Digestive System- Accessory Organs Flashcards
Digestive System Accessory Organs
- Pancreas
- Liver
- Gall bladder
The Pancreas
- The pancreas is an elongated organ, light tan or pinkish in color, that lies in close proximity to the duodenum
- Pancreatic duct connects with the duodenum
- It is covered with a very thin connective tissue capsule which extends inward dividing the gland into lobules
Histology of the Pancreas
- Partitions of connective tissue divide the interior of the pancreas into distinct lobes
- Blood vessels of the pancreatic ducts are located within these connective tissue septum
- In each lobule:
– Ducts branch repeatedly->
ending in pockets called pancreatic acini - Each acinus is lined with simple cuboidal epithelium
- Pancreatic islets (endocrine tissues) are scattered among the acini
The Pancreas- Exocrine
- Secrete into ducts
- Secretes “pancreatic juice”
- Acinar cells; enzymatic secretions
- Duct cells; aqueous NaHCO3 (sodium bicarbonate)
The Pancreas- Endocrine
- Release secretory products (hormones)-> blood
- Glycaemic homeostasis
- Islets of Langerhan
- Insulin/glucagon
Exocrine Cells
Acinar cells
Duct cells
Acinar cells
- Produce small volume of primary
secretion - Water, electrolytes, and digestive
enzymes
Duct cells
- Produce bicarbonate-rich fluid
- Large volume
Endocrine- Pancreatic islets
Alpha cells
Beta cells
Delta cells
F cells
Alpha cells
glucagon; glucagon raises blood glucose levels
Beta cells
insulin; lowers blood glucose levels
Delta cells
somatostatin; suppresses release of glucagon and insulin; slows rate of food absorption and enzyme secretion along digestive tract
F cells
pancreatic polypeptide; inhibits gallbladder contractions and regulates production of pancreatic enzymes
Pancreatic Juice
composed of 2 secretory products critical to proper digestion:
– Digestive enzymes (secreted from Acinar cells)
– Bicarbonate (a base) (secreted from epithelial cells lining the ducts)
Types of Digestive Enzymes
Proteases
Pancreatic Lipase
Amylase
Proteases
Digestion of proteins is initiated by pepsin in the stomach, but the bulk of protein digestion is due to the pancreatic proteases.
2 major pancreatic proteases: Trypsin and Chymotrypsin
Pancreatic Lipase
- Major form of dietary fat is triglyceride, or neutral lipid.
– A triglyceride molecule cannot be directly absorbed across the intestinal mucosa.
– It must first be digested into a 2-monoglyceride and two free fatty acids. - The enzyme that performs this is Pancreatic lipase, which is delivered into the lumen of the gut in pancreatic juice.
- Bile salts are also needed –from liver
Amylase
- The major dietary carbohydrate is starch.
- the enzyme that hydrolyses starch to maltose.
- also present in saliva.
Bicarbonate and Water
- Epithelial cells in pancreatic ducts are the source of the bicarbonate and water secreted by the pancreas.
- In pancreatic duct cells, the bicarbonate is secreted into the lumen of the duct and hence into pancreatic juice– buffering the duodenum and preventing damage from stomach acid
The Liver
- lies in the abdominal cavity, in contact with diaphragm
- Its mass is divided into several lobes, the number and size of which vary among species – four lobes in humans
- In most mammals, a greenish sac - the gallbladder - is seen attached to the liver
- The common bile duct delivers bile from the liver and gallbladder into the duodenum
Gross anatomy of the liver
- Tough fibrous capsule
- Covered by layer of visceral peritoneum
- Right & left lobes
- Further subdividing lobes
- Anteriorly:
– falciform ligament divides left/right lobes becoming round ligament/ligamentum teres - Posteriorly:
– depression for inferior vena cava - Caudate & quadrate lobes also visible
Liver Hepatocytes
- Cuboidal epithelia
- Two hepatocytes forming plates/cords
- Bile canaliculus between hepatocytes
- Sinusoids lined with fenestrated capillaries between plates
- Basal surface has microvilli that project into sinusoids
- Apical surface forms canaliculi
Liver Hepatic lobules
- Each lobe is divided into lobules
- 100,000 liver lobules = functional units of liver
- Adjacent lobules separated by interlobular septum (connective tissue)
- Hepatocytes form irregular plates arranged in a hexagon shape
– Plates are one cell thick - Within a lobule, sinusoids between adjacent plates empty into central vein.
– Sinusoids contain Kupffer cells- phagocytic cells
– Blood enters liver sinusoids from small branches of the hepatic portal vein (Venous blood) and hepatic artery (arterial blood)
Histological Organisation of the Liver
- Hepatic lobes consist of +100,000 lobules
- Lobules are separated by connective tissue at the interlobular septum
- Central vein
- Hepatocytes radiate outwards (cords/plates)
- Sinusoids spaces between plates lined with fenstrated capillaries
- Bile canaliculi channels arising between hepatocytes
Liver Portal area of the lobule
- There are 6 portal areas in each lobule
- Portal area contains:
- branch of hepatic portal vein
- branch of hepatic artery
- small branch of bile duct
Liver- Metabolic Regulation
– Metabolism – Carbohydrate, Fat & Protein
– Detoxification – toxins, ammonia, etc.
– Storage – Vitamins (B12), carbohydrates etc. e.g. glycogen
Liver- Haematological Regulation
– Secretory – bile – Bile acids, salts & pigments
– Excretory – Bilirubin
– Synthesis – Albumin, coagulation factors
Liver- Bile Functions
– Emulsification of lipids in duodenum
– Excretion of lipid soluble waste
Liver- Non-digestive functions (hepatobiliary system)
- Synthesis of plasma proteins
- Synthesis of clotting factors
- Synthesis of the inactive angiotensinogen (blood pressure)
- Phagocytosis of damaged red blood cells
- Breakdown of circulating hormones (insulin and epinephrine) and immunoglobulins (IgA etc)
- Inactivation of lipid-soluble drugs
Liver- Digestive & metabolic functions (hepatobiliary system)
- Synthesis and secretion of bile
- Storage of glycogen and lipid reserves
- Maintaining normal blood glucose, amino acid and fatty acid concentrations within blood
- Synthesis and release of cholesterol bound to transport proteins
- Inactivation of toxins
- Storage of iron
- Storage of fat-soluble vitamins
Liver- Vitamin & Mineral Storage
- Fat soluble vitamins (A,D, E and K) absorbed from the blood and stored in the liver (Vit B 12 also stored but not fat soluble)
- These reserves are called on when the diet contains inadequate amounts of those vitamins
- Liver converts iron reserves to ferritin (essential for haemoglobin) and stores this protein-iron complex
Liver- Removal of Waste products
- Removal of ammonia from the body by synthesis of urea.
– Ammonia is very toxic and if not rapidly and efficiently removed from the circulation, will result in central nervous system disease. - Other waste products, circulating toxins and drugs are also removed from the blood for inactivation, storage or excretion.
Liver- Drug Inactivation
- Liver removes and breaks down circulating drugs, limiting drug duration.
- Rate at which liver removes drug is important.
- If absorbed quickly, drug must be administered every few hours to keep plasma concentrations at therapeutic levels.
Liver Hepatic Vascular system
- The circulatory system of the liver is unlike that seen in any other organ:
– The majority (75%) of the liver’s blood supply is venous blood - Roughly 75% of the blood entering the liver is from the portal vein.
– All of the venous (waste) blood returning from the small and large intestine, stomach, pancreas and spleen enters this vein - The remaining 25% of the blood supply to the liver is arterial blood from the hepatic artery.
– This provides the liver tissue with oxygen and nutrients - Converging at the hilus (porta hepatis)
- Venous return via hepatic veins-inferior vena cava
- The hepatic portal vein and hepatic artery empty together and mix as they enter sinusoids in the liver.
- Sinusoids are vascular channels (leaky capillaries) lined with highly fenestrated or “holey” endothelial cells and surrounded by hepatocytes.
- As blood flows through the sinusoids:
– plasma is filtered into the space between endothelium and hepatocytes, providing a major fraction of the body’s lymph
– Plasma proteins are secreted
– Solutes are absorbed from the plasma
Hepatic veins
carry blood AWAY from liver
Hepatic Portal vein
carry blood TO liver
The Biliary System
- a series of channels and ducts that conveys bile - a secretory and excretory product of hepatocytes - from the liver into the lumen of the small intestine
- Biliary canal is the space between the apical surfaces of the hepatocytes that form cords
- Hepatocytes secrete bile into the canals, and those secretions flow parallel to the sinusoids,– but in the opposite direction that blood flows – towards the portal area.
*Small bile ducts, join into larger and larger ducts, eventually forming the common bile duct, which dumps bile into the duodenum.
*The gall bladder is another important structure in the biliary system.
*This is a sac-like structure adhering to the liver.
*During periods of time when bile is not flowing into the intestine, it is diverted into the gall bladder, where it is dehydrated and stored until needed.
The Gallbladder
- Hollow, pear-shaped organ
- Stores, modifies, and concentrates bile
- There are two important functions of bile:
- Bile acids are critical for digestion and absorption of fats and fat soluble vitamins in the small intestine.
- Many waste products are eliminated from the body by secretion into bile and subsequent elimination in faeces
Liver disease
- Any condition that severely damages the liver represents a serious threat to life
- The liver can regenerate itself after injury, but liver function will not recover fully unless normal vascular patterns return
- Conditions such as Cirrhosis and Hepatitis interfere with liver function
– they cause structural changes to occur
Alcoholic Fatty Liver
yellow appearance
Can be Reversible
Cirrhosis
End stage liver disease
Irreversible
