Animal Physiology

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

The cardiovascular system is the body's transport network, pumping blood through the heart and vessels. It delivers oxygen and nutrients to tissues while removing waste products, playing a crucial role in maintaining homeostasis and supporting vital functions. Understanding the heart's structure, blood vessel types, and circulation patterns is essential for grasping cardiovascular physiology. Key concepts include cardiac cycle regulation, blood pressure control mechanisms, and how the system adapts to various physiological demands and environmental challenges.

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


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© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.