BIOLOGY • PHYSIOLOGY • IMAT
Lesson 18: Digestive, Excretory & Endocrine Systems
A single integrated physiology map: digest nutrients → absorb them → process them in the liver → filter and regulate plasma in the kidney → coordinate homeostasis with hormones.
DigestionAbsorption
Liver & MetabolismKidney & Nephron
Hormonal Regulation
Endocrine Control
Introduction: Processing Nutrients and Removing Waste
This lesson follows the same sequence used in the video masterclass: food is mechanically and chemically digested, nutrients cross the intestinal epithelium, the liver processes absorbed material and detoxifies nitrogen, the kidneys filter and fine-tune the internal environment, and endocrine feedback coordinates long-term homeostasis.
Core Functions: Ingestion, Digestion (Hydrolysis), Absorption, Assimilation, and Egestion.
1 • DIGESTMouth → stomach → duodenum
→
2 • ABSORBVilli → portal blood / lacteals
→
3 • PROCESSLiver → bile, storage, detox, urea
→
4 • FILTERNephron → water, ions, pH, waste
→
5 • CONTROLPituitary → glands → feedback
Part 1: The Digestive System
DIGESTIVE SYSTEM — one map, one job per region
| Region | Main event | Key secretion / enzyme | What leaves this region | IMAT anchor |
|---|---|---|---|---|
| Mouth | Mastication + start carbohydrate hydrolysis | Salivary amylase | Bolus containing partially digested starch | Amylase works near neutral pH |
| Stomach | Acidification, protein denaturation, mixing | HCl + pepsin; intrinsic factor | Acidic chyme | Parietal ≠ chief cell |
| Duodenum | Neutralize acid + major chemical digestion | HCO₃⁻, bile, pancreatic enzymes | Small absorbable molecules | Secretin → HCO₃⁻; CCK → enzymes/bile |
| Jejunum / Ileum | Absorption through villi and microvilli | Brush-border enzymes + transporters | Sugars/amino acids → blood; lipids → lymph | Lacteal carries chylomicrons |
| Colon | Water/electrolyte recovery + microbiome | — | Feces | Vitamin K + water balance |
1.1 The Oral Cavity & Swallowing
Digestive tract anatomy: follow the continuous lumen from mouth to colon. Accessory organs contribute secretions but food does not pass through them.
Mechanical Digestion (Mastication):
- Incisors: Cutting.
- Canines: Tearing.
- Molars/Premolars: Grinding.
Chemical Digestion (Saliva): Secreted by parotid, submandibular, and sublingual glands.
- Salivary Amylase (Ptyalin): Hydrolyzes starch to maltose (pH ~7).
- Lysozyme: Antibacterial agent.
- Mucus: Lubricates food into a bolus.
Deglutition (Swallowing):
Swallowing is a complex reflex involving three phases:
- Oral Phase (Voluntary): The tongue pushes the bolus against the hard palate and moves it to the back of the throat.
- Pharyngeal Phase (Involuntary): The Epiglottis closes the trachea, the soft palate seals the nasal cavity, and breathing stops momentarily.
- Esophageal Phase (Involuntary): Peristalsis pushes the bolus toward the stomach.
Food moves down the esophagus via Peristalsis (wave-like muscle contractions of circular and longitudinal muscles).
Swallowing & Peristalsis
Read the image left → right: circular muscle contracts behind the bolus, the segment ahead relaxes, and the resulting pressure gradient pushes contents forward. Longitudinal shortening assists propulsion.
High Yield: Histology & Regulation
1.2 Stomach Histology & Function
Learn the four cell → secretion pairs first. Then use the two images to anchor where those cells sit and what they release.
Parietal cellHCl + intrinsic factorAcidifies lumen; B12 absorption depends on intrinsic factor.
Chief cellPepsinogenInactive zymogen → pepsin in acidic lumen.
Mucous cellMucus + HCO₃⁻Protects epithelium from acid and pepsin.
G cellGastrinEndocrine signal that promotes gastric acid secretion.
Gastric Pits: where the secretory cells sit
The key exam distinction is not simply “stomach cell,” but which cell secretes which product. Intrinsic Factor belongs to parietal cells; pepsinogen belongs to chief cells.
Regulation of Gastric Secretion
Gastric juice secretion is not continuous; it occurs in three distinct phases:
- Cephalic Phase (Brain): Sight, smell, or thought of food stimulates the Vagus nerve → G-cells → Gastrin → Acid release (30% of secretion).
- Gastric Phase (Stomach): Food stretches the stomach (distension) and protein presence raises pH → Massive Gastrin release (60% of secretion).
- Intestinal Phase (Duodenum): Chyme enters duodenum. Initially stimulates, but then Enterogastric Reflex inhibits stomach via hormones (Secretin, CCK) to prevent overloading the intestine.
Gastric hormonal control — anatomy reference
Use this image after the three phases above: gastrin drives gastric activity, while duodenal secretin and CCK coordinate pancreatic/biliary responses and slow gastric output.
1.3 The Small Intestine & Accessory Organs
SMALL INTESTINE — digestion and absorption are different jobs
| Segment / structure | What happens | Why it matters |
|---|---|---|
| Duodenum | Receives acidic chyme, bile and pancreatic juice; bicarbonate raises luminal pH. | Creates the correct environment for pancreatic enzymes. |
| Jejunum | Major bulk absorption of digested nutrients. | Villi + microvilli maximize surface area. |
| Ileum | Completes absorption; recovers vitamin B12 and bile salts. | Links intrinsic factor from stomach to B12 uptake. |
| Capillary | Receives glucose, amino acids and other water-soluble products. | These enter hepatic portal blood. |
| Lacteal | Receives chylomicrons assembled inside enterocytes. | Most dietary lipid enters lymph before blood. |
Anatomy of a Villus (Diagram)
Villus → Microvillus hierarchy
The video lesson emphasizes scale: folds → villi → microvilli. The brush-border membrane is not passive; it contains enzymes such as maltase and lactase.
Villus architecture: capillaries receive water-soluble nutrients while the central lacteal receives chylomicrons.
The Villus: Specialized for absorption. Glucose/Amino acids enter blood; Fats enter the Lacteal.
IMAT CHECKPOINTSmall-intestine structureNow test villi, epithelium and absorptive anatomy.
IMAT Challenge
Question 264 Challenge
2011 - Q49
Which of the following features apply to the walls of the small intestine in a healthy human?
1 contain epithelial tissue
2 contain smooth muscle
3 contain villi to increase surface area
1 contain epithelial tissue
2 contain smooth muscle
3 contain villi to increase surface area
The Liver: The Chemical Factory
Bile pathway: liver makes it, gallbladder stores it
Do not confuse production with storage: hepatocytes synthesize bile; the gallbladder concentrates and releases it after a fatty meal.
Blood arrives via the Hepatic Portal Vein (nutrient-rich) and Hepatic Artery (Oxygen-rich). It leaves via the Hepatic Vein.
- Bile Production: Bile salts (derived from cholesterol) emulsify lipids. Stored in Gallbladder.
- Carbohydrate Metabolism: Regulates blood glucose (Glycogenesis, Glycogenolysis, Gluconeogenesis).
- Protein Metabolism: Deamination (removing amino groups), Urea synthesis, Plasma protein synthesis (Albumin, Fibrinogen).
- Lipid Metabolism: Synthesizes lipoproteins and cholesterol.
- Detoxification: SER breaks down toxins, alcohol, and drugs.
- Storage: Vitamins A, D, E, K, B12, Iron, Glycogen.
Portal circulation: why absorbed nutrients reach the liver first
This diagram shows how nutrient-rich blood from the digestive tract is filtered through the liver before returning to the systemic circulation via the hepatic vein.
IMAT CHECKPOINTLiver & bileProduction, storage, location and core liver functions.
IMAT Challenge
Question 261 Challenge
2016 - Q30
Which option shows the structure in humans that produces bile and then the structure that stores bile?
IMAT Challenge
Question 259 Challenge
2020 - Q34
Which of the following statements about bile in the digestive system are correct?
1 It emulsifies fat globules.
2 It dilutes the contents of the small intestine.
3 It lowers the activation energy of lipase.
4 It helps to reduce the acidity of the contents leaving the stomach.
1 It emulsifies fat globules.
2 It dilutes the contents of the small intestine.
3 It lowers the activation energy of lipase.
4 It helps to reduce the acidity of the contents leaving the stomach.
IMAT Challenge
Question 266 Challenge
2013 - Q32
Which one of the following is found below the diaphragm in a human?
IMAT Challenge
Question 263 Challenge
2012 - Q59
Which of the following are correct about the liver in a healthy human?
1 It breaks down haemoglobin.
2 It stores glycogen.
3 It produces insulin.
1 It breaks down haemoglobin.
2 It stores glycogen.
3 It produces insulin.
The Pancreas
Pancreatic juice: neutralize first, digest second
HCO₃⁻Neutralizes acidic chyme
TrypsinogenProtein digestion after activation
LipaseTriglycerides
AmylaseStarch
NucleasesDNA / RNA
- Exocrine (Acini cells): Secrete Pancreatic Juice (HCO₃⁻ + Enzymes: Trypsin, Lipase, Amylase) into the duodenum via the pancreatic duct.
- Endocrine (Islets of Langerhans): Insulin (β-cells) and Glucagon (α-cells) regulate blood sugar.
Zymogen Activation (Protection):
Proteases are secreted as inactive precursors to prevent autodigestion (eating the pancreas itself).
Zymogen Activation Cascade
Why pancreatic proteases are released inactive
Enterokinase/enteropeptidase activates trypsinogen at the duodenal brush border. Trypsin then activates other zymogens—a protective cascade that prevents pancreatic autodigestion.
This diagram details the complex cascade where inactive pro-enzymes from the pancreas (e.g., trypsinogen) are activated within the intestinal lumen to prevent autodigestion of the pancreas.
🧪 Digestive Enzyme Master Chart
CarbohydratesAmylases start starch hydrolysis; brush-border disaccharidases finish it to monosaccharides.
ProteinsPepsin begins digestion; pancreatic proteases and brush-border peptidases complete it.
LipidsBile emulsifies but is not an enzyme. Pancreatic lipase performs the major chemical hydrolysis.
Nucleic acidsPancreatic nucleases generate nucleotides that are further processed at the intestinal surface.
How to use the chart: first identify where digestion occurs, then match the enzyme to its substrate and final absorbable product. This prevents the common IMAT error of confusing bile with a digestive enzyme.
Glucose Homeostasis:
IMAT CHECKPOINTPancreas & duodenal pHBicarbonate, bile/amylase distinctions and pancreatic function.
IMAT Challenge
Question 260 Challenge
2018 - Q34
Which of the following structures make a solution that can raise the pH in the digestive system of a healthy human?
1 gall bladder
2 pancreas
3 stomach
1 gall bladder
2 pancreas
3 stomach
IMAT Challenge
Question 258 Challenge
2021 - Q40
The diagram shows the relationship between the features of two substances involved in digestion: bile and amylase. (X = both, Y = bile only, Z = amylase only)
| row | X | Y | Z |
|---|---|---|---|
| 1 | assimilates lipid | stored in the gall bladder | digests maltose |
| 2 | emulsifies lipid | produced by the pancreas | digests starch |
| 3 | emulsifies lipid | produced by the pancreas | produces maltose |
| 4 | digests lipid | works in the small intestine | digests maltose |
| 5 | digests lipid | works in the small intestine | digests starch |
Advanced Histology
1.3a The Hepatic Lobule
Hepatic lobule map
Portal triads sit at the corners; sinusoidal blood flows inward toward the central vein.
The functional unit of the liver is the Lobule, a hexagonal structure centered around a Central Vein.
Hepatic lobule: blood flows inward, bile flows outward
This diagram illustrates the portal triad (portal vein, hepatic artery, and bile duct), the blood flow towards the central vein, and the retrograde flow of bile within the liver lobule.
Advanced Physiology
1.4 Absorption — which nutrients enter blood vs lymph?
Water-soluble nutrients — transporter logic
One rule: SGLT1 works because the Na⁺ gradient exists; the Na⁺/K⁺ ATPase creates that gradient indirectly. This is why glucose uptake is secondary active transport, not primary active transport.
Macronutrient structures matter because digestion must reduce polymers and large lipids into forms that can cross the epithelial barrier.
Before memorizing transporters:Water-soluble nutrients → blood capillariesMost dietary lipids → lacteals / lymph
Absorption occurs mainly in the small intestine via specific transport mechanisms:
A. Carbohydrates & Proteins (Enter Blood)
- Glucose/Galactose: Secondary Active Transport (SGLT1). Co-transport with Na⁺. Sodium gradient is maintained by the Na+/K+ pump on the basolateral membrane.
- Fructose: Facilitated Diffusion (GLUT5).
- Amino Acids: Secondary Active Transport (Co-transport with Na⁺).
B. Lipids (Enter Lymph)
Fat absorption is complex because lipids are non-polar:
- Emulsification: Bile salts break large drops into droplets.
- Digestion: Lipase hydrolyzes Triglycerides → Fatty Acids + Monoglycerides.
- Micelle Formation: Bile salts surround fatty acids to ferry them to the epithelial surface.
- Diffusion: Fatty acids diffuse into the cell.
- Chylomicron Formation: ER resynthesizes Triglycerides and packages them with proteins (Lipoproteins).
- Exocytosis: Chylomicrons enter Lacteals (Lymphatic system), NOT blood capillaries. They eventually drain into the venous blood via the thoracic duct.
Lipid Digestion and Absorption
Micelle formation
Bile emulsifies; lipase hydrolyses; micelles ferry fatty acids/monoglycerides to the brush border.
Chylomicron route
Inside enterocytes, lipids are rebuilt and packaged as chylomicrons, which are too large for ordinary capillaries and therefore enter lacteals.
Triglyceride ester bonds are hydrolysed during lipid digestion; enterocytes later rebuild triglycerides before chylomicron packaging.
This visual guide shows the process of emulsification by bile salts, micelle formation, chylomicron synthesis within the cell, and subsequent transport into the lacteals.
Carbohydrate and Protein Digestion Pathway
This visual map traces the step-by-step activation of digestive enzymes from the oral cavity to the duodenum and the breakdown of macronutrients into their final absorbable products.
IMAT CHECKPOINTDigestion + absorption integrationUse the villus and transporter logic you just learned.
IMAT Challenge
Question 265 Challenge
2011 - Q50
Which of the following molecules are absorbed by the small intestine and enter the circulatory system?
1 glucose
2 fatty acids
3 glycerol
1 glucose
2 fatty acids
3 glycerol
IMAT Challenge
Question 262 Challenge
2012 - Q51
Which of the following statements about the digestive system is/are correct?
1 Pepsin breaks down protein into amino acids.
2 The pH of the stomach is maintained by the bicarbonate ions secreted by the pancreas.
3 The small intestine is the main site for the absorption of digested food.
1 Pepsin breaks down protein into amino acids.
2 The pH of the stomach is maintained by the bicarbonate ions secreted by the pancreas.
3 The small intestine is the main site for the absorption of digested food.
1.5 The Large Intestine & Gut Microbiome
Colon — recovery and microbial metabolism, not major macronutrient digestion
Colon microbiome — what it contributes
This diagram illustrates the role of gut microbiota in fermenting dietary fiber into short-chain fatty acids (SCFA) and maintaining the mucosal barrier and tight junctions.
Part 2: The Excretory System
The excretory system has three linked jobs: convert nitrogen into excretable urea, filter plasma without losing cells/proteins, and adjust water/ions/pH before urine leaves the body.
2.1 Nitrogenous Waste: The Liver's Role
Nitrogenous waste strategy depends on water availability
Fish: ammonia. Mammals: urea. Birds/reptiles: uric acid. The trade-off is toxicity versus water needed for excretion.
Urinary system: liver-generated urea travels in blood to the kidneys; urine then passes through ureters, bladder and urethra.
IMAT CHECKPOINTKidney input vs outputCompare renal artery and renal vein after orienting the urinary system.
IMAT Challenge
Question 275 Challenge
2011 - Q45
Which one of the following is correct about the composition of plasma in the renal artery and renal vein of a healthy human?
Before excretion, excess amino acids must be processed in the Liver (Deamination).
Ornithine Cycle
2NH₃ + CO₂ → urea + H₂O
Nitrogenous waste varies by animal (Ammonia=Fish, Urea=Mammals, Uric Acid=Birds/Reptiles). Urea is water-soluble and less toxic than ammonia, allowing it to be concentrated in urine.
Urea Cycle (Ornithine Cycle)
This flowchart details the conversion process in the liver where highly toxic ammonia is transformed into safe, water-soluble urea for excretion.
The metabolic pathway in the liver cells converting Ammonia to Urea.
IMAT CHECKPOINTNitrogenous wasteTest the liver → urea → kidney connection immediately.
IMAT Challenge
Question 257 Challenge
2025 - Q25
Which of the following metabolic processes occurs mainly in the liver?
Comparative physiology of excretion.
High Yield: Nephron Physiology
2.2 Detailed Nephron Physiology
NEPHRON ROADMAP — follow filtrate in this order
Read the nephron in sequence. The next sections use this same route: filtration at the renal corpuscle → bulk reabsorption in PCT → medullary-gradient generation in the Loop of Henle → hormone-sensitive final adjustment distally.
NEPHRON SEGMENTS — the high-yield logic in one place
| Segment | Main transport / permeability | Net result | IMAT distinction |
|---|---|---|---|
| Glomerulus | Pressure filtration across endothelium → basement membrane → podocyte slits | Water and small solutes enter Bowman space; cells and large proteins remain in blood | Filtration, not selective reabsorption |
| PCT | Na⁺-linked nutrient uptake; bulk Na⁺/water recovery; bicarbonate recovery | Nearly all filtered glucose/amino acids and ~65% water/salts reclaimed | Na⁺ gradient powers secondary active transport |
| Descending limb | High water permeability; little salt movement | Water exits → tubular fluid concentrates | “Water out” limb |
| Ascending limb | Water-impermeable; Na⁺/K⁺/Cl⁻ removed | Tubular fluid dilutes; medulla becomes hypertonic | “Salt out, no water” limb |
| DCT / collecting duct | Hormone-sensitive fine tuning | Final Na⁺, K⁺, H⁺ and water balance | ADH changes water permeability; aldosterone changes Na⁺/K⁺ handling |
GFR autoregulation: keep filtration near constant
Myogenic mechanismHigh pressure stretches the afferent arteriole → smooth muscle constricts → glomerulus protected.
Tubuloglomerular feedbackMacula densa senses high distal NaCl delivery → paracrine signals constrict the afferent arteriole → GFR falls.
Kidney cross-section and nephron position
Nephrons begin in the cortex and extend into the medulla. Their geometry matters because the Loop of Henle builds the medullary osmotic gradient.
A. Ultrafiltration (The Barrier)
Glomerular filtration barrier — three layers, one purpose
Result: normal filtrate resembles plasma without cells and with very little large protein.
Occurs in Bowman's Capsule. Hydrostatic pressure forces fluid out. The filter has 3 layers preventing cells and proteins from entering urine:
- Fenestrated Endothelium: Capillary wall with pores.
- Podocytes: Epithelial cells with "foot processes" forming filtration slits.
Basement Membrane: Mesh of collagen and negative charge (The effective filter for proteins).
Glomerular Filtration Barrier
Three-layer filtration barrier
Fenestrated endothelium blocks cells; the negatively charged basement membrane is the major barrier to large plasma proteins; podocyte slit diaphragms provide the final size-selective layer.
This diagram shows the three-layer molecular filter composed of fenestrated endothelium, basement membrane, and podocytes that prevents blood cells and large proteins from entering the filtrate.
Autoregulation of GFR: The kidney maintains constant filtration despite BP changes via:
- Myogenic Mechanism: Arteriole smooth muscle contracts when stretched (high BP).
- Tubuloglomerular Feedback: Macula Densa cells sense high NaCl in filtrate and constrict the afferent arteriole.
The three layers of the filtration barrier.
IMAT CHECKPOINTGlomerular filtrationPressure, barrier structure and filtrate composition.
IMAT Challenge
Question 274 Challenge
2013 - Q37
Which of the following statements is/are correct about the glomerulus?
1 It is supplied with blood at high pressure.
2 It is found in the cortex of the kidney.
3 It contains a network of capillaries.
1 It is supplied with blood at high pressure.
2 It is found in the cortex of the kidney.
3 It contains a network of capillaries.
IMAT Challenge
Question 272 Challenge
2015 - Q37
In a healthy human, which of the following is/are correct about the composition of blood and filtrate at the glomerulus?
1 Plasma protein concentration is lower in the filtrate than the blood.
2 Water potential is higher in the filtrate than the blood.
3 Urea concentration is higher in the filtrate than the blood.
1 Plasma protein concentration is lower in the filtrate than the blood.
2 Water potential is higher in the filtrate than the blood.
3 Urea concentration is higher in the filtrate than the blood.
IMAT Challenge
Question 267 Challenge
2022 - Q33
The diagram shows a glomerulus and part of a nephron in the kidney of a healthy human. Which of the following cause(s) water to move from the glomerulus into the tubule?
1 An increase in pressure caused by the left ventricle of the heart.
2 The effect of blood proteins on the direction of osmosis.
3 An increase in pressure of the liquid in the tubule.
1 An increase in pressure caused by the left ventricle of the heart.
2 The effect of blood proteins on the direction of osmosis.
3 An increase in pressure of the liquid in the tubule.
IMAT Challenge
Question 137 Challenge
2023 - Q15
Which statement correctly describes the function of the glomerulus in the kidney?
B. Selective Reabsorption (PCT)
PCT — bulk reclamation driven by the sodium gradient
1Basolateral Na⁺/K⁺ ATPaseUses ATP to keep intracellular Na⁺ low.
2Apical cotransportNa⁺ moving down its gradient drives glucose/amino-acid uptake from filtrate.
3Basolateral exitNutrients leave the tubule cell toward peritubular blood.
4Water followsBulk solute reabsorption creates osmotic water reabsorption; the video lesson emphasizes ~65% recovery here.
IMAT trap: the cotransporter is secondary active transport; ATP is spent by the Na⁺/K⁺ pump, not directly by the glucose cotransporter.
Proximal Tubule Cellular Reabsorption
Cell-level PCT view: sodium-linked apical uptake is sustained by basolateral Na⁺/K⁺ ATPase; water follows solute reabsorption osmotically.
C. The Counter-Current Multiplier (Loop of Henle)
LOOP OF HENLE — compare permeability before memorizing arrows
| Descending limb | Ascending limb | |
|---|---|---|
| Water | Permeable → water exits | Impermeable → water cannot follow |
| Salt | Relatively retained | Na⁺/K⁺/Cl⁻ removed from tubular fluid |
| Tubular fluid | Becomes more concentrated | Becomes more dilute |
| Medulla | Opposing limb properties establish the corticomedullary osmotic gradient. | |
| Why it matters | The collecting duct can then reclaim water efficiently when ADH increases its water permeability. | |
One-line rule: descending = water out; ascending = salt out, water stays.
IMAT CHECKPOINTNephron segment logicFilter → reabsorb → concentrate: identify the job of each segment.
IMAT Challenge
Question 273 Challenge
2014 - Q35
Which of the following processes occur in the human kidney?
1 Ultrafiltration
2 Selective reabsorption
3 Hormonal control of water reabsorption
1 Ultrafiltration
2 Selective reabsorption
3 Hormonal control of water reabsorption
IMAT Challenge
Question 269 Challenge
2019 - Q35
The diagram shows a nephron with a collecting duct from a healthy human. Which of the regions (1-4) contain a liquid with no urea?
IMAT Challenge
Question 271 Challenge
2015 - Q33
Which row correctly identifies the function of the structures listed in a healthy human?
Countercurrent multiplier: descending and ascending limbs must be read together
Descending: water-permeable, salt-impermeable → filtrate concentrates. Ascending: water-impermeable, salt is removed → filtrate dilutes while the medulla becomes hypertonic.
IMAT CHECKPOINTWater handlingApply permeability rules to the nephron.
IMAT Challenge
Question 270 Challenge
2020 - Q37
Which one of the structures listed is NOT involved in the reabsorption of water in a healthy human?
Advanced: Homeostasis
2.3 Kidney in Acid-Base Balance
RENAL ACID–BASE CONTROL — preserve bicarbonate, excrete acid
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
Secrete H⁺Tubular cells move acid into urine.
Reclaim HCO₃⁻Prevents loss of the major extracellular buffer.
Trap H⁺ as NH₄⁺Ammonia buffers secreted H⁺ and permits net acid excretion.
Acid trapping: NH₃ + H⁺ → NH₄⁺
The kidneys regulate blood pH (7.35-7.45) slower but more powerfully than lungs.
- Reabsorption of Bicarbonate (HCO₃⁻): Occurs mainly in PCT. Bicarbonate acts as a buffer.
- Secretion of Hydrogen Ions (H⁺): H⁺ is actively secreted into the filtrate in PCT and DCT.
- Ammoniagenesis: Generation of ammonia to buffer H⁺ in urine (NH₃ + H⁺ → NH₄⁺).
Advanced Hormonal Control
2.4 RAAS & ADH
FINAL URINE CONTROL — distinguish the trigger and the variable being changed
| Signal | Trigger | Kidney action | Physiological result |
|---|---|---|---|
| ADH | High plasma osmolarity / dehydration | Increases collecting-duct water permeability via aquaporins | Less water lost; urine becomes concentrated |
| RAAS → Aldosterone | Low renal perfusion / low pressure | Promotes Na⁺ reabsorption and K⁺ secretion distally | Water follows Na⁺; blood volume/pressure rise |
| ANP | Atrial stretch / high volume | Promotes Na⁺ and water excretion; opposes RAAS | Blood volume/pressure fall |
ADH and aquaporins
ADH is released by the posterior pituitary when blood becomes too concentrated. It increases collecting-duct water permeability by inserting aquaporins.
ADH responds to increased plasma osmolarity and increases collecting-duct water permeability through aquaporin-2.
ADH (Vasopressin)
- Trigger: High blood osmolarity (Dehydration). Detected by hypothalamus osmoreceptors.
- Action: Inserts Aquaporins (water channels) into Collecting Duct.
- Result: Water reabsorption, Concentrated urine, Decreased osmolarity.
Aldosterone (RAAS)
- Trigger: Low Blood Pressure (detected by JGA) → Renin → Angiotensin I → Angiotensin II → Aldosterone.
- Action: Increases Na⁺ reabsorption and K⁺ secretion in DCT/Collecting Duct.
- Result: Water follows salt (osmosis) → Increased Blood Volume/Pressure.
Mechanism of ADH action on collecting duct cells via Aquaporin insertion.
Hormonal Regulation of the Distal Nephron
This diagram visualizes the control of urine concentration via Aldosterone (Na+ recovery) and ADH (water reabsorption) in the distal tubule and collecting duct.
The RAAS Pathway
RAAS is activated by low renal perfusion: renin → angiotensin II → aldosterone, increasing sodium retention and blood pressure.
IMAT CHECKPOINTADH & water balanceOnly after learning ADH/RAAS should this question appear.
IMAT Challenge
Question 268 Challenge
2022 - Q34
Drug Q reduces the release of vasopressin (ADH) into the bloodstream of a healthy human. Which organ releases ADH into the blood, and what is the effect of drug Q on the concentration of urea in the urine?
Part 3: Endocrine System & Homeostasis — Complete Master Edition
Study this chapter as control systems, not as a list of names. Every endocrine question can be reduced to trigger → source → hormone → target → response → feedback. The master atlas below stores the facts once; the sections after it explain only the mechanisms and high-yield connections, so the same point is not repeated again and again.
1MASTER ATLASsource • hormone • action • class
→
2SIGNALINGmembrane vs intracellular receptor
→
3CENTRAL AXEShypothalamus → pituitary → gland
→
4HOMEOSTASISglucose • water • BP • Ca²⁺ • temperature
→
5REPRODUCTIONFSH • LH • gonads • cycles
3.0 Before Hormones — Nervous vs Endocrine vs Exocrine
NERVOUSSignal: action potential + neurotransmitterSpeed: millisecondsDuration: usually shortTarget: highly localised synaptic target
ENDOCRINESignal: hormone released directly into bloodSpeed: slowerDuration: often prolongedTarget: any cell carrying the matching receptor
EXOCRINESignal: not a blood-borne hormone systemRoute: secretion passes through a ductExamples: saliva, sweat, pancreatic digestive enzymesExam trap: endocrine glands are ductless
THE ONE CONTROL LOOP
IMAT CHECKPOINTWhat counts as a hormone?Distinguish endocrine secretion from duct-based exocrine secretion.
IMAT Challenge
Question 292 Challenge
2012 - Q51
Which one of the following is NOT true of human hormones?
3.1 Complete Hormone Master Atlas — learn the facts once
This is the central reference for the whole endocrine chapter. Later diagrams show how these signals work; they do not re-list every hormone again.
PeptideAmineSteroidEnzyme/Peptide
| Source / Gland | Hormone | Core function | Class |
|---|---|---|---|
| Hypothalamus | TRH, CRH, GnRH, GHRH | Stimulate anterior pituitary | Peptide |
| Somatostatin | Inhibits GH and TSH release | Peptide | |
| Dopamine | Inhibits prolactin release | Amine | |
| Pituitary (Anterior) | GH | Growth, protein synthesis | Peptide |
| TSH | Stimulates thyroid gland | Peptide | |
| ACTH | Stimulates adrenal cortex | Peptide | |
| FSH | Follicle/Sperm development | Peptide | |
| LH | Ovulation, testosterone production | Peptide | |
| Prolactin | Milk production | Peptide | |
| MSH | Melanin synthesis | Peptide | |
| Pituitary (Posterior) | ADH (Vasopressin) | Water reabsorption in kidneys | Peptide |
| Oxytocin | Uterine contractions, milk ejection | Peptide | |
| Pineal Gland | Melatonin | Circadian rhythm (sleep-wake cycle) | Amine |
| Thyroid | T3 & T4 (Thyroxine) | Increase metabolic rate | Amine |
| Calcitonin | Lowers blood Ca²⁺ (inhibits osteoclasts) | Peptide | |
| Parathyroid | PTH (Parathormone) | Raises blood Ca²⁺ (stimulates osteoclasts) | Peptide |
| Adrenal Cortex | Cortisol (Glucocorticoids) | Stress response, gluconeogenesis | Steroid |
| Aldosterone (Mineralocorticoids) | Na⁺ reabsorption, K⁺ excretion | Steroid | |
| Androgens | Secondary sex characteristics | Steroid | |
| Adrenal Medulla | Epinephrine & Norepinephrine | Fight-or-flight (SNS response) | Amine |
| Pancreas | Insulin (Beta cells) | Lowers blood glucose | Peptide |
| Glucagon (Alpha cells) | Raises blood glucose | Peptide | |
| Somatostatin (Delta cells) | Inhibits insulin and glucagon | Peptide | |
| Kidney | Erythropoietin (EPO) | Stimulates RBC production in bone marrow | Peptide |
| Renin | Starts RAAS (increases blood pressure) | Enzyme/Peptide | |
| Calcitriol | Active Vitamin D, increases Ca²⁺ absorption | Steroid | |
| Heart (Atria) | ANP | Excretes Na⁺ and water, lowers blood pressure | Peptide |
| GI Tract | Gastrin | Stimulates HCl secretion in stomach | Peptide |
| Secretin | Stimulates bicarbonate release from pancreas | Peptide | |
| CCK | Stimulates bile and enzyme release | Peptide | |
| Thymus | Thymosin | T-lymphocyte maturation | Peptide |
| Gonads (Ovaries/Testes) | Estrogen | Female characteristics, endometrium growth | Steroid |
| Progesterone | Maintains endometrium, pregnancy | Steroid | |
| Testosterone | Male characteristics, spermatogenesis | Steroid | |
| Inhibin | Inhibits FSH secretion | Peptide |
Important posterior-pituitary nuance: ADH and oxytocin are synthesized in the hypothalamus, then transported to and stored/released from the posterior pituitary. The table labels the clinically used release site.
Visual cross-check: major human hormones
Use the atlas above as the authoritative study structure; this image is a visual cross-check, not a second list to memorise.
Anatomical anchor: where the glands are
IMAT CHECKPOINTPineal glandMelatonin and circadian rhythm are now immediately tested.
IMAT Challenge
Question 212 Challenge
2023 - Q21
Which of the following anatomical structures is responsible for regulating the circadian rhythm?
3.2 Signaling Logic — chemistry tells you where the receptor is
Question to ask first: Can the hormone cross the lipid bilayer? If no, expect a membrane receptor and second messenger. If yes, expect an intracellular receptor and altered gene transcription.
| Class | Synthesis / storage | Blood transport | Receptor | Typical timing |
|---|---|---|---|---|
| Peptide / protein | Amino-acid derived; synthesized in advance and commonly stored in secretory vesicles | Water-soluble; usually circulates freely | Cell-surface receptor | Fast onset; often shorter half-life |
| Steroid | Cholesterol-derived; generally synthesized on demand rather than stored in vesicles | Lipid-soluble; commonly carried by plasma proteins | Cytoplasmic / nuclear receptor | Slower onset; often longer-lasting |
| Thyroid T3/T4 | Tyrosine/iodine-derived and stored extracellularly in thyroid colloid | Mostly protein-bound in plasma | Nuclear receptor | Slow, prolonged genomic effect |
PEPTIDE / MOST AMINEShydrophilic→membrane receptor→G protein / cAMP / kinase→rapid protein activity change
STEROIDSlipid-soluble→cross membrane→cytoplasmic / nuclear receptor→DNA transcription → new protein
T3 / T4amine-derived→lipid-soluble behaviour→nuclear receptor→gene transcription
Peptide vs steroid receptor mechanism
Steroidogenesis: cholesterol is the common precursor
IMAT trap: thyroid hormone is chemically an amine derivative, but its receptor mechanism is much closer to a steroid hormone than to adrenaline.
3.3 Central Control — Hypothalamus → Pituitary → Target Gland
The hypothalamus is the neural–endocrine interface. It sends releasing/inhibiting hormones to the anterior pituitary; it also synthesizes ADH and oxytocin, which travel down axons for release from the posterior pituitary.
HPT AXISHypothalamus TRH→Anterior pituitary TSH→Thyroid T3/T4metabolic rate
HPA AXISHypothalamus CRH→Anterior pituitary ACTH→Adrenal cortex Cortisollong-term stress
HPG AXISHypothalamus GnRH→Anterior pituitary FSH/LH→Gonads sex steroids / inhibinreproduction
GH AXISGHRH (+) / Somatostatin (−)→Anterior pituitary GH→Liver IGF-1growth
PRL CONTROLHypothalamic Dopamine (−)→Anterior pituitary Prolactin→Mammary glandmilk production
ANTERIOR PITUITARYGlandular tissue; synthesizes hormones. Major outputs: GH, TSH, ACTH, FSH, LH, prolactin, MSH (and endorphin peptides).
POSTERIOR PITUITARYNeural extension; does not synthesize its two classic hormones. Stores/releases hypothalamic ADH and oxytocin.
Hypothalamus–pituitary connection
Pituitary anatomy
Major pituitary outputs
GH regulation: GHRH vs somatostatin
Multiple endocrine feedback loops
IMAT CHECKPOINTHypothalamus & pituitaryAnterior vs posterior pituitary, osmoregulation and reproductive consequences.
IMAT Challenge
Question 287 Challenge
2011 - Q47
In a healthy human, which of the following is/are correct about the pituitary gland?
1 It is under the control of the hypothalamus.
2 It produces follicle stimulating hormone (FSH).
3 It produces anti-diuretic hormone (ADH).
1 It is under the control of the hypothalamus.
2 It produces follicle stimulating hormone (FSH).
3 It produces anti-diuretic hormone (ADH).
IMAT Challenge
Question 282 Challenge
2014 - Q42
Which of the following is/are correct about the pituitary gland in a healthy human?
1 It is directly involved in osmoregulation.
2 It secretes releasing hormones.
3 It directly controls the heart rate.
1 It is directly involved in osmoregulation.
2 It secretes releasing hormones.
3 It directly controls the heart rate.
IMAT Challenge
Question 278 Challenge
2017 - Q29
Which of the following could be a consequence of a reduction in pituitary function?
1 more urine produced in a male or female
2 infertility in a female
3 infertility in a male
1 more urine produced in a male or female
2 infertility in a female
3 infertility in a male
3.4 Peripheral Control Systems — group hormones by the variable they control
After the central axis, stop thinking gland-by-gland. IMAT questions are easier when you ask which variable is being corrected?
Insulin vs glucagon feedback
Insulin receptor signaling and GLUT4
Blood-glucose regulation reference
STRESS, VOLUME & BLOOD PRESSUREadrenal + kidney + heart
Acute stresssympathetic nervesadrenal medulla → epinephrine/norepinephrineHR, BP, glucose availability ↑fight-or-flight
Long-term stressCRH → ACTHadrenal cortex → cortisolgluconeogenesis ↑ / immune suppressionenergy mobilisation
Low renal perfusionkidney renin → RAASaldosteroneNa⁺ retention; K⁺ secretionvolume/BP ↑
Atrial stretchheart → ANPnatriuresis + water lossrenin/aldosterone opposedvolume/BP ↓
Thyroid, parathyroid and adrenal locations
Adrenal cortex vs medulla
IMAT CHECKPOINTAdrenal gland & adrenalineCortex vs medulla and fight-or-flight.
IMAT Challenge
Question 284 Challenge
2013 - Q39
Which of the following is/are correct about the adrenal gland?
1 It is located above the kidney.
2 It secretes adrenaline.
3 It produces glucocorticoids.
1 It is located above the kidney.
2 It secretes adrenaline.
3 It produces glucocorticoids.
IMAT Challenge
Question 255 Challenge
2012 - Q57
Which of the following are increased when the level of adrenaline rises in a human?
1 heart rate
2 breathing rate
3 impulse rate in a sensory neurone
1 heart rate
2 breathing rate
3 impulse rate in a sensory neurone
CALCIUM HOMEOSTASISPTH + calcitonin + calcitriol
OTHER ENDOCRINE SOURCESfrequent one-line exam facts
PinealMelatonin → circadian / sleep–wake timing.
KidneyEPO → RBC production; renin → RAAS; calcitriol → intestinal Ca²⁺ absorption.
GI tractGastrin → HCl; secretin → pancreatic bicarbonate; CCK → bile + pancreatic enzymes.
ThymusThymosin → T-lymphocyte maturation.
3.5 Homeostasis — one logic applied to six variables
| Variable | Sensor / trigger | Main signal | Effector response | Return toward set point |
|---|---|---|---|---|
| Temperature | Hypothalamic thermoreceptors | Autonomic + endocrine coordination | Hot: vasodilation/sweating; Cold: vasoconstriction/shivering | Heat loss or heat production changes |
| Blood glucose | Pancreatic islets | Insulin / Glucagon | Uptake & storage / hepatic release | Glucose falls / rises |
| Plasma osmolarity | Hypothalamic osmoreceptors | ADH | Collecting-duct aquaporins ↑ | Water retained; osmolarity falls |
| Blood volume / BP | Renal perfusion / atrial stretch | RAAS–Aldosterone / ANP | Na⁺ retention / natriuresis | Volume rises / falls |
| Blood Ca²⁺ | Parathyroid / thyroid C cells | PTH / Calcitonin (+ calcitriol) | Bone, kidney and gut handling changes | Ca²⁺ rises / falls |
| Stress | Hypothalamus + sympathetic input | CRH→ACTH→Cortisol; catecholamines | Fuel mobilisation + cardiovascular response | Negative feedback after stressor resolves |
Negative feedback axis
NEGATIVE FEEDBACK = defaultFinal hormone/effect suppresses upstream drive.
POSITIVE FEEDBACK = exceptionLH surge before ovulation; oxytocin during labour.
3.6 Reproductive Endocrinology — put FSH and LH on their target cells
1 • Follicular phaseFSH supports follicular growth → estrogen rises → endometrium proliferates.
2 • Positive feedback switchSustained high estrogen → large LH surge.
3 • OvulationLH surge ruptures the mature follicle around mid-cycle.
4 • Luteal phaseCorpus luteum → progesterone (plus estrogen) → endometrium maintained.
5 • No pregnancyCorpus luteum degenerates → progesterone/estrogen fall → menstruation.
FEMALE FSHfollicular development + estrogen production
FEMALE LHovulation + corpus luteum
MALE FSHSertoli cells → spermatogenesis
MALE LHLeydig cells → testosterone
INHIBINgonadal peptide → inhibits FSH
IMAT CHECKPOINTFSH & LH targetsApply the same gonadotropins to both sexes.
IMAT Challenge
Question 285 Challenge
2013 - Q43
Which one of the following hormones is directly involved in both male and female reproduction?
Female reproductive anatomy
Follicular development → ovulation → corpus luteum
Menstrual-cycle hormone pattern
Anchor points: estrogen rises before ovulation; LH surge triggers ovulation; progesterone dominates the luteal phase.
IMAT CHECKPOINTFemale cycleUse the timeline above: estrogen rise → LH surge → ovulation → progesterone.
IMAT Challenge
Question 289 Challenge
2020 - Q36
Which one of the following statements about the female reproductive system is correct for a healthy human?
IMAT Challenge
Question 279 Challenge
2015 - Q23
The changes in concentration of hormones in a healthy woman's blood were monitored during several menstrual cycles. During which stage of the menstrual cycle was the concentration of oestrogen falling, the concentration of luteinising hormone (LH) and follicle-stimulating hormone (FSH) maximal and the progesterone concentration rising?
Male reproductive anatomy
Spermatogenesis and seminiferous tubules
PLACENTA EXCHANGE — maternal and fetal blood stay separate
MATERNAL BLOODO₂ • glucose • amino acids
→ diffusion / transport →
PLACENTAL BARRIERlarge exchange surface; thin diffusion distance
← CO₂ • urea ←
FETAL BLOODumbilical vein carries oxygenated blood toward fetus
IMAT CHECKPOINTPlacental exchangeThese questions now follow the exchange model they depend on.
IMAT Challenge
Question 291 Challenge
2020 - Q41
Which one of the following statements about the human placenta is correct?
IMAT Challenge
Question 280 Challenge
2016 - Q39
Which row correctly identifies a molecule which diffuses across the placenta from the fetus to the mother, and a molecule which diffuses across the placenta from the mother to the fetus?
ENDOCRINE EXAM CORE — only five rules to carry forward
- Receptor location follows chemistry: peptides → membrane; steroids → intracellular; thyroid hormone is the key amine exception.
- Anterior pituitary synthesizes; posterior pituitary stores/releases hypothalamic ADH and oxytocin.
- Feedback is usually negative. LH surge and labour oxytocin are the classic positive-feedback exceptions.
- Insulin lowers glucose; glucagon raises it. ADH retains water; aldosterone retains Na⁺ and secretes K⁺; ANP opposes volume retention.
- FSH/LH must be tied to targets: follicle/Sertoli vs ovulation–corpus luteum/Leydig.
Final Cross-System Integration — the three stories
DIGESTIONMacromolecule → enzymatic hydrolysis → absorbable unit → blood or lymphMouth / stomach / pancreatic–brush-border enzymes
EXCRETIONBlood → filtration → selective reabsorption/secretion → concentrated urineGlomerulus → PCT → loop → DCT/collecting duct
ENDOCRINETrigger → hormone → receptor-bearing target → response → feedbackUse the complete Hormone Master Atlas in Part 3 rather than memorising duplicate lists here.
GI ↔ endocrine: gastrin, secretin, CCKKidney ↔ endocrine: renin, EPO, calcitriol, ADH, aldosterone, ANPCalcium: PTH + calcitonin + calcitriol connect endocrine, bone, kidney and gut