Splanchnic Circulation Physiology Notes

Splanchnic Circulation Introduction

The splanchnic or visceral circulation constitutes three portions:

1. Mesenteric circulation supplies blood to the Gl tract
2. Splenic circulation supplies blood to the spleen
3. Hepatic circulation supplies blood to the liver.

The unique feature of splanchnic circulation is that the blood from the mesenteric bed and spleen forms a major amount of blood flowing to the liver. Blood flows to the liver from the GI tract and spleen through the portal system.

Read And Learn More: Medical Physiology Notes

Mesenteric Circulation

Distribution Of Blood Flow

• Stomach — 35 mL/100 gm/minute
• Intestine — 50 mL/100 gm/minute
• Pancreas — 80 mL/100 gm/minute

Regulation Of Mesenteric Blood Flow

Mesenteric blood flow is regulated by the following factors:

1. Mesenteric blood flow Local autoregulation

It is the primary factor regulating blood flow through the mesenteric bed.

2. Mesenteric blood flow Activity of the Gl Tract

• Contraction of the wall of the Gl tract reduces blood flow due to compression of blood vessels.
• And, relaxation of the wall of the Gl tract increases the blood flow due to the removal of compression on the vessel wall.

3. Mesenteric blood flow Nervous Factor

• Mesenteric blood flow is regulated by sympathetic nerve fibers. An 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.
• The parasympathetic nerves do not have any direct action on the mesenteric blood vessels.
• But these nerves increase the contraction of the Gl tract which compresses the blood vessels resulting in a reduction in blood flow.

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 Gl hormones, digestive products of food substances such as glucose and fatty acids also cause vasodilatation and increase the mesenteric blood flow.

Splenic Circulation

Importance Of Splenic Circulation

• The spleen is the main reservoir for blood. Due to the dilatation of blood vessels, a large amount of blood is stored in the spleen.
• And the constriction of blood vessels by sympathetic stimulation releases blood into circulation.

Storage Of Blood

• In the spleen, two structures are involved in the storage of blood namely, splenic venous sinuses and splenic pulp.
• The small arteries and arterioles open directly into the venous sinuses. When the spleen distends, the sinuses swell and a large quantity of blood is stored.
• The capillaries of splenic pulp are highly permeable. So, most of the blood cells pass through the capillary membrane and are stored in the pulp.
• The venous sinuses and the pulp are lined with reticuloendothelial cells.

Regulation Of Blood Flow To Spleen

The blood flow to the spleen is regulated by sympathetic nerve fibers.

Hepatic Circulation

Bloodvessels

The liver receives blood from two sources:

1. A hepatic artery from the aorta
2. A portal vein from mesenteric and splenic vascular bed.

Normal Blood Flow

• The liver receives the 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.
• The blood flow to the liver is 1,500 mL/minute, which forms 30% of cardiac output. It is about 100 mL/100 g of tissue/minute.
• Normally, about 1100 mL of blood flows through the portal vein and the remaining 400 mL of blood flows through the hepatic artery. However, the portal vein carries only about 25% of oxygen to the liver.
• It is because it carries the blood, which has already passed through the blood vessels of the Gl tract where oxygen might have been used. The hepatic artery transports 75% of oxygen to the liver.

Regulation Of Blood Flow To Liver

The blood flow to the liver is regulated by the following factors:

1. Blood Flow To Liver ystemic Blood Pressure

It is the important factor responsible for blood flow to the liver and hepatic blood flow is directly proportional to systemic blood pressure.

2. Blood Flow To Liver Splenic Contraction

During splenic contraction, blood flow to the liver increases.

3. Blood Flow To Liver Movements of Intestine

The motility of the intestine increases hepatic blood flow.

4. Blood Flow To Liver Chemical Factors

Chemical factors that increase the blood flow to the liver by vasodilatation are:

1. Excess carbon dioxide
2. Oxygen lack
3. Increase in hydrogen ion concentration.

5. Blood Flow To Liver Nervous Factors

• Sympathetic fibers to the liver cause vasoconstriction in the liver and decrease blood flow.
• Sympathetic fibers to the liver and other portions of splanchnic circulation pass through a splanchnic nerve.
• The role of parasympathetic fibers in hepatic circulation is not known.