i want 30 mcq with their model answer below each question about Objectives Identify types of arteriovenous connections Compare between histological types of blood capillariesArteriovenous connections
Capillaries
Arteriovenous
anastomosisCapillaries
➢The smallest blood vessels that conduct blood from
arterioles to postcapillary venules.
➢Always function in groups or networks called capillary beds.
➢Its density depends on the metabolic activity of the tissues.
✓Tissues with high metabolic rates (kidney, liver and cardiac
muscle) have rich capillary networks.
✓Tissues with low metabolic rates (dense connective tissue)
have less extensive capillary networks.Structure of blood capillaries:
Capillaries are composed of:
➢ A single layer of endothelial cells
that rest on a basal lamina & are
surrounded by pericytes.
TS of a capillary
LS of a capillary
E
Endothelial cell nuclei
PericytePericytes
➢ Definition: they are perivascular contractile cells.
➢ Site: surrounding capillaries and postcapillary venules.
➢ Origin: mesenchymal.
➢ Structure: have long cytoplasmic processes that surround the
capillary endothelium.
They are surrounded by a basal lamina that is continuous
with the basal lamina of the capillary endothelium.
Functions:
➢ They have contractile ability (have well-developed networks
of myosin & actin)
dilate & constrict capillaries
regulate
blood flow through capillaries in some organs as brain.
➢ They act as stem cells so can differentiate into endothelial
cells, smooth muscle cells, fibroblasts after tissue injury.
➢ Provide vascular support of capillaries and postcapillary
venules.Basal lamina of
pericyte
Nucleus
Endothelial
cells
Basal lamina of
endothelium
Nucleus
Cytoplasmic process
of pericyteFunction of capillaries :
Capillaries allow metabolic exchange between
blood and surrounding tissues.
Factors that help capillaries adapt to their
function:
How do capillaries adapt to their function?
➢ Their thin walls
➢ The nucleus is curved to accommodate the
very small tubular structure.
➢ Extensive surface area (network).
➢ Slow pulsatile blood flow.
Pericyte
Nucleus of
Endothelial cellTypes of capillaries
According to the continuity of the endothelial cells & continuity of the basal laminae,
capillaries are divided into 3 histologic types:
1.Continuous capillaries
2.Fenestrated capillaries
3.Sinusoidal capillariesContinuous capillaries
➢ The most common type.
➢ Diameter: 4-10 μm (allows passage of
one blood cell at a time)
➢ Characterized by:
- continuous endothelium
- continuous basal lamina
➢ Surrounded by pericytesTEM of continuous capillaries:
✓The usual organelles (Golgi, RER,
mitochondria...).
✓Numerous pinocytotic vesicles.
✓Tight, occluding junctions between
endothelial cells.TEM of a continuous capillary
RBC
Tight J.
Lumen
Endothelium
EN
Pericyte
ENSites of continuous capillaries
Skeletal ms
fibers
▪ Connective tissue.
▪ All muscle tissues (cardiac, skeletal, and
smooth)
▪ Blood-tissue barriers, such as
✓ blood-brain barrier (central nervous
system).
✓ blood-air barrier (lungs).
✓ blood-thymic barrier (thymus).
✓ blood-ocular barrier.
✓ blood-testis barrier.
Blood brain barrier
Astrocyte foot
processesFenestrated capillaries
➢Diameter: 4-10 μm
➢Endothelial cells:
are penetrated by fenestrations (holes).
➢ Basal lamina:
is continuous
➢Pericytes: are rarely found.Fenestrations:
• Are small circular openings within the
endothelial cells.
• Approximately 80 nm in diameter.
• Some fenestrations are covered by very thin
non-membranous diaphragms, others not
covered by diaphragm.
• Fenestrated capillaries without diaphragms
are found in the glomeruli of the kidney
(glomerular capillaries).
SEM of glomerular
capillary showing
fenestrations
(arrows)Basal lamina
Fenestration
covered by
diaphragmTEM of fenestrated capillaries:
✓The usual organelles (Golgi, RER,
mitochondria.....).
✓Numerous pinocytotic vesicles.
✓Tight, occluding junctions between
endothelial cells.
✓Thinner cytoplasm than continuous
capillaries.
✓ Presence of fenestrae (with or without
diaphragm).Fenestrated capillary
numerous pinocytotic
vesicles
Endothelial cell
cytoplasm
BL
Fenestrations covered by
diaphragm
(thin electron dense line)
FenestrationsSites of Fenestrated capillaries
Because fenestrated capillaries are highly
permeable, they are found in areas engaged in fluid
transport and secretion of hormones, such as:
✓Kidneys.
✓Intestine.
✓Endocrine glands.Sinusoidal capillaries
(Discontinuous capillaries or sinusoids)
➢ Diameter: 30 - 40 μm
(much larger & more irregular than other capillaries).
➢ Endothelial cells:
▪ Discontinuous (having large perforations without
diaphragms)
▪ Cells are separated by wide, irregular, intercellular
gaps that allow passage of blood plasma proteins &
blood cells.
➢ Basal lamina: highly discontinuous and supported by
reticular fibers.Sites of sinusoidal capillaries
TEM of a liver sinusoid
They are found in:
➢ Liver.
➢ Bone marrow.
➢ Spleen.
Sinusoids are specialized for
❑Exchange of macromolecules (as plasma proteins).
❑Easy movement of blood cells across the
endothelium.
Lumen of
sinusoidLiver
Sinusoidal capillaries
Spleen
Carry plasma proteins to blood
Carry stored blood cells to circulation
Bone marrow
Carry formed blood cells to circulation
Adipocytes
Sinusoids
Haemopoietic
cells
SinusoidMacrophages are associated with sinusoidal capillaries
(to engulf foreign bodies & defective or old erythrocytes)
Kupffer cells in liver
Macrophages in spleen
Reticular fibers
Macrophages in bone marrow
Macrophages
Macrophage
Endothelial cellsDiameter
Continuous capillaries
4-10 μm
Regularity of the
wall
Endothelium
Basal lamina
Tight junction
between endothelial
cells
Associated cells in
the wall
Sites
Regular
Continuous
Continuous
present
pericyte
C.T., Muscle, barriers
Fenestrated capillaries
4-10 μm
Regular
Fenestrations with
diaphragm except in
glomerular capillaries
Continuous
present
pericyte
Kidney, intestine,
endocrine glands
Sinusoidal capillaries
30 - 40 μm
Irregular
Large perforations
without diaphragm
Discontinuous
Wide intercellular gaps
macrophages
Liver, spleen, bone
marrowArteriovenous (AV) anastomoses
(AV shunts)
▪ Definition: direct routes between arterioles and venules
bypassing capillaries.
▪ Structure: The shunt has three segments:
✓ Initial arterial segment (typical of an arteriole).
✓ Intermediate segment has:
thicker tunica media acting as a sphincter
thicker tunica adventitia acting as a capsule
It is richly innervated by autonomic fibers .
It may be coiled.
✓Terminal venous segment (typical of a postcapillary venule).▪ Sites of AV shunts:
➢ They are located throughout the body, but commonly found
in the skin of the fingertips, nose, and lips, and in the
erectile tissue of the penis and clitoris.
▪ Mechanism of action:
➢ When the AV shunt is closed, due to contraction of the
smooth muscle in the intermediate segment, blood flows
normally through the capillary bed.
➢ When the shunt is open, due to relaxation of the smooth
muscle in the intermediate segment, blood bypasses the
capillary bed.
N.B. thoroughfare channels, whose proximal segment is called
a metarteriole is considered AV shunt as they allow some blood to
pass directly from arteriole to postcapillary venule.
open shunt
Closed shuntFunction of Av shunts in the skin
AV shunts of skin serve in thermoregulation at the body surface.
Hot weather sympathetic innervation
closing AV shunt in the skin flow through the capillary bed
dissipation. Cold weather
increases blood more heat
opening AV shunt in the skin reduced capillary blood flow conserves heat.