Week 3 Flashcards
What is the digestive system composed of?
Gastrointestinal tract
Accessory digestive organs
Where is the liver located?
RUQ & part of LUQ
All of Rt hypochondrium, most of epigastrium, some of Lt hypochondrium
Separated from thoracic cavity by the diaphragm
Explain the surfaces of the liver
Diaphragmatic surface
Smooth, dome shaped, convex
Related to the concave, inf. aspect of the diaphragm
Covered with peritoneum except posteriorly where the bare area of the liver lies in contact with diaphragm
Visceral surface of the liver
Inferior & posterior aspect
Is covered with peritoneum except near the gall bladder and the porta hepatis
What is the porta hepatis?
Transverse fissure on the visceral surface of the liver between the caudate & quadrate lobes
Gives passage to the:
Portal vein
Hepatic artery
Hepatic nerve plexus
Hepatic ducts
Lymphatic vessels
What is the lesser omentum?
Double fold of peritoneum which connects the lesser curvature of the stomach & the proximal duodenum to the liver
What are the two ligaments of the lesser omentum?
Hepatoduodenal ligament: a thickened region on the free edge
Encloses the portal triad (bile duct, hepatic a., portal v.) along with lymph nodes, lymphatic vessels and the hepatic plexus of nerves
Hepatogastric ligament: the remaining, sheet-like aspect of the lesser omentum
Describe the peritoneal ligaments of the liver
Falciform ligament
Connects liver to anterior abdominal wall & diaphragm
Round ligament (Ligament teres) Contained within the free edge of the falciform ligament
Fibrous remnant of the umbilical vein (carried well-oxygenated, nutrient-rich blood from placenta to foetus)
Ligamentum Venosum
Fibrous remnant of the ductus venosus
In the foetus, connects the left umbilical vein to the IVC (allows blood to bypass the liver)
The above 3 ligaments divide the liver into Rt and Lt lobes
Coronary ligament
Peritoneal reflection from the liver to the diaphragm
Demarcates the bare area of the liver
Triangular ligaments
Peritoneal folds that connect the liver to the diaphragm
Rt triangular ligament: situated at the Rt extremity of the bare area
Lt triangular ligament: continuous with falciform ligament anteriorly
What is the arterial supply to the liver?
Portal Vein (70%) Formed by SMV & splenic v. just post. to the neck of pancreas
Conveys nutrient-rich blood from stomach and intestines
Hepatic Artery (30%) Branch of Celiac a. – provides oxygenated blood
At or close to porta hepatis, both vessels divide into Lt + Rt branches (supply corresponding part of liver)
What is the venous supply to the liver?
Hepatic veins (open into the IVC just inf. to diaphragm)
What is the lymphatic drainage of the liver?
Superficial lymphatics drain to hepatic & phrenic nodes
- Hepatic nodes celiac nodes cisterna chyli (dilated sac at inf. end of thoracic duct)
- Phrenic nodes posterior mediastinal nodes thoracic duct or Rt lymphatic duct
Deep lymphatics drain to hepatic nodes
What is the nerve supply of the liver?
Hepatic plexus consisting of: Sympathetic fibres (T5-9 via celiac plexus) & parasympathetic fibres (CNX Vagus)
What is the role of the liver?
Synthesis
Bile (~1L daily)
Plasma proteins e.g. clotting factors, albumin
Hormones e.g. angiotensinogen, erythropoietin
Haemopoiesis (foetal development)
Detoxification
Drugs, alcohol
Conversion of ammonia to urea
Metabolism & storage
Fat, carbohydrates, amino acids, vitamins & minerals (iron, vitamins A, B12 & D)
Excretion
Bilirubin in bile (erythrocyte breakdown)
Immune function Phagocytic activity (Kupffer cells)
What is the functional unit of the liver?
Liver lobules
What is the direction of flow from lobules (histology)?
Blood flows from portal tract to the central vein (via sinusoids)
Bile flows from central areas to portal tracts (via bile canaliculi)
What is an acinus?
a kernel-shaped area of liver parenchyma that is centred upon a portal tract
Divided into three zones:
Zone 1 is closest to the portal tract (receives the most O2)
What is the structure of the biliary tree?
Blood passing into central v. eventually enters hepatic vv IVC
Bile entering the bile ducts eventually enters the Lt and Rt hepatic ducts, which drain the Lt and Rt lobes of the liver, respectively. These unite to form the common hepatic duct
Common hepatic duct merges with the cystic duct of the gall bladder to form the bile duct (formerly called the common bile duct)
Hepatopancreatic ampulla is formed by union of bile duct & major pancreatic duct
What is the gallbladder?
Pear-shaped muscular sac
What is the function of the gallbladder?
Storage & concentration of bile (capacity ~50 ml)
What are the parts of the gallbladder?
Fundus: wide end, projects from the inferior border of the liver
Body: contacts the visceral surface of the liver
Neck: narrow tapered region, makes an S-shaped bend and is continuous with the cystic duct
What is the arterial supply of the gallbladder?
Cystic a. (off Rt hepatic a.)
What is the Venous drainage of the gallbladder?
Cystic vv directly into liver or into portal vein
What is the lymphatic drainage of the gall bladder?
Cystic nodes ->Hepatic nodes ->Celiac nodes
What is the nerve supply of the gallbladder?
Parasympathetic: CNX Vagus
Sympathetic: Splanchnic nn T5-9 (via celiac plexus)
Sensory: Rt Phrenic n.
Explain the histology of the gallbladder
Mucosa:
Simple columnar epithelium that possess microvilli (absorption)
When empty the mucosa is thrown into honeycomb-like folds
Note: the GB wall lacks a muscularis mucosae and submucosa
Muscularis:
Contains smooth m., collagen & elastin
Contracts in response to cholecystokinin & vagal stimulation
Adventitia/Serosa:
Surface attached to liver has an adventitia
Peritoneal surface - serosa
Describe the pancreas
Elongated tadpole-shaped gland, ~ 15cm long
Found in the epigastric & Lt hypochondriac regions
Stretches transversely behind stomach, in between spleen & duodenum
Retroperitoneal
Does not have a distinct fibrous capsule, instead has a very thin capsule that invaginates into gland (forms septae)
What is the function of the pancreas?
Functions as both an exocrine (chemicals) & endocrine gland (hormones)
What are the regions of the pancreas?
Head of the pancreas
Expanded part of the gland found within the duodenum
Firmly attached to medial aspect of D2 & D3
Bile duct situated in a groove posterior/superior to the head or sometimes within the parenchyme itself
Uncinate process: lower part of the head that projects upwards to the left (L. ‘hook’)
Neck of the pancreas
Short region (1.5 - 2cm) between the head and body
Anteriorly: groove for the gastroduodenal a.
Posteriorly: union of the SMV and Splenic vein (forming the Portal vein)
Body of the pancreas
Crosses the aorta (level of L2)
Anteriorly: covered with peritoneum
Posteriorly: aorta & origin of SMA; Lt adrenal gland, Lt kidney & renal vessels, left crus of the diaphragm, splenic vein
Tail of the pancreas
Anterior to the Lt kidney
Closely related to the hilum of the spleen & Lt colic flexure
Passes between layers of the splenorenal ligament (peritoneal ligament)
What are the pancreatic ducts?
MAIN PANCREATIC DUCT
Begins in the tail of the pancreas and runs to the head
Found more in the posterior aspect of the pancreas
Unites with bile duct to form the hepatopancreatic ampulla (opens into D2: major duodenal papilla)
ACCESSORY PANCREATIC DUCT
Drains the inferior part of the head
Opens into the duodenum (minor duodenal papilla)
In 60% of people, it communicates with the main pancreatic duct
What are the sphincters of the pancreas?
Smooth m. sphincters control the flow of bile & pancreatic juice
Sphincter of the pancreatic duct
around the terminal part of the pancreatic duct
Sphincter of the bile duct
around terminal part of the bile duct
Hepatopancreatic sphincter (sphincter of Oddi) around the hepatopancreatic ampulla
Sphincters are closed when no digestion is occurring (bile secreted by liver gets stored in GB)
What is the arterial supply to the pancreas?
Mainly from branches (~10) of the splenic artery
Anterior & posterior superior pancreaticoduodenal aa (from gastroduodenal a)
Anterior & posterior inferior pancreaticoduodenal aa (from SMA)
What is the venous drainage of the pancreas?
Pancreatic veins (most are tributaries of the splenic vein, some the SMV)
What is the lymphatic drainage of the pancreas?
Mostly the pancreaticosplenic nodes
Efferents from these drain into the celiac, hepatic & superior mesenteric nodes
What is the nerve supply of the pancreas?
Parasympathetic: CNX Vagus
Sympathetic: abdominopelvic splanchnic nerves (T5-9) via the celiac ganglion
Describe the histology of the pancreas
EXOCRINE PORTION
Acinar cells – form the bulk of the pancreas
Acinar cells possess abundant rough ER and zymogen granules (inactive enzymes)
Secrete enzyme-rich pancreatic juice into the ductal system
Ductal cells also secrete water & HCO3-
ENDOCRINE PORTION
Pancreatic islets are scattered among the acinar cells
Contain hormone-producing cells (endocrine function)
What is digestion?
catabolic process that breaks down complex food molecules to monomers
What is absorption?
passage of digested end products from the lumen of the GIT through mucosal cells into the blood or lymph
What do digestion and absorption rely on?
bile and pancreatic secretions
What are the components of bile?
Bile salts*
Phospholipids (lecithin)*
Bile pigments (bilirubin)
Cholesterol, triglycerides
Electrolytes
*Only bile salts and phospholipids assist in digestion & absorption
95% of bile salts are recycled by the enterohepatic circulation: reabsorbed in the ileum, returned to the liver (portal vein) & resecreted in newly formed bile
What are the functions of bile?
DIGESTION & ABSORPTION
Emulsification of large fat particles into small droplets (bile salts have detergent properties)
This increases SA available for lipases to act upon the fat molecules
Bile salts & phospholipids interact with the breakdown products of fat digestion – forming micelles
Micelles “ferry” free fatty acids & monoglycerides to the intestinal mucosa for absorption
EXCRETION OF WASTE PRODUCTS
Bilirubin & excess cholesterol
Pathologies that interfere with bilirubin excretion can result in jaundice
What are the key contents of pancreatic juice?
enzymes, water & electrolytes (HCO3-)
What are the pancreatic enzymes?
Proteases: digest proteins
Amylase: digests starch
Lipases: digest emulsified fats
Nucleases: digest nucleic acids
What is enteropeptidase?
(brush border enzyme) - converts pancreatic proteases to their active form
Trypsinogen –>trypsin
Chymotrypsinogen –>chymotrypsin
Describe the regulation of bile and pancreatic secretion
HORMONAL REGULATION
CCK & secretin released by duodenal enteroendocrine cells, in response to fatty, acidic or protein-rich chyme
CCK: pancreatic secretion (enzyme component), GB contraction & relaxation of hepatopancreatic sphincter
Secretin: pancreatic secretion (H2O & HCO3- component), bile secretion
Return of bile salts from enterohepatic circulation: bile secretion
NERVOUS REGULATION
Vagal stimulation: pancreatic secretion & GB contraction
Describe the chemistry of digestion
Digestion involves hydrolysis: water is added to each molecular bond that is broken
DIGESTIVE ENZYMES
Are produced by various organs of the GIT
Act as organic catalysts (accelerate chemical reactions without appearing in final product)
Each enzyme possesses an active site for a specific substrate
Function optimally at a specific pH
Most are synthesised as zymogens (proenzymes)
What are absorption methods?
Small intestine receives ~9L of water daily and absorbs ~95% via osmosis (remaining fluid absorbed in large intestine)
Glucose & amino acids
Enter epithelial cells through cotransport with Na+
Enter blood via facilitated diffusion
Fatty acids & monoglycerides
Diffuse from micelles into epithelial cells
Are repackaged with other lipids & proteins into chylomicrons
Chylomicrons exit epithelial cells via exocytosis and enter lacteals
Vitamins
Fat-soluble vitamins (A, D, E & K) also rely on micelles for ‘ferrying’ to epithelial cells (these enter via diffusion)
Vitamin B12 needs to be bound to intrinsic factor
The B12-IF complex triggers endocytosis
What is an example of a food component that cannot be digested and is excreted in faeces?
Cellulose
