446 Chapter 13
- This reaction is catalyzed by the enzyme thrombin.
- Thrombin is derived from prothrombin, its inactive
precursor, by either an intrinsic or an extrinsic pathway.
a. The intrinsic pathway, the longer of the two, requires
the activation of more clotting factors.
b. The shorter extrinsic pathway is initiated by the
secretion of tissue thromboplastin. - The clotting sequence requires Ca^2 1 as a cofactor and
phospholipids present in the platelet cell membranes.
G. Dissolution of the clot eventually occurs by the action of
plasmin, which cleaves fibrin into split products.
13.3 Structure of the Heart 418
A. The right and left sides of the heart pump blood through the
pulmonary and systemic circulations, respectively.
- The right ventricle pumps blood to the lungs. This blood
then returns to the left atrium. - The left ventricle pumps blood into the aorta and systemic
arteries. This blood then returns to the right atrium.
B. The heart contains two pairs of one-way valves. - The atrioventricular valves allow blood to flow from the
atria to the ventricles, but not in the reverse direction. - The semilunar valves allow blood to leave the ventricles
and enter the pulmonary and systemic circulations, but
they prevent blood from returning from the arteries to
the ventricles.
C. Closing of the AV valves produces the first heart sound, or
“lub,” at systole. Closing of the semilunar valves produces
the second heart sound, or “dub,” at diastole. Abnormal
valves can cause abnormal sounds called murmurs.
13.4 Cardiac Cycle 422
A. The heart is a two-step pump. The atria contract first, and
then the ventricles.
- During diastole, first the atria and then the ventricles fill
with blood. - The ventricles are about 80% filled before the atria contract
and add the final 20% to the end-diastolic volume. - Contraction of the ventricles ejects about two-thirds of
their blood, leaving about one-third as the end-systolic
volume.
B. When the ventricles contract at systole, the pressure within
them first rises sufficiently to close the AV valves and then
rises sufficiently to open the semilunar valves. - Blood is ejected from the ventricles until the pressure within
them falls below the pressure in the arteries. At this point, the
semilunar valves close and the ventricles begin relaxation. - When the pressure in the ventricles falls below the
pressure in the atria, a phase of rapid filling of the
ventricles occurs, followed by the final filling caused by
contraction of the atria.
13.5 Electrical Activity of the Heart and the
Electrocardiogram 425
A. In the normal heart, action potentials originate in the SA
node as a result of spontaneous depolarization called the
pacemaker potential.
1. When this spontaneous depolarization reaches a
threshold value, opening of the voltage-regulated
Na^1 gates and fast Ca^2 1 channels produces an action
potential.
2. Repolarization is produced by the outward diffusion
of K^1 , but a stable resting membrane potential is not
attained because spontaneous depolarization once again
occurs.
3. Other myocardial cells are capable of spontaneous
activity, but the SA node is the normal pacemaker
because its rate of spontaneous depolarization is the
fastest.
4. When the action potential produced by the SA node
reaches other myocardial cells, they produce action
potentials with a long plateau phase because of the slow
inward diffusion of Ca^2 1.
5. The long action potential and long refractory period of
myocardial cells allows the entire mass of cells to be in
a refractory period while it contracts. This prevents the
myocardium from being stimulated again until after it
relaxes.
B. The electrical impulse begins in the sinoatrial node and
spreads through both atria by electrical conduction from one
myocardial cell to another.
1. The impulse then excites the atrioventricular node,
from which it is conducted by the bundle of His into the
ventricles.
2. The Purkinje fibers transmit the impulse into the
ventricular muscle and cause it to contract.
C. The regular pattern of conduction in the heart produces a
changing pattern of potential differences between two points
on the body surface.
1. The recording of this changing pattern caused by the
heart’s electrical activity is called an electrocardiogram
(ECG).
2. The P wave is caused by depolarization of the atria; the
QRS wave is caused by depolarization of the ventricles;
and the T wave is produced by repolarization of the
ventricles.13.6 Blood Vessels 431
A. Arteries contain three layers, or tunics: the interna, media,
and externa.
1. The tunica interna consists of a layer of endothelium,
which is separated from the tunica media by a band of
elastin fibers.
2. The tunica media consists of smooth muscle.
3. The tunica externa is the outermost layer.
4. Large arteries, containing many layers of elastin, can
expand and recoil with rising and falling blood pressure.
Medium and small arteries and arterioles are less
distensible, and thus provide greater resistance to blood
f l o w.
B. Capillaries are the narrowest but the most numerous of the
blood vessels.
1. Capillary walls consist of just one layer of endothelial
cells. They provide for the exchange of molecules
between the blood and the surrounding tissues.