Chapter 20: The HeartBiol 141 A& P

R.L. Brashear-Kaulfers

How are the cardiovascular system and heart organized?

The Heart: AnatomyPLAYPLAY

Figure 20–1

Organization of the Cardiovascular System

The Pulmonary Circuit

• Carries blood to and from gas exchange surfaces of lungs

The Systemic Circuit• Carries blood to and from the body Alternating Circuits• Blood alternates between

pulmonary circuit and systemic circuit

3 Types of Blood Vessels

• Arteries:– carry blood away from heart

• Veins:– carry blood to heart

• Capillaries:– networks between arteries and veins

Capillaries

• Also called exchange vessels • Exchange materials between blood

and tissues• Dissolved gases, nutrients, wastes

4 Chambers of the Heart

• 2 for each circuit:– left and right: ventricles and atria

• Right atrium:– collects blood from systemic circuit

• Right ventricle:– pumps blood to pulmonary circuit

4 Chambers of the Heart

• Left atrium:– collects blood from pulmonary circuit

• Left ventricle:– pumps blood to systemic circuit

Anatomy of the Heart

• Located directly behind sternum

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Figure 20–2aPLA

Figure 20–2c

Anatomy of the Heart

• Great veins and arteries at the base• Pointed tip is apex

Relation to Thoracic Cavity

Figure 20–2b

Surrounded by pericardial sacBetween 2 pleural cavities In the mediastinum

Figure 20–2c

The Pericardium

• Double lining of the pericardial cavity

2 Layers of Pericardium

1. Parietal pericardium:– outer layer– forms inner layer of pericardial sac

2. Visceral pericardium:– inner layer of pericardium

Structures of Pericardium

• Pericardial cavity:– Is between parietal and visceral layers – contains pericardial fluid

• Pericardial sac: – fibrous tissue– surrounds and stabilizes heart

PericarditisAn infection of the pericardium

Superficial Anatomy of the Heart

• 4 cardiac chambers• Atria - Thin-walled• Expandable outer auricle • Coronary sulcus:

– divides atria and ventricles

• Anterior and posterior interventricular sulci:– separate left and right

ventricles– contain blood vessels of

cardiac muscle

Figure 20–3

The Heart Wall

Figure 20–4

3 Layers of the Heart Wall• Epicardium:- outer layer • Visceral pericardium , Covers the heart

• Myocardium: middle layer, Muscular wall • Concentric layers of cardiac muscle tissue• Atrial myocardium wraps around great

vessels• 2 divisions of ventricular myocardium

• Endocardium: inner layer

Cardiac Muscle Cells

Figure 20–5

Intercalated discs:interconnect cardiac muscle cellssecured by desmosomes linked by gap junctionsconvey force of contraction propagate action potentials

Characteristics of Cardiac Muscle Cells

1. Small size2. Single, central nucleus3. Branching interconnections

between cells4. Intercalated discs

Cardiac Cells vs. Skeletal Fibers

Table 20-1

What is the path of blood flow through the heart, and what are the major

blood vessels, chambers, and heart valves?

Internal Anatomy

3D Panorama of the HeartPLAYPLAY

Figure 20–6a

Atrioventricular (AV) Valves

• Connect right atrium to right ventricle and left atrium to left ventricle

• Permit blood flow in 1 direction: – atria to ventricles

Septa – • Interatrial septum:

– separates atria

• Interventricular septum:– separates ventricles

The Vena Cava

• Delivers systemic circulation to right atrium • Superior vena cava:

– receives blood from head, neck, upper limbs, and chest

• Inferior vena cava: – receives blood from trunk, and viscera, lower

limbs

Coronary Sinus• Cardiac veins return blood to coronary sinus • Coronary sinus opens into right atrium

Foramen Ovale

• Before birth, is an opening through interatrial septum

• Connects the 2 atria• Seals off at birth, forming fossa

ovalis

Cusps - Fibrous flaps that form bicuspid (2) and tricuspid (3)

valves- Prevent valve from opening

backward Right Atrioventricular (AV) Valve• Also called tricuspid valve• Opening from right atrium to right

ventricle • Has 3 cusps• Prevents backflow

The Pulmonary Circuit

• Conus arteriosus (superior right ventricle) leads to pulmonary trunk

• Pulmonary trunk divides into left and right pulmonary arteries

• Blood flows from right ventricle to pulmonary trunk through pulmonary valve

• Pulmonary valve has 3 semilunar cusps

Return from Pulmonary Circuit

• Blood gathers into left and right pulmonary veins

• Pulmonary veins deliver to left atrium

• Blood from left atrium passes to left ventricle through left atrioventricular (AV) valve

• 2-cusp bicuspid valve or mitral valve

The Left Ventricle• Holds same volume as right ventricle• Is larger; muscle is thicker, and more

powerful• Similar internally to right ventricle,

but does not have moderator band Systemic circulation:

– blood leaves left ventricle through aortic valve into ascending aorta

– ascending aorta turns (aortic arch) and becomes descending aorta

Left and Right Ventricles

• Have significant structural differences

• Right ventricle wall is thinner, develops less pressure than left ventricle

• Right ventricle is pouch-shaped, left ventricle is round

Figure 20–7

The Heart Valves• One-way valves prevent

backflow during contraction

• (AV) Valves- between atria and ventricles

• Blood pressure closes valve cusps during ventricular contraction

• Papillary muscles tense chordae tendineae:– prevent valves from swinging

into atria• Regurgitation -Failure of

valves• Causes backflow of blood

into atria Figure 20–8

Semilunar Valves

• Pulmonary and aortic tricuspid valves

• Prevent backflow from pulmonary trunk and aorta into ventricles

• Have no muscular support• 3 cusps support like tripod

• Aortic Sinuses - at base of ascending aorta

• Prevent valve cusps from sticking to aorta

• Origin of right and left coronary arteries

Carditis - An inflammation of the heart• Can result in valvular heart disease

(VHD): e.g., rheumatic fever

KEY CONCEPT • The heart has 4 chambers:

– 2 for pulmonary circuit:• right atrium and right ventricle

– 2 for systemic circuit:• left atrium and left ventricle

• Left ventricle has a greater workload• Is much more massive than right ventricle,

but the two chambers pump equal amounts of blood

• AV valves prevent backflow from ventricles into atria

• Semilunar valves prevent backflow from aortic and pulmonary trunks into ventricles

Connective Tissue Fibers of the Heart

1. Physically support cardiac muscle fibers

2. Distribute forces of contraction3. Add strength and prevent

overexpansion of heart4. Elastic fibers return heart to

original shape after contraction

Blood Supply to the Heart• Coronary circulation

Figure 20–9

Coronary Circulation

• Coronary arteries-Left and right Originate at aortic sinuses• High blood pressure, elastic rebound

force blood through coronary arteries between contractions

• cardiac veins• Supplies blood to muscle tissue of heart

Right Coronary Artery

• Supplies blood to:– right atrium– portions of both ventricles– cells of sinoatrial (SA) and

atrioventricular nodes – marginal arteries (surface of right

ventricle)– posterior interventricular artery

Left Coronary Artery

• Supplies blood to:– left ventricle– left atrium– interventricular septum

• 2 main branches:– circumflex artery – anterior interventricular artery

Cardiac Veins• Great cardiac vein:

– drains blood from area of anterior interventricular artery into coronary sinus

• Anterior cardiac vein:– empties into right atrium

• Posterior cardiac vein, middle cardiac vein, and small cardiac vein:– empty into great cardiac vein or coronary

sinus

Figure 20–11

The Cardiac CycleThe HeartbeatA single contraction of the heartThe entire heart contracts in series:

first the atriathen the ventricles

2 Types of Cardiac Muscle Cells• Conducting system:

– controls and coordinates heartbeat

• Contractile cells:– produce contractions

* The Cardiac Cycle begins with action potential at SA node– transmitted through conducting system– produces action potentials in cardiac muscle

cells (contractile cells)

• Electrical events in the cardiac cycle can be recorded on an electrocardiogram (ECG)

Figure 20–12

The Conducting System

The Conducting System

• A system of specialized cardiac muscle cells:– initiates and distributes electrical

impulses that stimulate contraction

• Automaticity:– cardiac muscle tissue contracts

automatically

Structures of the Conducting System

• Sinoatrial (SA) node• Atrioventricular (AV) node • Conducting cells

Conducting Cells

• Interconnect SA and AV nodes• Distribute stimulus through

myocardium• In the atrium:

– internodal pathways

• In the ventricles:– AV bundle and bundle branches

Prepotential

• Also called pacemaker potential• Resting potential of conducting

cells:– gradually depolarizes toward

threshold

• SA node depolarizes first, establishing heart rate

Heart Rate

• SA node generates 80–100 action potentials per minute

• Parasympathetic stimulation slows heart rate

• AV node generates 40–60 action potentials per minute

Figure 20–13

Impulse Conduction through the Heart

The Sinoatrial (SA) Node

• In posterior wall of right atrium• Contains pacemaker cells• Connected to AV node by

internodal pathways• Begins atrial activation (Step 1)

The Atrioventricular (AV) Node

• In floor of right atrium• Receives impulse from SA node

(Step 2)• Delays impulse (Step 3)• Atrial contraction begins

The AV Bundle

• In the septum• Carries impulse to left and right bundle

branches:– which conduct to Purkinje fibers (Step 4)

• And to the moderator band:– which conducts to papillary muscles 4. The Purkinje Fibers

• Distribute impulse through ventricles (Step 5)

• Atrial contraction is completed• Ventricular contraction begins

Abnormal Pacemaker Function

• Bradycardia:– abnormally slow heart rate

• Tachycardia:– abnormally fast heart rate

Ectopic Pacemaker:• Abnormal cells • Generate high rate of action potentials• Bypass conducting system• Disrupt ventricular contractions

The Electrocardiogram

Figure 20–14b

Electrocardiogram (ECG or EKG)• A recording of electrical events in

the heart• Obtained by electrodes at specific

body locations• Abnormal patterns diagnose damage Features of an ECG :• P wave: atria depolarize• QRS complex: ventricles depolarize• T wave: ventricles repolarize

Time Intervals• P–R interval:

– from start of atrial depolarization– to start of QRS complex

• Q–T interval:– from ventricular depolarization– to ventricular repolarization Cardiac Arrhythmias – Abnormal patterns of cardiac electrical

activity

KEY CONCEPT • Heart rate is normally established by cells

of SA node• Rate can be modified by autonomic

activity, hormones, and other factors• From the SA node, stimulus is conducted

to AV node, AV bundle, bundle branches, and Purkinje fibers before reaching ventricular muscle cells

• Electrical events associated with the heartbeat can be monitored in an electrocardiogram (ECG)

What events take place during an action

potential in cardiac muscle?

Action Potentials in Skeletal and Cardiac Muscle

Figure 20–15

Resting Potential

• Of a ventricular cell:– about —90 mV

• Of an atrial cell:– about —80 mV

3 Steps of Cardiac Action Potential

1. Rapid depolarization: – voltage-regulated sodium channels

(fast channels) open

3 Steps of Cardiac Action Potential

2. As sodium channels close:– voltage-regulated calcium channels

(slow channels) open– balance Na+ ions pumped out– hold membrane at 0 mV plateau

3 Steps of Cardiac Action Potential

3. Repolarization: – plateau continues– slow calcium channels close– slow potassium channels open– rapid repolarization restores resting

potential

The Refractory Periods

• Absolute refractory period:– long – cardiac muscle cells cannot respond

• Relative refractory period:– short– response depends on degree of

stimulus

Timing of Refractory Periods

• Length of cardiac action potential in ventricular cell:– 250–300 msecs

• 30 times longer than skeletal muscle fiber

• long refractory period prevents summation and tetany

Contraction of a cardiac muscle cell is produced by an

increase in calcium ion concentration around

myofibrils 1. 20% of calcium ions required for a

contraction:– calcium ions enter cell membrane

during plateau phase

2. Arrival of extracellular Ca2+:– triggers release of calcium ion

reserves from sarcoplasmic reticulum

Intracellular and Extracellular Calcium

• As slow calcium channels close:– intracellular Ca2+ is absorbed by the

SR– or pumped out of cell

• Cardiac muscle tissue:– very sensitive to extracellular Ca2+

concentrations

The Cardiac Cycle

• The period between the start of 1 heartbeat and the beginning of the next

• Includes both contraction and relaxation

2 Phases of the Cardiac Cycle:• Within any 1 chamber:

– systole (contraction)– diastole (relaxation)

Blood Pressure

• In any chamber:– rises during systole– falls during diastole

• Blood flows from high to low pressure:– controlled by timing of contractions– directed by one-way valves

Phases of the Cardiac Cycle

Figure 20–16

4 Phases of the Cardiac Cycle

1. Atrial systole2. Atrial diastole3. Ventricular systole 4. Ventricular diastole

Cardiac Cycle and Heart Rate

• At 75 beats per minute:– cardiac cycle lasts about 800 msecs

• When heart rate increases:– all phases of cardiac cycle shorten,

particularly diastole

Pressure and Volume in the Cardiac Cycle

• 8 steps in the cardiac cycle

Figure 20–17

8 Steps in the Cardiac Cycle1. Atrial systole:

– atrial contraction begins– right and left AV valves are open

2. Atria eject blood into ventricles:– filling ventricles

3. Atrial systole ends: – AV valves close– ventricles contain maximum volume– end-diastolic volume (EDV)

4. Ventricular systole:– isovolemic ventricular contraction– pressure in ventricles rises– AV valves shut

8 Steps in the Cardiac Cycle5. Ventricular ejection:

– semilunar valves open– blood flows into pulmonary and aortic

trunks

• Stroke volume (SV) = 60% of end-diastolic volume

6. Ventricular pressure falls:– semilunar valves close– ventricles contain end-systolic volume

(ESV), about 40% of end-diastolic volume

8 Steps in the Cardiac Cycle

7. Ventricular diastole: – ventricular pressure is higher than

atrial pressure– all heart valves are closed– ventricles relax (isovolumetric

relaxation)

8. Atrial pressure is higher than ventricular pressure:

– AV valves open– passive atrial filling – passive ventricular filling– cardiac cycle ends

Heart Failure

• Lack of adequate blood flow to peripheral tissues and organs due to ventricular damage

Heart Sounds

Figure 20–18b

How do heart sounds relate to specific events in the cardiac cycle?

4 Heart Sounds

• S1:– loud sounds– produced by AV valves

• S2:– loud sounds– produced by semilunar valves

• S3, S4:– soft sounds– blood flow into ventricles and atrial

contraction

Figure 20–18a

Positioning the Stethoscope

• To detect sounds of each valve

• Heart Murmur-• Sounds produced

by regurgitation through valves

Aerobic Energy of Heart

• From mitochondrial breakdown of fatty acids and glucose

• Oxygen from circulating hemoglobin

• Cardiac muscles store oxygen in myoglobin

Stroke Volume

• Volume (ml) of blood ejected per beat

Figure 20–19

Cardiac Output

• Cardiac output (CO) ml/min = • Heart rate (HR) beats/min • Stroke volume (SV) ml/beat

Overview: Control of Cardiac Output

Figure 20–20 (Navigator)

Adjusting to Conditions

• Cardiac output:– adjusted by changes in heart rate or

stroke volume• Heart rate:

– adjusted by autonomic nervous system or hormones

• Stroke volume:– adjusted by changing EDV or ESV

What variables influence heart rate? Autonomic Innervation

Figure 20–21 (Navigator)

Autonomic Pacemaker Regulation

Figure 20–22

Autonomic Pacemaker Regulation

• Sympathetic and parasympathetic stimulation:– greatest at SA node (heart rate)

• Membrane potential of pacemaker cells:lower than other cardiac cells

• Rate of spontaneous depolarization depends on:– resting membrane potential– rate of depolarization

• ACh (parasympathetic stimulation):– slows the heart

• NE (sympathetic stimulation):– speeds the heart

Atrial Reflex

• Also called Bainbridge reflex• Adjusts heart rate in response to

venous return• Stretch receptors in right atrium:

– trigger increase in heart rate– through increased sympathetic

activity

Hormonal Effects on Heart Rate

• Increase heart rate (by sympathetic stimulation of SA node):– epinephrine (E)– norepinephrine (NE)– thyroid hormone

Factors Affecting Stroke Volume• Changes in EDV or ESV

Figure 20–23 (Navigator)

2 Factors Affect EDV-1. Filling time:

duration of ventricular diastole

2. Venous return: rate of blood flow during ventricular diastole

What variables influence stroke volume?

Preload

• The degree of ventricular stretching during ventricular diastole

• Directly proportional to EDV• Affects ability of muscle cells to

produce tension

EDV, Preload, and Stroke Volume

• At rest:– EDV is low– myocardium stretches less– stroke volume is low

• With exercise:– EDV increases– myocardium stretches more– stroke volume increases

• As EDV increases, stroke volume increases

Physical Limits

• Ventricular expansion is limited by:– myocardial connective tissue– the fibrous skeleton– the pericardial sac

End-Systolic Volume (ESV)• The amount of blood that remains

in the ventricle at the end of ventricular systole is the ESV

3 Factors that Affect ESV

1. Preload:– ventricular stretching during diastole

2. Contractility:– force produced during contraction, at a

given preload

3. Afterload:– tension the ventricle produces to open

the semilunar valve and eject blood

Contractility is affected by:autonomic activity &

hormones : • Sympathetic stimulation:

– NE released by postganglionic fibers of cardiac nerves

– epinephrine and NE released by adrenal medullae

– causes ventricles to contract with more force– increases ejection fraction and decreases ESV

• Parasympathetic activity:– acetylcholine released by vagus nerves– reduces force of cardiac contractions

Hormones and Contractility

• Many hormones affect heart contraction

• Pharmaceutical drugs mimic hormone actions: – stimulate or block beta receptors– affect calcium ions e.g., calcium

channel blockers

Afterload

• Is increased by any factor that restricts arterial blood flow

• As afterload increases, stroke volume decreases

Factors Affecting Heart Rate and Stroke Volume

Figure 20–24

Autonomic nervous system: sympatheti

c and parasympathetic

Circulating hormones

Venous return and stretch receptors

Stroke Volume Control Factors

• EDV:– filling time– rate of venous return

• ESV:– preload– contractility– Afterload

Cardiac Reserve-• The difference between resting and

maximal cardiac output

KEY CONCEPT

• Cardiac output:– the amount of blood pumped by the

left ventricle each minute– adjusted by the ANS in response to:

• circulating hormones• changes in blood volume • alterations in venous return

• Most healthy people can increase cardiac output by 300–500%

The Heart and Cardiovascular System

• Cardiovascular regulation:– ensures adequate circulation to body

tissues • Cardiovascular centers:

– control heart and peripheral blood vessels

• Cardiovascular system responds to:– changing activity patterns– circulatory emergencies

SUMMARY (1)• Organization of cardiovascular system:

– pulmonary and systemic circuits

• 3 types of blood vessels:– arteries, veins, and capillaries

• 4 chambers of the heart:– left and right atria– left and right ventricles

• Pericardium, mediastinum, and pericardial sac

• Coronary sulcus and superficial anatomy of the heart

• Structures and cells of the heart wall

SUMMARY (2)• Internal anatomy and structures of the

heart:– septa, muscles, and blood vessels

• Valves of the heart and direction of blood flow

• Connective tissues of the heart• Coronary blood supply• Contractile cells and the conducting

system:– pacemaker calls, nodes, bundles, and

Purkinje fibers

SUMMARY (3)

• Electrocardiogram and its wave forms• Refractory period of cardiac cells• Cardiac cycle:

– atrial and ventricular– systole and diastole

• Cardiodynamics:– stroke volume and cardiac output

• Control of cardiac output