William A Beresford MA, D Phil ©
Professor of Anatomy
Anatomy Department, West Virginia University, Morgantown, USA
A GENERAL FUNCTIONS AND ASPECTS OF THE CIRCULATORY SYSTEM.
Fig. 6 | GAS EXCHANGE |
| (Lungs) |
| PUMPING |
/ --------------------- \
/ (Heart) \
HORMONES __ / \ __ FOOD & (Gut)
(Endocrine | | WATER
Glands) | |
WATER | |
WASTE & HEAT /\ |__ STORAGE &
( Kidneys, Gut \ / PROCESSING
Lungs, Skin) \ / (Liver)
BLOOD CELLS \ _______________________ /
& / | | ~ \ CLEANING (Spleen, Liver,
ANTIBODIES | | ~ Marrow)
(Red Marrow | | ~
Lymphoid Organs) | | ~
| | ~
| | Lymphatic drainage
| | ~
DETERMINING CELL ENVIRONMENTS ~
1. By forming special fluids, 2. Forming tissue fluids in
e.g., CSF, aqueous extracellular space by
humour, synovial fluid more widespread diffusion
and transport, serving, e.g.,
CT, epithelia, muscles.
l Closed system of tubes, through which blood is forced by the
pumping action of the four-chambered, contractile heart.
2 Tubular walls are permeable so that exchange of materials can take
place between the system of small blood vessels and their environment - cells,
or tissue spaces.
3 Lymphatic system collects fluid and colloids and crystalloids from
the tissue spaces and returns them to the bloodstream.
4 There is a balance whereby materials are lost, e.g., from kidneys,
lungs, skin, and replenished by the intake of foodstuffs, air and water.
B SYSTEMIC BLOOD VESSELS
l Blood capillaries
l Very numerous, anastomosing, delicate tubes of diameter 7-9 µm.
2 Total cross-sectional area of the capillary bed is very great, thus
blood flows slowly under low pressure.
3 Wall is made up of curved, thin, plate-like endothelial cells lying
on a BL and oriented with the tube's long axis.
4 Type l unfenestrated capillaries have complete endothelial cells,
e.g., in muscle and skin: type ll capillaries have endothelial cells
with fenestrations/pores through them (not between them), e.g., in
kidney and choroid plexus.
5 Endothelial cells have serrated margins where they attach by
glycocalyx and gap junctions to each other, and by occluding junctions
where more of a barrier is needed, e.g., in the brain. Continuous capillaries
have no gaps between the endothelial cells, in contrast to discontinuous
6 Transport is controlled by the cells, with diffusion and facilitated
transport for small molecules, and transcytotic vesicles or passage
through the pores for larger materials.
7 Some capillaries have the occasional pericapillary cell - pericyte -
imbedded within the BL, perhaps playing a contractile role.
8 Show transitions at both ends: to arterioles (by acquiring
smooth muscle cells), or venules (by widening and taking on more
9 Endothelial cells secrete vasoconstrictor, vasodilator, and
mitotic agents, and their own BL; they interact with blood, leucocytes and
platelets, vary their permeability, and proliferate. Despite their lack of
presence in routine light microscopy, they keep very busy, and are
specialised for each organ that they serve.
10 Selectins are molecules expressed on the endothelial cells of small
vessels, and on white blood cells. They bond intermittently with the sugars of
a glycoprotein on the corresponding cell to cause the WBC to roll to a
stop attached to the endothelium, before squeezing through the vessel wall
into the connective tissues for defence. Sometimes the selectin is on
endothelium, the ligand on the WBC, at other times the reverse achieves a
11 von Willebrand factor (vWF) also has a dual distribution, being
present in Weibel-Palade storage granules of endothelial cells and alpha granules of
platelets. Vascular injury releases vWF from endothelium to cause platelet
activation, aggregation, binding to subendothelial collagen, and blood
clotting - processes of haemostasis.
l Have wider, more irregular lumens than capillaries.
2 Some of the lining cells are phagocytic.
3 Basal lamina may deficient or absent so that lining endothelial and
phagocytic cells lie directly on reticular fibres and other cells, as in the liver.
3 Sinusoidal capillaries
l Have wide irregular lumens and a continuous, but fenestrated,
2 are the usual smallest vessel in endocrine tissue.
l Have three main layers composed of several tissues:
... (a) Endothelial lining on a BL
... (b) Subendothelial CT
... (c) Internal elastic lamina (fenestrated)
... (d) Smooth muscle cells (tightly spiralling or 'circular')
... (e) Sparse reticular and elastic fibres
... (f) External elastic lamina (interrupted)
... (g) Collagenous and elastic CT (mostly longitudinal)
2 Arterioles, less than 0.5 mm wide, have (a),(c),(d),(e) and (g) of
3 Small and medium-sized arteries (muscular/distributing) have all elements.
4 Large arteries (elastic/conducting) differ significantly:
... (a) Endothelium on a BL
... (b) Subendothelial CT
... (c) Innermost fenestrated elastic lamina
... (d) Many fenestrated elastic laminae interspersed with
... (e) smooth muscle cells and collagen fibres
... (f) Collagenous CT with vessels and nerves
The larger arteries and veins have nutrient vessels and nerves (of vessels)
in the adventitia - vasa vasorum and nervi vasorum.
In atherosclerosis, the arterial smooth muscle cell (SMC) changes its
phenotype from static and contractile to proliferative, migratory, and synthetic.
The converted SMC is further delinquent in invading the territory of the
intima, where it lays down matrix and encourages the deposition of lipid,
which, aside from narrowing the lumen, attracts platelets and macrophages.
Their activation carries worse implications for blood flow, clotting, and
deterioration of the vessel wall.
l Venules have an endothelial lining, BL and a collagenous outer
sheath. Pericytes are numerous. The wall is thin enough to permit transport through it. White
blood cells can squeeze between endothelial cells (transmigration/
diapedesis) and escape into the tissues. Lymphocytes may migrate actually
through the interior of the endothelial cell.
[Emperipolesis is the migration of a cell into (and out of) another cell,
while remaining intact: high endothelial cells, megakaryocytes, and thymic
epithelio-reticular cells are hosts for such activity.]
2 Small veins acquire an additional thin media of smooth muscle and a
thicker adventitia of collagen and elastic fibres.
3 No distinct elastic laminae are seen, but sparse elastic networks are
found throughout the wall.
4 Many veins have valves - leaf-like projections of the intima,
usually in a bicuspid form.
5 Large veins (e.g., vena cava) have bundled longitudinal smooth
muscle in the CT adventitia and intima, whilst the media is thin or absent.
6 Comparison between a vein and its companion muscular artery
Both are tubes lined by endothelium and may contain RBCs.
(a) Shape less deformed (a) Flattened
(b) Thick wall (b) Thin wall
(c) Intima crinkled (c) Intima smooth
(d) Three distinct layers (d) Layering indistinct
(media prominent) (media weak)
(e) Internal elastic lamina (e) No internal elastic lamina
7 Exceptional vascular structures
l Cerebral, retinal and osseous veins have no valves and no
media. Veins in general are very variable in their structure.
2 Cerebral arteries are thin walled and have no external elastic layer.
3 Umbilical vein is very muscular; and the umbilical arteries have
little elastic, and a media with distinct longitudinal and circular muscle
4 Arterial intimal cushions are present in arteries to erectile tissue,
5 Some vessels have a high protruding endothelium, e.g., fetal stem arteries.
8 Exceptions to the vascular pattern of arteries, arterioles, capillary
bed, venules, veins, heart
9 Morphology in relation to physical factors in various vessels of the
- Arteriovenous anastomoses - bypassing the bed (e.g., in the skin
and gut) with thick muscle to close the bypass.
- Arterial anastomoses, e.g., circle of Willis to the brain.
- In the periphery, arteries and veins run together with nerves bound in
CT as a neurovascular bundle. In the brain, arterial and venous
distributions are separate.
- Vasa vasorum are blood vessels serving the adventitia and media of
larger arteries and veins.
- Portal systems exist to the liver and pituitary gland, where
venous blood drained from one organ is fed as a supply to the sinusoids or
capillaries of another.
- Sinusoids may take the place of a capillary bed. Thus, for
instance, sinusoidal capillaries permit blood to pass slowly by and influence
chemosensitive epithelioid cells in the carotid body/glomus caroticum.
- Venous sinuses are endothelium-lined CT spaces where blood can
collect for purposes other than metabolic exchange, thus, as part of the
venous collecting system, e.g, coronary and dural sinuses, and in erectile
tissue. (Caution: sinus is also a term for the pocket behind a venous valve -
a site causing problems, when veins are grafted to substitute for arteries.)
- Large elastic or conducting arteries - collagen fibres and
elastic laminae predominate for strength, and elastic distensibility provides
for elastic recoil during diastole thus damping the pulsatile flow resulting
from the intermittent contractions of the heart. Endothelium provides a
smooth lining to facilitate flow and prevent clotting.
- Muscular distributing arteries - lumens of a controllable size are
narrowed by smooth muscular contraction to direct blood flow appropriately
for the needs of various regions; mainly muscular media, strong CT adventitia
with vasa vasorum, and autonomic nerve fibres to the muscle; elastic tissue
limits distension of the lumen, and acts with the muscle.
- Arterioles - smooth muscle provides for a great reduction, by
vasoconstriction, of the blood flow to a region; they maintain an adequate
arterial pressure, but reduce blood pressure to an acceptable level for:
- Capillaries - pressure is low so can be thin-walled to
permit exchange of gases, minerals, carbohydrates and small molecules + water
The wall serves to keep back in the capillary most of the colloidal proteins
of the plasma; the presence of these then encourages the return of fluids at
the venous end of the capillary.
- (a) diffusion or carrier-mediated transport through and between endothelial cells;
- (b) active transcytotic transport through endothelial cells; then
passage through the basal lamina.
- Veins - low, even pressure so have large lumens, thin collagenous
walls, and valves to prevent backflow; larger veins acquire some muscle,
circular and longitudinal, in the media and adventitia. In general, elastic
is not needed for recoil, for variations in pressure between diastole and
systole are insignificant, but the vena cava has significant numbers of
1 Thick-walled, hollow, muscular pumping, and endocrine, organ.
2 Heart structures
Systemic Veins Pulmonary Veins
| Coronary sinus |
| / |
| / |
RIGHT ATRIUM/ LEFT ATRIUM
| . . . . . . . . . |
Tricuspid Valve . . . . . Annuli fibrosi . . . . . Mitral Valve
| . . |
| . Trigona fibrosa. |
| . . |
| . . |
RIGHT VENTRICLE . Septum . LEFT VENTRICLE
| . membranaceum . |
| . . |
Pulmonary Valve . . . . . Annuli fibrosi . . . . Aortic Valve
| . . . . . . . . . |
| . |
Pulmonary artery . Aorta
| . |
Fig. 7 Heart structures.
3 Heart wall`s three layers
4 Cardiac skeleton of dense fibrous CT, with a tendency to
turn into fibrocartilage. Elements are listed above in Fig. 7.
- Endocardium (innermost)
- (a) Lined by endothelium on a basal lamina.
- (b) Subendothelial layer of collagenous and elastic fibres, fibroblasts
and some smooth muscle cells.
- (c) Subendocardial layer of CT with blood and lymphatic vessels, nerve
fibres, and Purkinje fibres of the heart's conducting system. A layer worth
calling a subendocardium is not everywhere present.
- (a) Cardiac muscle fibres, bundled and wound in spiralling sheets,
thickest in the left ventricle, thinnest in the atria.
- (b) Blood vessels and lymphatics and fine CT.
- Epicardium (visceral pericardium) and subepicardium
- (a) Outer mesothelial sheet and BL on
- (b) loose subepicardial CT of fat cells and collagen fibres with
- (c) blood vessels (coronary), lymphatics and nerves to the heart nodes.
- Pericardium (parietal)
CT membrane of fibres supporting a mesothelium. This faces the
epicardium across the pericardial cavity containing a small amount
of lubricating fluid.
5 Heart valves
1 Atrio-ventricular valves
... (a) Leaflets are covered with endothelium on a
... (b) core of dense CT fused to the supporting annulus.
... (c) Cords of CT (chordae tendineae) connect the valve to
... (d) the papillary muscles in the ventricular wall.
2 Semi-lunar valves
... (a) Deploy three leaflets.
... (b) Thinner than the atrio-ventricular valves.
... (c) Lack chordae tendineae.
... (d) Fibrous core enlarges to the nodule of Arantius at the free margin.
6 Impulse-conducting system (coordinates myocardial contractions)
l Sino-atrial node of thin, modified, cardiac muscle fibres,
influenced by parasympathetic (ganglionic neurons are found in the heart)
and sympathetic autonomic nerve fibres, initiates contraction (pacemaker).
2 Contraction spreads through the atrial myocardium to the
3 atrio-ventricular node (Tawara's) consisting of a tangled
plexus of modified cardiac fibres in the medial wall of the right atrium.
4 These fibres enlarge into Purkinje fibres and continue through the septal
CT as the bundle of His, which then branches.
5 Purkinje fibres are rich in sarcoplasm and glycogen, but poor in
myofilaments. They lack T-tubules, and are connected by intermediary
transitional cells with ordinary myocardial fibres, whose contraction they can
thus evoke in many regions of the ventricles.
7 In ungulates, Purkinje fibres are very large, pale and easily recognized: in
man, the system is less obvious.
7 Endocrine role of heart
Atrial myocytes synthesize atrial natriuretic factor (ANF), which relaxes
blood vessels and increases the excretion of sodium and water by the kidney.
ANF is thus a partial counterweight to the renin-angiotensin system.
D LYMPHATIC VESSELS
l Lymphatic capillaries
l Network of blindly ending or anastomosing tubes, 5-50 µm wide.
2 The wall is made of an endothelial tube, with a discontinuous basal
lamina and fine anchoring fibrils.
3 The wall permits the capillary to collect water, solutes and
macromolecules from the tissue spaces.
4 Capillaries (i.e., a lymphatic drainage) are absent from the CNS, bone
marrow, eye, and parts of the spleen.
2 Collecting vessels
l Lymph passes from capillaries into larger lymphatic vessels with
very thin walls of endothelium, basal lamina and collagen, and numerous
2 Lymph is led to small protective ovoid bodies - lymph nodes - through
whose tissues it must filter before going further.
3 Lymph collects in the thoracic duct before entering the circulating
blood at the left innominate vein; the right lymphatic duct also collects
lymph for return to the bloodstream.
4 Thoracic duct
- (a) Intima of endothelium, BL, CT, some longitudinal smooth muscle
and an elastic lamina.
- (b) Thick media of mixed longitudinal and circular smooth muscle.
- (c) Thin adventitia of collagen and a little longitudinal smooth
muscle, vasa vasorum and nerve fibres.
- (d) A valve is at the venous exit.
l Adds to the blood proteins leaked from blood capillaries, new and
recirculated lymphocytes, and antibodies, fat droplets (chylomicrons), etc.
2 Fat is collected from the gut in blind lymphatic capillaries lying
centrally in intestinal villi. The fat-whitened lymph (chyle) gives these
vessels a milky colour, hence their name lacteal.
4 Oedema and its causes
Oedema is an excessive accumulation of tissue fluid, involving mainly
the extracellular space (except in CNS), and making the tissue swollen and
puffy. It is caused by:
- Venous obstruction, e.g., from cardiac incompetence, which
.. (a) raises intracapillary hydrostatic pressure, thus forcing more fluid
into the tissues;
.. (b) reduces the volume of blood collected from the capillaries.
- Injured capillary walls, e.g., from heat, become permeable
permitting greater egress of fluid, solutes and colloids.
- Resulting reduction in intracapillary colloid at the venous end of
the blood capillary lessens the osmotic attraction for tissue fluid
to come back into the capillary.
- Lowering of systemic plasma colloids (proteins), from
.. (a) proteinuria (excretion of protein in the urine),
.. (b) protein starvation, or
.. (c) exudation from burnt skin surfaces,
will likewise reduce the osmotic return of extracellular fluid to capillary
- Obstruction of lymphatic vessels receiving the lymph drained from
tissue fluid by lymph capillaries, for instance, blockage by metastatic cancer
cells. The tropical filaria parasites often block the lymphatics of their host
causing gross swelling (elephantiasis) of affected extremities.
E VESSEL DEVELOPMENT
l Blood and lymphatic vessels (except sinusoids) form initially as simple
endothelial tubes developed from mesenchymal cells - angioblasts.
2 Larger vessels and systems start independently of one another.
3 Their tunics with muscle and CT are added from mesenchymal condensations
around the endothelium.
4 Capillaries of the adult can multiply or regenerate by extending
cords of endothelial cells, which arrange themselves into a tubule. Cords can
fuse with one another to build an anastomosing network.
5 Various cytokines promote or inhibit angiogenesis, e.g., vascular
endothelial growth factor (VEGF).