BIO 224: Ch. 20 The Heart

Capillaries

(aka exchange vessels) are networks between arteries and veins

Epicardium

(outer layer) is the same as the visceral pericardium

Myocardium

- (middle layer) is the muscular wall of the heart that forms the atria and ventricles -Contains cardiac muscle tissue, blood vessels, nerves, connective tissue

Myocardial infarction (MI)

- A heart attack - Part of the coronary circulation becomes blocked, and cardiac muscle cells die from lack of oxygen -Commonly result from severe coronary artery disease (CAD)

Cardiac Contraction

- A heart beat - the entire heart contracts in series; first atria then ventricles

Atrioventricular (AV) valves

- bicuspid valve and the tricuspid valves are located between the upper chambers (atria) and the lower chambers (ventricles) -Blood pressure closes valve cusps during ventricular contraction -Papillary muscles tense chordae tendineae to prevent valves from swinging into atria

Endocardium

-(inner layer) is simple squamous ET that covers the inner surfaces -Continuous with the endothelium of the attached vessels

The Electrocardiogram (ECG or EKG)

-A recording of electrical events in the heart -EKG patterns are different than the action potentials that occur along individual cell membranes -Obtained by electrodes at specific body locations -Abnormal patterns diagnose damage

The Conducting System

-A system of specialized cardiac muscle cells initiates and distributes electrical impulses that stimulate contraction -Automaticity → cardiac muscle tissue contracts automatically

Sympathetic and parasympathetic neurotransmitters effects on the SA Node

-Alter the membrane permeability of conducting cells -ACh (parasympathetic stimulation) Slows the heart by opening chemically gated K+ channels -NE (sympathetic stimulation) Speeds the heart by opening Na+/Ca2+ channels

Atrial Reflex

-An increase in heart rate due to an increase in central venous pressure -Stretch receptors in the right atrium trigger an increase in heart rate in response to increased venous return -Through increased sympathetic activity

The cardiac cycle: Atrial Systole phase 2

-Atria eject blood into ventricles -Complete the filling of ventricles (begin 70% full due to passive blood flow)

Atrial Diastole/Ventricular Systole

-Atrial diastole and ventricular systole begin at the same time -As the ventricles begin contracting, the AV valves close -Atria fill during atrial diastole

The cardiac cycle: Atrial Diastole phase 3

-Atrial systole ends -AV valves close and ventricles contain maximum blood volume -Known as end-diastolic volume (EDV) → amount of blood at the end of ventricular diastole

The chordae tendineae

-Attach to free edges of the valve at one end and to papillary muscles at the other end -Prevent valve from opening backward → no backflow

Heart Rate Control Factors

-Autonomic nervous system (S & P) Circulating hormones Venous return and stretch receptors

The Cardiac Skeleton

-Bands of tough elastic tissue that encircle the valves as well as the base of the pulmonary trunk and aorta -Stabilize valve positions -Electrically insulate ventricular cells from atrial cells

The Cardiac Cycle

-Begins with an action potential at the SA node that is transmitted through conducting system -Produces action potentials in contractile cells

Pulmonary Veins

-Blood travels from the lungs to the heart via the left and right pulmonary veins -Pulmonary veins empty into the left atrium

Cardiac reflexes

-Cardiac centers (acceleratory & inhibitory) adjust cardiac activity based on changes in -Blood pressure (baroreceptors) Arterial oxygen and carbon dioxide levels (chemoreceptors)

Autonomic Innervation adjusts Heart Rate

-Cardiac plexus innervates the heart -Vagus nerves carry parasympathetic preganglionic fibers to small ganglia in cardiac plexus -Sympathetic neurons are located in the cervical and upper thoracic ganglia

Cardiac centers of medulla oblongata

-Cardioacceleratory center controls sympathetic neurons (increases heart rate) -Cardioinhibitory center controls parasympathetic neurons (slows heart rate)

The Pulmonary Circuit

-Carries deoxygenated blood away from the heart, to the lungs, and returns oxygenated (oxygen-rich) blood back to the heart. -Blood is deposited into the left ventricle once oxygenated, and is then transported back to the body through the aorta via Systemic Circulation

The Role of Calcium Ions in Cardiac Contractions

-Contraction is produced by an increase in calcium ion concentration around myofibrils - Cardiac muscle is highly sensitive to changes in [Ca2+] in extracellular fluid

Autonomic tone

-Dual innervation maintains resting tone by releasing ACh and NE -Fine adjustments meet needs of other systems

The EDV and Stroke Volume: At rest

-EDV is low → myocardium stretches little and stroke volume is low -Sarcomeres are short and don't generate much tension when they contract

Stroke Volume Control Factors

-EDV: filling time and rate of venous return -ESV: preload, contractility, afterload

The Purkinje fibers

-Extend into the ventricular myocardium -Distribute stimulus to contractile cells -At this point, atrial contraction is completed and ventricular contraction begins -Ventricles contract in a wave from the apex towards the base -Entire process requires 225 msec

The Cardiac Veins

-Great cardiac vein drains blood from anterior interventricular area -Anterior cardiac veins empty into right atrium -Posterior cardiac vein, middle cardiac vein, and small cardiac vein empty into great cardiac vein or coronary sinus

Semilunar valves

-Have no muscular support -The valve supports itself (like a tripod) when closed and opens under pressure -The two semilunar (SL) valves, the aortic valve and the pulmonary valve, which are in the arteries leaving the heart.

The Left Ventricle

-Holds same volume as the right ventricle -Muscle is thicker and more powerful to generate a greater pressure and push blood a greater distance -Internal structures include trabeculae carnae, papillary muscles, chordae tendinae, left AV valve

Atrioventricular (AV) Node

-In the floor of the right atrium -Receives impulse from SA node -Impulse takes about 100 msec to pass through AV node -Atrial contraction begins during this delaye

Sinoatrial (SA) Node

-In the posterior wall of the right atrium Contains pacemaker cells -SA node generates 80-100 action potentials per minute however parasympathetic stimulation slows heart rate

P wave

-Indicates atrial depolarization Atrial contraction begins about 25 msec later

QRS complex

-Indicates ventricular depolarization -Ventricular contraction begins just after the R peak -Atrial repolarization occurs at this time but is masked by electrical events in the ventricles

Factors Affecting End-Diastolic Volume

-Influenced by filling time and venous return -Filling time is the duration of ventricular diastole and depends entirely on heart rate -Venous return is the rate of blood flow during ventricular diastole -Preload is the degree of ventricular stretching during ventricular diastole; affects ability of muscle cells to produce tension

Intercalated discs

-Interconnect cardiac muscle cells -Secured by desmosomes (adhesions arranged on the lateral sides of plasma membranes, binding muscles to one another)→ transfer force of contraction -Linked by gap junctions → propagate action potentials

Arterial anastomoses

-Interconnections between arteries -Small branches supplied by the anterior interventricular artery are also supplied by the posterior interventricular artery -Stabilize blood supply to cardiac muscle

Absolute refractory period

-Is long (200 msec) -Cardiac muscle cells cannot respond → Na+ channels are already open or inactivated (remain inactivated until -60 mV)

Relative refractory period

-Is short (50 msec) -Response depends on degree of stimulus

Timing of Refractory Periods (cardiac vs skeletal)

-Length of cardiac action potential in ventricular cell is 250-300 msec -30 times longer than skeletal muscle fiber -Long refractory period prevents summation and tetany

The AV Bundle

-Located in the interventricular septum -Carries impulse to left and right bundle branches -Bundle branches conduct impulse to Purkinje fibers

Two phases of the cardiac cycle

-Occur within any one chamber 1. Systole (contraction) - pressure in the blood change goes up 2. Diastole (relaxation) -pressure in blood goes down, heart is relaxed, blood is filling in the chambers

Isovolumetric Contraction

-Occurs in early systole during which the ventricles contract with no corresponding volume change (isovolumetrically)-This short-lasting portion of the cardiac cycle takes place whilst all heart valves are closed.

Left Coronary Artery

-Originates at the base of the aorta -Gives rise to Anterior interventricular artery & Circumflex artery

Right Coronary Artery

-Originates at the base of the aorta -Gives rise to Marginal arteries (surface of right ventricle) & Posterior interventricular artery

The Action Potential in Cardiac Muscle Cells

-Resting potential for a ventricular contractile cell is -90 mV -An action potential begins when threshold (-75 mV) is passed -The typical stimulus is the excitation of an adjacent muscle cell → threshold normally reached near intercalated disc

Structural Differences between the Left and Right Ventricles

-Right ventricle wall is thinner, less pressure is required to pump blood through the pulmonary circuit -Right ventricle is pouch-shaped → squeezes blood against the the thick wall of the left ventricle as it contracts -Left ventricle is thick-walled and round 4 to 6 times as much pressure is required to pump blood through the systemic circuit -When the left ventricle contracts, it shortens and narrows

Blood Pressure

-Rises during systole -Falls during diastole

Heart Sounds

-S1 → loud sound produced by AV valves closing -S2 → loud sound produced by semilunar valves closing -S3, S4 → soft sounds as blood flows into ventricles and atrial contraction

Interventricular septum

-Separates the ventricles -Each septum is a muscular partition

The Right Atrium

-Superior vena cava receives blood from head, neck, upper limbs, and chest -Inferior vena cava receives blood from trunk, viscera, and lower limbs -Coronary sinus receives blood from cardiac veins

Coronary circulation

-Supplies blood to muscle tissue of the heart

The Cardiac Cycle

-The period between the start of one heartbeat and the beginning of the next -Includes both contraction (systole) and relaxation (diastole) -Fluids move from an area of higher pressure to an area of lower pressure

The Right Ventricle

-The right atrioventricular (AV) valve (aka tricuspid valve) regulates flow from the right atrium into the right ventricle -The chordae tendineae prevent valve from opening backward → no backflow

The Pulmonary Circuit: The right ventricle

-The right ventricle pumps blood through the pulmonary semilunar valve into the pulmonary trunk -The pulmonary trunk divides into left and right pulmonary arteries -Pulmonary arteries supply the lungs

The Energy for Cardiac Contractions

-Uses aerobic metabolism: -Mitochondrial breakdown of fatty acids and glucose -Oxygen from circulating hemoglobin -Cardiac muscles store oxygen in myoglobin

The cardiac cycle: Ventricular Systole phase 4

-Ventricles contract isometrically and build pressure -AV and semilunar valves are closed, causing isovolumetric contraction

The cardiac cycle: Ventricular Systole phase 5

-Ventricular ejection occurs during isotonic contraction -Ventricular pressure exceeds vessel pressure opening the semilunar valves and allowing blood to leave the ventricle - Amount of blood ejected is called the stroke volume (SV)

The cardiac cycle: Ventricular Diastole phase 7

-Ventricular myocardium is relaxing -All heart valves are closed -Ventricular pressure is higher than atrial pressure -Ventricles relax (isovolumetric relaxation)

The cardiac cycle: Ventricular Systole phase 6

-Ventricular pressure falls and semilunar valves close -Ventricles contain end-systolic volume (ESV), about 40% of end-diastolic volume

The cardiac cycle: Ventricular Diastole phase 8

-Ventricular pressure falls below atrial pressure -AV valves open -Passive atrial filling leads to -Passive ventricular filling

The EDV and Stroke Volume: with Exercise

-With exercise, EDV increases → myocardium stretches more and stroke volume increases -Sarcomeres are stretched to a better length and generate more tension when they contract "More in, more out"

Ca2+ stored

-Within the sarcoplasmic reticulum (80%) is released in response to the influx of extracellular Ca2+

Foramen Ovale

-connect the two atria before birth -seals off at birth, forming the fossa ovalis

The Systemic Circuit

-the circulation of the blood to all parts of the body except the lungs. -transports oxygenated blood away from the heart to the rest of the body, and returns oxygen-depleted blood back to the heart -Blood alternates between pulmonary circuit and systemic circuit

Three Types of Blood Vessels

1. Arteries 2. Veins 3. Capillaries

Factors affecting heart rate

1. Atrial Reflex 2. Autonomic Intervention 3. Hormones

Systemic circulation: Left Ventricle

1. Blood leaves the left ventricle through the aortic semilunar valve into ascending aorta 2. Ascending aorta turns (aortic arch) and becomes descending aorta

The Left Atrium

1. Blood travels from the lungs to the heart via the pulmonary veins 2. Pulmonary veins empty into the left atrium 3. Blood from left atrium passes into the left ventricle through the left atrioventricular (AV) valve -A two-cusped bicuspid valve or mitral valve

The Heart and Cardiovascular System

1. Cardiovascular regulation -Ensures adequate circulation to body tissues 2.Cardiovascular centers -Control heart rate, blood pressure, and peripheral blood vessels 3.Cardiovascular system responds to -Changing activity patterns -Circulatory emergencies

Two types of Cardiac Muscle Cells

1. Conducting system controls and coordinates heartbeat 2. Contractile cells produce contractions that propel blood

Layers of the Heart Wall

1. Epicardium (outer layer) 2. Myocardium (middle layer) 3. Endocardium (inner layer)

Connective Tissue Function

1. Physically support cardiac muscle fibers, blood vessels, and nerves 2. Distribute forces of contraction 3. Add strength and prevent overexpansion of heart 4.Provide elasticity that helps return heart to original size and shape after contraction

Factors That Affect End-Systolic Volume (ESV)

1. Preload is the degree of ventricular stretching during diastole -affected by venous return and filling time 2. Contractility is the force produced during contraction, at a given preload -Normally altered by autonomic innervation or hormones 3. Afterload is the tension the ventricle must produce to open the semilunar valve and eject blood -is increased by restricting arterial blood flow

Oxygenated blood flow to the heart from the lungs

1. Pulmonary Veins → carry oxygenated blood in from the lungs 2. Left Atrium 3. Bicuspid Valve 4. Left Ventricle 5. Aortic Semilunar Valve 6. Aorta 7. To the body → th ebody consumes oxygenated blood and becomes deoxygenated

Steps in Cardiac Muscle Action Potential

1. Rapid depolarization → voltage-gated sodium channels (fast sodium channels) open for a few msec 2. Plateau → membrane potential remains near 0 as -Na+ channels close and Na+ is pumped out of the cell -Voltage-gated calcium channels (slow calcium channels) open and Ca2+ slowly leaks in for ~175 msec 3. Repolarization → slow calcium channels close and slow potassium channels open

Four Chambers of the Heart

1. Right atrium collects blood from systemic circuit 2. Right ventricle pumps blood to pulmonary circuit 3. Left atrium collects blood from pulmonary circuit 4. Left ventricle pumps blood to systemic circuit

Cardiac versus skeletal muscle cells

1. Similarities: -An action potential leads to the appearance of Ca2+ among the myofibrils -Ca2+ binding to troponin initiates contraction 2. Differences -The events (ion movements) of the action potential -The source of Ca2+ -Contraction duration

Characteristics of Cardiac Muscle Cells

1. Small size 2. Single, central nucleus 3. Branching interconnections between cells 4. Intercalated discs

The Cardiovascular System

A pump (the heart moves ~8000 liters/day) A conducting system (blood vessels) A fluid medium (blood)

auricle

AKA atrial appendage -Outer portion of the thin-walled atria that becomes a wrinkled flap when empty

Coronary artery disease (CAD)

Areas of partial or complete blockage of coronary circulation

Phases of the cardiac cycle

Atrial systole, Atrial diastole, Ventricular systole, Ventricular diastole

Valves in the left heart

Atrioventricular (AV) valve: The mitral (bicuspid) Semilunar valve: aortic

Valves in the right heart

Atrioventricular (AV) valve: The tricuspid Semilunar valve: The pulmonary

The Heart

Base: great veins and arteries Apex: pointed tip Pericardial sac: surrounds the heart Mediastinum: sits between two pleural cavities

The cardiac cycle: Atrial systole phase 1

Begins with atrial systole: -Atrial contraction begins, blood pressure rises -Right and left AV valves are open

Factors Affecting the Stroke Volume

Changes in either EDV or ESV will change SV

SA and AV nodes

Conducting cells of the SA and AV nodes exhibit prepotential (pacemaker potential) → they spontaneously depolarize SA node is the pacemaker because it depolarizes the fastest

Connective Tissues

Connective tissues include large numbers of collagen and elastic fibers → each cardiac muscle cell is wrapped in a strong, yet flexible, sheath

Deoxygenated blood flow to the heart from the body

Deoxygenated Blood 1. Superior & Inferior Vena Cava 2. Right Atrium 3. Tricuspid Valve 4. Right Ventricle 5. Pulmonary Semilunar Valve 6. Left Pulmonary Artery → Left Lung Right Pulmonary Artery → Right Lung 7. Blood becomes oxygenated in the lungs

Pericardium

Double lining (serous membrane) of the pericardial cavity

Electrocardiogram (ECG or EKG)

Electrical events in the cardiac cycle can be recorded on an electrocardiogram

Extracellular Ca2+

Enters during the plateau phase and provides about 20% of the Ca2+ required for contraction

Factors Affecting Cardiac Output

Heart rate is adjusted by autonomic nervous system or hormones Stroke volume is adjusted by changing EDV or ESV

Hormonal Effects on Heart Rate

Increase heart rate by effecting the SA node -Epinephrine (E) -Norepinephrine (NE) -Thyroid hormone

Contractile Cells

Purkinje fibers distribute the stimulus to the contractile cells, which make up most (99%) of the cells in the heart

Anterior and Posterior interventricular sulcus

Separate left and right ventricles Contain blood vessels serving cardiac muscle

Cardiodynamics

The movement and force generated by cardiac contractions

Cardiac Output (CO)

The volume pumped by left ventricle in 1 minute CO = HR × SV CO = cardiac output (mL/min) HR = heart rate (beats/min) SV = stroke volume (mL/beat)

Atrioventricular valves

are folds of fibrous tissue that permit blood flow in only one direction, from the atria into the ventricles

Trabeculae carneae

are muscular ridges on internal surface of the right (and left) ventricle

Arteries

carry blood away from the heart

Veins

carry blood towards the heart

Coronary sulcus

divides atria and ventricles

Pericardial sac

fibrous tissue that surrounds and stabilizes the heart

T wave

indicates ventricular repolarization

Stroke volume (SV)

is the amount of blood pumped out of each ventricle during a single beat SV = EDV - ESV

End-systolic volume (ESV)

is the amount of blood remaining in each ventricle at the end of systole

Pectinate muscles

prominent muscular ridges on the anterior atrial wall and inner surfaces of the right auricle

Protection against overexpansion

provided by: -myocardial connective tissues -the cardiac skeleton -the pericardial sac

Interatrial septum

separates the atria

Heart murmur

sounds produced by regurgitation through valves

End-diastolic volume (EDV)

the amount of blood in each ventricle at the end of ventricular diastole

Visceral pericardium

the inner layer of pericardium

Parietal pericardium

the outer layer Forms inner layer of pericardial sac

Ejection fraction

the percentage of EDV represented by SV, typically 60% at rest

Pericardial cavity

the space between parietal and visceral layers Contains pericardial fluid → lubrication