Spleen
Anatomy
and physiology
The spleen arises by mesenchymal
differentiation along the left side of the dorsal mesogastrium in the 8-mm
embryo. The weight of the normal adult spleen is between 75 and 150 g. It is
shaped like a segment of an orange, its convexity directed backwards and to the
left. It lies between the tenth and eleventh ribs posteriorly. It abuts the
diaphragm superiorly, anteriorly there is the gastric impression, posteriorly it
is in contact with the kidney and the colon lies inferiorly. Within the hilum is
the tail of the pancreas and the splenic vessels. It forms the apex of the
lesser sac. Within the anterior leaf of the lesser sac the short gastrics pass
to the greater curve of the fundus of the stomach. The splenic artery divides at
the hilus into branches which run along the trabeculae of the spleen into the
white pulp, where they give off branches which are almost perpendicular to the
central trunk (Fig. 53.1). This produces a ‘skimming effect’ by which
plasma tends to pass down the branches to the white pulp and most of the red
cells pass in the trabecular artery to the red pulp. The white pulp has an
immune function, whereas the red pulp filters abnormal red cells from the
circulation. Phagocytosis of blood-borne particles occurs in both areas. On
cutting the spleen areas of red pulp, within which can be seen pale ovoid
nodules of white, are apparent.
The
white pulp consists of a central trabecular artery surrounded by nodules with
germinal centres and periarterial lymphatic sheaths which provide a framework
filled with lymphocytes and macrophages. At the edge of the white pulp is the
marginal zone into which pass arteries from both the central artery and from the
peripheral ‘penicillar’ arteries. Plasma-rich blood which has passed through
the central lymphatic nodules is filtered as it passes through the sinuses
within the marginal zone, and particles are phagocytosed. Immunoglobulins
produced in the lymphatic nodules enter the circulation through the sinuses in
the marginal zone. Beyond the marginal zone is the red pulp which consists of
cords and sinuses. Cell-concentrated blood passes in the trabecular artery
through the centre of the white pulp to the red pulp cords. To pass from the
cords to the sinuses, the red cell must elongate and become thinner. This
filters abnormally shaped or rigid cells out of the circulation.
Ninety
per cent of blood passing through the spleen moves through an open circulation
in which blood flows from arteries to cords and thence to sinuses. Thus, splenic
pulp pressure reflects pressure throughout the portal venous system. The
remaining 10 per cent bypasses the cords and sinuses by direct arteriovenous
connections. The overall flow rate through the spleen is about 300 ml/minute.
Functions
of the spleen
In the past the spleen was considered
dispensable because it was found not to be essential to life. The surgeon
therefore
•
Filter function —
macrophages in
the reticulum capture cellular and noncellular material from the blood and
plasma; this material includes bacteria, especially pneumococci.
•
Removal of effete, platelets and red cells —
this process is
called ‘cullin
•
Pitting — this is the process of removing particulate inclusions from red cells
and returning the repaired red cell to the circulation. Red blood cell nuclei or
malarial parasites can be removed by this process without destroying the cell.
•
Iron reutilisation —
the phagocytic
reticular cells remove iron from ingested degraded haemoglobin during red cell
culling and return the iron to the plasma.
•
Pooling — up to 30—40 per cent of blood platelets are sequestered within the
spleen. In splenomegaly up to 80 per cent of the platelet pool can be
sequestered in the spleen and this, together with accelerated platelet destruction,
can result in thrombocytopenia.
•
Reservoir function —
in animals,
especially the dog, up to one-third of the blood volume may be sequestered in
the
spleen
during sleeping and returned to the circulation on waking. This does not occur
in humans.
Haematopoiesis
— this
only occurs up to the fifth intrauterine month and thereafter in certain
disease states.
Investigation
of the spleen
Conditions that cause splenomegaly can be
diagnosed by history, physical signs and laboratory examination. For instance
in haemolytic anaemia a full blood count, reticulocyte count and tests for
haemolysis will determine the type of anaemia. Similarly, splenomegaly
associated with portal hypertension caused by hepatic cirrhosis is diagnosed by
the physical signs of liver dysfunction, abnormal liver function tests and
evidence of oesophageal varices. Many conditions that cause splenomegaly also
cause lymphadenopathy. The cause of the splenomegaly should then be determined
from investigations for diseases known to cause lymphadenopathy and splenomegaly.
A lymph node biopsy can be helpful in this respect.
Plain
radiography
This plays little part in investigation now,
although calcification in the spleen may suggest an old infarct or hydatid
disease. Multiple areas of calcification would suggest splenic tuberculosis.
Imaging
Ultrasonography
of the spleen
(Fig. 53.2) is of value in determining its size and consistency, and
whether or not cysts are present. It can be used for diagnosis of a ruptured
spleen. However, in this case a computerised tomography (CT) scan is more
usually undertaken to exclude other intra-abdominal
problems. Similarly, a spiral CT scan with
contrast enhancement will be preferable in the diagnosis of splenomegaly to
determine both the extent of the disease and the associated problems such as the
extent of lymphadenopathy.
Radioisotope
scans
Technetium-99m (99mTc)-labelled
colloid can provide information about the position and size of the spleen and,
with appropriately labelled red cells, the life and place of their destruction
can he determined.