Download Chapter 17 The cardivascular system I the heart

Chapter 17 / The Cardiovascular System: The Heart
I. INTRODUCTION
A. The heart is the center of the cardiovascular system. This chapter explores the
design
of the heart and the unique properties of cardiac muscle that permit a lifetime of
pumping with never a minute's rest.
B. The study of the normal heart and diseases associated with it is known as
cardiology.
II. OVERVIEW OF THE CIRCULATION
A. Once blood has been oxygenated in the lungs it passes through a series of channels
via
the heart and body before returning to the heart, all together called the systemic
circulation ,which includes;
1. pulmonary veins,
2. left atrium and ventricle,
3. aorta and progressively smaller arterial branches,
4. capillaries (where exchange with the tissues occurs),
5. venules,
6. progressively larger veins, and
7. vena cava.
B. The deoxygenated blood containing high levels of carbon dioxide from the tissues
returned to the right heart passes through the pulmonary circulation, which
includes;
1. the right atrium and ventricle,
2. pulmonary artery, and
3. arteries and capillaries of the lungs.
III. LOCATION AND SIZE OF THE HEART
A. The heart is situated between the lungs in the mediastinum.
B. About two-thirds of its mass is to the left of the midline.
C. The heart is about 12 cm long, 9 cm wide, and 6 cm thick.
IV. PERICARDIUM
A. The heart is enclosed and held in place by the pericardium.
1. The pericardium consists of an outer fibrous pericardium and an inner serous
pericardium.
2. The serous pericardium is composed of a parietal layer and a visceral layer.
3. Between the parietal and visceral layers of the serous pericardium is the
pericardial
cavity, a potential space filled with pericardial fluid that reduces friction
between the
two membranes.
B. An inflammation of the pericardium is known as pericarditis. Associated bleeding
into
the pericardial cavity compresses the heart (cardiac tamponade) and is potentially
lethal.
V. HEART WALL
A. The wall of the heart has three layers: epicardium, myocardium, and endocardiurn
B. The epicardium consists of mesothelium and connective tissue, the myocardium is
composed of cardiac muscle tissue, and the endocardiurn consists of endothelium
and
connective tissue.
VI. CHAMBERS OF THE HEART
A. The chambers of the heart include two upper atria and two lower ventricles.
B. An interatrial septum separates the atria; an interventricular septum separates the
ventricles.
VII. VALVES OF THE HEART
A. Valves, composed of dense connective tissue covered by endothelium, prevent
backflow of blood in the heart.
1. Atrioventricular (AV) valves, between the atria and their ventricles, are the
tricuspid
valve on the right side of the heart and the bicuspid (mitral) valve on the left.
2. The chordae tendineae and their papillary muscles keep the flaps of the valves
pointing in the direction of the blood flow and stop blood from backing into the
atria.
3. Semilunar valves prevent blood from flowing back into the heart as it leaves the
heart for the lungs (pulmonary semilunar valve) or for the rest of the body
(aortic
semilunar valve).
B. Rheumatic fever is precipitated by infection with group A, alphahemolytic strains
of
Streptococcus pyogenes bacteria. The ultimate result is damage to valves of the
heart,
most commonly the bicuspid and aortic semilunar valves.
VIII. BLOOD FLOW THROUGH THE PULMONARY AND SYSTEMIC
CIRCULATIONS
A. Blood flows through the heart from the superior and inferior venae cavae and the
coronary
sinus to the right atrium, through the tricuspid valve to the right ventricle, through
the
pulmonary trunk and pulmonary arteries to the lungs, through the pulmonary veins
into the
left atrium, through the bicuspid valve to the left ventricle, and out through the
aorta.
B. Divisions of the aorta are the ascending aorta, arch of the aorta, thoracic aorta, and
abdominal aorta.
C. Once blood has been oxygenated in the lungs it passes through a series of channels
via
the heart and body before returning to the heart, all together called the systemic
circulation, which includes;
1. pulmonary veins,
2. left atrium and ventricle,
3. aorta and progressively smaller arterial branches,
4. capillaries (where exchange with the tissues occurs),
5. venules,
6. progressively larger veins, and
7. vena cava.
D. The deoxygenated blood containing high levels of carbon dioxide from the tissues
returned
to the right heart passes through the pulmonary circulation, which includes;
1. the right atrium and ventricle,
2. pulmonary artery, and
3. arteries and capillaries of the lungs.
IX. HEART BLOOD SUPPLY
A. The flow of blood through the many vessels that pierce the myocardium of the
heart is
called the coronary (cardiac) circulation; it delivers oxygenated blood and
nutrients to the
myocardiurn and removes carbon dioxide and wastes from it.
B. The principal arteries, branching from the ascending aorta and carrying
oxygenated blood,
are the right and left coronary arteries; deoxygenated blood returns to the right
atrium
primarily via the principal vein, the coronary sinus.
C. Most heart problems result from faulty coronary circulation due to blood clots,
fatty
atherosclerotic plaques, or spasms of the smooth muscle in coronary artery walls.
Complications of this system include angina pectoris (severe pain that accompanies
reduced blood flow, or ischernia, to the myocardium) and myocardial infarction
(MI, or heart attack, in which there is death of an area of the myocardium due to
an
interruption of the blood supply; it may result from a thrombus or embolus).
D. Whenever a disease or injury deprives a tissue of oxygen, reestablishing the blood
flow (reperfusion) may damage the tissue further due to the formation of oxygen
free
radicals; these can destabilize the molecular structure of proteins,
neurotransmitters,
nucleic acids, and phospholipids of plasma membranes.
X. CONDUCTION SYSTEM AND PACEMAKER
A. The conduction system consists of tissue specialized for generation and conduction
of
spontaneous action potentials that stimulate the cardiac muscle fibers (cells) to
contract.
B. Components of this system are the sinoatrial (SA) node (pacemaker),
atrioventricular
(AV) node, atrioventricular (AV) bundle (bundle of His), right and left bundle
branches,
and the conduction myofibers (Purkinje fibers)
C. Signals from the autonomic nervous system and hormones, such as epinephrine, do
modify the heartbeat (in terms of rate and strength of contraction), but they do not
establish the fundamental rhythm.
D. An artificial pacemaker may be used to restore cardiac rhythm due to disruption of
some
component of the conduction system.
XI. PHYSIOLOGY OF CARDIAC MUSCLE CONTRACTION
A. An impulse in a ventricular contractile fiber is characterized by rapid
depolarization,
plateau, and repolarization.
B. The refractory period of a cardiac muscle fiber (the time interval when a second
contraction cannot be triggered) is longer than the contraction itself.
XII. ELECTROCARDIOGRAM
A. Impulse conduction through the heart generates electrical currents that can be
detected at the surface of the body. A recording of the electrical changes that
accompany each cardiac cycle (heartbeat) is called an electrocardiogram (ECG or
EKG).
B. A normal ECG consists of a P wave (atrial depolarization-spread of impulse from
SA node over atria), QRS complex (ventricular depolarization-spread of impulse
through ventricles), and T wave (ventricular repolarization).
C. The P-Q (PR) interval represents the conduction time from the beginning of atrial
excitation to the beginning of ventricular excitation.
D. The S-T segment represents the time when ventricular contractile fibers are fully
depolarized, during the plateau phase of the impulse.
E. The ECG is invaluable in diagnosing abnormal cardiac rhythms and conduction
patterns and following the course of recovery from a heart attack.
XIII. CARDIAC CYCLE
A. A cardiac cycle consists of the systole (contraction) and diastole (relaxation) of
both
atria, rapidly followed by the systole and diastole of both ventricles.
B. The phases of the cardiac cycle are (1) the relaxation (or quiescent) period, (2)
ventricular filling, and (3) ventricular systole.
C. With an average heart rate of 75 beats/riiin, a complete cardiac cycle requires 0.8
sec.
D. The act of listening to sounds within the body is called auscultation, and it is
usually
done with a stethoscope. The sound of a heartbeat comes primarily from the
turbulence in blood flow caused by the closure of the valves, not from the
contraction
of the heart muscle.
1. The first heart sound (S1 --lubb) is created by blood turbulence associated with
the
closing of the atrioventricular valves soon after ventricular systole begins.
2. The second heart sound (S2--dupp) represents the closing of the semilunar
valves
close to the end of the ventricular systole.
3. A heart murmur is an abnormal sound that consists of a flow noise that is heard
before, between, or after the lubb-dupp or that may mask the normal sounds
entirely.
Some murmurs are caused by turbulent blood flow around valves due to
abnormal
anatomy or increased volume of flow.
a. Not all murmurs are abnormal or symptomatic, but most indicate a valve
disorder.
b. Among the valvular disorders that may contribute to murmurs are mitral
stenosis,
mitral insufficiency, aortic stenosis, aortic insufficiency, and mitral valve
prolapse
(MVP).
XIV. CARDIAC OUTPUT
A. Since the body's need for oxygen varies with the level of activity, the heart's
ability to
discharge oxygen-carrying blood must also be variable. Body cells need specific
amounts of blood each minute to maintain health and life.
B. Cardiac output (CO) is the amount of blood ejected by the left ventricle (or right
ventricle)
into the aorta (or pulmonary trunk) per minute. It is calculated as follows:
CO = stroke volume x beats per minute.
1. Stroke volume (SV) is the amount of blood ejected by a ventricle during each
systole.
a. Stroke volume depends on how much blood enters a ventricle during
diastole, that
is, the stretch on the heart before it contracts (end-diastolic volume, or EDV,
also
called preload), and how much blood is left in a ventricle following its
systole (endsystolic volume, or ESV). EDV averages 120-130 ml and SV is about 70 ml;
ESV,
then, is approximately 50-60 ml.
b. Stroke volume also is related to contractility (forcefulness of contraction at
any
given time) and afterload (pressure that must be exceeded before ventricular
ejection
can begin); factors that increase the stroke volume or heart rate tend to
increase CO
and vice versa.
c. Cardiac reserve is the ratio between the maximum cardiac output a person
can
achieve and the cardiac output at rest.
d. According to the Frank-Starling law of the heart, a greater preload (stretch)
on
cardiac muscle fibers just before they contract increases their force of
contraction
during systole.
e. Congestive heart failure (CHF) results when the heart cannot supply the
oxygen
demands of the body; it is characterized by diminished blood flow to the
various
tissues of the body and by accumulation of excess blood in the various organs
because the heart is unable to pump out the blood returned to it by the great
veins.
Causes include chronic hypertension and myocardial infarction (heart attack).
2. Cardiac output depends on heart rate as well as stroke volume. Changing heart
rate is
the body's principal mechanism of short-term control over cardiac output and
blood
pressure; several factors contribute to regulation of heart rate.
a. Nervous system control of the cardiovascular system stems from the
cardiovascular
center in the medulla
1) Sympathetic impulses increase heart rate and force of contraction;
parasympathetic
impulses decrease heart rate.
2) Baroreceptors (pressure receptors) are nerve cells that respond to changes in
blood pressure and relay the information to the cardiovascular center;
important
baroreceptors are located in the arch of the aorta and carotid arteries.
b. Heart rate is also affected by hormones (epinephrine, norepinephrine, thyroid
hormones), ions (Na+, K+, Ca2+), age, gender, physical fitness, and
temperature.
C. Help for Failing Hearts
1. Although a variety of drugs are helpful in the earlier stages of heart disease, at
some
point they are no longer effective because there is too little functional cardiac
muscle left.
2. Researchers are investigating a wide variety of devices and techniques to aid a
failing
heart, including heart transplants, artificial hearts, and cardiac assist devices.
XV. RISK FACTORS IN HEART DISEASE
A. Risk factors in heart disease that can be modified include high blood cholesterol
level,
high blood pressure, cigarette smoking, obesity, and lack of regular exercise.
B. Other factors include diabetes mellitus; genetic predisposition; male gender; high
blood
levels of fibrinogen, renin, and uric acid; and left ventricular hypertrophy.
XVI. PLASMA LIPIDS AND HEART DISEASE
A. A strong risk factor for developing heart disease is high blood cholesterol level.
The
reason is that high blood cholesterol promotes growth of fatty plaques that build
up in
the walls of arteries.
B. Most lipids are transported in the blood in combination with proteins as
lipoproteins.
1. Three classes of lipoproteins are called low-density lipoproteins (LDLs), high
density lipoproteins (HDLs), and very low-density lipoproteins (VLDLs).
2. Whereas HDLs remove excess cholesterol from circulation, high levels of
LDLs
are associated with the formation of fatty plaques in arteries. VLDLs also
contribute
to increased fatty plaque formation.
C. There are two sources of cholesterol in the body: some is present in foods we
ingest, but
most is synthesized by the liver.
D. For adults, desirable levels of blood cholesterol are TC (total cholesterol) under
200 mg/dl, LDL under 130 mg/dl, and HDL over 40 mg/dl; normally,
triglycerides are in
the range of 10- 190 mg/dl.
E. Among the therapies used to reduce blood cholesterol level are exercise, diet, and
drugs.
XVII. EXERCISE AND THE HEART
A. Sustained exercise increases oxygen demand in muscles.
B. Among the benefits of aerobic exercise (any activity that works large body
muscles for at
least 20 minutes, preferably 3-5 times per week) are increased cardiac output,
increased
HDL and decreased triglycerides, improved lung function, decreased blood
pressure, and
weight control.
XVIII. DEVELOPMENTAL ANATOMY OF THE HEART
A. The heart develops from mesoderm before the end of the third week of gestation.
B. The endothelial tubes develop into the four-chambered heart and great vessels of
the heart.
XIX. DISORDERS: HOMEOSTATIC IMBALANCES
A. Coronary artery disease (CAD), or coronary heart disease (CHD), is a condition
in which
the heart muscle receives an inadequate amount of blood due to obstruction of its
blood
supply. It is the leading cause of death in the United States each year. The
principal causes
of narrowing include atherosclerosis, coronary artery spasm, or a clot in a
coronary artery.
1. Atherosclerosis is a process in which smooth muscle cells proliferate and fatty
substances, especially cholesterol and triglycerides (neutral fats), accumulate in
the walls
of medium-sized and large arteries in response to certain stimuli, such as
endothelial
damage.
2. Treatment of coronary artery disease varies with the nature and urgency of
symptoms.
Among the treatment options are drug therapy (nitroglycerin, beta blockers, and
thrombolytic agents) and various surgical and nonsurgical procedures.
Examples include
coronary artery bypass grafting (CABG), percutaneous transluminal coronary
angioplasty (PTCA), percutaneous balloon valvuloplasty, laser angioplasty,
balloon-laser
welding, and catheter arthrectomy.
3. Coronary artery spasm is a condition in which the smooth muscle of a coronary
artery
undergoes a sudden contraction, resulting in narrowing of a blood vessel.
B. Congenital Defects
1. A congenital defect is a defect that exists at birth, and usually before birth.
2. Congenital defects of the heart include coarctation of the aorta , patent ductus
arteriosus, septal defects (interatrial or interventricular; valvular stenosis, and
tetralogy of Fallot.
a. Some congenital defects are not serious or remain asymptomatic; others heal
themselves.
b. A few congenital defects are life threatening and must be corrected
surgically.
Fortunately, surgical techniques are highly refined for most of the defects
listed.
C. Arrhythmias
1. An arrhythmia is an abnormality or irregularity in the heart rhythm resulting
from a
disturbance in the conduction system of the heart, due either to faulty
production of
electrical impulses or to poor conduction of impulses as they pass through the
system.
2. Examples of arrhythmias include heart block (most commonly atrioventricular
block,
flutter, fibrillation (atrial and ventricular), and premature ventricular
contraction (PVC).
a. Some are caused by the effects of such factors as caffeine, nicotine, alcohol,
anxiety, certain drugs, hyperthyroidism, potassium deficiency, and certain
heart
diseases.
b. Most can be controlled if detected and treated early enough.