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Lecture 8

BIOC63H3 Lecture Notes - Lecture 8: Lymphatic Vessel, Microcirculation, Smooth Muscle Tissue


Department
Biological Sciences
Course Code
BIOC63H3
Professor
Ivana Stehlik
Lecture
8

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The microcirculation and the lymphatic system
Figure 1. Microcirculation and the Lymphatic System
The microcirculation comprises the smallest arterioles, and the exchange vessels, including
the capillaries and the post-capillaries venules. Then transfer of gases, water, nutrients, waste
materials and other substances between the blood and body tissues carried out by the exchange
vessels is ultimate function of the cardiovascular system.
Organization of the microcirculation
Blood enters the microcirculation via small arterioles, the walls of which contain smooth muscle
cells. These vessels are densely innervated by the sympathetic system, particularly in the
splanchnic and cutaneous vascular beds. Sympathetically mediated constriction of each small
arteriole reduces the blood-flow to many capillaries.
In some cases tissues (e.g. mesentery) capillaries branch form thoroughfare vessels which run
from small arterioles to venules (Figure 1a, right). The proximal (arteriolar) end of such a vessel
is termed metarteriole, and it is wrapped intermittently in smooth muscles cells. The capillaries
have a ring of smooth muscle called a precapillary sphincter at the origin, but thereafter lack

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smooth muscle cells. Constriction of the precapillary sphincter controls the flow of blood
through that capillary.
Most tissues, however, lack metarterioles or precapillary sphincters per se. Instead, the smallest
or terminal arterioles divide to give rise to sets of capillaries (Figure 1a, left). The terminal
arteriole itself acts as a functional precapillary sphincter for its entire cluster of capillaries.
Terminal arterioles are not innervated, and their tone is controlled by local metabolic factors.
Under basal conditions, terminal arterioles constrict and relax periodically. This vasomotion
causes the flow of blood through the cluster of capillaries to fluctuate.
The capillaries join to form postcapillary venules, which also lack smooth muscle cells. These
merge to form venules, which contain smooth muscles cells and are sympathetically innervated.
Movement of solutes across the capillary wall
Water, gases and solutes (e.g. electrolytes, glucose, proteins) cross the walls of exchange vessels
mainly by diffusion, a passive process by which substances move down their concentration
gradients. O2 and CO2 can diffuse through the lipid bilayers of the endothelial cells. These and
other lipophilic substances (e.g. general anesthetics) therefore cross the capillary wall very
rapidly. However, the lipid bilayer is impermeable to electrolytes and small hydrophilic (lipid-
insoluble) molecules such as glucose, which therefore cross the walls of continuous capillaries
(Figure 1b, bottom) 1000-10 000 times more slowly than does O2. Hydrophilic molecules cross
the capillary wall mainly by diffusing between the endothelial cells. This process is slowed by
tight junctions between the endothelial cells which impede diffusion through the intercellular
clefts. Diffusion is also retarded by the glycocalyx and basal lamina dense networks of fibrous
macromolecules coating the luminal and abluminal sides of the endothelium, respectively. This
tortuous diffusion pathway (the small pore system) acts as a sieve which admits molecules of
molecular weight less than 10 000.
Even large proteins (e.g. albumin, MW 69 000) can cross the capillary wall, albeit very slowly.
This suggests that the capillary wall also contains a small number of large pores, although these
have never been directly visualized. It has been proposed that large pores exist transiently when
membrane invaginations on either side of the endothelial cell fuse, temporarily creating a
channel through which large molecules diffuse.
The endothelial cells of fenestrated capillaries (found in kidneys, intestines, and joints) contain
pores called fenestrae (Figure 1b, upper right). Fenestrated capillaries are about 10 times more
permeable than are continuous capillaries to small hydrophilic molecules, because these can
move through the fenestrae. Sinusoidal or discontinuous capillaries (liver, bone marrow,
spleen) are very highly permeable, because they have wide spaces between adjacent endothelial
cells through which proteins and even erythrocytes can pass (Figure 1b, upper left).
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Figure 2. Schematic diagram showing the relationship of the lymphatic system and to the
cardiovascular system
The lymphatic system (Figure 2)
Approximately 8 L of fluid containing solutes and plasma proteins is filtered from the
microcirculation into the tissues spaces each day. This returns to the blood via the lymphatic
system. Most body tissues contain lymphatic capillaries (Figure 1a). These are blind-ended
bulbous tubes 15-75 μm in diameter, with walls formed of monolayer of endothelial cells.
Interstitial fluid, plasma proteins and bacteria can easily enter the lymphatic capillaries via the
gaps between these cells, the arrangement of which then prevents these substances from
escaping. These vessels merge to form collecting lymphatic, the walls of which contain smooth
muscle cells and one-way valves (as do the larger lymphatic vessels). The sections between these
valves constrict strongly, forcing the lymph towards the blood. Lymph is also propelled by
compression of the vessels by muscular contraction, body movement, and tissue compression.
Lymph then enters the larger afferent lymphatics, which flow into the lymph nodes. Here,
foreign particles initiate immune reactions. Much of the lymph fluid s returned to the blood here
via capillary absorption. The remaining fluid enters efferent lymphatics, most of which
eventually merge into the thoracic duct. This duct empties into the left subclavian vein in the
neck. Lymphatics from parts of the thorax, and the right sides of the head and neck merge
forming the right lymph duct, which enters the right subclavian vein. The lymphatic system is
also important in the absorption of lipids from the intestines. The lacteal lymphatics are
responsible for transporting about 60% of digested fat into venous blood.
Fluid filtration in the microcirculation
The term “microcirculation” refers to the functions of the smallest blood vessels, the capillaries
and the neighboring lymphatic vessels. Delivery of blood to and from the capillaries is critically
important because the capillaries are the site of exchange of nutrients and waste products in the
tissues, as well as the site of fluid exchange between the vascular and interstitial compartments.
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