Histology 2016-2017
Department of Anatomy &Histology:
Dr.Rajaa Ali
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BLOOD VESSELS II
CAPILLARIES
GENERAL FEATURES
 Arterioles break up into small blood vessels called capillaries.
 Capillaries are often referred to as exchange vessels, because they are
involved in exchange of gases, nutrients and metabolites between
blood and tissue.
 Tissues with high metabolic rates have abundant capillary network
(e.g. kidney, liver, cardiac muscle).
STRUCTURE
The lumen of a typical capillary is about 7–9 μm wide (equal to the
diameter of an erythrocyte) and is lined by endothelial cells, which are
two or three in number on cross section of vessel and form its tunica
intima.
The margin of endothelial cells are held together by tight and gap
junctions. Numerous pinocytotic vesicles are seen in the cytoplasm.
They are involved in transporting material across the endothelial
lining in either direction.
Pericytes or adventitial cells are occasionally seen within the
basement membrane of the endothelium constituting the tunica
media. These cells contain contractile fi laments in the cytoplasm and
can transform into other cells.
A thin layer of collagen fi bres that surround the capillaries form the
tunica adventitia.
Capillaries are divided into following three types depending on the
nature of the endothelium:
1. Continuous or somatic capillary (Fig. 10.3)
It is the commonest type of capillary present in connective tissue,
muscle, brain, lung, etc.
The endothelial cells form a continuous lining of the capillary.
Fig. : continuous capillary
2. Fenestrated or visceral capillary (Fig. 10.4)
This is characterised by the presence of tiny pores in the endothelial
cells.
These pores are often closed by a thin diaphragm (thinner than the cell
membrane) and allows dissolved substances and macromolecules to
pass through slowly.
The permeability of fenestrated capillary is much greater than that of
continuous capillary. So they are found in tissues in which rapid
exchange of substances occur between tissues and blood, e.g. kidney,
intestinal villi, endocrine glands, etc.
Fig. : fenestrated capillary
3. Sinusoidal capillary
It is found in liver and haemopoietic organs like red bone marrow and
spleen.
It is a thin walled tortuous blood vessel having a large irregular lumen
(30–40 μm).
Lumen is lined by discontinuous endothelium (the basal lamina is
discontinuous). There are gaps between the endothelial cells that
permit the passage of blood cells and macromolecules.
Phagocytic cells may be seen in its wall (e.g. Kupffer’s cells in liver).
Fig. : sinusoidal capillary
FUNCTIONS OF CAPILLARY ENDOTHELIUM
Permeability:
Capillary endothelium allows exchange of water, oxygen, CO2 and
metabolites between blood and tissue.
It also allows migration of leucocytes from blood to tissue
(diapedesis), which is increased during infl ammation.
It forms blood brain barrier – the tight junction between the
endothelial cells and absence of pinocytotic vesicles in the cytoplasm
suggest the presence of blood brain barrier.
Metabolic function: Capillary endothelial cells can metabolise a wide
variety of substances:
Activation – converts angiotensin I to angiotensin II.
Inactivation – converts some active substances (like bradykinin,
serotonin, prostaglandin, norepinephrine, thrombin)
into inactive/inert compounds.
Lipolysis – breaks down lipoprotein into triglycerides and cholesterol.
Nonthrombogenic function: Platelets do not normally adhere to an
intact endothelium. This is due to the ability of endothelial
cells to release prostacyclin, which is a powerful inhibitor of platelet
aggregation and thus prevent, clot formation.
GENERAL FEATURES
 Venules receive blood from capillaries.
 They have a larger diameter (0.5–1 mm) than arterioles.
 Small venules (postcapillary venules) take part in exchange of
metabolites between blood and tissue and permit leucocyte
 migration as do capillaries. The post capillary venules in mucosa
associated lymphoid tissue (MALT) are lined by tall
 cuboidal endothelial cells and are called high endothelial venules
(HEV). Venules are sensitive to infl ammatory agents
 resulting in leakage of fl uids and defensive cells).
STRUCTURE
 The wall is thin with a large collapsed lumen.
 A venule is composed of the following three layers:
(a) Tunica intima
 It is composed of endothelium.
(b) Tunica media
 It is composed of one or two layers of smooth muscle fi bres.
(c) Tunica adventitia
 It is thick and composed of connective tissue rich in collagen fi bres.
VEINS
GENERAL FEATURES
 Veins are thin-walled blood vessels that carry blood from capillaries
to heart.
 Large veins are formed by union of smaller veins like tributaries of a
river.
 They are often provided with valves which serve to prevent the refl ux
of the blood.
STRUCTURE
 The veins are subdivided into the following types based on the size.
1. Medium-sized vein (Box 10.3)
Medium-sized Vein.
Presence of:
 Thin tunica media with few smooth muscle fi bres and less elastic fi
bres;
 Large collapsed lumen.
Medium-sized vein differs from medium-sized artery in having
A collapsed lumen,
Thin wall with tunica media containing fewer smooth muscle and less
elastic fi bres,
No internal elastic lamina,
Presence of valves to prevent back fl ow of blood.
It is composed of the following three layers:
(a) Tunica intima
It is made of endothelium supported by a thin layer of subendothelium.
It does not have internal elastic lamina.
(b) Tunica media
It is composed of few circularly arranged smooth muscle fi bres embedded
in connective tissue predominantly made of collagen fi bres. Elastic fi bres
are few.
(c) Tunica adventitia
This comprises loose fi broelastic connective tissue carrying vasa vasorum
and nerve fi bres.
2. Large vein, e.g. superior vena cava (SVC), inferior vena cava (IVC;
It is made of the following three layers:
(a) Tunica intima
This layer is well developed.
It is formed by endothelium with subendothelial connective tissue.
(b) Tunica media
It is either thin or absent.
(c) Tunica adventitia
It is well developed and is the thickest coat.
It is made of many longitudinal bundles of smooth muscle fi bres embedded
in connective tissue.
Large Vein
Presence of:
Thick tunica adventitia with longitudinally oriented bundles of
smooth muscle fi bres;
Poorly developed tunica media.