Outline the features of the hepatic portal system.
- The portal circulatory systems differ from the typical circulatory route in that blood passes through two sets of smaller vessels before returning to the heart.
- Blood from the first set of capillaries collects in portal vessels (sometimes called portal veins) which then begin to branch again to supply a capillary network to a second location before entering a series of veins which will lead to the heart.
What travels to the portal vein?
- Water soluble vitamins
- Carbohydrates (monosaccharides)
- Proteins – amino acids, dipeptides and tripeptides
- Intestinal hormones, particularly pancreatic hormones
- Toxins, including ammonia
In terms of the Krebs cycle, why is the liver important?
- The liver is the only organ in the body in which the complete urea cycle (Krebs cycle) is expressed.
- It converts ammonia to urea which can then be excreted in the urine.
What doesn't travel through the portal vein?
I. Dietary lipid is mostly triglycerides, cholesterol and phospholipids
II. Digested by lipases, this process needs bile acids to form micelles
III. Lipids are taken into the cells and processed into chylomicrons
IV.Chylomicrons are taken up by lymphatics, called lacteals. Lacteals contain chyle
- Fat soluble vitamins (Vitamin ADEK)
Outline the functions of the liver.
- Storage: Iron; Vitamin, B12, D, Vitamin K and Glycogen
III. Numerous coagulation factors
- Catabolism/breakdown/ toxin degradation
I. Drugs and poisons (cytochrome P450)
II. Hormones - Insulin, Glucagon, Oestrogen and Progesterone
IV. Can take over removal of aged red cells after splenectomy
- Filtering function – macrophages are critical for the execution of this function
Outline the endocrine and exocrine functions of the liver.
- Exocrine function – bile is an exocrine secretion of the liver via the bile duct
- Endocrine function - it produces angiotensinogen, thrombopoetin, insulin like growth factor 1 (IGF – 1)
Outline the structure of hepatocytes
- Constitute 80% of liver cell population
- Can regenerate remarkably!
- Compared to other cells contain:
I. Numerous mitochondria
II. A lot of peroxisomes (organelles full of oxidising agents)
III. Numerous free ribosomes
IV. A lot of rER
V. A lot of sER
VI. Numerous Golgi complexes
VII. Glycogen deposits
Describe the structure and function of Kupffer cells.
- Kupffer cells are monocyte derived specialist macrophages that form part of the lining of the sinusoids.
- They patrol tiny vessels in the liver called sinusoids, recycling old red blood cells and ingesting pathogens.
- The endothelium of these vessels is perforated with large holes, allowing the Kupffer cells to migrate into liver tissue at sites of inflammation and damage.
Describe the structure of Stellate (Ito) cells.
Stellate (Ito) cells are full of cytoplasmic vacuoles containing Vitamin A.
What happens to stellate cells in the case of liver cirrhosis.
- In liver cirrhosis, hepatic stellate cells lose their vitamin A storage capability and differentiate into myofibroblasts.
- That synthesize and deposit collagen within the perisinusoidal space, resulting in liver fibrosis.
- This collagen surrounds the central vein, constricting it and leading to portal hypertension
What is a sinusoid?
- A sinusoid is an irregular tubular space for the passage of blood, taking the place of capillaries and venules in the liver, spleen and bone marrow.
- Its endothelial cells gas large gaps (perforated)
Describe the features of a liver acinus.
Describe the features of a liver lobule.
Describe the location of lymphatics in the liver.
Lymphatics arise from the periportal space of Mall and drain to the liver hilum and then onto the hepatic duct.
Identify the components of the alimentary canal.
Identify and describe the 4 layers of the gut wall.
- Mucosa (innermost)
I. Muscularis mucosae
II. Lamina propria
- Submucosa - a layer of connective tissue bearing glands, arteries, veins and nerves
- Two external muscle layers (muscularis externae) - circular and longitudinal muscle
- Serosa - a serous membrane
Identify the functions of the gastrointestinal tract.
- To provide a port of entry for food into the body
- To mechanically disrupt the food
- To temporarily store the food
- To chemically digest the food
- To kill pathogens in the food
- To move the food along the tract
- To absorb nutrients from the resultant solution
- To eliminate residual waste material
What is digestion?
Digestion: is the conversion of what we eat by physical and chemical disruption into a solution that is relatively sterile, neutral in pH and isotonic; from which we can absorb our nutrients (a few sugars, a few fatty acids, approx. 20 amino acids, some minerals and vitamins).
What is the role of saliva?
- Starts digestion (amylase and lipase)
- Bacteriostatic (contains Immunoglobin A antibody (IgA))
- High calcium (protects teeth)
- Assist swallowing
- Protects the mouth
- Maintains moisture
Discuss physical and chemical disruption in the mouth and stomach.
- Begins in the mouth:
I. Physical - by action of teeth, tongue, and muscles of mastication
II. Chemical - by action of salivary amylase and lipase
- Continues in the stomach:
I. Physical - by churning (3 muscle layers and mucosal rugae)
II. Chemical - by acid (HCl) and enzymes (e.g. pepsin)
Outline the role of the oesophagus.
- Mouth forms a bolus which enters the oesophagus
- Upper end oesophagus – voluntary control (some striated skeletal muscle)
- Lower end oesophagus – involuntary control (solely smooth muscle)
- Rapid peristaltic transport, which works even when upside down, transports bolus to stomach (8-9 seconds)
Outline the oesophageal mucosa.
- Epithelium – stratified squamous non-keratinized (withstands abrasion).
- Lamina propria – loose connective tissue bearing blood & lymph vessels, some smooth muscle cells & many cells of immune system.
- Muscularis mucosae – thin layer of smooth muscle cells
Outline the structure of the other layers of the oesophageal wall.
- Submucosa – subtending layer of connective tissue containing mucus-secreting glands.
- Muscularis externa – smooth muscle layers (inner – circular; outer – longitudinal) which move food by peristalsis.
- Adventitia – thin outermost layer of connective tissue (no peritoneal enfolding of this portion of GI tract).
What is the role of the stomach?
- Acts as a necessary food store (we can eat faster than digest).
- Wall relaxes so pressure doesn’t rise (called receptive relaxation).
- Contracts rhythmically to mix and disrupt
- Secretes acid and proteolytic enzymes to break down tissues and disinfect.
- Protects its epithelium by secreting mucus.
- Produces hypertonic chyme by combined action of acid, enzymes and agitation.
- Delivers incompletely digested chyme slowly, and in a controlled way, to the duodenum.
What are the 3 layers of smooth muscle in the stomach (part of the muscularis externa)
What are rugae?
Folds of gastric mucosa forming longitudinal ridges in the empty stomach
Identify and describe the components of the gastric gland.
- The gastric pit is lined by mucus secreting cells, very much like goblet cells.
- The isthmus is the region in which stem cells divide to populate the gland by upward or downward migration
- Parietal cells secrete H+ ions into the lumen and HCO3- ions into nearby capillaries, which move it to surface mucous cells
- Chief cells secret pepsinogens which are converted into pepsin which partly hydrolyse proteins
- Enteroendrocrine cells include G cells which secrete gastrin. The wider gastric mucosa responds to this hormone by secreting acid.
Outline the role of surface mucous cells in the gastric pit.
- Surface mucous cells are abundant in the gastric pits.
- Their secreted mucus is resistant to pepsin (enzymatic) degradation.
- The mucus is released in response to distension, stomach contents, and acid secretion from gastric glands.
- Alcohol or aspirin can damage the mucous cells but they are quickly replaced by mitosis in deeper cells in the neck of the gastric pit
- The secreted mucus contains HCO3- ions which neutralise the effect of H+ ions and thereby protect the stomach lining.
Describe the structure of the duodenum
- Proximal portion of small intestine
- Curves around head of pancreas
- Walls contain Brunner’s glands which secrete bicarbonate–rich mucus, to neutralise acidic chyme.
Outline the dilution and neutralisation of chyme.
- Water drawn in from ECF to render hypertonic chyme, isotonic
- Liver releases bile (generally via the gall bladder). bile contains: water, alkali, bile salts (to emulsify fat)
- Pancreas & liver secrete alkali to neutralise acidic chyme (precisely controlled)
- Pancreas, liver and intestine secrete specific enzymes which act, with bile, to complete digestion of chyme (enzymes come to lie in ‘unstirred layer’)