Circulation of blood through the abdominal viscera.
It has mainly three components –
1. Circulation through the GIT proper and the mesenteric attachments.
2. Circulation through the liver.
3. Circulation through the spleen.
INTESTINAL CIRCULATION
•Major function of the intestine is digestion and absorption of nutrients.
•Normal intestinal blood flow is 20% of cardiac output at rest and 50% after a meal.
BLOOD SUPPLY OF INTESTINE
•ARTERIAL SUPPLY –
It is supplied mainly by three arteries-
Celiac trunk or artery
Superior mesenteric artery
Inferior mesenteric artery
•Superior mesenteric artery is the largest branch of aorta. It supplies to many parts of small and large intestine.
•The branches of mesenteric arteries form an extensive vascular network in the submucosa of GIT.
•The branches from these arteries penetrate deep into the muscle layers and form the arterioles. The arterioles supply the tip of the villi.
•Intestinal mucosa receives 60-70% of total intestinal blood flow.
Venous drainage
• Capillaries
↓↓
Venules
↓↓
Mucosal Venules
↓↓
Veins
•Arterioles and venules form typical counter current exchange system.
•Supply of oxygen to the mucosal cells at the tip of the villus is reduced.
Values of normal blood flow
•Gastric blood flow – 40mL/100gm of tissue/minute
•Intestinal blood flow – 60mL/100gm of tissue/minute
•Pancreatic blood flow- 80mL/100gm of tissue/minute
REGULATION OF INTESTINAL CIRCULATION
•NEURAL REGULATION
It is mainly achieved by the sympathetic system.
•Stimulation of the sympathetic fibers results in constriction of the mesenteric arteries and arterioles and greatly reduces the blood flow.
•During exercise, the sympathetic vasoconstriction mediated via alpha receptors shifts blood from the mesenteric vascular bed to the skeletal muscle.
AUTOREGULATION
•It is not well developed.
•Mainly occurs by metabolic and myogenic mechanisms.
•Metabolic regulation –
• Adenosine – Vasodilation and ↑↑ blood flow
•Osmolality - ↑ osmolality of blood passing through intestine occurs following ingestion, causes vasodilation.
•Potassium – causes vasodilation.
Regulation by GI activity
•GI blood flow increases with increase in GI activity occurs during digestion of food.
•Increased absorption of food affects intestinal blood flow.
•Chyme , glucose and fatty acids are potent vasodilators.
APPLIED PHYSIOLOGY
•Intestinal necrosis is common in shock.
•Do not perform strenuous exercise after meals because it impairs GI circulation.
HEPATIC CIRCULATION
•Liver is an important vital organ as it performs major metabolic activities of the body.
•Hepatic blood flow is about 28% of cardiac output.
BLOOD SUPPLY OF LIVER
It is mainly by two vessels
Portal vein
Hepatic artery
Portal vein – 75% of blood flow to the liver and this blood have less oxygen concentration.
Hepatic artery – 25% 0f blood flow to the liver
and it is fully saturated with oxygen.
•Blood flow to liver is about 1500mL/min which is 28% of cardiac output.
REGULATION OF HEPATIC BLOOD FLOW
•Hepatic arterial buffer response – An interactive relationship exists between the regulation of portal venous and hepatic arterial blood flow.
•Alteration in hepatic arterial blood flow occurs reciprocally with portal venous blood flow.
•It compensates for change in portal blood flow.
NEURAL REGULATION
Both hepatic artery and portal vein are innervated by sympathetic vasoconstrictor fibers
METABOLIC REGULATION
Various metabolites like CO2 and H+ , Adenosine cause vasodilation and increase hepatic vasodilation.
AUTO REGULATION
Portal venous system doesn’t auto regulate its blood flow.
REGULATION BY INTESTINAL ACTIVITY
GIT activities increase blood flow to the liver via portal system.
CLINICAL IMPORTANCE OF HEPATIC CIRCULATION
•Liver filters the blood and permits into systemic circulation.
•Toxins are detoxified in the liver.
•Nutrients are absorbed through the liver.
•When CVP is elevated in CHF , hydrostatic pressure increases and leads to tender hepatomegaly. This is an important feature of heart failure.
•Extensive fibrosis of liver as occurs in hepatic cirrhosis causes increased hepatic vascular resistance.
↓
Increased capillary hydrostatic pressure
Transudation of fluid into abdominal cavity
↓
Ascites
•In portal hypertension , eosophageal varices occurs. To prevent hemetemesis , portocaval shunt is performed.
•Liver stores about 15% of the total blood volume.
•Liver is an important reservoir of blood.
SPLENIC CIRCULATION
•Main source of blood supply to spleen is splenic artery.
•Splenic artery is supplied by the sympathetic vasoconstrictors.
CLINICAL IMPORTANCE
•Spleen functions as a reservoir of blood.
•Splenic contraction releases adequate amount of blood into general circulation.
•Protective mechanism in exercise and shock.
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