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Chapter 14: Cardiovascular system: The Heart
Location and Surface of the Heart: The heart is situated in the middle mediastinum and is divided
into right and left sides by an obliquely placed, longitudinal septum. Each side consists of an atrium, which
receives blood from the pulmonary veins, and a ventricle, which propels the blood into the arteries. The heart is
situated more in the left side of the thorax than in the right. The adjective cardiac is derived from the Greek
kardia, heart. The cardiac wall consists, from outside inward, of epicardium (visceral pericardium),
myocardium, and endocardium. The epicardium contains variable amounts of fat that tends to aggregate along
vessels and in the grooves on the surface of the heart.
Structure and Function of the Heart: The heart is enclosed in a fibroserous sac termed the
pericardium, which occupies the middle mediastinum. The pericardium and its fluid lubricate the moving
surfaces of the heart. The fibrous pericardium is the outermost layer, and it is firmly bound to the central tendon
of the diaphragm. Extrapericardial fat, which may be visible radiographically, is often found in the angles
between the pericardium and diaphragm on each side. The pericardium is attached to the sternum (by the
sternopericardial ligaments) and is adherent to the mediastinal pleura except where the two are separated by the
phrenic nerves. The serous pericardium is a closed sac, the parietal layer of which lines the inner surface of the
fibrous pericardium and is reflected onto the heart as the visceral layer, or epicardium. The potential space
between the parietal and visceral layers contains a thin film of fluid and is known as the pericardial cavity.
Layers of the Heart Wall: Three layers of tissue form the heart wall. The outer layer of the heart wall
is the epicardium, the middle layer is the myocardium, and the inner layer is the endocardium. Endocarditis
refers to an inflammation of the endocardium and typically involves the heart valves. Most cases are caused by
bacteria (Bacterial endocarditis). The walls of the heart are largely made from myocardium, which is a special
kind of muscle tissue. This muscle is so constructed that it is able to perform the 60 to 70 contractions which the
healthy adult human heart undergoes every minute. On the inside this muscle is provided with a lining of flat
cells called the endocardium, which is direct contact with the blood within the heart. Myocarditis is an
inflammation of the myocardium that usually occurs as a complication of a viral infection, rheumatic fever, or
exposure to radiation or certain medications.
Chambers of the Heart:
Right Atrium: Each atrium has an auricle, which is on the side of each aorta and pulmonary trunk, is
an ear-shaped appendage. (In clinical usage, an atrium is sometimes still called an auricle and an auricle an
auricular appendage.) The inner surfaces of both auricles present muscular ridges termed the pectinate muscles,
whereas the inner aspects of the atria are mostly smooth. The right atrium receives the openings of the superior
and inferior venae cavae. The valve of the inferior vena cava (Eustacian valve) is a variable semilunar fold in
anterior aspect of the orifice. The valve of the coronary sinus (Thebesian valve) is also variable. A ridge, the
crista terminalis, separates the rough-walled part of the left ventricle from the auricular appendage, containing
pectinate muscles. The interatrial septum, seen from the right side, presents an ovoid depression termed the
fossa ovalis, which is bounded (most noticeably on the superior aspect) by a fold called the limbus fossae
ovalis. The upper part of the fossa may be separated from the limbus by a variable aperture, the foramen ovale,
which is the persistence of a fetal interatrial opening. The right atrioventricular orifice is guarded by the
tricuspid valve, which usually can admit three fingers. The atrioventricular (A-V) node is located within the
wall near the center of a triangle (the so-called triangle of Koch) which has a base formed by the attachment of
the septal cusp of the tricuspid valve and an apex at the osteum of the coronary sinus. Damage to the atrium in
this area can result in A-V conduction block.
Right Ventricle: The right ventricle lies anterior to the right atrium, the plane of the tricuspid opening is nearly
vertical, and the blood flows horizontally from the atrium to the ventricle. The tricuspid (right atrioventricular)
valve has three cusps: anterior, posterior, and septal. The associated papillary muscles are classified also as
anterior, posterior, and septal. The septomarginal trabecula (or moderator band) is a bridge-type trabecula that
extends from the interventricular septum to the base of the anterior papillary muscle. Its importance lies in the
fact that it contains Purkinje fibers from the right limb of the atrioventricular bundle. The cavity of the right
ventricle is U-shaped, but with the U on its side. The lower limb of the U, which receives blood from the right
atrium, is the venous, or inflowing, part of the ventricle. The limbs of the U are separated by a muscular ridge
termed the supraventricular crest. The upper limb, or conus arteriosus (or infundibulum), is the arterial, or
outflowing, part of the ventricle, and it ends in the pulmonary trunk. The walls of the conus are usually smooth.
The junction of the conus and pulmonary trunk contains dense fibrous tissue that encircles the pulmonary valve
and is continuous with the cardiac skeleton.
Left Atrium: The left atrium is prolonged on each side as pouches for the openings of the pulmonary veins.
The fossa ovalis appears as a translucent area in the interatrial septum. The upper edge of this region is called
the valve of the foramen ovale. The left atrioventricular orifice is guarded by the mitral valve, which usually can
admit two fingers.
Left Ventricle: The left ventricle is usually characterized by (1) a bicuspid valve, (2) a smooth septal surface,
and (3) the absence of a crest or conus. (In some anomalies of alignment, a morphological left ventricle may be
present on the right side of the heart.) Because the arterial pressure in the systemic circulation is much higher
than in the pulmonary circulation, the left ventricle performs more work; hence the wall of the left ventricle is
usually more than twice as thick as that of the right. The left ventricle lies mostly anterior to the left atrium, the
plane of the mitral opening is nearly vertical, and blood flows obliquely forward from the atrium to the
ventricle. The mitral (left atrioventricular) valve has two major cusps: anterior and posterior. The lower, or
inflowing, part of the left ventricle receives blood from the left atrium. The upper part, which has mainly
fibrous walls, is known as the aortic vestibule, and it ends in the aorta.
Valves of the Heart:
The heart has two types of valves that keep the blood flowing in the correct direction. The valves between the
atria and ventricles are called atrioventricular valves (also called cuspid valves), while those at the bases of the
large vessels leaving the ventricles are called semilunar valves. The right atrioventricular valve is the tricuspid
valve. The left atrioventricular valve is the mitral valve (also called the bicuspid valve). The valve between the
right ventricle and pulmonary trunk is the pulmonary valve. The valve between the left ventricle and the aorta is
the aortic valve. Both the pulmonary and aortic valves are semilunar valves. When the ventricles contract,
atrioventricular valves close to prevent blood from flowing back into the atria. When the ventricles relax,
semilunar valves close to prevent blood from flowing back into the ventricles.
To summarize:
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The tricuspid valve regulates blood flow between the right atrium and right ventricle
The pulmonary valve controls blood flow from the right ventricle into the pulmonary arteries, which
carry blood to the lungs to pick up oxygen
The mitral valve lets oxygen-rich blood from the lungs pass from the left atrium into the left ventricle.
The aortic valve opens the way for oxygen-rich blood to pass from the left ventricle into the aorta, the
body's largest artery, where it is delivered to the rest of your body
Circulation of Blood:
The Systemic and Pulmonary Circulations:
The superior and inferior venae cava and the intrinsic veins of the heart discharge venous blood
into the right atrium via the coronary sinus. The blood then enters the right ventricle, from which it is
ejected into the pulmonary trunk, which, by way of the right and left pulmonary arteries, delivers blood
to the lungs. The pulmonary veins return blood to the left atrium. The blood then enters the left ventricle
and is ejected into the aorta. Four important valves are encountered: the right and left atrioventricular,
known as the tricuspid and mitral valves, respectively, and the right and left semilunar, known as the
pulmonary and aortic valves, respectively. Insufficient closure results in an incompetent valve and reflux
of blood.
Coronary Circulation:
The myocardium of the heart wall is a working muscle that needs a continuous supply of oxygen
and nutrients to function with efficiency. For this reason, cardiac muscle has an extensive network of blood
vessels to bring oxygen to the contracting cells and to remove waste products.
The right and left coronary arteries, branches of the ascending aorta, supply blood to the walls of the
myocardium. After blood passes through the capillaries in the myocardium, it enters a system of cardiac
(coronary) veins. Most of the cardiac veins drain into the coronary sinus, which opens into the right atrium.
Coronary Arteries:
Either of two arteries – the right coronary artery and the left coronary artery – that arise one from
the right and one from the left side of the aorta immediately above the semilunar valves. The coronary
arteries wrap around the surface of the heart and supply the heart with essential oxygen and nutrients.
They form no effective anastomoses (communicating openings) with other arteries, so that any blockage
of a coronary artery is immediately life-threatening. The coronary arteries spring from dilations
(narrowings) of the aorta called the sinuses of the aorta, of which there is one anterior and two
posterior. The right coronary artery arises from the anterior sinus, and the left from the left posterior
sinus.
The right coronary artery passes forward between the pulmonary trunk and the auricle of the right atrium, and
runs downward, in the atrio-ventricular groove, to the lower part of the right margin of the heart, round which it
curves. It then proceeds to the left, in the posterior part of the atrio-ventricular groove, and ends by
anastomosing with the left coronary artery. During its course, it gives branches to the walls of the right atrium
and ventricle. The most constant of these are: (1) a slender marginal artery, which runs from right to left along
the lower margin of the front of the heart; and (2) an interventricular branch, which runs forward on the
diaphragmatic surface.
The left coronary artery runs for a short distance toward the left behind the pulmonary trunk, and then forward
to appear between the pulmonary trunk and the auricle of the left atrium; it next curves backward and
downward in the left part of the atrio-ventricular groove to reach the lower border of the base of the heart;
where it ends by anastomosing with the right coronary. Its branches supply the roots of the aorta and pulmonary
trunk and the walls of the left atrium and ventricle. The left coronary artery typically runs for 1 to 25 mm before
bifurcating into the anterior interventricular artery (also called left anterior descending (LAD) artery) and
the left circumflex artery (LCX). Occasionally, an additional artery arises at the division of the left main
artery, which is known as the intermediate artery.
Coronary veins:
After blood passes through the arteries by coronary circulation, it then goes into capillaries
where it sends oxygen and nutrients to the heart muscle and gathers carbon dioxide and wastes.
After going through the capillaries, the blood then enters into veins. The majority of the
deoxygenated blood from the myocardium gets deposited into the coronary sinus which empties
into the right atrium. The main tributaries carrying blood into the coronary sinus include the
following:
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Great cardiac vein: Begins with the anterior interventricular sulcus, and gets drained in
the areas of the heart supplied by the left coronary artery
Middle cardiac vein: Starts with the posterior interventricular sulcus, and drains the
areas supplied by the posterior interventricular branch of the right coronary artery.
Small cardiac: vein in the coronary sulcus, that gets deposited into the right atrium and
right ventricle
Anterior cardiac veins: drains into the right ventricle and opens into the right atrium.
Cardiac Conduction System:
The conducting system consists of specialized (i.e., impulse-conducting) muscle fibers that connect
certain "pacemaker" regions of the heart with ordinary "working" cardiac muscle fibers. The intrinsic, rhythmic
contractions of cardiac muscle fibers are regulated by pacemakers, and the intrinsic rhythmicity of the
pacemakers is regulated in turn by nerve impulses from vasomotor centers in the brain stem. If the conducting
system between the atria and ventricles is destroyed (complete heart block), the atria and ventricles beat at
different rates.
The conducting system comprises the sinuatrial (S.A.) node, the atrioventricular (A.V.) node, and the A. V.
bundle, with its two limbs and the subendocardial plexus of Purkinje fibers. The impulse begins at the S.A.
node, activates the atrial musculature, and is thereby conveyed to the A. V. node. Special bundles (internodal
tracts) of atrial muscle fibers pass more or less directly from the S.A. to the A.V. node, but their functional
significance is unclear.
The S.A. node is the usual pacemaker for the heart. It is situated anterolaterally at the junction of the superior
vena cava and the right atrium, near the upper end of the sulcus terminalis and immediately beneath the
epicardium. It consists of a network of specialized cardiac muscle fibers, which are continuous with the atrial
muscle fibers. The node is supplied by autonomic fibers and by a branch of the right (sometimes the left)
coronary artery.
The Hearts electrical system:
Each electrical signal in the heart begins in a group of cells called the sinus node or sinoatrial (SA) node.
The SA node is located in the right atrium, which is the upper right chamber of the heart. In a healthy adult
heart at rest, the SA node fires off an electrical signal to begin a new heartbeat 60 to 100 times a minute.
From the SA node, the signal travels to the right and left atria. This causes the atria to contract and pump blood
into the heart's two lower chambers, the ventricles. This is recorded as the P wave on an EKG.
The signal passes between the atria and ventricles through a group of cells called the atrioventricular (AV)
node. The signal slows down as it passes through the AV node. This slowing allows the ventricles time to finish
filling with blood. On an EKG, this is the flat line between the end of the P wave and beginning of the Q wave.
The electrical signal then leaves the AV node and travels along a pathway called the bundle of His. From there
the signal travels into the right and left bundle branches. On an EKG, this is the Q wave.
As the signal spreads across the right and left ventricles, they contract and pump blood out to the lungs and the
rest of the body. On an EKG, R marks the contraction of the left ventricle and S marks the contraction of the
right ventricle.
The ventricles then relax (shown as the T wave on an EKG). This entire process continues over and over with
each new heartbeat.
Heart Sounds:
The rhythmic noises accompanying heartbeat are called heart sounds. Normally, two distinct sounds are
heard through the stethoscope: a low, slightly prolonged “lub” (first sound) occurring at the beginning of
ventricular contraction, or systole, and produced by closure of the mitral and tricuspid valves, and a sharper,
higher-pitched “dup” (second sound), caused by closure of aortic and pulmonary valves at the end of systole.
Occasionally audible in normal hearts is a third soft, low-pitched sound coinciding with early diastole and
thought to be produced by vibrations of the ventricular wall. A fourth sound, also occurring during diastole, is
revealed by graphic methods but is usually inaudible in normal subjects; it is believed to be the result of atrial
contraction and the impact of blood, expelled from the atria, against the ventricular wall.
Exercise and the Heart:
Whether a person is conditioned or de-conditioned, a person’s cardiovascular fitness can be improved
regardless of age with regular exercise. Doing aerobics for at least 20 minutes that focuses on large muscle
groups will elevate cardiac output and accelerates metabolic rate. During intense activity, a trained athlete
could potentially achieve a cardiac output double that of a sedentary person because training, especially
cardiovascular/cardio-respiratory causes the heart to enlarge. Interestingly enough, a heart of a well-trained
athlete is bigger but the resting cardiac output is about the same as in a healthy untrained person. Exercising on
a regular basis greatly lowers blood pressure, anxiety, and depression; control weight; and will help increase the
body’s ability to dissolve blood clots.
Coronary Artery Disease (CAD):
CAD is a serious medical problem that annually affects about 7 million people. Responsible for nearly
three quarters of a million deaths in the United States each year, it is the leading cause of death for both men
and woman. CAD is defined as the effects of accumulation of atherosclerotic plaques (described shortly) in
coronary arteries that lead to a reduction in blood flow to the myocardium.
When plaque builds up in arteries, the condition is called atherosclerosis. Over time, plaque hardens and
narrows the arteries. This limits the flow of oxygen-rich blood to your organs and other parts of your body.
Atherosclerosis can affect any artery in the body. For example, when plaque builds up in the coronary arteries, a
heart attack can occur. When plaque builds up in the carotid arteries, a stroke can occur. A stroke also can occur
if blood clots form in the carotid arteries. This can happen if, over time, the plaque in an artery cracks or
ruptures. Blood cells called platelets stick to the site of the injury and may clump together to form blood clots.
Blood clots can partly or fully block a carotid artery.
Cholesterol travels in the bloodstream in small packages called lipoproteins. Two major kinds of lipoproteins
carry cholesterol throughout your body: Low-density lipoproteins (LDL). LDL cholesterol is sometimes called
"bad" cholesterol. This is because it carries cholesterol to tissues, including your heart arteries. The higher the
level of LDL cholesterol in your blood, the greater your risk for heart disease. High-density lipoproteins (HDL).
HDL cholesterol is sometimes called "good" cholesterol. This is because it helps remove cholesterol from your
arteries. A low HDL cholesterol level raises your risk for heart disease. A number of factors affect your blood
cholesterol levels. For example, after menopause, women's LDL cholesterol levels tend to rise and their HDL
cholesterol levels tends to fall. Other factors, such as age, gender, and diet, also affect your cholesterol levels.
Healthy levels of both LDL and HDL cholesterol will prevent plaque from building up in your arteries. Routine
blood tests can show whether your blood cholesterol levels are healthy.