2. INTRODUCTION
• Splanchnic or visceral circulation constitutes
three portions:
1. Mesenteric circulation supplying blood to GI
tract
2. Splenic circulation supplying blood to spleen
3. Hepatic circulation supplying blood to liver.
• Unique feature of splanchnic circulation is that
the blood from mesenteric bed and spleen
forms a major amount of blood flowing to liver.
Blood flows to liver from GI tract and spleen
through portal system.
4. REGULATION OF MESENTERIC BLOOD
FLOW
• Mesenteric blood flow is regulated by the
following factors:
1. Local Autoregulation
• Local autoregulation is the primary factor
regulating blood flow through mesenteric bed.
1. Activity of Gastrointestinal Tract
• Contraction of the wall of the GI tract reduces
blood flow due to compression of blood vessels.
And relaxation of wall of GI tract increases the
blood flow due to removal of compression on
the vessel wall.
5. 3. Nervous Factor
• Mesenteric blood flow is regulated by
sympathetic nerve fibers. Increase in sympathetic
activity as in the case of emotional conditions or
‘fight and flight reactions’ constrict the
mesenteric blood vessels.
• So, more blood is diverted to organs like skeletal
muscles, heart and brain, which need more blood
during these conditions. Parasympathetic nerves
do not have any direct action on the mesenteric
blood vessels.
• But these nerves increase the contraction of GI
tract which compresses the blood vessels,
resulting in reduction in blood flow.
6. 4. Chemical Factors – Functional Hyperemia
• Functional hyperemia is the increase in
mesenteric blood flow immediately after food
intake. It is mainly because of gastrin and
cholecystokinin secreted after food intake.
• In addition to these two GI hormones,
digestive products of food substances such as
glucose and fatty acids also cause
vasodilatation and increase the mesenteric
blood flow.
7. SPLENIC CIRCULATION
IMPORTANCE OF SPLENIC CIRCULATION
• Spleen is the main reservoir for blood. Due to
the dilatation of blood vessels, a large amount
of blood is stored in spleen. And the
constriction of blood vessels by sympathetic
stimulation releases blood into circulation.
8. STORAGE OF BLOOD
• In spleen, two structures are involved in storage
of blood, namely splenic venous sinuses and
splenic pulp.
• Small arteries and arterioles open directly into
the venous sinuses. When spleen distends,
sinuses swell and large quantity of blood is
stored. Capillaries of splenic pulp are highly
permeable.
• So, most of the blood cells pass through capillary
membrane and are stored in the pulp. Venous
sinuses and the pulp are lined with
reticuloendothelial cells.
9. REGULATION OF BLOOD FLOW TO SPLEEN
• Blood flow to spleen is regulated by
sympathetic nerve fibers.
11. „
NORMAL BLOOD FLOW
• Liver receives maximum amount of blood as
compared to any other organ in the body since,
most of the metabolic activities are carried out in
the liver.
• Blood flow to liver is 1,500 mL/minute, which
forms 30% of the cardiac output.
• It is about 100 mL/100 g of tissue/minute.
• Normally, about 1,100 mL of blood flows through
portal vein and remaining 400 mL of blood flows
through hepatic artery
12. REGULATION OF BLOOD FLOW TO
LIVER
1. Systemic Blood Pressure
• Systemic blood pressure is the important factor
responsible for blood flow to liver and hepatic
blood flow is directly proportional to systemic
blood pressure.
2. Splenic Contraction
• During splenic contraction, blood flow to liver
increases.
3. Movements of Intestine
• Motility of intestine increases hepatic blood flow
13. 4. Chemical Factors
• Chemical factors which increase the blood flow to
liver by vasodilatation are:
i. Excess carbon dioxide
ii. Lack of oxygen
iii. Increase in hydrogen ion concentration.
5. Nervous Factors
• Sympathetic fibers to liver cause vasoconstriction
in liver and decrease the blood flow. Sympathetic
fibers to liver and other portions of splanchnic
circulation pass through splanchnic nerve. Role of
parasympathetic fibers in hepatic circulation is
not known.
15. ARCHITECTURE OF CUTANEOUS
BLOOD VESSELS
• Architecture of cutaneous blood vessels is formed
in the following manner:
1. Arterioles arising from the smaller arteries reach
the base of papillae of dermis
2. Then, these arterioles turn horizontally and give
rise to meta-arterioles
3. From meta-arterioles, hairpin-shaped capillary
loops arise. Arterial limb of the loop ascends
vertically in the papillae and turns to form a
venous limb, which descends down.
16. 4. After reaching the base of papillae, few
venous limbs of neighboring papillae unite to
form the collecting venule
5. Collecting venules anastomose with one
another to form the subpapillary venous
plexus 6. Subpapillary plexus runs horizontally
beneath the bases of papillae and drain into
deeper veins.
17. FUNCTIONS OF CUTANEOUS
CIRCULATION
• Cutaneous blood flow performs two functions:
1. Supply of nutrition to skin
2. Regulation of body temperature by heat loss.
18. NORMAL BLOOD FLOW TO SKIN
• Under normal conditions, the blood flow to
skin is about 250 mL/square meter/minute.
• When the body temperature increases,
cutaneous blood flow increases up to 2,800
mL/square meter/minute because of
cutaneous vasodilatation.
19. REGULATION OF CUTANEOUS BLOOD
FLOW
• Cutaneous blood flow is regulated mainly by
body temperature. Hypothalamus plays an
important role in regulating cutaneous blood
flow.
• When body temperature increases, the
hypothalamus is activated.
• Hypothalamus in turn causes cutaneous
vasodilatation by acting through medullary
vasomotor center
20. • Now, blood flow increases in skin. Increase in
cutaneous blood flow causes the loss of heat
from the body through sweat.
• When body temperature is low,
vasoconstriction occurs in the skin.
• Therefore, the blood flow to skin decreases
and prevents the heat loss from skin.
21. LEWIS TRIPLE RESPONSE
• Lewis triple response is the vascular response of
skin that includes three consecutive reactions of
blood vessels of skin to a mechanical stimulus.
• It was discovered by Lewis Sir Thomas in 1927.
• He noticed that the vascular reactions of skin to
various injuries occur in three stages and named
these reactions as triple response.
• Three reactions of this response:
1. Red reaction
2. Flare
3. Wheal.
22. 1. Red Reaction
• Red reaction is the appearance of a red line
when a pointed instrument is drawn firmly
over the surface of the skin. This reaction
occurs over the line of the stroke. Red reaction
appears within 15 seconds after the stroke.
• It obtains the maximum intensity at the end
of 1 minute and disappears later gradually.
Red reaction is because of dilatation of
capillaries due to mechanical stimulus.
• This reaction is purely a local response.
23. • It occurs due to the release of histamine-like
substance from the tissues damaged by the
stimulus.
• Lewis called it ‘H’ substance. Red reaction
does not depend upon nervous factors. It
occurs even after the sectioning or
degeneration of nerves of skin.
24. 2. Flare
• If the stroke is applied with little more force or
if the stroke is repeated on the same line, the
red reaction spreads around the line of stroke.
• It spreads for about 10 cm from the line of
stroke, depending upon the force applied. This
is called flare or spreading flush.
• Flare appears within 30 seconds after
appearance of red line. It also disappears later.
Flare is due to dilatation of arterioles.
• It depends upon nervous mechanism and is
due to axon reflex.
25. Axon Reflex
• Axon reflex or antidromic reflex is the process
by which the impulses are conducted in a
direction opposite to the normal direction.
• Normally, the impulses produced by a
cutaneous pain receptor pass through sensory
nerve fiber towards the nerve cell body in
posterior nerve root ganglion.
26. • Some of these impulses pass through the
other branches of the same fiber in the
opposite direction and reach the blood vessels
supplied by these branches.
• Impulses now dilate the blood vessels. This is
called the antidromic or axon reflex. Nerve
fibers transmitting the impulses in the
opposite direction are called antidromic
vasodilator fibers.
• Flare occurs if the main trunk of nerves is cut.
It does not occur when the nerves
degenerate.
27.
28. 3. Wheal
• When intensity of stimulus is severe, the surface
of skin on the line of stroke is interrupted. A small
elevation or swelling is seen in the surrounding
area up to a height of 2 mm.
• It is called wheal or local edema. Wheal appears
within 3 minutes after the stimulus and it
replaces the red line.
• Maximum height is obtained within 5 minutes
and it disappears after several hours. Wheal
appears due to the leakage of fluid from
capillaries.
• The permeability of capillary membrane is
increased. Wheal does not depend upon nervous
mechanism.
29. Dermographism
• The process of embossing signs over skin is
called dermographism. It is also called writing
on skin. Some letters or designs can be
embossed upon the skin over back or in the
forearm in the same manner by which the
wheal is produced.
