Other organs that are part of the digestive system include the liver, gall bladder and pancreas.
The liver is the largest gland in the body; it is multifunctional. To understand the function of the liver it is necessary to understand the blood supply to the liver. It is supplied by the hepatic artery in the typical manner but it is the only digestive organ drained by the inferior vena cava. Other digestive organs such as the small intestine, parts of the large intestine, stomach and pancreas are drained by the hepatic portal system which takes the blood directly to the liver. Thus, the liver receives oxygen poor, nutrient rich blood from the hepatic portal system and oxygen rich blood from the hepatic artery.
Functions of the liver
Digestive and Metabolic Functions
synthesis and secretion of bile
storage of glycogen and lipid reserves
maintaining normal blood glucose, amino acid and fatty acid concentrations
synthesis and release of cholesterol bound to transport proteins
inactivation of toxins
storage of iron reserves
storage of fat-soluble vitamins
synthesis of plasma proteins
synthesis of clotting factors
synthesis of the inactive angiotensinogen
phagocytosis of damaged red blood cells
storage of blood
breakdown of circulating hormones (insulin and epinephrine) and immunoglobulins
inactivation of lipid-soluble drugs
General organization. Structurally the liver is divided into lobules by loose connective tissue septae. These septae are more prominent in some domestic animals than in others; the pig has the most prominent septae and they are readily apparent grossly. For a long time the lobule as defined by these septae was thought to be the basic functional unit of the liver but now it seems that another unit, i.e., the hepatic acinus, might better represent the functional unit of the liver. Both the hepatic lobule and the hepatic acinus will be described but first the basic histology of the liver will be described.
At low magnification the liver looks relatively homogeneous and on first examination little organization can be discerned. A closer look reveals the presence of "lobules" or groups of hepatocytes arranged around a blood vessel, the central vein, and defined by loose connective tissue in which the portal canals are found. This type of organization is most easily seen in the pig liver.
Hepatocytes are one of the primary functional cells of the liver. They are located in flat irregular plates that are arranged radially like the spokes of a wheel around a branch of the hepatic vein, called the central vein or central venule since it really has the structure of a venule.
Portal canal: Three structures are found gouped together in the loose connective tissue surrounding the plates of hepatocytes. These include branches of the hepatic artery, the hepatic portal vein (venule) and the intralobular bile ductule. This group of three structures has been called a portal triad but now is called a portal canal.
of pig liver (Lab slide 61)
Sinusoids are larger than conventional capillaries and less regular in shape. They are lined by thin endothelial cells (E). Also residing on the sinusoidal walls are macrophages called Kupffer cells (K). These are phagocytic cells that remove particulate material and old red blood cells from circulation. Kupffer cells are members of the mononuclear phagocyte system
Hepatocytes are arranged in rows that radiate out from the central vein. These rows are one cell wide and are surrounded by sinusoidal capillaries or sinusoids. This arrangement ensures that each hepatocyte is in very close contact with blood flowing through the sinusoids, i.e. bathed in blood.
The endothelial cells lining sinusoids are fenestrated and in most species lack a basal lamina. Gaps are also present between the endothelial cells. Taken together these two properties make the sinusoids extremely leaky and allow for the extremely close contact between the blood and the surface of hepatocytes. Many materials in the blood, except for whole blood cells, can pass between the spaces in the sinusoidal lining.
Although sinudoidal endothelial cells lie very close to hepatocytes, they do not actually make contact. A narrow space is present between the surface of the hepatocyte and the surface of the endothelial cell. This is called the space of Disse; it is filled with numerous microvilli from the hepatocytes. As in other areas of the body, these structures serve to increase the surface area of the cell membrane that comes in contact with the blood facilitating exchange of molecules between hepatocytes and the blood.
What is the basic functional unit of the liver?
|Secretion of bile in the liver
Bile is produced and secreted by hepatocytes into a special "duct" called a bile canaliculus. This "duct" is actually just a space formed between two hepatocytes that is separated from the connective tissue space around the hepatocytes by the presence of tight junctions. The bile canaliculi empty into branches of the bile ductules which eventually empty into the hepatic duct that carries the bile out of the liver to the gall bladder for concentration and storage. In the duct system, bile flows in the direction opposite to the flow of blood in the sinusoids.
Pig liver (Lab slide 61). HPV = hepatic portal vein; HA = hepatic artery; BD= bile ductule. Note that bile flows in the direction of the arrows, from its production by hepatocytes through the bile canaliculi toward the bile ductule.
The gall bladder receives bile from the liver. Bile is composed of bile salts that emulsify fats forming water-soluble complexes with lipids (micelles) to facilitate the absorption of fat. Bile salts in the small intestine also activates lipases in the intestine.
Functions of the gall bladder.
storage of bile
concentration of bile
acidification of bile
send bile to the duodenum in
response to cholecystokinin
secreted by from enteroendocrine cells in small intestine;
horse does not have a gall bladder and bile is continuously received from the liver
Gall bladder structure. The gall bladder is a sac that is lined with a simple columnar epithelium and has a tunica muscularis containing smooth muscle that is innervated by both the parasympathetic and sympathetic branches of the autonomic nervous system.
Tunics (layers) of the Gall Bladder
The pancreas contains both exocrine and endocrine components that secrete digestive enzymes and peptide hormones respectively. These two components are very different structurally and functionally but are intermingled within the gland. However, the organization of the exocrine part into acini make it fairly easy to recognize in histological sections as does the organization of the endocrine part around areas of high vascularity.
Organization of the pancreas.
The bulk of the pancreas by volume consists of exocrine cells that secrete an alkaline solution of digestive enzymes. This secretion moves through a duct system that eventually leads to the pancreatic duct. Only about 5% of the volume of the pancreas consists of endocrine cells. These cells secrete peptide hormones that play a role in controlling carbohydrate metabolism. The endocrine cells are closely associated with large numbers of blood capillaries into which they secrete the peptide hormones.
The exocrine pancreas. The exocrine portion of the pancreas is a compound acinar gland. It has many small lobules, each of which is surrounded by connective tissue septa through which run blood vessels, nerves, lymphatics, and interlobular ducts. Exocrine secretion by the pancreas is controlled by hormones and nerves. When the hormone, secretin, is released from neuroendocrine cells in the duodenum of the small intestine, the pancreas secretion is watery and rich in bicarbonate. This "basic" secretion helps to neutralize the acidic chyme as it comes into the small intestine. In addition, when cholecystokinin-pancreozymin (CCK) is released by neuroendocrine cells in the duodenum the pancreas secretes a product rich in enzymes that breakdown proteins, carbohydrates, lipids and nucleic acids in the lumen of the small intestine. Gastrin which is secreted by pyloric neuroendocrine cells also results in a pancreatic secretion rich in digestive enzymes. Two of the digestive enzymes secreted by the pancreas are trypsin and chymotrypsin; they are secreted as non-active, pro- or zymogen forms and are subsequently activated by enterokinase in the lumen of the duodenum to avoid digestion of the pancreatic acinar cells.
A compound acinar gland.
Micrograph of rabbit pancreas (Lab slide U). C=center
of acinus. Note pale-stained centroacinar cell that forms the
beginning of the lining of the intercalated or interlobular duct.
Micrograph of rabbit pancreas (Lab slide U).
Note the smaller intralobular duct as compared with the much larger
The endocrine pancreas.
|The cells of the endocrine portion of the
pancreas are arranged either in round-to-oval shaped areas rich in blood
vessels known as the islets of Langerhans or they may be
scattered throughout the exocrine portions of the pancreas near the
acini or ducts. There are several different types of cells in the
islet or other regions, each secreting a different peptide
hormone. It is not possible to distinguish among these cells with
routine hematoxylin and eosin stain used for histological
preparations. Immunocytochemistry is necessary to identify which
cells are secreting a particular peptide. This is done by staining
with an antibody made to the specific peptide that is combined with a
label that can be visualized at the light microscopic level such as
Examples of peptide hormones secreted by the endocrine pancreas:
Micrograph of rabbit pancreas (Lab slide U) showing a typical islet of Langerhans, the endocrine part of the pancreas.
Copyright 2002 Charlotte L. Ownby
Histology Part 2 Index