Alimentary System

Question 1

What are the functions of the Liver

Answer 1

The liver is the second largest organ of the body and weighs about 1.5kg. The liver processes and stores nutrients absorbed from the GIT. It also neutralises and eliminates toxic wastes. Bile is produced by the liver and is both an excretory product and also plays an important role in lipid metabolism. The liver produces plasma proteins; it is therefore an exocrine and endocrine organ.

Question 2

Describe the blood supply of the liver

Answer 2

The liver has a dual blood supply.
The hepatic portal vein brings nutrient rich but oygen depleted blood from the GIT and the hepatic artery supplies oxygen rich blood from the aorta.

These two bloods mix in the portal regions and then flow in sinusoidal capillaries past the hepatocytes of the liver. The blood is then collected in central veins and eventually back to the heart via the inferior vena cava.

Question 3

How is the liver arranged? with respect to blood supply

Answer 3

The liver is arranged in lobules (hexagonal) around the central veins. The hepatocytes are arranged in sheets or cords separated by the liver sinusoids. Around the periphery of the liver lobules are found the PORTAL AREAS- where the branches of the HEPATIC PORTAL VEIN, HEPATIC ARTERY and BILE DUCTS are found.

Question 4

Describe the structure of the liver!

Answer 4

1. Cells are arranged in columns or cords of cells radiating out from a central vein (CV)
2. The liver cords are separated by sinusoids through which the blood flows towards the central vein.
3. This arrangement creates a liver with many hexagonal lobules each with a central vein and a series of PORTAL REGIONS at the periphery of the lobule
4. The portal region contains an HEPATIC ARTERY, HEPATIC PORTAL VEIN, BILE DUCT, LYMPHATIC CHANNEL. Can be recognised and distinguished by histology

Question 5

Sinusoids

Answer 5

The sinusoids (a type of capillary) are lined by a specialised endothelium consisting of DISCONTINUOUS SIMPLE SQUAMOUS EPITHELIAL CELLS . - 'leaky'
Present are KUPFFER CELLS- liver macrophages- removal of aged blood cells
SPACE OF DISSE- function is to allow hepatocytes to have microvilli for absorption- origin of lymph within the liver.  * ADD PHOTO

Question 6

Kupffer Cells

Answer 6

Can be seen with INDIA INK
Derived from monocytes
Phagocytic cells lining sinusoids

Question 7

Cuboidal Cells

Answer 7

Cuboidal cells are hepatocytes.
Microvilli extending into the space of Disse
Organelles- indicative of a secretory cell (numerous GOLGI, RER, sER, mitochondria and GLYCOGEN granules).

The lateral walls have BILE CANALICULI- these are UNLINED or have NO endothelium. The cannaliculi channels carry bile produced by the hepatocytes.

Gaps between the endothelial lining cells of the sinusoids that allows unrestricted access of sinusoidal content to the hepatocytes.

Question 8

SPACE OF DISSE

Answer 8

Between the sinusoidal lining cells and the hepatocytes. Microvilli from the hepatocytes are found in this space. RBCs are seen in the sinusoid.

ITO CELLS (hepatic stellate cells, HSC) which are thought to produce VITAMIN A are also found in the SPACE OF DISSE.
They are involved in liver fibrosis and recent data suggests they are the resident antigen presenting cells for T lymphocytes.

It is thought that lymph originates from the Space of Disse. Its flow in the Space is opposite to that of the BLOOD in the sinusoid.

Question 9

Portal region

Answer 9

Each lobule has a number of these. Each portal region services more than one lobule

Components:
BILE DUCT
HEPATIC ARTERY
HEPATIC PORTAL VEIN
LYMPHATIC VESSEL

Question 10

What are the three different ways of describing the liver structure?

Answer 10

CLASSIC LOBULE- hexagonal shape
PORTAL LOBULE- triangular
LIVER ACINUS- makes it an oval shape with the central veins

Question 11

What is the path of bile flow from the liver into the duodenum?

Answer 11

Hepatocytes -> Bile Capillaries -> small bile ducts -> right and left hepatic ducts -> common hepatic duct –> common bile duct (or cystic duct for storage in gallbladder) –> hepatopancreatic ampulla (ampulla of Vater)

Question 12

GALL BLADDER what epithelium lines it? what is the mucosa?

Answer 12

NO GOBLET CELLS
The gall bladder, like the liver is an outgrowth of the duodenum. It is lined by SIMPLE COLUMNAR EPITHELIUM and shows evidence of a BRUSH BORDER. It’s mucosa is very folded and when actively resorbing water shows evidence of intercellular spaces in the basal regions of the epithelium.

It has a weird tunnelling unique to the gall bladder- often called tunnelling of the mucosa

Function of the gall bladder is storage, also absorbs a lot of water.

Extensive lamina propria of loose connective tissue. with TUNNELING OF THE EPITHELIUM. Aband of smooth muscle completes the structure of the wall.

Question 13

How do you tell the difference between the GALL BLADDER and SMALL INTESTINE?

Answer 13

NO SUBMUCOSA in GALL BLADDER

Question 14

Electron Micrograph

Answer 14

Dome shaped cells with small mirovilli on the surface. When ABSORBING (concentrating bile) there are massive intercellular spaces - PHOTO

Question 15

Explain HORMONAL CONTROL of EXOCRINE Liver Function

Answer 15

1. Parasympathetic impulses along the vagus (X) nerves stimulate bile production by the liver
2. Fatty acids and amino acids in chyme entering the duodenum stimulate the secretion of cholesystokinin (CCK) into blood.
3. CCK causes contraction of the gall bladder
4. Secretin enhances flow of bile rich HCO3- from the liver

Question 16

PANCREAS HAS EXOCRINE AND ENDOCRINE COMPONENTS

Answer 16

Endocrine = Islets of Langerhans- alpha, beta, delta pancreatic cells, alpha cells secrete GLUCAGON, beta cells secrete INSULIN
Exocrine pancreas looks very similar to the parotid glands- BUT NO STRIATED DUCTS (difference)

Question 17

High Power of the pancreas- What is an ACINUS?

Answer 17

ACinus is a group of secretory cells.
Each acinus has a basophilic basal region and an eosinophilic part towards the apical or secretory aspect of the cells. This is the typical appearance of an active protein secreting cell. The basophilia is caused by the RER and the eosinophilia by the zomogen granules (precursers of enzymes).

Question 18

The pancreatic acinus

Answer 18

The centroacinar cells of the intercalated duct extend into the lumen of the acinus. This is NOT SEEN in salivary glands. The zymogenic cells secrete the digestive enzymes whilst the duct cells secrete a bicarbonate rich fluid. These secretions are under the control of CCK (enzymes) and secretin (bicarbonate) from enteroendocrine cells of the duodenum.

Question 19

STOMACH, DUODENUM, PANCREATIC ACINUS REFRESHER

Answer 19

1. Secretin and cholecystokinin are secreted into th e blood by enteroendocrine cells of the duodenum when acidic gastric chyme enters the small intestine
2. Acinar pancreatic cells secrete the inactive forms of enzymes trypsin, chymotrypsin, and carboxylpeptidases. Active amylase, lipase, cholesterol esteraase, and phospholipase are also secreted. Acinar pancreatic cells secrete trypsin inhibitor, which prevents the activation of trypsin and other proteolytic enzymes within the acinar lumen and ducts. Epithelial cells of the intercalated duct secrete water and bicarbonate ions

3.The secretion of bicarbonate ions and water is regulated by SECRETIN