Introductory Anatomy: Circulatory System & Blood
Perhaps the most important thing to remember about the circulatory
system is its variability. This is because of the way the circulatory
system develops: the early embryo is essentially a blood filled
sponge - intricate network. Most of this is removed during development,
leaving remnants governed by highest flow and pressure. Its plasticity
is also shown by migratory organs: if an organ moves during development
eg the kidney it will take its nerve supply with it, but acquire
a new local blood supply - successive segmental arteries of the
kidney as it rises may persist as 'abnormal' renal arteries.
Sensible place to start is the heart, complicated at first
glance so let's reduce it to a diagram: four chambered, atria
and ventricles in one way communication. Right half circulates
blood from body to lungs, left half circulates blood from lungs
around body. This has immediate implications: the pulmonary circulation
is rather small, little peripheral resistance, so low pressures
needed and the walls of the right ventricle are rather thinner.
The left side pumps the same volume against greater peripheral
resistance, so the left ventricular walls are more muscular.
There is functional specialisation in the vessels too.
1. Distribution system
Low volume, high pressure system of large arteries which leave
the heart: we speak of the arterial tree, trunk, branches, twigs.
The branches are always smaller than the trunk and so on. Several
branches may come off at a single level, but more often a main
trunk gives off branches and continues. Structure is also graded
as you pass away from the heart. Large trunks have much elastic
tissue in their walls. Repeated distention and contraction of
this tends to even out heart contractions into a steady, though
pulsatile flow. Smaller branches have less elastic tissue and
more smooth muscle offering better control of flow to suit temperature,
2. Resistance vessels
Control blood flow at more intimate level - muscular arterioles
and precapillary sphincters provide principle resistance to blood
flow which governs pressure in arterial tree.
3. Exchange vessels
Capillaries are often composed of a single wrapped cell at any
given level. Across their walls occurs exchange between blood
and tissue fluids, oxygen, CO2, nutrients, water, inorganic ions,
vitamins, hormones, metabolic products, immune substances, even
immune competent cells. Capillaries may be plain, fenestrated
or sinusoidal - to slow blood flow.
4. Capacitance vessels.
After capillary beds blood is collected in venules which are
tributaries of veins. These vessels provide a low pressure blood
reservoir through which blood returns to the heart. Veins have
the same basic histological structure as arteries, but tend to
be greater in cross sectional area at any given level, because
of slower flow rate. For the same reason arteries are often accompanied
by paired veins, vena combatants.
Veins often have a dead space around them to allow for dilation,
so not sheathed as arteries are, but run in loose connective
tissue. Because only a little external pressure would stop flow
veins are confined to the dorsum of the foot and the back of
the hand, and often run on the flexor aspects of joints.
The tendency for gravity to stop or reverse the flow of blood
in veins is countered by valves - pockets in the walls, usually
in twos or threes. Reflex blood pours into these pockets, filling
them and stopping the flow. They are found where a tributary
joins a larger vein, and at intervals along main veins. Most
frequent in lower limb, where the effect of gravity is greatest,
diminishing as we move superiorly and virtually absent above
the heart, where gravity acts with venous return, not against
it. Return of blood is ensured by several factors. Smaller veins
are continually filled by capillaries, larger veins (especially
in the lower limb) are continually squeezed by muscular action,
valves controlling the direction of flow.
Exceptions to the rule
1. Portal circulations
These differ from those already described in having two capillary
beds. The largest portal system covers the spleen, pancreas,
stomach, small intestine. Blood supplying these, having passed
through their capillary network ends up in the hepatic portal
vein, which drains into the liver and through a hepatic capillary
bed, where products of digestion are removed, processed and stored.
A second, smaller but equally important portal system is found
connecting the brain and pituitary gland.
Arteries do not always end up as capillaries. They often anastomose
with other arteries, either of equal (in the brain) or unequal
size (elsewhere). In limbs anastomoses are most common around
joints: quite clearly they form an alternative route when the
main road is obstructed by, say, flexion of the elbow: they will
also allow equalisation of pressure. They become more frequent
as you get further from the heart i.e. as the arteries get smaller,
so that arterioles tend to form a network. If you cut an artery
which forms part of an anastomosis it will bleed from both ends.
Anastomoses form the basis of collateral circulation. This is
important if an artery is gradually furring up: alternative arteries
will enlarge to restore an adequate supply. Sudden blockage may
lead to the death of the area supplied (avascular necrosis) if
collaterals are absent or inadequate. This is seen in the head
of the femur, scaphoid after fracture and central artery of the
retina, and in coronary arteries supplying heart muscle - end
3. Vascular shunts.
These are important shortcuts from artery to vein cutting out
capillary bed. These are seen as
a. preferential thoroughfares - areas of a capillary bed
are deferentially supplied with blood under different demands.
Areas are then restricted by closing precapillary sphincters.
b. artero-venous anastomoses - direct connections between
arteries and veins, by coiled vessels with a thick muscular coat,
which may allow complete closure. These, found in skin of nose,
ear, tongue, erectile sexual tissue, hands and feet are temperature
control mechanisms, cutting off blood supply to a capillary bed
to reduce heat loss, or encouraging it to encourage it (panting
dog). Regulation of blood pressure? Maybe.
Absent in newborn, atrophy in old age - so wrap up newborns and
Lymphatic circulation As well as blood vessels we can also
find a system of lymphatic capillaries and larger vessels. Lymph
capillaries coexist with blood capillaries in capillary beds,
have fenestrated walls and can thus exchange anything from liquids
to cells. Most of the fluid which leaks from blood capillaries
into tissues returns, but 10-20% doesn't, and would therefore
gradually flood the tissues if left (oedema). This is mopped
up by the lymphatics,, which shadow the veins and eventually
dump lymph, usually via one or more lymph nodes into the blood
stream, via the thoracic duct and right lymphatic duct which
open into veins in the neck.
As well as fluid this system contains cells, notably phagocytic
dustman cells which circulate in the blood, exit via the capillaries,
spend time in the tissues collecting anything from dead cells
to bacteria, then return via the lymphatic system. Because of
this behaviour the phagocytic cells are often laden with bacteria,
viruses or diseased cells. Lymph nodes are thus prone to infection
and swelling and the lymphatic system is important in the spread
of cancerous cells - metastasis.
While we are considering the circulatory system it is logical
to look at blood. Blood is a liquid with suspended cells 30-50%
by volume. These cells are of three basic types, erythrocytes
or rbcs, leucocytes or wbcs and thrombocytes or platelets. Two
of these three cell types are odd, in being anucleate. All originate
in the bone marrow, although the number of primary stem cell
types is unresolved.
By far the commonest blood cells (4-6m/mm3) erythrocytes are
classically biconcave enucleate discs 7-8* in diameter. They
are red because the cytoplasm is packed with haemoglobin which
transports oxygen. They also transport carbon dioxide. Their
shape is variable: they are able to deform to squeeze through
capillaries. Because they lack a nucleus they have a short lifespan
(c 120d) after which their components are recycled (iron) or
excreted (bilirubin). Young erythrocytes - reticulocytes (1%)
contain a net made up of the remains of the RNA used in haemoglobin
Less common than rbcs (5-10,000/mm3), two main types distinguished
according to the presence or absence of cytoplasmic granules.
The nuclei of granulocytes are multilobed and cells also called
polymorphonuclear leucocytes or polymorphs. The cytoplasm contains
two sorts of granules: primary granules - lysosomes - present
in all polymorphs and associated with their phagocytic nature
secondary or specific granules: which allow identification because
they take up specific stains.
We thus distinguish
neutrophils - commonest (50-70%) 10-12* diameter, lobed
nucleus, secondary granules unstained. Phagocytic against micro-organisms
eosinophils - (1-4%), 10-12* diameter, lobed nucleus,
secondary granules stained pink by eosin. Eat antigen/antibody
basophils - (<1%), 9-10* diameter, lobed nucleus usually
hidden by blue staining secondary granules. Not so actively phagocytic
-blue granules are heparin and histamine which are released in
inflammation and immune responses.
Lack granules, and have a rounded nucleus. Lymphocytes are the
commonest (40%), 6-8* diameter, with a large nucleus all but
obscuring the cytoplasm . Monocytes (4-8%) larger, 12-16*m diameter
with a relatively smaller nucleus.
Small (2-3*m) purple staining cell components, non nucleated
(In background, monocyte slide). They tend to aggregate in clumps.
Involved in wound repair and blood clotting. Formed by the disintegration
of huge (150*m) megakaryocytes in the bone marrow.
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