The peculiarities that have been encountered in the splenic circulation are the presence of both ‘open’ and ‘closed’ circulation. The pulp acts as filter for separating R.B.C. from plasma. Phagocytes of the pulp remove the foreign particles from the blood.
Pulp also acts as blood reservoir yet its function as such in human beings is not clear but in dogs and cats, it is fully settled. During emergency, such as exercise, shock haemorrhage, asphyxia, etc., there is profound contraction of the spleen causing discharge of a large amount of blood rich in R.B.C. in the general circulation. However, its role in human beings is not fully known because splenectomy has got no effect.
The circulation of spleen has a capacity of about 500 ml. When sympathetic stimulation occurs, these capacities of about 300 ml are added to general circulation. This added blood possesses a very high concentration of erythrocytes. During splenic relaxation, R.B.C. are filtered out of the circulation and stored in the Pulp. On the other hand, the splenic contraction causes return of R.B.C. to the circulation.
Rate of Blood Flow:
Most of the investigators have concentrated in studying the volume change rather than its rate of blood flow. Accordingly, flow pattern of the spleen has been mostly considered by the volume change of the spleen.
Increase of volume as dilatation of splenic blood vessels causing increase of flow. Reverse is the case when the volume is decreased. However Grim (1963) has described the rate of blood flow in dogs of 15 Kg body weight and 130 mm of Hg arterial blood pressure to be 70 ml/min/100 gm tissue.
Control of Blood Flow:
Pressure/flow relationship of the spleen indicates that the resistance is greatly decreased during increase of perfusion pressure. This decrease in resistance is considered to be the cause of opening of the vascular sinuses.
Factors that alter the splenic blood flow are described below:
i. Neurogenic Factor:
Neurogenic factors seem to have an important role in the regulation of blood flow in the spleen. Stimulation of the splanchnic nerve causes rapid and intensive rhythmical contraction of the spleen along with decrease of flow. During contraction it gives into the general circulation a considerable amount of blood.
Green and his associates (1960) have considered the splenic-emptying mechanisms being under the sympathetic control. Splenic nerve stimulation causes decrease of arteriolar inflow, decrease of splenic volume and weight and increase of venous outflow.
ii. Reflex Control:
Stimulation of baroreceptors in the carotid sinus and aortic arch baroreceptors causes reflex modification of the splenic volume, splenic weight and splenic circulation. Profound increase or the splenic volume following increased pressure in the isolated carotid sinus of dogs has been observed by Hey-mans (1929).
Reflex dilatation of the splenic vessels is possibly the cause of this response. On the contrary during increase of pressure in the sinus causes reflex decrease of splenic volume through the activation of the sympathetic nerve.
iii. Humoral Control:
Adrenaline and noradrenaline both decrease the splenic volume. Splenic rhythmical contraction is increased following administration of adrenaline or noradrenaline. It is claimed that these hormones play an important role in emptying mechanisms of spleen. The noradrenaline helps by arteriolar constriction whereas the adrenaline helps by increasing the venous outflow through venular dilatation or through contraction of the splenic smooth muscle.
Acetylcholine and methacholine generally increase the arterial and venous outflow. The volume of the spleen is also slightly decreased. But topical application of acetylcholine, adrenaline, noradrenaline and histamine use arteriolar constriction.
During haemorrhage, when the systemic blood pressure falls, the contractions of the spleen along with arteriolar constriction, decrease of splenic weight and increase of venous outflow are observed due to reflex sympathetic activity through the stimulation of chemoreceptors.
During exercise, the contraction of the spleen is increased. This is due to redistribution of blood from the inactive area to the active area. Increased sympathetic activity during exercise is the possible cause.
vi. Distention of Viscera:
The splenic vein and the superior mesenteric veins provide the major blood supply to the liver through the portal vein. Thus the splenic venous drainage to is through the portal vein. So if there is any distention of liver vascular bed due to increase of sinusoidal resistance or due to pathological conditions then splenic drainage may be affected.
Besides this, recently it has been observed that if the hollow viscera like stomach is distended then redistribution of blood occurs there. Under such state the hepatic blood flow is decreased whereas splenic and mesenteric blood flows are increased.