Cardiovascular System: Heart Structure and Function

• The cardiovascular system is a transport system where blood is pumped by the heart in a closed circuit through vessels

General Functions

• Delivers and removes substances through fluid circulation
• Provides living cells with necessary materials like O2 and nutrients
• Removes cell metabolism products like CO2
• Substances pass indirectly via interstitial (extracellular) fluid

Organs of the Cardiovascular System

Heart

• A muscular organ in the thorax with an inverted cone shape
• Adult heart is about the size of a fist and weighs around 300 grams
• Pumps blood into arteries, specifically the aorta or pulmonary artery

Heart Structure

• Four chambers: two atria (right and left) and two ventricles (right and left)
• The right atrium and right ventricle receive deoxygenated blood from systemic veins and pump it to the lungs
• The left atrium and left ventricle receive oxygenated blood from the lungs and pump it to the systemic vessels
• Interatrial and interventricular septa separate the left and right sides of the heart and are continuous
• Blood flows from atria to ventricles through atrioventricular orifices controlled by heart valves
• Heart valves open/close rhythmically

Heart Valves

• Separate atria from ventricles, and ventricles from great vessels
• Each valve has two or three leaflets (cusps)
• Cusps open for blood flow in one direction, then close to prevent backflow
• Chordae tendineae prevent backward prolapse of cusps
• Two sets of valves exist

Atrioventricular Valves

• Prevent backflow from ventricles to atria
• The right atrioventricular valve (tricuspid valve) is between the right atrium and right ventricle
• The left atrioventricular valve (bicuspid/mitral valve) is between the left atrium and left ventricle

Semilunar Valves

• Prevent backflow from great vessels into ventricles
• The pulmonary semilunar valve is between the right ventricle and pulmonary artery and has three cusps
• The aortic semilunar valve is between the left ventricle and aorta and has three cusps

Arteries

• Arteries supply the body with oxygenated blood
• Pulmonary and umbilical arteries are exceptions and carry deoxygenated blood
• Blood flows from arteries to arterioles to capillaries, where gaseous exchange occurs

Types of Arteries

Elastic Arteries

• Largest in diameter with large lumen and low resistance
• Expand and recoil to accommodate blood volume changes

Muscular Arteries

• Smaller arteries that regulate local blood flow
• Deliver blood to individual organs and have more smooth muscle but less elastin compared to elastic arteries

Arterioles

• Smallest arteries near capillaries, comprised of a single smooth muscle layer
• Regulate blood flow to capillaries

Capillaries

• Smallest branches connecting arterioles to venules
• Thin layer allows nutrient, gas, and waste exchange
• Blood flow is determined by arteriole diameter
• Capillary flow increases via arteriolar vasodilation

Veins

• Transport blood back to the heart with valves that prevent backflow

Vein Characteristics

• Thin, elastic vessels acting as a blood reservoir
• Require less elastin and smooth muscle due to low-pressure blood transport
• Have a large lumen and valves for unidirectional blood flow

Cardiac Cycle

• Heart pumps oxygenated blood from left ventricle to aorta for systemic circulation
• Blood returns deoxygenated to right atrium after supplying cells
• Deoxygenated blood flows from right atrium to right ventricle
• Heart pumps deoxygenated blood from right ventricle to pulmonary arteries for pulmonary circulation
• Blood travels to lungs, exchanges carbon dioxide for oxygen, and returns to left atrium
• Oxygenated blood moves from the left atrium to the left ventricle to begin systemic circulation again

Human Circulatory System Sections

Systemic Circulation

• Left ventricle pumps 85% of circulation at high intravascular pressure

Pulmonary Circulation

• Right ventricle pumps 10% of circulation at low intravascular pressure
• Remaining 5% is in the heart
• Pulmonary circulation allows blood oxygenation
• Systemic circulation provides oxygenated blood/nutrients

Physiology of Cardiac Muscle

Cardiac Muscle Cell Types

Ordinary Myocardial Contractile Fibers

• 99% of cells in atria and ventricles
• Responsible for contractions that pump blood

Specialized Myocardial Conducting Fibers

• 1% of cells
• Form the heart's conduction system, similar to neurons
• Initiate and propagate action potentials, triggering contractions

Physiological Properties of Cardiac Muscle

Rhythmicity (Automaticity)

• Ability to spontaneously depolarize without external stimulation

Conductivity

• Ability of cardiac muscle fibers to conduct impulses from the SA node (pacemaker)

Excitability

• Ability to respond to stimuli by generating an action potential

Contractility

• Ability to convert electrical energy into mechanical work

Cardiac Rhythmicity

Cardiac Rhythmicity/Automaticity

• Ability of cardiac muscle to initiate its own electrical impulse
• Triggers mechanical contraction, pumping blood regularly without nervous/endocrine control

Primary Pacemaker

• The sinoatrial node (SA node) is a group of cells in the right atrium near the superior vena cava
• SA node cells are modified cardiac myocytes
• Cells depolarize spontaneously, generating ~100 action potentials per minute
• Sympathetic and parasympathetic nerves modify the rate
• Average resting cardiac rate in adults is ~70 beats per minute

Secondary Pacemaker

• If the SA node fails, another group of cells becomes the pacemaker (ectopic)
• The atrioventricular node (AV node) lies beneath the endocardium on the lower right posterior portion of the interatrial septum
• It lies near the coronary sinus above the tricuspid valve
• AV node cells discharge at 40-60 beats per minute

Tertiary Pacemaker

• The bundle of His, left/right branches, and Purkinje fibers produce spontaneous action potentials at 30-40 bpm if SA/AV nodes fail
• The SA node controls the entire heart because its action potentials are the most rapid