The Gastrointestinal System

What are the four histological layers of the GI tract?

Mucosa, submucosa, muscularis externa, serosa/adventitia.

What type of epithelium lines the oesophagus?

Moist stratified squamous epithelium.

Which sphincter prevents gastric reflux into the oesophagus?

Lower oesophageal (cardiac) sphincter.

What are rugae in the stomach?

Mucosal folds allowing expansion.

What is the primary function of the small intestine?

Nutrient digestion and absorption (90% of nutrients).

Which glands are found in the submucosa of the duodenum?

Brunner’s glands (mucus-secreting).

What are Peyer’s patches and where are they located?

Lymphoid follicles in the ileum submucosa (immunity).

What structures increase surface area in the small intestine?

Plicae circulares and villi.

What is the predominant cell type in large intestine crypts?

Goblet cells (mucus secretion).

What are taeniae coli in the large intestine?

Three bands of longitudinal muscle forming haustra.

What is the function of haustra in the large intestine?

Permit expansion and elongation.

Which sphincter in the anal canal is voluntary?

External anal sphincter (skeletal muscle).

What is the role of the ileocecal valve?

Prevents backflow from large intestine to ileum.

Which part of the GI tract is primarily retroperitoneal?

Duodenum (except superior part).

What are epiploic appendages in the large intestine?

Fat-filled pouches on the serosa.

What are the main functions of GI secretions?

Chemical digestion, lubrication, signalling, protection, enzyme activation, waste excretion.

What enzyme in saliva begins carbohydrate digestion?

Salivary amylase.

What are symptoms of xerostomia (dry mouth)?

Difficulty swallowing, speech issues, increased dental caries, oral infections.

Which gastric cell secretes HCl and intrinsic factor?

Parietal cell.

At what pH is pepsinogen activated to pepsin?

~2 (acidic, via HCl).

What is the role of gastric mucus?

Forms a protective barrier against acid and pepsin.

What neutralizes stomach acid in the duodenum?

Pancreatic bicarbonate (NaHCO₃, pH 7.1–8.2).

How is trypsinogen activated in the duodenum?

By enteropeptidase (brush border enzyme).

Which pancreatic hormone promotes glucose uptake?

Insulin (β cells).

What tissue type are hepatocytes (bile-producing cells)?

Epithelial.

What is the primary function of bile salts?

Emulsify lipids for digestion and absorption.

How does the gallbladder concentrate bile?

Absorbs water and ions.

Which brush border enzyme activates trypsinogen?

Enteropeptidase.

What triggers the cephalic phase of secretion?

Sight, smell, taste, or thought of food.

What effect do CCK and secretin have on gastric secretions?

Inhibit gastric acid and pepsin secretion.

What are the types of passive transport?

Simple diffusion (e.g., oxygen, fatty acids), facilitated diffusion (e.g., sodium, glucose).

What drives secondary active transport?

Na⁺ gradient created by Na⁺/K⁺ ATPase.

How are glucose and galactose absorbed?

Secondary active transport via Na⁺-glucose cotransporter (SGLT1)

Which monosaccharide uses facilitated diffusion?

Fructose (via GLUT5).

Symptoms of lactase deficiency?

Bloating, diarrhoea, gas.

Why are carbohydrates digested at the brush border?

Ensures monosaccharides are produced near absorption sites.

How are amino acids absorbed?

Na⁺-dependent cotransporters or facilitated diffusion.

What enzyme digests most lipids in adults?

Pancreatic lipase.

Why do chylomicrons enter lymph?

Too large for blood capillaries; lymphatics transport to bloodstream.

How are fat-soluble vitamins absorbed?

Via micelles, simple diffusion.

How is vitamin B12 absorbed?

Binds intrinsic factor, absorbed by endocytosis in ileum.

How does bile duct blockage cause vitamin E deficiency?

Reduces micelle formation, impairing fat-soluble vitamin absorption.

How is calcium absorption regulated?

By vitamin D and parathyroid hormone (PTH).

How is water absorbed?

Osmosis, following solute gradients (95% in small intestine).

What causes malabsorption in celiac disease?

Gluten damages villi/brush border; treated with gluten-free diet.

What is the liver’s dual blood supply?

Hepatic artery (oxygen-rich, ~25%), hepatic portal vein (nutrients/toxins, ~75%).

How is blood drained from the liver?

Via 3 hepatic veins into inferior vena cava.

What is the hepatic portal system’s role?

Delivers nutrients/toxins from GI tract to liver for processing without returning to heart.

What structures form the portal triad?

Hepatic artery, portal venule, bile duct.

What are sinusoids?

Specialised capillaries carrying blood from hepatic artery/portal vein to central vein; fenestrated for metabolite transfer.

What do Kupffer cells do?

Macrophages in sinusoids; phagocytose pathogens, debris, old RBCs.

What is the role of stellate cells?

Store vitamin A; produce collagen in disease, causing fibrosis.

What happens in Phase I detoxification?

Cytochrome P450 enzymes add polar groups (oxidation, reduction, hydrolysis); may produce toxic metabolites.

How is bilirubin processed?

Conjugated with glucuronic acid in liver, becomes water-soluble, excreted in bile.

What causes neonatal jaundice?

Immature liver lacks glucuronyl transferase, leading to unconjugated bilirubin buildup.

How does the liver maintain blood glucose?

Glycogenesis (glucose → glycogen), glycogenolysis (glycogen → glucose), gluconeogenesis (from amino acids/triglycerides).

What hormones regulate carbohydrate metabolism?

Insulin (anabolic, glycogenesis), glucagon (catabolic, glycogenolysis/gluconeogenesis).

What lipids does the liver synthesise?

Triglycerides (stored/transported as VLDL), cholesterol (hormones, membranes), phospholipids.

What is the urea cycle’s purpose?

Converts toxic ammonia to urea for excretion.

What are jaundice symptoms?

Yellow skin/eyes, dark urine, pale stools.

What are the main functions of the urinary system?

Waste disposal, osmoregulation, blood volume/pressure/pH regulation, hormone production.

Where are the kidneys located?

Posterior abdominal wall, retroperitoneal, ~12th rib, left slightly higher.

What is the blood supply to the kidneys?

Renal arteries (from aorta, 20-25% cardiac output); drain via renal veins to inferior vena cava.

What is the nephron’s role?

Functional unit; filters blood, reabsorbs/excretes to form urine.

What structures form the renal corpuscle?

Glomerulus (capillary network), Bowman’s capsule (visceral/parietal layers).

What can pass through the filtration membrane?

Water, sodium, glucose, chloride; blocks RBCs, large proteins.

What is the role of the proximal convoluted tubule?

Reabsorbs ~65% of filtrate (glucose, sodium, amino acids, water).

How does the Loop of Henle function?

Counter-current multiplier; descending limb reabsorbs water, ascending limb reabsorbs sodium/potassium/chloride.

What does ADH do in the kidneys?

Inserts aquaporins in collecting duct, increases water reabsorption, concentrates urine.

What is the other name for ADH, and its effect?

Vasopressin; causes vasoconstriction, increases blood pressure.

Outline the RAAS pathway.

Renin converts angiotensinogen to angiotensin I; ACE converts to angiotensin II; stimulates aldosterone, vasoconstriction.

What is the effect of RAAS on blood pressure?

Increases via vasoconstriction and sodium/water reabsorption.

How does the sympathetic nervous system affect kidneys?

Reduces sodium/water excretion, increases angiotensin II, constricts afferent arterioles.

What do natriuretic peptides do?

Increase sodium/water excretion, reduce blood volume/pressure (ANP, BNP, CNP).

How much filtrate is produced vs. urine excreted daily?

~180 L filtered, ~2 L urine excreted.