Page 403 - Atlas of Histology with Functional Correlations

Fenestrated capillaries are characterized by openings or fenestrations

(pores) in the cytoplasm of endothelial cells designed for rapid exchange of
molecules between blood and tissues. Fenestrated capillaries are found in those
organs/tissues where enhanced exchange of substances occurs between tissues
and blood. Endocrine tissues and glands, the small intestine, the kidney
glomeruli, and the choroid plexus in the brain ventricles are organs that exhibit

fenestrated capillaries.

Sinusoidal (discontinuous) capillaries are blood vessels that exhibit
irregular, tortuous paths. Their much wider diameters slow down the flow of
blood. Endothelial cell junctions are rare in sinusoidal capillaries, and wide gaps

exist between individual endothelial cells. Also, because a basement membrane
underlying the endothelium is either incomplete or absent, direct exchange of
molecules occurs between blood contents and cells. Sinusoidal capillaries are
found in the liver, spleen, and bone marrow.

LYMPHATIC VASCULAR SYSTEM

The lymphatic vascular system is closely associated with the circulatory system.
It is composed of vascular channels that drain extracellular fluid called lymph
from the tissues. The lymphatic system consists of lymph capillaries and lymph
vessels that originate as blind-ending tubules or lymphatic capillaries in the
connective tissue of organs. The lymph capillaries lie close to the blood

capillaries and collect the excess interstitial fluid (lymph) from the tissues. The
collected lymph is returned to the venous blood via the large lymph vessels, the
thoracic duct, and the right lymphatic duct after it is filtered through numerous

lymph nodes located throughout the body. Also, the walls of lymph vessels show
more permeability than the walls of blood capillaries because the endothelium
in lymph capillaries is extremely thin. The structure of larger lymph vessels is
similar to that of veins except that their walls are much thinner.

Lymph movement in the lymphatic vessels is similar to that of venous blood.

In larger lymph vessels, the contraction of smooth muscles in their walls moves
the lymph forward. In addition, external factors such as the contractions of
surrounding skeletal muscles, arterial pulsations, and compression of tissues also
assist in the lymph flow. Similar to the veins, numerous valves in the lymph

vessels prevent backflow of the collected lymph. Lymph vessels are found in all
tissues except in the CNS, cartilage, bone and bone marrow, thymus, placenta,

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