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INTRODUCTION

This chapter will focus on general considerations concerning the hepatic system and its related physiology. The hepatic system plays a critical role in many essential functions of the body including normal metabolism, hemostasis, and drug breakdown.

GENERAL PHYSIOLOGICAL CONSIDERATIONS

Hepatic Physiology

The digestive system may seem to play a less critical role during anesthesia than the circulatory and respiratory systems.1,2 While digestion and processing of nutrients are major functions of the liver and gastrointestinal tract, their roles in hemostasis, drug metabolism, and regurgitation (with possible pulmonary aspiration of gastric contents) illustrate their importance perioperatively.

Hepatic Anatomy

The largest solid organ in the body, the liver, occupies the upper portion of the abdomen immediately below the right diaphragm.3 Based on functional anatomy, the liver is divided into four lobes mostly on the right side of the upper abdominal cavity underlying the diaphragm. The lobes are further subdivided into 50,000 to 100,000 lobules.

Hepatic Blood Supply

The liver receives blood from two sources: the hepatic artery and the hepatic portal vein.4 The hepatic artery is a branch of the celiac artery, which is the first major branch of the aorta below the diaphragm. The hepatic portal vein drains blood from several abdominal organs (see Fig. 39-1), then delivers it to the liver for modifying and detoxifying substances in that blood. (liver/GI lect 1)

The adult liver receives about 500 ml/min of systemic arterial blood from the hepatic artery. The stomach, spleen, pancreas, small intestine, and most of the colon contribute blood to the hepatic portal vein, which delivers about 1300 ml/min. This blood is less well oxygenated than the hepatic arterial blood because it has already perfused the abdominal organs where oxygen has been extracted and nutrients, antibodies, toxins, and bacteria added. Oxygen delivery to the liver via these two vascular inflows is nearly equal (45−50% via the hepatic artery and 50−55% via the portal vein).

There is an interdependence of these two blood flows to the liver. Portal vein blood flow undergoes little active control, changing rather passively in response to fasting, feeding, and changes in cardiac output. In contrast, changes in hepatic arterial tone can produce up to a doubling in hepatic artery blood flow in times of reduced portal venous perfusion.

Hepatic Physiology

The liver performs multiple essential metabolic and excretory functions. Related to its position between the gastrointestinal tract and the systemic circulation, the liver is the site where ingested nutrients and other substances from the GI tract are processed (hepatic first-pass) before entering the body.1,2 Useful substances absorbed from the gut can be modified and stored while potentially harmful material can be detoxified and excreted. Digested carbohydrate, protein, and lipid molecules are processed by the multiple ...

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