs) are used to treat arrhythmias
  • Pacemakers generate electrical impulses to maintain a normal heart rhythm
  • ICDs detect and correct life-threatening arrhythmias by delivering electrical shocks
• Cardiovascular drugs target various aspects of the cardiovascular system
  • Antihypertensive medications (ACE inhibitors, beta-blockers, diuretics) lower blood pressure
  • Anticoagulants and antiplatelet drugs prevent blood clots and reduce the risk of stroke or heart attack
  • Statins lower cholesterol levels and slow the progression of atherosclerosis