Liver
It is a solid organ of dark reddish-brown color and the largest gland in the body.
In the average adult, the liver weighs about 3 lb. It is situated high in the abdomen and below diaphragm. It has two lobes, in which right lobe is larger containing gallbladder. It is composed of (different parts)-
q Hepatocytes – major functional cells of liver
n It has wide variety of metabolic, secretory, and endocrine functions – secrete bile (excretory product and digestive secretion)
q Bile canaliculi – ducts between hepatocytes that collect bile
n It exits livers as common hepatic duct, joins cystic duct from gallbladder to form common bile duct.
q Hepatic sinusoids – highly permeable blood capillaries receiving oxygenated blood from hepatic artery and deoxygenated nutrient-rich blood from hepatic portal vein
q SPACE OF DISSE
It is the perisinusoidal space between basal surfaces of the endothelial cells and the surfaces of the hepatocytes.
Why are liver capillaries called sinusoids?
These vessels are unusual in that their lining cells contain “sieve plates” which allow proteins to pass through into the Space of Disse and there contact the underlying hepatocytes. Because of this they are termed sinusoids.
Three different ways to organize units
1. Hepatic acinus – preferred method
n Hepatocytes arranged in 3 zones around short axis with no sharp boundaries
2. Portal
3. Classical- basic functional unit of liver
Portions of liver
1. EXOCRINE PORTION
It synthesizes and secretes bile via a system of ducts that is essential for digestion in the intestine.
2. ENDOCRINE PORTION
It synthesizes and secretes numerous plasma proteins into the bloodstream: (albumin, fibrinogen, prothrombin, lipoproteins, etc).
Blood Supply to the Liver
The liver has a dual blood supply. Only 20-25% of the blood derives from the hepatic artery, the remainder comes from the portal veins which drain the stomach and all the intestines except the rectum. The liver hence can remove damaging materials before they reach the systemic circulation. Blood leaves the liver via the hepatic veins which join with the ascending vena cava.
Circulatory Zones in the Liver
n As blood flows from the portal tract to the hepatic venules it gradually loses oxygen. Although the hepatocytes in the well oxygenated “periportal zone” look similar to hepatocytes around hepatic veins (the centrilobular zone) their enzymology is different. Cells in the intermediate (midzonal) area are - intermediate
n Synthesis of blood proteins and gluconeogenesis occur principally in the periportal zone.
Hepatic blood flow
n Hepatic artery carrying oxygenated blood
n Hepatic portal vein carrying deoxygenated blood with newly absorbed nutrients and possibly drugs, microbes or toxins from GI tract
Functions of liver
Carbohydrate Metabolism
n Turn proteins into glucose
n Turn triglycerides into glucose
n Turn excess glucose into glycogen & store in the liver
n Turn glycogen back into glucose as needed
Protein Metabolism
n Deamination = removes NH2 (amine group) from amino acids
n Converts resulting toxic ammonia (NH3) into urea for excretion by the kidney
n Synthesizes plasma proteins utilized in the clotting mechanism and immune system
n Convert one amino acid into another
Lipid Metabolism
n Synthesize cholesterol
n Synthesize lipoproteins----HDL and LDL (used to transport fatty acids in bloodstream)
n Stores some fat
n Breaks down some fatty acids
Other functins
n Detoxifies the blood by removing or altering drugs & hormones (thyroid & estrogen)
n Releases bile salts help digestion by emulsification
n Stores fat soluble vitamins-----A, B12, D, E, K
n Stores iron and copper
n Phagocytizes worn out blood cells & bacteria
n Activates vitamin D (the skin can also do this with 1 hr of sunlight a week)
Gallbladder
It is defined as the pear-shaped reservoir for the bile.
Layers of gallbladder
It has three layers.
1) MUCOSA
-lined with tall columnar epithelium and underlying basal lamina & lamina propria
- mucosa highly folded and irregular
- no muscularis mucosa or submucosa
2) MUSCULARIS EXTERNA
- layers of smooth muscle with irregular orientation
3) ADVENTITIA or SEROSA
Bile Production
Bile is made up of the bile salts, bile pigments, and other substances dissolved in an alkaline electrolyte solution that resembles pancreatic juice. It is yellowish green in color and having pH 7.6 to 8.6.
One quart of bile/day is secreted by the liver. [1 quart = 1.14L (British)
= 0.94L (USA)]
Flow of Bile
n Bile capillaries
n Hepatic ducts connect to form common hepatic duct
n Cystic duct from gallbladder & common hepatic duct join to form common bile duct
n Common bile duct & pancreatic duct empty into duodenum
Role and composition of bile
q Hepatocytes secrete 800-1000mL of bile daily
q It contains mostly water, bile salts, cholesterol, lecithin, bile pigments and several ions.
· Partially excretory product/ partially digestive secretion
· Bilirubin – principal bile pigment
o Derived from heme of recycled RBCs
o Breakdown product stercobilin gives feces brown color
q Bile salts play role in emulsification
o Also aid in absorption of lipids following digestion
Composition of liver and gall bladder bile:
Constituents Liver Bile Gall Bladder Bile
Water 97.5 g/dl 92 g/dl
Bile Salts 1.1 g/dl 6 g/dl
Bilirubin 0.04 g/dl 0.3 g/dl
Cholesterol 0.1 g/dl 0.3 to 0.9 g/dl
Fatty acid 0.12 g/dl 0.3 to 1.2 g/dl
Lecithin 0.04 g/dl 0.3 g/dl
Na+ 145 mcg/lit 130 mcg/lit
K+ 5 mcg/lit 12 mcg/lit
Ca+ 5 mcg/lit 23 mcg/lit
Cl- 100 mcg/lit 25 mcg/lit
HCO3- 28 mcg/lit 10 mcg/lit
Function of Bile
1. Digestive function
Bile helps in the digestion of fat and to a lesser extent of proteins and carbohydrate.
2. Absorptive function
Bile helps in the absorption of fat and other substances like fat soluble vitamins, iron, Ca++ etc.
3. Laxative function
Bile salts increases peristalsis and thereby help in defecation.
4. Excretory function
Certain substance are excreted through bile
a) Heavy metals – Cu, Zn, Hg etc
b) Certain drugs
c) Bile pigments
d) Cholesterols
e) Toxin, Bacteria
f) Acid-Base balance: Mucin of bile acts as buffer and help in acid-base balance
BILE ACIDS
§ Cholic acid
§ Deoxy cholic acid
§ Cheno-deoxy cholic acid
§ Lethocholic acid
§ Enolic acid
Bile Salts
-Sodium tauro-cholate
-Potassium tauro-cholate
-Sodium glycholate
-Potassium glycholate
Functions of bile salts
-Bile salts help in the digestion of fat
-Bile salts help in the absorption of fatty acids, monoglycerides, cholesterol and other lipids.
Gallbladder Diseases
Cholelithiasis & Cholecystitis
1. Cholecystitis = inflammation of GB
2. Cholelithiasis = Stone(s) in GB
Cholelithiasis
n GB shows likely sites of stone formation/deposition.
Histology of the Liver
n Hepatocytes arranged in lobules
n Sinusoids in between hepatocytes are blood-filled spaces
n Kupffer cells are fixed star shaped macrophages which live within the sinusoids and throw processes across the vessel. Their job is to remove particulate materials picked up in the intestine and worn out red blood cells. They are also very heavily involved in message passing
1. Review of medical Physiology, Guyton, 22nd edition, W. B. Sander’s Company, Philadelphia
2. Dictionary of Pharmacy, Md Ali, Tara Publishers, Delhi
3. Anatomy & Physiology, F. Evans, 2nd edition, Prentice Hall Inc., New Jersey
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