Liver Functions

Liver Functions

Functional unit of liver consists of lobules. Each hepatic lobule has cells radiating centrifugally from a central vein. The periphery of the lobule shows branches of hepatic artery, portal vein and bile duct. These three structures form the portal triad. There are large sinusoids present between the cells, which contain blood from portal vein and hepatic artery. The sinusoids drain blood into the hepatic vein. The endothelial cells lining the sinusoids have Kupffer’s cells and tissue macrophages. Biliary canaliculi are present between adjacent layers of cells. They collect bile and drain into the bile duct.
Each lobe of liver is drained by one hepatic duct. The right and left hepatic ducts join together to form the common hepatic duct. It joins with the cystic duct from gallbladder and forms the common bile duct. This opens at the second part of duodenum, together with the pancreatic duct (ampulla of Vater). The opening at the second part of duodenum is guarded by the sphincter of Oddi.

Liver has many functions which include metabolic, synthetic, storage, excretory, and detoxification. Carbohydrate metabolism Liver helps in glucose homeostasis. It shows glycogenesis, gluconeogenesis and glycogenolysis. It is the site for the conversion of galactose to glucose and fructose to glucose.

Protein metabolism The formation of urea from ammonia, deamination and transamination reactions take place in the liver. Fat metabolism Lipogenesis, lipolysis, β oxidation of fatty acids, synthesis of lipoproteins, synthesis and esterification of cholesterol, formation of bile acids from cholesterol, ketogenesis, take place in the liver.

Liver Functions
Formation of blood coagulation factors,
prothrombin synthesis in the presence of vit K,
synthesis of plasma proteins, formation of bile
salts takes place in the liver. Liver excretes bile pigments, drugs, metals and
dyes (BSP). Liver stores glycogen, vit A, D, B12 and iron. Inactivation of hormones, drugs, toxic substances
occurs in the liver. The Kupffer cells and macrophages in the liver
destroy blood cells.In the early foetal stage, liver is the site of
hemopoiesis.

It is the yellow colouration of skin and mucous membrane, due to increased bile pigments level in plasma. This condition occurs, when bilirubin level in the plasma exceeds 2 mg%. Depending on the cause, three types of jaundice can occur. They are hemolytic (prehepatic) and hepatic
Obstructive (posthepatic)
The hemolytic jaundice is caused by increased destruction of red cells, arising from intrinsic and extrinsic defects in RBCs. Hepatic jaundice occurs, due to hepatitis caused by virus. There are several forms of this type, i.e. Hepatitis A, B, C, etc. Obstructive type is caused by gallstones in the bile ducts and tumors of biliary tree.

Source: P. McKee, J. Calonje – McKee’s Pathology of the Skin (Elsevier)


Deglutition

Deglutition

It refers to the act of swallowing. It consists of oral, pharyngeal and oesophageal stages. The first stage is voluntary, second and third stages are involuntary.

Swallowing, or deglutition, is divided into three phases:

  • The buccal phase occurs voluntarily in the mouth when the tongue forces the bolus of food toward the pharynx.
  • The pharyngeal phase occurs involuntarily when food enters the pharynx, as follows:
    1. The soft palate and uvula fold upward and cover the nasopharynx to prevent the passage of food up and into the nasal cavity.
    2. The epiglottis, a flexible cartilaginous flap at the top of the larynx, folds down as the larynx rises. As a result, the opening to the trachea is covered, and food can pass only into the esophagus.

Deglutition

  • The esophageal phase occurs involuntarily in the esophagus. The esophageal sphincter, normally closed, opens to allow food to pass when the larynx rises during swallowing. When food reaches the lower end of the esophagus, the lower esophageal sphincter opens to allow the food to enter the stomach.

Movements of stomach

Movements of stomach

Receptive relaxation

The stomach shows receptive relaxation, accommodating large volume of food. The receptors for this are present in the wall of pharynx and esophagus. The function of fundus and body of the stomach is to store the food (storage function).
The afferent and efferent impulses for receptive relaxation are carried by the vagus (vagovagal reflex) and causes the myenteric plexus to secrete VIP. This transmitter causes relaxation of the wall of the stomach. Vagotomy decreases the receptive relaxation, though, not completely abolishes, because, the intrinsic nerve plexus is responsible for the receptive relaxation.

Mixing of food (digestive peristalsis)

The distal part of the stomach shows digestive peristalsis. The distention of the wall of the distal part of body and antrum stimulates the intrinsic plexus. The smooth muscle in the wall, shows slow waves, which are nonpropagatory depolarization waves. They are also called basic electrical rhythm (BER). The distention of the wall or the activity of vagus causes development of trains or spikes on the peak of slow waves. They are action potentials, developed, when the slow waves reach the threshold level of firing. The entry of Na+ and Ca++ into the cell causes depolarization. Once the action potential spikes are developed, it becomes propagatory in the form of peristalsis. Vagal stimulation, acetylcholine, gastrin, cause development of spikes or action potentials on the peak of slow waves, which results in peristalsis.

Movements of stomach

The digestive peristalsis travel towards the pylorus, pushing the food forwards. The peristalsis is the wave of contraction followed by relaxation. The frequency of digestive peristalsis in the stomach is 3 to 5/min (20 sec rhythm). The food when reaches the pylorus is retropulsed into the antrum, due to the pyloric sphincter closure. The sphincter closes as the peristalsis arrives at the pylorus. This is necessary to prevent the entry of food into the duodenum without thorough mixing and forming acid chyme. The propulsion, mixing and retropulsion in the pylorus breaks down the food into smaller particles (chyme) and helps thorough mixing with the gastric juice. Each time the peristalsis arrives at the pylorus, only 2 to 3 ml of chyme is emptied into the duodenum.

Source: Textbook of Physiology, 3E (Chandramouli) (2010)


Phases of gastric secretion

Phases of gastric secretion

There are three phases namely:

  • Cephalic
  • Gastric
  • Intestinal.

Cephalic phase
Conditioned reflexes like sight, smell, thought of food causes secretion of the gastric juice. Presence of food in the mouth also causes secretion in the stomach. The cephalic phase occurs by the activity of the vagus. Sham feeding experiments in animals like dogs, gives a good example for the cephalic secretion of gastric juice. The quantity of juice secreted in this phase is less when compared to gastric phase.

Phases of gastric secretion

Gastric phase
The arrival of food and distention of stomach causes the secretion of gastric juice. The secretion in this phase involves the activity of vagus and the hormone gastrin. During this phase, maximal secretion of gastric juice occurs.

Intestinal phase
The arrival of food and products of food digestion in the small intestine also stimulates gastric juice secretion. The quantity of juice secreted is very less. However, the presence of products of food digestion and acid in duodenum inhibits the secretion of gastric juice. This kind of inhibition is mediated through the enterogastric reflex. The presence of acid releases secretin and fat in the duodenum releases CCK. There is also secretion of GIP, VIP hormones from small intestine. All of them cause inhibition of gastric juice.
The eroding of mucosa in the stomach or in the duodenum by HCl and pepsin is called peptic ulcer. It occurs in various conditions and the important ones are:
Breakdown of acid mucosal barrier due to

  • infection by Helicobacter pylori
  • ingestion of aspirin, NSAID (nonsteroidal anti-inflammatory drugs
  • Zollinger-Ellison syndrome (gastrinomas especially from pancreas)
  • Chronic alcoholism
  • Chronic exposure to stress.

Treatment of peptic ulcer involves administering H2 blockers cimetidine, ranitidine, etc. blocking of H+- K+ ATPase by omeprazole.

Source: Textbook of Physiology, 3E (Chandramouli) (2010)