HEPATIC PORTAL CIRCULATION

 

OBJECTIVES OF THE TOPIC:  By the end of this discussion, a medical student will be able to;

1.  Outline the blood vessels that supply and drain the liver.

2.  Describe the importance of blood shunting through the liver from the GIT.

3.  Give examples of molecules moderated by the liver enzyme system.

NEW TERMS

1.  Portal vein:  This is a blood vessel that is shared between two organs in a series connection.  This scenario occurs majorly in two instances in the human body; between the intestines and the liver, and between the hypothalamus and pituitary gland.

INTRODUCTION: Hepatic portal circulation is a subdivision of systemic circulation in which blood from the abdominal digestive organs and spleen circulates through the liver before returning to the heart.  This is to ensure that all contents of the digestion from the intestines pass through the liver for regulation and modulation.

FORMATION OF THE PORTAL VEIN: Blood from the capillaries of the stomach, small intestine, colon, pancreas, and spleen flows into two large veins, the superior mesenteric vein and the splenic vein, which unite to form the portal vein. The portal vein takes blood into the liver, where it branches extensively and empties blood into the sinusoids, the capillaries of the liver.

BLOOD FLOW IN THE PORTAL VEIN: From the sinusoids, blood flows into hepatic veins, to the inferior vena cava and back to the right atrium. In this pathway, there are two sets of capillaries, and it is in capillaries that exchanges take place.

IMPORTANCES OF THE PORTAL CIRCULATION: Example 1: Glucose from carbohydrate digestion is absorbed into the capillaries of the small intestine; after a big meal this may greatly increase the blood glucose level. If this blood were to go directly back to the heart and then circulate through the kidneys, some of the glucose might be lost in urine. However, blood from the small intestine passes first through the liver sinusoids, and the liver cells remove the excess glucose and store it as glycogen. The blood that returns to the heart will then have a blood glucose level in the normal range.

Example 2: Alcohol is absorbed into the capillaries of the stomach. If it were to circulate directly throughout the body, the alcohol would rapidly impair the functioning of the brain. Portal circulation, however, takes blood from the stomach to the liver, the organ that can detoxify the alcohol and prevent its detrimental effects on the brain. Of course, if alcohol consumption continues, the blood alcohol level rises faster than the liver’s capacity to detoxify, and the well known signs of alcohol intoxication appear.

In summery: The portal circulation pathway enables the liver to modify the blood from the digestive organs and spleen. Some nutrients may be stored or changed, bilirubin from the spleen is excreted into bile, and potential poisons are detoxified before the blood returns to the heart and the rest of the body.

RELATED;

1.  THE LIVER

2.  FUNCTIONS OF THE LIVER

3.  PITUITARY GLAND

4.  ANATOMY AND PHYSIOLOGY

REFERENCES