lecture 42

are the pumps that eject blood into the arteries and keep it flowing
around the body. The right ventricle constitutes most of the anterior
portion of the heart

are the pumps that eject blood into the arteries and keep it flowing
around the body. while the left ventricle forms the apex and
inferoposterior portion

The right side of the heart serves the pulmonary circuit. It receives
blood that has circulated through the body and pumps it into
a large artery, the pulmonary trunk. From there, the oxygen-poor
blood is distributed to the lungs, where it unloads carbon dioxide
and picks up a fresh load of oxygen

The beginning of the pulmonary circuit

pulmonary
trunk, a large vessel that ascends diagonally from the right ventricle
and branches into the right and left pulmonary arteries. As it approaches
the lung, the right pulmonary artery branches in two, and
both branches enter the lung at a medial indentation called the
hilum

A valve that consists of crescentshaped
cusps, including the aortic and pulmonary
valves of the heart and valves of the veins and
lymphatic vessels
The
pulmonary valve controls the opening from the right ventricle into
the pulmonary trunk
Each has three cusps shaped
somewhat like shirt pockets. There are no tendinous cords on the
semilunar valves.

The ascending aorta arises from the left ventricle
and immediately gives off the two
coronary arteries to the heart wall. It continues
as the aortic arch, which gives off
three large arteries to the neck, head, and
upper limbs: the brachiocephalic trunk, left
common carotid artery, and left subclavian
artery.

and the aortic valve controls the opening
from the left ventricle into the aorta. Each has three cusps shaped
somewhat like shirt pockets. There are no tendinous cords on the
semilunar valves

listening to heart sounds with a stethoscope

Cardiac muscle is striated like skeletal muscle but otherwise differs
from it in many structural and physiological ways. Cardiac myocytes,
or cardiocytes, are relatively short, thick cells, typically 50 to
100 m long and 10 to 20 m wide (fig. 20.14). The ends of the cell
are slightly branched, like a log with notches in the end. Through its
different end branches, each cardiocyte contacts several other cells,
so collectively they form a network throughout a heart chamber

Cardiac myocytes are said to be autorhythmic15 because they electrically
discharge, or depolarize, spontaneously at regular time
intervals. Some of the myocytes lose the ability to contract and
become specialized, instead, for generating and conducting these
electrical signals

Cardiocytes are joined end to end by thick connections called
intercalated (in-TUR-ku-LAY-ted) discs, which appear as dark
lines (thicker than the striations) in properly stained tissue sections.
An intercalated disc is a complex steplike structure with three
distinctive features not found in skeletal muscle:
1. Interdigitating folds. The plasma membrane at the end of the
cell is folded somewhat like the bottom of an egg carton. The
folds of adjoining cells interlock with each other and increase
the surface area of intercellular contact.
2. Mechanical junctions. The cells are tightly joined by two
types of mechanical junctions—the fascia adherens and
desmosomes. The fascia adherens18 (FASH-ee-ah ad-HEERenz)
is the most extensive. It is a broad band in which the
actin of the thin myofilaments is anchored to the plasma
membrane, and each cell is linked to the next by way of
transmembrane proteins. Thus, the moving myofilaments
of a contracting cell are able to pull indirectly on the
neighboring cells. The fascia adherens is interrupted here and
there by desmosomes. Described in more detail on page 59,
desmosomes are weldlike mechanical junctions between cells.
They prevent the contracting cardiocytes from pulling apart.
3. Electrical junctions. The intercalated discs also contain gap
junctions, which form channels that allow ions to flow from the
cytoplasm of one cell directly into the next (see p. 59 for their
structure). These junctions enable each myocyte to electrically
stimulate its neighbors. Thus, the entire myocardium of the
two atria behaves almost as if it were a single cell, as does the
entire myocardium of the two ventricles. This unified action is
essential for the effective pumping of a heart chamber.

Alternating light and dark bands in
skeletal and cardiac muscle produced by the
pattern of overlapping myofi laments

Cardiocytes have especially large mitochondria,
which make up about 25% of the cell volume, compared to
skeletal muscle mitochondria, which are much smaller and only 2%
of the cell volume.

the heart is autorhythmic because the cardiac myocytes electrically discharge, depolarize spontaneously at regular time intervals

Some of the myocytes lose the ability to contract and
become specialized, instead, for generating and conducting these
electrical signals. These cells constitute the cardiac conduction system,
which controls the route and timing of stimulation to ensure
that the four heart chambers are coordinated with each other. Electrical
signals arise and travel through the conduction system in the
following order (fig. 20.13)
SA node
AV node
AV bunder
Purkinje fibers

The sinoatrial (SA) node. This is a patch of modified
myocytes in the right atrium, just under the epicardium
near the superior vena cava. It serves as the pacemaker that
initiates each heartbeat and determines the heart rate.The sinoatrial (SA) node. This is a patch of modified
myocytes in the right atrium, just under the epicardium
near the superior vena cava. It serves as the pacemaker that
initiates each heartbeat and determines the heart rate.

The atrioventricular (AV) node. This is a similar patch of
modified myocytes located at the lower end of the interatrial
septum near the right AV valve. It acts as an electrical gateway
to the ventricles. All electrical signals traveling to the ventricles
must pass through the AV node because the fibrous skeleton
acts as an insulator that prevents currents from traveling to
the ventricles by any other route.

This
is a cord of modified myocytes that leaves the AV node and
travels to the interventricular septum. It forks into right and
left bundle branches, which enter the septum and descend
toward the apex of the heart.

These are nervelike processes
that arise from the lower end of the bundle branches and turn
upward to spread throughout the ventricular myocardium.
Purkinje fibers distribute the electrical excitation to the
myocytes of the ventricles. They form a more elaborate
network in the left ventricle than in the right.

The relaxation of any chamber is called diastole (dy-
ASS-toe-lee) and allows the chamber to refill

Electrical excitation of a heart chamber induces contraction,
or systole (SIS-toe-lee), which expels blood from the
chamber

an irregularity in the heartbeat

is a condition in which electrical signals cannot travel normally
through the cardiac conduction system because of disease and
degeneration of the conduction system fibers

A system of blood vessels that
serve the wall of the heart...the blood vessels of the heart wall

Immediately after the aorta leaves the left ventricle, it gives off a
right and left coronary artery. The orifices of these two arteries lie deep
in the pockets formed by two of the aortic valve cusps (see fig. 20.8a).
The left coronary artery (LCA) travels through the coronary sulcus
under the left auricle and divides into two branches
The right coronary artery (RCA) supplies the right atrium and sinoatrial
node (pacemaker), then continues along the coronary sulcus
under the right auricle and gives off two branches of its own

Coronary artery disease (CAD) is a narrowing of the coronary arteries resulting
in insufficient blood flow to maintain the myocardium. It is usually
caused by atherosclerosis, a vascular disorder in which fatty deposits
form in an arterial wall, causing arterial degeneration and obstructed
blood flow. The atherosclerotic plaque (atheroma) is composed of lipids,
smooth muscle, and scar tissue, and may progress to a calcified complicated
plaque, causing the arterial walls to become rigid. Myocardial infarction
(heart attack) can occur when the artery becomes so occluded
that cardiac muscle begins to die from lack of oxygen. Partial obstruction
of an artery can cause a temporary sense of heaviness and chest
pain called angina pectoris when the artery constricts.
There are multiple ways in which an atheroma can lead to heart
attack. The atheroma itself may block so much of the artery that blood
flow is insufficient to support the cardiac muscle (fig. 20.12), especially
during exercise when the metabolic need of the myocardium increases
sharply. Platelets often adhere to atheromas and produce blood clots. If
the vessel space (lumen) is already largely closed off by the atheroma,
a blood clot may finish the job. Furthermore, a clot can break free from
the atheroma and block a smaller coronary artery downstream

The thickening and hardening of the walls of the arteries, occurring typically in old age

A degenerative disease of the blood
vessels characterized by the presence of plaques
on the vessel wall composed of lipid, smooth
muscle, and macrophages; can lead to arterial
occlusion, loss of arterial elasticity, hypertension,
heart attack, kidney failure, and stroke.

An X-ray photograph of blood or lymph vessels, made by angiography

(heart attack) can occur when the artery becomes so occluded
that cardiac muscle begins to die from lack of oxygen.