612
UNIT FOUR
6. Regulation of the cardiac cycle
a. Physical exercise, body temperature, and
concentration of various ions affect heartbeat.
b. Branches of sympathetic and parasympathetic
nerve F
bers innervate the SA and AV nodes.
(1) Parasympathetic impulses decrease heart
action; sympathetic impulses increase heart
action.
(2) The cardiac center in the medulla oblongata
regulates autonomic impulses to the heart.
15.4
BLOOD VESSELS (PAGE 573)
The blood vessels form a closed circuit of tubes that
transports blood between the heart and body cells. The
tubes include arteries, arterioles, capillaries, venules, and
veins.
1. Arteries and arterioles
a. The arteries are adapted to carry blood under
relatively high pressure away from the heart.
b. The arterioles are branches of arteries.
c. The walls of arteries and arterioles consist of layers of
endothelium, smooth muscle, and connective tissue.
d. Autonomic F bers innervate smooth muscles in
vessel walls.
2. Capillaries
Capillaries connect arterioles and venules. The capillary
wall is a single layer of cells that forms a semipermeable
membrane.
a. Capillary permeability
(1) Openings in the capillary walls are thin slits
between endothelial cells.
(2) The sizes of the openings vary from tissue to
tissue.
(3) Endothelial cells of brain capillaries are tightly
fused, forming a blood-brain barrier through
which substances move by facilitated diffusion.
b. Capillary arrangement
Capillary density varies directly with tissue
metabolic rates.
c. Regulation of capillary blood fl ow
(1) Precapillary sphincters regulate capillary blood
fl ow.
(2) Precapillary sphincters open when cells are
low in oxygen and nutrients and close when
cellular needs are met.
d. Exchanges in the capillaries
(1) Gases, nutrients, and metabolic by-products
are exchanged between the capillary blood and
the tissue fl
uid.
(2) Diffusion provides the most important means
of transport.
(3) Diffusion pathways depend on lipid
solubilities.
(4) Plasma proteins generally remain in the blood.
(5) ±iltration, due to the hydrostatic pressure of
blood, causes a net outward movement of fl
uid
at the arteriolar end of a capillary.
(6) Osmosis due to colloid osmotic pressure
causes a net inward movement of fl
uid at the
venular end of a capillary.
(7) Some factors cause fl
uids to accumulate in the
tissues.
5. Skeleton of the heart
a. The skeleton of the heart consists of F
brous rings
that enclose the bases of the pulmonary artery,
aorta, and atrioventricular oriF
ces.
b. The F brous rings provide attachments for valves
and muscle F bers and prevent the oriF ces from
excessively dilating during ventricular contractions.
6. Path of blood through the heart
a. Blood low in oxygen and high in carbon dioxide
enters the right side of the heart from the venae
cavae and coronary sinus and then is pumped into
the pulmonary circulation.
b. After the blood is oxygenated in the lungs and some
of its carbon dioxide is removed, it returns to the left
side of the heart through the pulmonary veins.
c. ±rom the left ventricle, it moves into the aorta.
7. Blood supply to the heart
a. The coronary arteries supply blood to the
myocardium.
b. It is returned to the right atrium through the
cardiac veins and coronary sinus.
15.3
HEART ACTIONS (PAGE 564)
1. Cardiac cycle
a. The atria contract (atrial systole) while the
ventricles relax (ventricular diastole); the
ventricles contract (ventricular systole) while the
atria relax (atrial diastole).
b. Pressure in the chambers rises and falls in cycles.
2. Heart sounds
a. Heart sounds can be described as
lubb-dupp.
b. Heart sounds are due to the vibrations that the
valve movements produce.
c. The F rst part of the sound occurs as AV valves
close, and the second part is associated with the
closing of pulmonary and aortic valves.
3. Cardiac muscle F
bers
a. Cardiac muscle F
bers connect to form a functional
syncytium.
b. If any part of the syncytium is stimulated, the
whole structure contracts as a unit.
c. Except for a small region in the fl
oor of the right
atrium, the F
brous skeleton separates the atrial
syncytium from the ventricular syncytium.
4. Cardiac conduction system
a. This system, composed of specialized
cardiac muscle tissue, initiates and conducts
depolarization waves through the myocardium.
b. Impulses from the SA node pass slowly to the AV
node; impulses travel rapidly along the AV bundle
and Purkinje F
bers.
c. Muscle F bers in the ventricular walls form
whorls that squeeze blood out of the contracting
ventricles.
5. Electrocardiogram
a. An electrocardiogram (ECG) records electrical
changes in the myocardium during a cardiac cycle.
b. The pattern contains several waves.
(1) The P wave represents atrial depolarization.
(2) The QRS complex represents ventricular
depolarization.
(3) The T wave represents ventricular repolarization.
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