BIOL 4000 - LECTURE NOTES 8
SYSTEM TEXT A - POWERPOINT
SYSTEM TEXT B - POWERPOINT
The circulatory system makes
"large-scale" multicellularity possible. It provides a means of
conveying nutrients and oxygen to and wastes out of the internal cells that
make-up our bodies. Without a circulatory system, life, as we know it, would
I. There are two major components of
the circulatory system.
A. The cardiovascular system
B. The lymph vascular system
1. blind ended lymphatic capillaries that
collect lymph fluid from tissues
2. larger lymphatic vessels that connect with
one and other and finally empty collected lymph into large veins in the neck
where the lymphatic and cardiovascular systems merge.
II. General cardiovascular
A. Two major components
1. arterial system
2. venous system
a. portal systems
B. Together these two systems form two major
1. systemic loop
2. pulmonary loop
C. Diagram of general pattern of
III. Structure of vessels 1 -
General structure of larger blood vessels.
A. Blood vessels larger than capillaries are
encircled by 2 or more of the following tissue layers
1. tunica intima (often referred to as the
a. layer of simple squamous epithelium called
b. the endothelial layer is encircled by a
subendothelial layer of loose connective tissue containing a few smooth muscle
c. the intima often appears wrinkled in sections
due to contraction of the smooth muscle cells.
2. tunica media (or "media")
a. this layer encircles the intima
b. consists of circumferential smooth muscle
with extracellular matrix secreted by cells between themselves that consists of
collagen fibers and elastin, as well as various proteoglycans
c. in most arteries a layer of elastin called
the internal elastic lamina separates the intima and media.
d. in some larger arteries, an external elastic
lamina separates the media from the outer adventitial layer.
e. in large blood vessels, the media receives
nutrients from arterioles that branch off of arteries in the adventitia and
extend into it.
3. tunica aventitia (or "adventitia")
a connective tissue layer
containing many collagen and elastin fibers
b. this layer gradually becomes
continuous with the connective tissue of the organ the vessel is in
c. in larger vessels a network of
small blood vessels is present in the adventitia.
* these provide nutrients to cells
in the adventitia and media.
* called the vasa vasorum
IV. Structure of vessels 2 - General
relationship between tunics and arterial and venous vessels
V. Structure of vessels 3 - Arteries
A. Large elastic arteries (e.g. decending aorta and large branches
1. lots of elastin in media that give these
vessels yellow color
2. very thick tunica intima consisting of the
endothelium with a relatively thick loose connective tissue sub-layer. An
elastic lamina may or may not be present.
3. sub-endothelial basal lamina may not be
4. media consists of concentrically arranged,
perforated elastic membranes with smooth muscle cells and collagen fibers
between them. The number and thickness of these elastic layers increases with
5. tunica adventitia is usually thin with a vasa
vasorum (network of blood vessels)
B. Muscular arteries
1. essentially all the named arteries in the
body, except for the large elastic arteries, are muscular arteries. Also the
unnamed smaller arteries.
2. intima consists of endothelium and a
flattened subendothelial layer of collagenous and elastic fibers. A predominant
feature of the intima is an internal elastic lamina.
3. the media is the major identifying
characteristic. It consists of a thick smooth muscular layer with as many as 40
layers of smooth muscle encircling the artery.
4. larger muscular arteries may have an external
elastic lamina at the border of the media with the adventitia.
5. adventitia is well developed and usually
thinner than media.
6. spasmodic contraction of the media smooth
muscle helps prevent hemorrhaging during injury.
1. arterioles are small arteries, 0.04 - 0.4 mm
2. tunica intima has a very thin sub-endothelial
layer of loose connective tissue
3. media is muscular and composed of 1-3 layers
of smooth muscle
4. adventitia is fairly prominent
1. metarterioles connect small arterioles to
2. less than 40 um in diameter
3. consist of endothelium surrounded by a few
discontinuous smooth muscle fibers
4. act as sphincters that control the flow of
blood through capillary beds
VI. Structure of vessels 4 -
1. usually 7-9 um in diameter, but may be as
small as 5 um or as large as 12 um in diameter (sinusoidal capillaries may be
2. wall consists of a simple squamous epithelium
called endothelium. This is just the continuation of the endothelium that lines
the whole circulatory system, without the various additional connective tissue
and muscular layers that we find surrounding larger blood vessels.
3. this simple squamous epithelium is surrounded
by a basal lamina.
4. cells called pericytes may be sporadically
found between the basal lamina and the endothelial cells. The cells are able to
contract and thus constrict capillaries. Pericytes have their own basal lamina
which may fuse with the basal lamina of the endothelial cells that form the
5. In transverse (cross-) sections we find that
the circumferential wall of a capillary is generally formed by 2-3 squamous cells
held together by occluding or gap junctions. Nuclei of the cells buldge into
the capillary lumen.
6. The walls of certain types of capillaries may
have pores (or fenestrae) penetrating the squamous endothelial cells, or there
may be spaces between adjacent endothelial cells.
B. There are 3 types of capillaries.
a. no pores or spaces between cells in walls
b. most capillaries are of this type
2. fenestrated or perforated capillaries
a. pores penetrate the endothelial cells
b. found in tissues where rapid exchange of
substances occurs (e.g., in the villi of the intestinal wall and glomeruli of
3. sinusoidal capillaries
a. highly convoluted and with enlarged diameter
b. open spaces are present between endothelial
c. in some sinusoidal capillaries, many small
multiple fenestrations penetrate the endothelial cells
d. phagocytic cells are present in and around
the endothelial layer
e. the endothelium lacks a continuous basal
f. found mainly in liver and hematopoietic
such as bone marrow and spleen.
C. So, except for continuous capillaries, we can
see that these vessels are constructed to allow for the passage of materials
(macromolecules) and sometimes cells (e.g., monocytes, eosinophils) into and
out of the circulatory system.
D. This can occur through pores (in cells) or
spaces (between cells). In continuous capillaries, macromolecules can also be
passed into or out of the circulatory system by what might be called transcellular
pinocytosis. Cells can move into and out of continuous capillaries by
VII. Structure of vessels 5 –
A. These are circulatory structures that
resemble capillaries in that they connect arterioles with venules (bone marrow,
spleen, adrenyl cortex) or venules with venules (liver, adenohypophysis) and
have just an endothelial wall, but they are different in a number of ways.
B. form large, uneven channals 30 um or more
C. endothelium is incomplete and, in addition to
endothelial cells, contains fixed phagocytic cells that bulge into the lumen
VIII. Structure of vessles 6 - Veins
A. post capillary venules (those venules
directly connected to capillaries)
1. 30 um - 200 um in diameter
2. consist of endothelium with incomplete layer
of pericytes and a thin connective tissue investment - similar to capillaries,
primary site where leukocytes leave the circulatory system at sites of
1. 0.2 - 1 mm in diameter
2. intima lacks a subendothelial layer of
3. thin media consisting of a few smooth muscle
fibers (collecting venules).
As venule size increases, the amount of muscle increases (muscular venules).
4. well developed adventitia rich in collagen
5. despite tunic investments, blood cells such
as moncytes can still pass through the walls of venules
a. this process is called diapedesis - the act of a leucocyte squeezing between
C. small to medium sized veins
1. 1-9 mm in diameter
2. intima usually has a thin sub-endothelial
layer of connective tissue associated with endothelium
3. thin media
D. large veins
1. intima is well developed, but still thin
2. Media is muscular, but thin relative to that
of similar sized muscular arteries. A few circular layers of smooth muscle are
present amongst abundant connective tissue.
3. Adventitia is thickest and best developed
tunic. Comprises most of wall of vessel. May contain longitudinal bundles of
4. This arrangement of longitudinal muscle in
adventitia and circular in media helps to strengthen wall and help prevent
settling of blood in limb veins due to gravity.
5. In addition to this muscular arrangement,
valves are also present in large veins. These also help prevent back flow of
6. Skeletal muscle in conjunction with valves is
helpful in preventing backflow by propelling blood through veins in the correct
direction as the muscles contract.
A. The heart is composed of
1. 3 layers or tunics
2. a fibrous connective tissue skeleton
4. an electrical impulse conducting system
B. Tunics of the heart
1. endocardium - in a sense, simply a
continuation of tunica intima
a. consists of 3 parts
* endothelium resting on a basal lamina and
associated thin layer of collagen fibers
* a deeper, denser layer of connective tissue
containing elastic fibers and a few smooth muscle cells
* the subendocardial zone of loose connective tissue that contains small
blood vessels and nerves. In the ventricle, the subendocardial zone may contain
scattered Purkinje fibers
2. Myocardium - consists of 2 components
a. fascicles of cardiac muscle cells that
connect to the fibrous connective tissue skeleton of the heart
b. non-contactile, modified muscle cells that
form the impulse (action potentil) generating and conducting system of the
* this system is composed of the
atrioventricular and sinoatrial nodes, and the Purkinje fibers
* the cells of this system are muscle cells that
are specialized for impulse (action potential) generation and impulse conduction
* these cells generate and conduct action
potentials that synchronize the heartbeat
* (in the ventricle) Purkinje fibers are
scattered along the innermost portion of the myocardium, next to the
3. Epicardium (visceral pericardium) – epithelium
and connective tissue covering of heart
a. squamous to cuboidal epithelial external
lining - a continuation of the epithelium that lines the pericardial cavity
b. below this is a layer of connective tissue
with high concentration of elastic fibers
c. between the elastic layer and myocardium is
the subepicardial layer that consists of loose connective tissue containing
nerves, veins, and adipose tissue.
C. Valves - read in text
X. A few words about the lymphatic
A. System of thin walled vessels that runs throughout
B. These vessels are lined with endothelium.
C. They begin with blind-ended lymphatic
capillaries that originate in connective tissue of nearly all parts of body.
D. These lymphatic capillaries and associated
small lymph vessels converge into larger vessels that return fluid expressed
from arteries and veins into tissues to the venous component of the blood
E. The lymph vessels carry fluid that eventually
drains into 2 large ducts, the thoracic duct and the smaller right lymphatic
duct that connect to large veins in the neck.
F. Larger lymphatic vessels are similar in
structure to veins except with thinner walls and with no clear-cut separation
between the 3 tunics (intima, media, adventitia).
G. The lymph vessels are also a pathway by which
various cells of the immune system can enter or leave the blood circulatory
system and tissues.