Splanchnic circulation

cardiac

1. Splanchnic Circulation Maria Idrees; PT

2. INTRODUCTION Splanchnic or visceral circulation constitutes three portions: Mesenteric circulation supplying blood to GI tract Splenic circulation supplying blood to spleen 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.

3. MESENTERIC CIRCULATION DISTRIBUTION OF BLOOD FLOW Stomach : 35 mL /100 g/minute Intestine : 50 mL /100 g/minute Pancreas : 80 mL /100 g/minute.

4. REGULATION OF MESENTERIC BLOOD FLOW Mesenteric blood flow is regulated by the following factors: Local Autoregulation Local autoregulation is the primary factor regulating blood flow through mesenteric bed. 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 I n 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.

10. HEPATIC CIRCULATION BLOOD VESSELS Liver receives blood from two sources: Hepatic artery Portal vein.

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.

14. Cutaneous Circulation

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: Red reaction Flare 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. 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.

28. 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.