Page 419 - Atlas of Histology with Functional Correlations

Conversely, the secretion of endothelin proteins by endothelial cells

counteracts the nitrous oxide effects by causing vasoconstriction and
decreased blood flow. The endothelium also converts angiotensin I to
angiotensin II, a powerful vasoconstrictor that increases blood pressure.
Endothelium changes prostaglandins, bradykinin, and serotonin to
biologically inactive compounds, degrades lipoproteins, and produces growth

factors for fibroblasts, blood cell colonies, and platelets. The arterial
endothelial cytoplasm contains small membrane-bound, electron-dense
structures called Weibel-Palade bodies that store the procoagulant

glycoprotein von Willebrand factor. When the endothelium is damaged,
von Willebrand factor is released into the bloodstream to induce platelets
adhesion, blood coagulation, and blood clot formation.

Heart Wall

Pacemaker of the Heart

Cardiac muscle is involuntary and contracts rhythmically and
automatically. The impulse-generating and impulse-conducting portions of

the heart are specialized or modified cardiac muscle fibers located in the
sinoatrial (SA) node and the AV node in the wall of the right atrium of the
heart. The modified cardiac muscle fibers in these nodes exhibit spontaneous
rhythmic depolarization or impulse conduction, which sends a wave of

stimulation throughout the myocardium of the heart. Because the cardiac
muscle fibers in the SA node depolarize and repolarize faster than those in
the AV node, the SA node sets the pace for the heartbeat and is, therefore,
called the pacemaker.

Intercalated discs bind all cardiac muscle fibers while stimulatory
impulses from the SA node are conducted via gap junctions to the atrial
musculature, causing rapid spread of stimuli throughout the entire cardiac
muscle and their contraction. Impulses from the SA node travel through the
heart musculature via internodal pathways to stimulate the AV node in the

interatrial septum. From the AV node, the impulses spread along specialized
conducting cardiac fibers, called the AV bundle (of His), located in the
interventricular septum (between ventricles). The AV bundle then divides

into right and left bundle branches to stimulate both ventricles to contract.
Approximately halfway down the interventricular septum, the AV bundle
branches become the Purkinje fibers, which branch further to transmit the

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