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Organ Systems

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Organ Systems - Complete Interactive Lesson

Part 1: Cardiovascular System

Organ Systems for the MCAT

Part 1 of 7 — Cardiovascular System

Heart Structure

4 chambers: RA → RV → Lungs → LA → LV → Body
Right side: deoxygenated blood to lungs (pulmonary circulation)
Left side: oxygenated blood to body (systemic circulation)
LV has thickest walls (pumps against systemic resistance)

Cardiac Cycle

Phase	AV Valves	Semilunar Valves	What happens
Atrial systole	Open	Closed	Atria contract, blood into ventricles
Ventricular systole	Closed (S1)	Open	Ventricles contract, blood into arteries
Diastole	Open	Closed (S2)	Ventricles relax, fill with blood

Key Equations

$\text{Cardiac Output} = \text{HR} \times \text{Stroke Volume}$

$\text{Blood Pressure} = \text{CO} \times \text{Total Peripheral Resistance}$

Blood Vessel Types

Arteries: Thick walls, elastic, carry blood FROM heart
Veins: Thin walls, valves, carry blood TO heart
Capillaries: Single cell thick, site of exchange

Cardiac Conduction & the ECG

$\text{SA node} \to \text{AV node (delay)} \to \text{Bundle of His} \to \text{Purkinje fibers}$

SA node = pacemaker (~60–100 bpm intrinsic rate); sets sinus rhythm.
AV node delay (~0.1 s) lets atria finish emptying before ventricles contract.
ECG: P wave = atrial depolarization; QRS = ventricular depolarization (masks atrial repolarization); T wave = ventricular repolarization.

Pressure–Flow Relationships (Poiseuille & Resistance)

Flow through a vessel obeys an Ohm's-law analog, $Q = \Delta P / R$ , where resistance depends sharply on radius:

$R \propto \frac{\eta L}{r^4}$

Radius dominates: halving $r$ raises resistance 16×. This is why arterioles ("resistance vessels") control BP.
Velocity is slowest in capillaries because total cross-sectional area is largest ( $v = Q / A_{total}$ ) — maximizing exchange time.

Frank–Starling Mechanism & Pressure–Volume Loop

Frank–Starling law: greater end-diastolic volume (preload) → greater stretch → stronger contraction → larger stroke volume. The heart pumps what it receives.
Preload ↑ by venous return; afterload = the pressure the ventricle must overcome (≈ aortic/systemic pressure); contractility ↑ by sympathetic/catecholamines independent of preload.

Capillary Exchange (Starling Forces)

$J_v \propto (P_c - P_i) - (\pi_c - \pi_i)$

Arteriolar end: hydrostatic pressure dominates → net filtration out.
Venular end: oncotic pressure dominates → net reabsorption in.
Excess filtered fluid returns via the lymphatics. Lymphatic blockage or low plasma albumin → edema.

Cardiovascular 🎯

Worked Examples — Cardiovascular Physiology

<details> <summary>Example 1: Compute cardiac output and mean arterial pressure</summary>

Question: HR = 75 bpm, end-diastolic volume = 120 mL, end-systolic volume = 50 mL, total peripheral resistance such that systolic BP = 120 and diastolic = 80 mmHg. Find stroke volume, cardiac output, and mean arterial pressure (MAP).

Solution:

Stroke volume = EDV − ESV = 120 − 50 = 70 mL
Cardiac output = HR × SV = 75 × 70 = 5250 mL/min ≈ 5.25 L/min ✓
MAP ≈ DBP + ⅓(SBP − DBP) = 80 + ⅓(40) = 93 mmHg

MCAT note: Ejection fraction = SV/EDV = 70/120 ≈ 58% (normal). A falling ejection fraction signals systolic heart failure.

</details> <details> <summary>Example 2: Predict the effect of increased afterload</summary>

Question: A patient develops aortic stenosis (narrowed aortic valve), raising the pressure the left ventricle must generate to eject blood. In the short term, what happens to stroke volume, and how does the heart compensate long-term?

Solution:

Higher afterload → the ventricle ejects against greater resistance → stroke volume falls acutely (end-systolic volume rises because less blood is ejected).
Frank–Starling: the leftover blood adds to the next preload, partially restoring SV via greater stretch.
Long-term compensation: concentric ventricular hypertrophy (thicker wall) to generate higher pressure — eventually maladaptive, leading to diastolic dysfunction. ✓

High-yield connection: Afterload ↑ → SV ↓; preload ↑ → SV ↑. Distinguish these on pressure–volume loops.

</details> <details> <summary>Example 3: Reason about capillary fluid movement</summary>

Question: At the arteriolar end of a capillary: $P_{c}$ , , , mmHg. At the venular end (others unchanged). Determine the direction of net fluid movement at each end.

</details>

Key Takeaways — Part 1

CO = HR $\times$ SV; BP = CO $\times$ TPR
Left ventricle = thickest (systemic pressure)
S1 = AV valves close (lub); S2 = semilunar valves close (dub)
Capillaries = site of gas/nutrient exchange (largest total cross-sectional area)

Part 2: Respiratory System

Organ Systems for the MCAT

Part 2 of 7 — Respiratory System

Gas Exchange

$\text{O}_2: \text{Alveoli} \to \text{Blood} \to \text{Tissues}$ $\text{CO}_2: \text{Tissues} \to \text{Blood} \to \text{Alveoli}$

Part 3: Renal & Excretory System

Organ Systems for the MCAT

Part 4 of 7 — Renal System (Kidneys)

Nephron Structure

$\text{Glomerulus} \to \text{PCT} \to \text{Loop of Henle} \to \text{DCT} \to \text{Collecting Duct}$

Key Functions by Segment

Segment	Function	Key Details
Glomerulus

Part 4: Digestive System

Organ Systems for the MCAT

Part 3 of 7 — Digestive System

GI Tract Order

Mouth → Esophagus → Stomach → Small intestine (duodenum → jejunum → ileum) → Large intestine → Rectum

Digestive Enzymes

Enzyme	Source	Substrate	Product
Salivary amylase	Mouth	Starch	Maltose
Pepsin	Stomach (chief cells)	Protein	Peptides
Trypsin, chymotrypsin	Pancreas	Protein	Peptides
Pancreatic lipase	Pancreas	Triglycerides	Monoglycerides + fatty acids
Pancreatic amylase	Pancreas	Starch	Maltose
Lactase, maltase	Small intestine brush border	Disaccharides	Monosaccharides

Stomach

Parietal cells: HCl (acidic pH ~2) + intrinsic factor (for B absorption)

Part 5: Endocrine System

Organ Systems for the MCAT

Part 5 of 7 — Endocrine System

Major Endocrine Glands & Hormones

Gland	Hormone(s)	Function
Hypothalamus	Releasing/inhibiting hormones	Controls anterior pituitary
Anterior pituitary	GH, TSH, ACTH, FSH, LH, Prolactin	Master gland
Posterior pituitary	ADH, Oxytocin	Stores/releases hypothalamic hormones
Thyroid	T3/T4, Calcitonin	Metabolism, lowers Ca $^{2+}$
Parathyroid

Part 6: Nervous System

Organ Systems for the MCAT

Part 6 of 7 — Nervous System

Organization

CNS: Brain + Spinal cord
PNS: Everything else
- Somatic: Voluntary (skeletal muscle)
- Autonomic: Involuntary
  - Sympathetic: Fight-or-flight (increases HR, dilates pupils, inhibits digestion)
  - Parasympathetic: Rest-and-digest (decreases HR, constricts pupils, promotes digestion)

Neuron Structure & the Action Potential (Figure)

The membrane potential trace over time:

Phase	Channel event	Ion movement	$V_m$ effect

Part 7: Review & MCAT Practice

Organ Systems for the MCAT

Part 7 of 7 — Immune System

Innate vs. Adaptive Immunity

Feature	Innate	Adaptive
Speed	Immediate (0-12 hours)	Slow (days to weeks)
Specificity	Broad	Highly specific (antigen-specific)
Memory	No	Yes
Components	Neutrophils, macrophages, NK cells, complement	T cells, B cells, antibodies

Adaptive Immunity

Humoral (B cells):

B cells → Plasma cells → Antibodies (immunoglobulins)
Antibodies neutralize, opsonize, activate complement
IgG: Most abundant, crosses placenta
IgM: First responder, pentamer
IgA: Mucosal surfaces (saliva, breast milk)
IgE: Allergies, parasites

Cell-mediated (T cells):

CD4+ (Helper T): Activate B cells and CD8+ cells
CD8+ (Cytotoxic T): Kill infected/cancer cells directly
T regulatory: Suppress immune responses (prevent autoimmunity)

Arteriolar end: net = $(35 - 0) - (25 - 0) = +10$ mmHg → filtration OUT. ✓
Venular end: net = $(15 - 0) - (25 - 0) = -10$ mmHg → reabsorption IN. ✓

Interpretation: Hydrostatic pressure drops along the capillary while oncotic pressure stays roughly constant, so fluid filters out early and is reabsorbed later. Net slight excess is cleared by lymphatics — block them and you get lymphedema.

CO_{2} : Tissues \to Blood \to Alveoli

Driven by PARTIAL PRESSURE gradients (Fick's law)
Alveoli maximize surface area for diffusion

Fick's Law of Diffusion

Gas flux across the alveolar membrane scales with surface area and partial-pressure gradient, and inversely with membrane thickness:

$V_{gas} \propto \frac{A \cdot D \cdot (P_1 - P_2)}{T}$

$A$ = surface area (huge in alveoli, ~70 m²); destroyed in emphysema → impaired exchange.
$T$ = membrane thickness; increased by pulmonary edema or fibrosis → impaired diffusion.
$(P_1 - P_2)$ = partial pressure gradient. Alveolar $P_{O_2}\approx 100$ mmHg vs. venous blood ~40 mmHg drives O₂ in.

98.5% bound to hemoglobin (Hb), 1.5% dissolved in plasma
Each Hb binds 4 O $_2$ molecules
Cooperative binding: Binding of first O $_2$ increases affinity for subsequent O $_2$ (sigmoidal curve)

The Oxygen–Hemoglobin Dissociation Curve (Figure)

The curve plots % Hb saturation (y) vs. $P_{O_2}$ (x) and is sigmoidal due to cooperativity:

$P_{O_2}$ (mmHg)	~% Saturation	Location
100	~98%	Lungs (loading plateau)
40	~75%	Resting venous blood / tissues
26 ( $P_{50}$ )	50%	Reference affinity point
20	~35%	Exercising muscle (steep unloading)

The flat upper plateau means modest drops in alveolar $P_{O_2}$ (altitude, mild lung disease) barely lower loading. The steep middle means small $P_{O_2}$ drops in tissue cause large O₂ release — efficient unloading exactly where metabolism is high.

The Bohr Effect (MCAT FAVORITE)

Conditions that RIGHT-shift the curve (raise $P_{50}$ , lower affinity, promote O $_2$ unloading):

Increased CO $_2$ (metabolically active tissue)
Decreased pH (acidic — more CO $_2$ /lactic acid)
Increased temperature
Increased 2,3-BPG (chronic hypoxia, high altitude)

Mnemonic: Right shift = Release O $_2$ to tissues. A LEFT shift (↑pH, ↓CO₂, ↓temp, ↓2,3-BPG; also fetal Hb and CO) raises affinity → O₂ held tightly.

CO $_2$ Transport & the Chloride Shift

70% as bicarbonate (HCO $_3^-$ )
23% bound to Hb (carbaminohemoglobin)
7% dissolved in plasma

In the tissues, CO₂ enters RBCs, and carbonic anhydrase catalyzes:

$\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$

HCO₃⁻ exits the RBC in exchange for Cl⁻ (the chloride shift); H⁺ binds Hb (driving the Bohr effect). In the lungs the entire reaction reverses, expelling CO₂.

Control of Ventilation (Feedback Loop)

The medullary respiratory center sets breathing rate. The dominant stimulus is CO₂/pH, not O₂:

$\uparrow P_{CO_2} \to \uparrow \text{H}^+ \text{ in CSF} \to \text{central chemoreceptors} \to \uparrow \text{ventilation} \to \downarrow P_{CO_2}$

Central chemoreceptors (medulla) sense CSF pH (a proxy for arterial CO₂) — the primary driver.
Peripheral chemoreceptors (carotid/aortic bodies) sense low $P_{O_2}$ (<60 mmHg), high CO₂, low pH — the backup hypoxic drive.
Hyperventilation blows off CO₂ → respiratory alkalosis; hypoventilation retains CO₂ → respiratory acidosis.

Respiratory 🎯

Worked Examples — Respiratory Physiology

<details> <summary>Example 1: Compute alveolar $P_{O_2}$ with the alveolar gas equation</summary>

Question: At sea level, $P_{atm} = 760$ mmHg, water vapor pressure = 47 mmHg, $F_{IO_2} = 0.21$ , arterial $P_{CO_2} = 40$ mmHg, respiratory quotient $R = 0.8$ . Estimate alveolar $P_{O_2}$ .

Solution:

Inspired $P_{O_2} = (760 - 47) \times 0.21 = 713 \times 0.21 \approx 150$ mmHg.

MCAT note: This ~100 mmHg matches the lung value in the dissociation table. At altitude, $P_{atm}$ falls → $P_{IO_2}$ falls → drops, triggering the hypoxic ventilatory response.

</details> <details> <summary>Example 2: Calculate O₂ delivered per minute</summary>

Question: Hb = 15 g/dL, each gram of Hb carries 1.34 mL O₂ at full saturation, arterial saturation = 98%, cardiac output = 5 L/min. Approximate O₂ delivery to tissues (ignore dissolved O₂).

Solution:

O₂ content $\approx 15 \times 1.34 \times 0.98 \approx 19.7$ mL O₂ / dL = 197 mL/L.
Delivery = content × cardiac output = $197 \times 5 \approx$ 985 mL O₂/min ✓

Interpretation: Anemia (lower Hb) cuts delivery proportionally even with normal saturation — this is why $SpO_2$ alone can mask poor O₂ delivery. The body compensates by raising cardiac output.

</details> <details> <summary>Example 3: Predict the acid–base effect of hyperventilation</summary>

Question: A panicking patient hyperventilates, dropping arterial $P_{CO_2}$ from 40 to 25 mmHg. Using the carbonic anhydrase equilibrium, predict the change in blood pH and the curve shift.

Solution:

$\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$ . Lowering CO₂ pulls the reaction LEFT → fewer H⁺ → (respiratory alkalosis). ✓

High-yield connection: Breathing into a bag re-raises CO₂, restoring pH and curve position. CO₂, not O₂, is the master regulator of ventilation.

</details>

Key Takeaways — Part 2

Gas exchange driven by partial pressure gradients (Fick's law: $V_{gas} \propto A(P_1-P_2)/T$ )
O $_2$ transport: 98.5% on hemoglobin (cooperative binding, sigmoidal curve)
Bohr effect: right shift = more O $_2$ release (higher CO $_2$ , lower pH, higher temp, ↑2,3-BPG)
CO $_2$ transport: mostly as bicarbonate (70%); chloride shift maintains charge balance
Ventilation is driven mainly by CO₂/pH via central chemoreceptors, not by O₂

Hormone	Source	Effect
ADH (vasopressin)	Posterior pituitary	Inserts aquaporins → H $_2$ O reabsorption in collecting duct
Aldosterone	Adrenal cortex	Na $^+$ reabsorption (+ K $^+$ secretion) in DCT
ANP	Heart (atria)	Na $^+$ excretion, opposes RAAS

$\text{GFR} \approx 180\;\text{L/day}$

But urine output is only ~1.5 L/day → 99% of filtrate is reabsorbed!

Net Filtration Pressure (Starling Forces at the Glomerulus)

Filtration at the glomerulus is governed by the balance of hydrostatic and oncotic pressures:

$P_{net} = (P_{GC} - P_{BS}) - (\pi_{GC} - \pi_{BS})$

$P_{GC}$ = glomerular capillary hydrostatic pressure (pushes fluid OUT, ~55 mmHg)
$P_{BS}$ = Bowman's space hydrostatic pressure (pushes IN, ~15 mmHg)
$\pi_{GC}$ = glomerular oncotic pressure (pulls IN, ~30 mmHg)
$\pi_{BS}$ ≈ 0 (essentially protein-free filtrate)

GFR is regulated by adjusting afferent vs. efferent arteriole tone — this is the single most testable renal concept.

Renal Clearance & the Filtration Equation

$C_x = \frac{U_x \cdot V}{P_x}$

where $U_x$ = urine concentration of substance x, $V$ = urine flow rate, $P_x$ = plasma concentration.

Inulin is freely filtered, not reabsorbed or secreted → its clearance EQUALS GFR.
PAH (para-aminohippurate) is filtered AND maximally secreted → its clearance estimates renal plasma flow (RPF).
If $C_x >$ GFR → net secretion. If $C_x <$ GFR → net reabsorption.

Countercurrent Multiplier (How Concentrated Urine Is Made)

The thick ascending limb actively pumps Na⁺/K⁺/2Cl⁻ out (impermeable to water), making the medullary interstitium hyperosmotic (up to ~1200 mOsm). The descending limb (water-permeable, solute-impermeable) loses water passively into that gradient. The vasa recta preserve the gradient via countercurrent EXCHANGE. ADH then lets water exit the collecting duct down this gradient → concentrated urine.

Renal System 🎯

Worked Examples — Renal Physiology

<details> <summary>Example 1: Compute net glomerular filtration pressure</summary>

Question: Given $P_{GC} = 55$ mmHg, $P_{BS} = 15$ mmHg, $\pi_{GC} = 28$ mmHg, $\pi_{BS} = 0$ . What is the net filtration pressure, and which direction does fluid move?

Solution: $P_{net} = (P_{GC} - P_{BS}) - (\pi_{GC} - \pi_{BS}) = (55 - 15) - (28 - 0) = 40 - 28 = +12 \text{ mmHg}$

Positive → net filtration OUT of the capillary into Bowman's space. ✓

MCAT twist: If a patient develops hypoalbuminemia (low plasma protein, e.g., nephrotic syndrome or liver failure), $\pi_{GC}$ falls. With $\pi_{GC} = 18$ : mmHg → GFR rises. Conversely, a ureteral stone raises and lowers GFR.

</details> <details> <summary>Example 2: Calculate renal clearance and classify handling</summary>

Question: A substance has plasma concentration $P_x = 2$ mg/mL, urine concentration $U_x = 60$ mg/mL, and urine flow rate mL/min. GFR (by inulin) = 120 mL/min. Is this substance secreted, reabsorbed, or neither?

Solution: $C_x = \frac{U_x \cdot V}{P_x} = \frac{60 \times 1}{2} = 30 \text{ mL/min}$

Clearance (30) < GFR (120) → the tubule reabsorbs most of the filtered substance. ✓

Interpretation: Filtered load = GFR × $P_x$ = 120 × 2 = 240 mg/min. Excreted = $U_x$ × V = 60 mg/min. Reabsorbed = 240 − 60 = 180 mg/min (75% reabsorbed). This is the kind of multi-step data problem the Bio/Biochem section loves.

</details> <details> <summary>Example 3: Predict the effect of a loop diuretic</summary>

Question: Furosemide blocks the Na⁺/K⁺/2Cl⁻ cotransporter in the thick ascending limb. Predict its effect on (a) the medullary osmotic gradient and (b) urine volume.

Solution:

The thick ascending limb normally pumps NaCl into the interstitium to build the medullary gradient.
Blocking the cotransporter → less NaCl deposited → the medullary gradient COLLAPSES.
Without a steep gradient, the collecting duct cannot extract water even when ADH is present.
Result: large volume of dilute urine (powerful diuresis), plus K⁺ wasting (K⁺ no longer recycled). ✓

High-yield connection: This is why loop diuretics are the strongest class — they attack the gradient itself, not just one segment's transport.

</details>

Key Takeaways — Part 4

Nephron: Glomerulus → PCT → Loop of Henle → DCT → Collecting duct
Descending loop: water out. Ascending loop: salt out (countercurrent multiplier).
ADH: water reabsorption. Aldosterone: Na $^+$ reabsorption + K $^+$ secretion.
180 L/day filtered but only ~1.5 L urine (99% reabsorbed!)

Chief cells: Pepsinogen → activated to pepsin by HCl

G cells: Gastrin (stimulates HCl secretion)

Mucous cells: Protect stomach lining from acid

Zymogen Activation Cascade (Why the Pancreas Doesn't Digest Itself)

Proteases are secreted as inactive zymogens and activated only in the gut lumen:

$\text{Enteropeptidase (duodenum)} \to \text{Trypsinogen} \to \textbf{Trypsin}$ $\textbf{Trypsin} \to \text{activates chymotrypsinogen, procarboxypeptidase, and MORE trypsinogen}$

Trypsin is the master activator (autocatalytic). Premature intra-pancreatic activation causes acute pancreatitis (the pancreas digests itself) — a classic clinical correlate.

Hormonal Control of Digestion (Figure / Feedback Loops)

Hormone	Trigger	Source	Main Action
Gastrin	Peptides, distension, vagal input	Stomach G cells	↑ HCl secretion by parietal cells
Secretin	Acidic chyme (low pH) entering duodenum	Duodenal S cells	↑ pancreatic bicarbonate → neutralizes acid; ↓ gastric acid
CCK (cholecystokinin)	Fats & amino acids in duodenum	Duodenal I cells	Gallbladder contraction (bile) + pancreatic enzyme release; slows gastric emptying

Flow: fatty/acidic chyme enters duodenum → secretin + CCK released → pancreas dumps bicarbonate (raises pH to the ~8 optimum for pancreatic enzymes) and enzymes; gallbladder ejects bile → fat emulsified and digested. This is a negative-feedback brake: duodenal contents signal back to slow the stomach until the small intestine catches up.

Duodenum: Iron, calcium
Jejunum: Most nutrients (amino acids, sugars, fatty acids)
Ileum: Bile salts (recycled via enterohepatic circulation), vitamin B $_{12}$
Large intestine: Water, electrolytes

Carbohydrate Absorption Mechanism

Glucose/galactose enter enterocytes via SGLT1 (secondary active transport powered by the Na⁺ gradient from the basolateral Na⁺/K⁺ ATPase), then exit to blood via GLUT2. Fructose enters by facilitated diffusion via GLUT5. This Na⁺-coupled uptake is why oral rehydration therapy pairs glucose with sodium.

Digestive System 🎯

Worked Examples — Digestive Physiology

<details> <summary>Example 1: Predict the response to a fatty meal</summary>

Question: A subject eats a high-fat meal. Trace the hormonal cascade and predict the effect on gastric emptying.

Solution:

Fats and amino acids reach the duodenum → I cells release CCK.
CCK → gallbladder contracts (bile ejected) + pancreas releases lipase; CCK also slows gastric emptying. ✓
Acidic chyme → secretin → pancreatic bicarbonate raises duodenal pH toward ~8 (optimum for lipase/trypsin).

MCAT note: Slowed gastric emptying after fat is adaptive — it prevents overwhelming the small intestine's limited digestive/absorptive capacity. Fatty meals therefore "sit heavy."

</details> <details> <summary>Example 2: Diagnose a malabsorption pattern</summary>

Question: A patient has chronic pancreatitis with destroyed exocrine pancreas. Which nutrients are most affected, and what stool finding appears?

Solution:

Loss of pancreatic lipase → triglycerides not digested → fat malabsorption.
Undigested fat in stool = steatorrhea (greasy, floating, foul stool). ✓
Fat-soluble vitamins (A, D, E, K) are also malabsorbed → e.g., vitamin K deficiency → bleeding tendency.

Interpretation: Because the pancreas supplies the major proteases, lipase, and amylase, its failure impairs all three macronutrient classes, but fat malabsorption is the most clinically obvious.

</details> <details> <summary>Example 3: Reason about Na⁺-coupled glucose uptake</summary>

Question: A drug blocks the basolateral Na⁺/K⁺ ATPase in enterocytes. Predict the effect on intestinal glucose absorption via SGLT1.

Solution:

SGLT1 is secondary active transport — it uses the inward Na⁺ gradient to drag glucose into the cell against its gradient.
The Na⁺/K⁺ ATPase pumps Na⁺ out the basolateral side to MAINTAIN that gradient.
Block the pump → intracellular Na⁺ rises → the Na⁺ gradient collapses → SGLT1 can no longer import glucose. ✓

High-yield connection: This is the textbook example of how a primary active pump powers a secondary active transporter — and why oral rehydration solutions combine glucose with sodium to maximize co-transport.

</details>

Key Takeaways — Part 3

Know all digestive enzymes with their sources and substrates
Stomach: HCl (parietal cells), pepsinogen (chief cells), gastrin (G cells)
Proteases secreted as zymogens; trypsin is the master activator (pancreatitis = self-digestion)
Hormones: gastrin ↑acid; secretin ↑bicarbonate; CCK → bile + enzymes + slows emptying
Bile emulsifies fat (liver-made, gallbladder-stored); B $_{12}$ + bile salts absorbed in ileum

Hormone Classes & Signaling (Mechanism)

Class	Examples	Solubility	Receptor location	Speed/Duration
Peptide	Insulin, glucagon, ADH, GH	Hydrophilic	Cell-surface (→ cAMP/IP₃ second messengers)	Fast, short
Steroid	Cortisol, aldosterone, sex hormones	Lipophilic	Intracellular/nuclear (→ alter gene transcription)	Slow, long
Amino-acid-derived	T3/T4 (lipophilic), epinephrine (hydrophilic)	Mixed	T3/T4 nuclear; catecholamines surface	Varies

Key principle: lipophilic hormones travel bound to carrier proteins, cross membranes, and change transcription (slow but lasting). Hydrophilic hormones can't cross membranes, so they use surface receptors and second messengers (fast but transient).

The HPA Axis (Three-Level Cascade + Feedback Loop)

$\text{Hypothalamus (CRH)} \to \text{Ant. pituitary (ACTH)} \to \text{Adrenal cortex (cortisol)}$

Cortisol exerts negative feedback on BOTH the hypothalamus (↓CRH) and pituitary (↓ACTH). The thyroid axis (TRH → TSH → T3/T4) works identically. This feedback logic lets you localize disease:

Pattern	Interpretation
↓ target hormone + ↑ tropic hormone	Primary (gland) failure — feedback intact, pituitary screaming
↓ target hormone + ↓ tropic hormone	Secondary (pituitary) failure
↑ target hormone + ↑ tropic hormone	Autonomous tropic (pituitary) tumor ignoring feedback
↑ target hormone + ↓ tropic hormone	Autonomous gland tumor (feedback intact, suppressing pituitary)

Blood Glucose Regulation (Antagonistic Feedback)

Insulin (β cells, fed state): ↑ glucose uptake (GLUT4 in muscle/fat), glycogenesis, lipogenesis → lowers blood glucose.
Glucagon (α cells, fasting): glycogenolysis + gluconeogenesis → raises blood glucose.
High glucose → insulin; low glucose → glucagon. This push–pull keeps glucose near ~90 mg/dL.

Calcium Regulation (Antagonistic Feedback)

PTH: ↑ bone resorption, ↑ renal Ca²⁺ reabsorption, ↑ activation of vitamin D (→ ↑ gut Ca²⁺ absorption) → raises Ca²⁺.
Calcitonin: inhibits osteoclasts → lowers Ca²⁺ (minor in humans).
Low serum Ca²⁺ → PTH release → restores Ca²⁺ (classic negative-feedback loop).

Feedback Loops Summary

Negative feedback (default): product inhibits its own production (T3/T4 ⊣ TSH; cortisol ⊣ ACTH). Positive feedback (rare): oxytocin in labor (contractions → more oxytocin → stronger contractions); the LH surge that triggers ovulation.

Worked Examples — Endocrine Physiology

<details> <summary>Example 1: Localize a thyroid disorder from lab values</summary>

Question: A patient is fatigued and cold-intolerant. Labs: low T3/T4, high TSH. Where is the lesion?

Solution:

Low T3/T4 → hypothyroid symptoms.
With negative feedback intact, low T3/T4 should DISinhibit TSH → TSH rises. The pituitary is responding correctly. ✓
High TSH + low T3/T4 ⇒ the thyroid gland itself cannot respond → primary hypothyroidism (e.g., Hashimoto's or iodine deficiency).

MCAT note: "Tropic hormone HIGH, target hormone LOW" almost always means the END gland failed (primary). Reverse both ⇒ pituitary failure (secondary).

</details> <details> <summary>Example 2: Predict the hormonal response to fasting</summary>

Question: A subject fasts 16 hours. Predict the changes in insulin and glucagon and the metabolic consequence.

Solution:

Falling blood glucose → β cells secrete less insulin, α cells secrete more glucagon. ✓
High glucagon/insulin ratio → hepatic glycogenolysis then gluconeogenesis, plus adipose lipolysis → ketone production.
Result: blood glucose is defended near normal while the body shifts to fat/ketone fuel.

Interpretation: Insulin and glucagon are antagonists; the MCAT cares about the RATIO, not absolute levels. A high insulin/glucagon ratio = storage; low = mobilization.

</details> <details> <summary>Example 3: Reason through a calcium feedback loop</summary>

Question: A patient's parathyroid glands are accidentally removed during thyroid surgery. Predict the change in serum Ca²⁺ and the symptom.

Solution:

No PTH → loss of bone resorption, less renal Ca²⁺ reabsorption, less vitamin-D activation → serum Ca²⁺ falls (hypocalcemia). ✓
Low Ca²⁺ increases neuromuscular excitability → tetany (muscle cramps/spasms, e.g., Chvostek/Trousseau signs).

High-yield connection: PTH is the dominant minute-to-minute Ca²⁺ regulator. Without it, calcitonin cannot compensate (calcitonin only lowers Ca²⁺), so hypocalcemia results. Hyperparathyroidism does the opposite: bone pain, kidney stones, "stones, bones, groans."

</details>

Key Takeaways — Part 5

Hypothalamus → Anterior pituitary → Target gland (3-level axis with negative feedback)
Use tropic vs. target hormone pattern to localize disease (primary vs. secondary)
Peptide hormones: surface receptors + second messengers (fast). Steroids: nuclear receptors + transcription (slow).
Insulin lowers glucose (fed); glucagon raises it (fasting) — judge by the RATIO.
PTH raises Ca $^{2+}$ ; Calcitonin lowers Ca $^{2+}$ (opposite effects)

All-or-none: once threshold is reached the spike is fixed in amplitude; stimulus strength is encoded by frequency, not size. The Nernst equation sets each ion's equilibrium potential:

$E_{ion} = \frac{61}{z} \log\frac{[\text{ion}]_{out}}{[\text{ion}]_{in}} \;\text{mV}$

$E_{K} \approx -90$ mV and $E_{Na} \approx +60$ mV; resting $V_m$ sits near $E_K$ because the membrane is most permeable to K⁺ at rest.

Refractory Periods (Why APs Go One Way)

Absolute refractory period: Na⁺ channels inactivated → no new AP regardless of stimulus. Ensures unidirectional propagation and caps maximum firing rate.
Relative refractory period: some Na⁺ channels recovered, but K⁺ efflux makes threshold harder → only a strong stimulus fires.

Saltatory Conduction

Myelin (Schwann cells in PNS, oligodendrocytes in CNS) insulates axons; APs regenerate only at nodes of Ranvier, "jumping" node to node. This speeds conduction ~10–100×. Multiple sclerosis demyelinates CNS axons → conduction slows or fails.

Synaptic Transmission (Flow)

$\text{AP reaches terminal} \to \text{Ca}^{2+}\text{ influx} \to \text{Vesicle fusion} \to \text{NT release} \to \text{Binds receptor}$

EPSP (e.g., glutamate → Na⁺ in) depolarizes; IPSP (e.g., GABA → Cl⁻ in / K⁺ out) hyperpolarizes.
Summation: temporal (rapid repeats from one synapse) + spatial (many synapses at once) determine whether threshold is reached at the axon hillock.

Key Neurotransmitters

ACh: NMJ (excitatory), parasympathetic; degraded by acetylcholinesterase
Norepinephrine: Sympathetic postganglionic
Dopamine: Reward, movement (low in Parkinson's)
Serotonin: Mood, sleep (targeted by SSRIs)
GABA: Main inhibitory NT in brain (Cl⁻ channels)
Glutamate: Main excitatory NT in brain

Nervous System 🎯

Worked Examples — Neurophysiology

<details> <summary>Example 1: Compute an equilibrium potential with the Nernst equation</summary>

Question: Extracellular [K⁺] = 5 mM, intracellular [K⁺] = 140 mM, at body temperature. Estimate $E_K$ .

Solution: $E_K = \frac{61}{+1}\log\frac{[K^+]_{out}}{[K^+]_{in}} = 61\log\frac{5}{140} = 61\log(0.0357)$ $= 61 \times (-1.45) \approx \textbf{-88 mV} \;✓$

MCAT note: Resting $V_m$ (~ $-70$ mV) sits close to $E_K$ because the membrane is most K⁺-permeable at rest. Hyperkalemia (raised external K⁺) makes less negative → resting cells partially depolarize → dangerous cardiac arrhythmias.

</details> <details> <summary>Example 2: Predict the effect of an acetylcholinesterase inhibitor</summary>

Question: An organophosphate pesticide inhibits acetylcholinesterase at the neuromuscular junction. Predict the effect on muscle.

Solution:

Acetylcholinesterase normally clears ACh from the synapse.
Inhibit it → ACh accumulates → receptors are continuously activated → sustained depolarization.
The motor end plate cannot repolarize/reset → depolarizing block → muscle fasciculations then paralysis. ✓

Interpretation: Too much "go" signal is as paralyzing as too little — the channels stay inactivated. This is why nerve-agent poisoning causes the "SLUDGE" cholinergic crisis plus respiratory muscle failure.

</details> <details> <summary>Example 3: Reason about synaptic summation</summary>

Question: A neuron has threshold at $-55$ mV and rests at $-70$ mV. A single EPSP depolarizes it by +8 mV; a single IPSP hyperpolarizes by $-5$ mV. If two EPSPs and one IPSP arrive nearly simultaneously, does the neuron fire?

Solution:

Net change = $2(+8) + 1(-5) = +16 - 5 = +11$ mV.
New $V_m = -70 + 11 = -59$ mV. ✓

High-yield connection: This is spatial summation at the axon hillock — the neuron integrates excitatory and inhibitory inputs. One more EPSP (+8) would push it to $-51$ mV and trigger a spike. Stimulus strength is then coded by firing FREQUENCY, not spike size.

</details>

Key Takeaways — Part 6

Sympathetic: fight-or-flight. Parasympathetic: rest-and-digest.
Action potential: resting → depolarization (Na $^+$ in) → repolarization (K $^+$ out); all-or-none, frequency-coded
Resting $V_m$ sits near $E_K$ (~ $-90$ mV by Nernst); refractory periods enforce one-way propagation
Myelin enables saltatory conduction (faster signaling); MS demyelinates CNS axons
Synapse: Ca²⁺ influx → vesicle fusion → NT release; EPSP/IPSP summation decides firing

MHC I: On ALL nucleated cells. Presents intracellular (endogenous) antigens → recognized by CD8+
MHC II: On APCs only (macrophages, dendritic cells, B cells). Presents extracellular (exogenous) antigens → recognized by CD4+

Antigen Presentation & Activation (Flow)

$\text{Pathogen} \to \text{Dendritic cell engulfs} \to \text{Presents on MHC II} \to \text{CD4+ Th activated} \to \text{Cytokines} \to \text{B cells \& CD8+ amplified}$

T-cell activation requires two signals: (1) TCR binds the peptide–MHC complex, and (2) a costimulatory signal (e.g., B7–CD28). Signal 1 without signal 2 → anergy (tolerance) — a built-in brake against autoimmunity.

Primary vs. Secondary Response (Figure)

Feature	Primary response (first exposure)	Secondary response (re-exposure)
Lag time	Long (~5–10 days)	Short (1–3 days)
Dominant antibody	IgM first, then IgG	IgG (class-switched, high affinity)
Magnitude	Lower antibody titer	Much higher, faster titer
Basis	Naïve B cells activating	Memory B cells

This memory curve is the entire logic of vaccination: a harmless primary exposure (antigen) generates memory cells so the real pathogen meets a fast, strong secondary response.

Clonal Selection & Antibody Maturation

A vast pre-existing repertoire of B/T cells (generated by V(D)J recombination) means an antigen "selects" the few lymphocytes whose receptors already fit; those clones proliferate. In germinal centers, B cells undergo somatic hypermutation + class switching (IgM → IgG/IgA/IgE), raising affinity and tailoring effector function.

Complement, Opsonization & NK Cells (Innate ↔ Adaptive Bridge)

Opsonization: antibody (IgG) or complement (C3b) coats a pathogen → tags it for phagocytosis ("opsonin = butter for the phagocyte").
Complement (membrane attack complex, MAC) punches pores in pathogen membranes → lysis.
NK cells kill cells with absent/low MHC I ("missing self") — many viruses and tumors downregulate MHC I to evade CD8+ T cells, but that very loss flags them for NK killing.

Immune System 🎯

Worked Examples — Immunology Reasoning

<details> <summary>Example 1: Interpret an antibody-titer graph</summary>

Question: A graph shows antibody titer vs. time. After the first antigen exposure, IgM rises slowly over ~7 days then falls. After a second exposure weeks later, a much taller, faster curve appears, dominated by IgG. Explain the two curves.

Solution:

First curve = primary response: naïve B cells need time to activate; IgM appears first (slow, low). ✓
Second curve = secondary response: memory B cells respond in 1–3 days with high-affinity, class-switched IgG (fast, tall).

MCAT note: "IgM = recent/first, IgG = past/memory" lets you date an infection from serology: high IgM ⇒ acute; high IgG with low IgM ⇒ prior exposure or vaccination.

</details> <details> <summary>Example 2: Predict the effect of losing CD4+ T cells (HIV)</summary>

Question: HIV destroys CD4+ helper T cells. Predict the impact on both humoral and cell-mediated immunity.

Solution:

CD4+ Th cells provide cytokine "help" that activates B cells AND boosts CD8+ cytotoxic T cells.
Lose CD4+ → B-cell antibody responses weaken AND CD8+ activation falters → both arms collapse. ✓
Result: opportunistic infections and certain cancers (e.g., by reactivated viruses) — defining AIDS.

Interpretation: The helper T cell is the central coordinator; removing it cripples the whole adaptive system, illustrating why CD4 count tracks immune competence.

</details> <details> <summary>Example 3: Reason about a transfusion / opsonization scenario</summary>

Question: A bacterium is coated with IgG and C3b. A macrophage encounters it. What process is occurring, and why is the bacterium cleared faster than an uncoated one?

Solution:

IgG and C3b are opsonins — they coat ("butter") the pathogen surface.
Macrophages bear Fc receptors (for IgG) and complement receptors (for C3b) → they bind the coated bacterium far more avidly.
This opsonization dramatically increases phagocytosis vs. an uncoated cell. ✓

High-yield connection: Opsonization links adaptive (antibody) and innate (complement, phagocyte) immunity. Asplenic patients clear encapsulated bacteria poorly because the spleen is a major site of opsonin-dependent clearance.

</details>

Organ Systems — Complete! ✅

From cardiovascular to immune, organ systems make up the bulk of MCAT biology.

Innate = fast, broad, no memory; adaptive = slow, specific, memory (the basis of vaccination)
MHC I (all nucleated cells) → CD8+; MHC II (APCs) → CD4+; T cells need two signals (or anergy)
Primary response = IgM, slow; secondary = IgG, fast, from memory B cells
NK cells kill "missing self" (low MHC I); opsonization (IgG/C3b) bridges adaptive and innate

Know the key structures, functions, and regulatory mechanisms for each system. Integration between systems (e.g., kidney + endocrine, nervous + cardiovascular) is frequently tested.

Alveolar gas equation:

P_{AO_2} = P_{IO_2} - P_{CO_2}/R = 150 - 40/0.8 = 150 - 50 =

100 mmHg ✓

Higher pH + lower CO₂ → LEFT shift of the O₂–Hb curve (reverse Bohr) → Hb holds O₂ more tightly → reduced tissue unloading, contributing to lightheadedness.

P_{n e t} = 40 - 18 = + 22

-59

mV has NOT reached the

-55

mV threshold → no action potential.

Organ Systems

Organ Systems - Complete Interactive Lesson

Part 1: Cardiovascular System

Organ Systems for the MCAT

Heart Structure

Cardiac Cycle

Key Equations

Blood Vessel Types

Cardiac Conduction & the ECG

Pressure–Flow Relationships (Poiseuille & Resistance)

Frank–Starling Mechanism & Pressure–Volume Loop

Capillary Exchange (Starling Forces)

Worked Examples — Cardiovascular Physiology

Key Takeaways — Part 1

Part 2: Respiratory System

Organ Systems for the MCAT

Gas Exchange

Part 3: Renal & Excretory System

Organ Systems for the MCAT

Nephron Structure

Key Functions by Segment

Part 4: Digestive System

Organ Systems for the MCAT

GI Tract Order

Digestive Enzymes

Stomach

Part 5: Endocrine System

Organ Systems for the MCAT

Major Endocrine Glands & Hormones

Part 6: Nervous System

Organ Systems for the MCAT

Organization

Neuron Structure & the Action Potential (Figure)

Part 7: Review & MCAT Practice

Organ Systems for the MCAT

Innate vs. Adaptive Immunity

Adaptive Immunity

Fick's Law of Diffusion

Oxygen Transport

The Oxygen–Hemoglobin Dissociation Curve (Figure)

The Bohr Effect (MCAT FAVORITE)

CO2_22​ Transport & the Chloride Shift

Control of Ventilation (Feedback Loop)

Worked Examples — Respiratory Physiology

Key Takeaways — Part 2

Hormonal Regulation

GFR

Net Filtration Pressure (Starling Forces at the Glomerulus)

Renal Clearance & the Filtration Equation

Countercurrent Multiplier (How Concentrated Urine Is Made)

Worked Examples — Renal Physiology

Key Takeaways — Part 4

Zymogen Activation Cascade (Why the Pancreas Doesn't Digest Itself)

Hormonal Control of Digestion (Figure / Feedback Loops)

Absorption

Carbohydrate Absorption Mechanism

Worked Examples — Digestive Physiology

Key Takeaways — Part 3

Hormone Classes & Signaling (Mechanism)

The HPA Axis (Three-Level Cascade + Feedback Loop)

Blood Glucose Regulation (Antagonistic Feedback)

Calcium Regulation (Antagonistic Feedback)

Feedback Loops Summary

Worked Examples — Endocrine Physiology

Key Takeaways — Part 5

Refractory Periods (Why APs Go One Way)

Saltatory Conduction

Synaptic Transmission (Flow)

Key Neurotransmitters

Worked Examples — Neurophysiology

Key Takeaways — Part 6

MHC Molecules

Antigen Presentation & Activation (Flow)

Primary vs. Secondary Response (Figure)

Clonal Selection & Antibody Maturation

Complement, Opsonization & NK Cells (Innate ↔ Adaptive Bridge)

Worked Examples — Immunology Reasoning

Organ Systems — Complete! ✅

CO $_2$ Transport & the Chloride Shift