Review
20.1 Anatomy of the Heart
1.
The heart is located in the mediastinum; about two‐thirds of its mass is to the left of the midline. It
is shaped like a cone lying on its side. Its apex is the pointed, inferior part; its base is the broad,
superior part.
2. The pericardium is the membrane that surrounds and protects the heart; it consists of an outer
fibrous layer and an inner serous pericardium, which is composed of a parietal and a visceral layer.
Between the parietal and visceral layers of the serous pericardium is the pericardial cavity, a
potential space filled with a few milliliters of lubricating pericardial fluid that reduces friction
between the two membranes.
3. Three layers make up the wall of the heart: epicardium (visceral layer of the serous pericardium),
myocardium, and endocardium. The epicardium consists of mesothelium and connective tissue, the
myocardium is composed of cardiac muscle tissue, and the endocardium consists of endothelium
and connective tissue.
4. The heart chambers include two superior chambers, the right and left atria, and two inferior
chambers, the right and left ventricles. External features of the heart include the auricles (flaps of
each atrium that slightly increase their volume), the coronary sulcus between the atria and
ventricles, and the anterior and posterior sulci between the ventricles on the anterior and posterior
surfaces of the heart, respectively.
5. The right atrium receives blood from the superior vena cava, inferior vena cava, and coronary
sinus. It is separated internally from the left atrium by the interatrial septum, which contains the
fossa ovalis. Blood exits the right atrium through the tricuspid valve.
6. The right ventricle receives blood from the right atrium. It is separated internally from the left
ventricle by the interventricular septum and pumps blood through the pulmonary valve into the
pulmonary trunk.
7. Oxygenated blood enters the left atrium from the pulmonary veins and exits through the bicuspid
(mitral) valve.
8. The left ventricle pumps oxygenated blood through the aortic valve into the aorta.
9. The thickness of the myocardium of the four chambers varies according to the chamber's function.
The left ventricle, with the highest workload, has the thickest wall.
10. The fibrous skeleton of the heart is dense connective tissue that surrounds and supports the valves
of the heart.
20.2 Heart Valves and Circulation of Blood
1. Heart valves prevent backflow of blood within the heart. The atrioventricular (AV) valves, which lie
between atria and ventricles, are the tricuspid valve on the right side of the heart and the bicuspid
(mitral) valve on the left. The semilunar (SL) valves are the aortic valve, at the entrance to the aorta,
and the pulmonary valve, at the entrance to the pulmonary trunk.
2. The left side of the heart is the pump for systemic circulation, the circulation of blood throughout
the body except for the air sacs of the lungs. The left ventricle ejects blood into the aorta, and blood
then flows into systemic arteries, arterioles, capillaries, venules, and veins, which carry it back to the
right atrium.
3. The right side of the heart is the pump for pulmonary circulation, the circulation of blood through
the lungs. The right ventricle ejects blood into the pulmonary trunk, and blood then flows into
pulmonary arteries, pulmonary capillaries, and pulmonary veins, which carry it back to the left
atrium.
Review
4. The coronary circulation provides blood flow to the myocardium. The main arteries of the coronary
circulation are the left and right coronary arteries; the main veins are the cardiac veins and the
coronary sinus.
20.3 Cardiac Muscle Tissue and the Cardiac Conduction System
1. Cardiac muscle fibers usually contain a single centrally located nucleus. Compared with skeletal
muscle fibers, cardiac muscle fibers have more and larger mitochondria, slightly smaller
sarcoplasmic reticulum, and wider transverse tubules, which are located at Z discs.
2. Cardiac muscle fibers are connected end‐to‐end via intercalated discs. Desmosomes in the discs
provide strength, and gap junctions allow muscle action potentials to conduct from one muscle fiber
to its neighbors.
3. Autorhythmic fibers form the conduction system, cardiac muscle fibers that spontaneously
depolarize and generate action potentials.
4. Components of the conduction system are the sinoatrial (SA) node (pacemaker), atrioventricular
(AV) node, atrioventricular (AV) bundle (bundle of His), bundle branches, and Purkinje fibers.
5. Phases of an action potential in a ventricular contractile fiber include rapid depolarization, a long
plateau, and repolarization.
6. Cardiac muscle tissue has a long refractory period, which prevents tetanus.
7. The record of electrical changes during each cardiac cycle is called an electrocardiogram (ECG). A
normal ECG consists of a P wave (atrial depolarization), a QRS complex (onset of ventricular
depolarization), and a T wave (ventricular repolarization).
8. The P–Q interval represents the conduction time from the beginning of atrial excitation to the
beginning of ventricular excitation. The S–T segment represents the time when ventricular
contractile fibers are fully depolarized.
20.4 The Cardiac Cycle
1. A cardiac cycle consists of the systole (contraction) and diastole (relaxation) of both atria, plus the
systole and diastole of both ventricles. With an average heartbeat of 75 beats/min, a complete
cardiac cycle requires 0.8 sec.
2. The phases of the cardiac cycle are (a) atrial systole, (b) ventricular systole, and (c) relaxation
period.
3. S1, the first heart sound (lubb), is caused by blood turbulence associated with the closing of the
atrioventricular valves. S2, the second sound (dupp), is caused by blood turbulence associated with
the closing of semilunar valves.
20.5 Cardiac Output
1. Cardiac output (CO) is the amount of blood ejected per minute by the left ventricle into the aorta (or
by the right ventricle into the pulmonary trunk). It is calculated as follows: CO (mL/min) = stroke
volume (SV) in mL/beat × heart rate (HR) in beats/min.
2. Stroke volume (SV) is the amount of blood ejected by a ventricle during each systole.
3. Cardiac reserve is the difference between a person's maximum cardiac output and his or her cardiac
output at rest.
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4. Stroke volume is related to preload (stretch on the heart before it contracts), contractility
(forcefulness of contraction), and afterload (pressure that must be exceeded before ventricular
ejection can begin).
5. According to the Frank–Starling law of the heart, a greater preload (end‐diastolic volume) stretching
cardiac muscle fibers just before they contract increases their force of contraction until the stretching
becomes excessive.
6. Nervous control of the cardiovascular system originates in the cardiovascular center in the medulla
oblongata.
7. Sympathetic impulses increase heart rate and force of contraction; parasympathetic impulses
decrease heart rate.
8. Heart rate is affected by hormones (epinephrine, norepinephrine, thyroid hormones), ions (
,
,
), age, gender, physical fitness, and body temperature.
20.6 Exercise and the Heart
1. Sustained exercise increases oxygen demand on muscles.
2. Among the benefits of aerobic exercise are increased cardiac output, decreased blood pressure, weight
control, and increased fibrinolytic activity.
20.7 Help for Failing Hearts
1. A cardiac (heart) transplant is the replacement of a severely damaged heart with a normal one.
2. Cardiac assist devices and procedures include the intra‐aortic balloon pump, the ventricular assist
device, cardiomyoplasty, and a skeletal muscle assist device.
20.8 Development of the Heart
1. The heart develops from mesoderm.
2. The endocardial tubes develop into the four‐chambered heart and great vessels of the heart.