Organ Systems

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

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: fastest intrinsic rate (~100 bpm); ~60–100 bpm at rest under vagal tone; 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

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:

1. Stroke volume = EDV − ESV = 120 − 50 = 70 mL
2. Cardiac output = HR × SV = 75 × 70 = 5250 mL/min ≈ 5.25 L/min ✓
3. 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.

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:

1. Higher afterload → the ventricle ejects against greater resistance → stroke volume falls acutely (end-systolic volume rises because less blood is ejected).
2. Frank–Starling: the leftover blood adds to the next preload, partially restoring SV via greater stretch.
3. 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.

Question: At the arteriolar end of a capillary: $P_c = 35$, $P_i = 0$, $\pi_c = 25$, $\pi_i = 0$ mmHg. At the venular end $P_c = 15$ (others unchanged). Determine the direction of net fluid movement at each end.

Solution:

• 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.

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)

Organ Systems for the MCAT

Part 2 of 7 — Respiratory System

Gas Exchange

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

• 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 $m^{2}$); 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 $O_{2}$ in.

Oxygen Transport

• 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:

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 $O_{2}$ release — efficient unloading exactly where metabolism is high.

Right-Shift Factors, incl. 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, ↓$CO_{2}$, ↓temp, ↓2,3-BPG; also fetal Hb and CO) raises affinity → $O_{2}$ 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, $CO_{2}$ enters RBCs, and carbonic anhydrase catalyzes:

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

$HCO_{3}^{-}$ 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 $CO_{2}$.

Control of Ventilation (Feedback Loop)

The medullary respiratory center sets breathing rate. The dominant stimulus is $CO_{2}/pH$, not $O_{2}$:

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

• Central chemoreceptors (medulla) sense CSF pH (a proxy for arterial $CO_{2}$) — the primary driver.
• Peripheral chemoreceptors (carotid/aortic bodies) sense low $P_{O_2}$ (<60 mmHg), high $CO_{2}$, low pH — the backup hypoxic drive.
• Hyperventilation blows off $CO_{2}$ → respiratory alkalosis; hypoventilation retains $CO_{2}$ → respiratory acidosis.

Respiratory 🎯

Worked Examples — Respiratory Physiology

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:

1. Inspired $P_{O_2} = (760 - 47) \times 0.21 = 713 \times 0.21 \approx 150$ mmHg.
2. Alveolar gas equation: $P_{AO_2} = P_{IO_2} - P_{CO_2}/R = 150 - 40/0.8 = 150 - 50 =$ 100 mmHg ✓

MCAT note: This ~100 mmHg matches the lung value in the dissociation table. At altitude, $P_{atm}$ falls → $P_{IO_2}$ falls → $P_{AO_2}$ drops, triggering the hypoxic ventilatory response.

Question: Hb = 15 g/dL, each gram of Hb carries 1.34 mL $O_{2}$ at full saturation, arterial saturation = 98%, cardiac output = 5 L/min. Approximate $O_{2}$ delivery to tissues (ignore dissolved $O_{2}$).

Solution:

1. $O_{2}$ content $\approx 15 \times 1.34 \times 0.98 \approx 19.7$ mL $O_{2}$ / dL = 197 mL/L.
2. Delivery = content × cardiac output = $197 \times 5 \approx$ 985 mL $O_{2}/min$ ✓

Interpretation: Anemia (lower Hb) cuts delivery proportionally even with normal saturation — this is why $SpO_2$ alone can mask poor $O_{2}$ delivery. The body compensates by raising cardiac output.

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:

1. $\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$. Lowering $CO_{2}$ pulls the reaction LEFT → fewer $H^{+}$ → pH rises (respiratory alkalosis). ✓
2. Higher pH + lower $CO_{2}$ → LEFT shift of the $O_{2}$–Hb curve (reverse Bohr) → Hb holds $O_{2}$ more tightly → reduced tissue unloading, contributing to lightheadedness.

High-yield connection: Breathing into a bag re-raises $CO_{2}$, restoring pH and curve position. $CO_{2}$, not $O_{2}$, is the master regulator of ventilation.

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)
• Right-shift factors (incl. the 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 $CO_{2}/pH$ via central chemoreceptors, not by $O_{2}$

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

Hormonal Regulation

GFR

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

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. Dilating the afferent arteriole (or constricting the efferent one) raises glomerular capillary pressure and GFR; constricting the afferent arteriole lowers both.

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

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 $\pi_{GC} = 18$: $P_{net} = 40 - 18 = +22$ mmHg → GFR rises. Conversely, a ureteral stone raises $P_{BS}$ and lowers GFR.

Question: A substance has plasma concentration $P_x = 2$ mg/mL, urine concentration $U_x = 60$ mg/mL, and urine flow rate $V = 1$ 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.

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:

1. The thick ascending limb normally pumps NaCl into the interstitium to build the medullary gradient.
2. Blocking the cotransporter → less NaCl deposited → the medullary gradient COLLAPSES.
3. Without a steep gradient, the collecting duct cannot extract water even when ADH is present.
4. Result: large volume of dilute urine (powerful diuresis), plus $K^{+}$ wasting (more $Na^{+}$ and fluid reach the collecting duct, where aldosterone-driven $Na^{+}$ reabsorption is coupled to $K^{+}$ secretion). ✓

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

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!)
• Clearance: inulin clearance = GFR; PAH (filtered and maximally secreted) clearance ≈ renal plasma flow

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

Stomach

• Parietal cells: HCl (acidic pH ~2) + intrinsic factor (for B$_{12}$ absorption)
• 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)

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.

Absorption

• 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. This $Na^{+}$-coupled uptake is why oral rehydration therapy pairs glucose with sodium.

• Fructose's apical facilitated-diffusion transporter is GLUT5.

Digestive System 🎯

Worked Examples — Digestive Physiology

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

Solution:

1. Fats and amino acids reach the duodenum → I cells release CCK.
2. CCK → gallbladder contracts (bile ejected) + pancreas releases lipase; CCK also slows gastric emptying. ✓
3. 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."

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

Solution:

1. Loss of pancreatic lipase → triglycerides not digested → fat malabsorption.
2. Undigested fat in stool = steatorrhea (greasy, floating, foul stool). ✓
3. 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.

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

Solution:

1. SGLT1 is secondary active transport — it uses the inward $Na^{+}$ gradient to drag glucose into the cell against its gradient.
2. The $Na^{+}/K^{+}$ ATPase pumps $Na^{+}$ out the basolateral side to MAINTAIN that gradient.
3. 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.

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

• Low-yield extras: fructose enters enterocytes through the GLUT5 transporter

Organ Systems for the MCAT

Part 5 of 7 — Endocrine System

Major Endocrine Glands & Hormones

Hormone Classes & Signaling (Mechanism)

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:

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 $Ca^{2+}$ reabsorption, ↑ activation of vitamin D (→ ↑ gut $Ca^{2+}$ absorption) → raises $Ca^{2+}$.
• Calcitonin: inhibits osteoclasts → lowers $Ca^{2+}$ (minor in humans).
• Low serum $Ca^{2+}$ → PTH release → restores $Ca^{2+}$ (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.

Endocrine 🎯

Worked Examples — Endocrine Physiology

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

Solution:

1. Low T3/T4 → hypothyroid symptoms.
2. With negative feedback intact, low T3/T4 should DISinhibit TSH → TSH rises. The pituitary is responding correctly. ✓
3. 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).

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

Solution:

1. Falling blood glucose → β cells secrete less insulin, α cells secrete more glucagon. ✓
2. High glucagon/insulin ratio → hepatic glycogenolysis then gluconeogenesis, plus adipose lipolysis → ketone production.
3. 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.

Question: A patient's parathyroid glands are accidentally removed during thyroid surgery. Predict the change in serum $Ca^{2+}$ and the symptom.

Solution:

1. No PTH → loss of bone resorption, less renal $Ca^{2+}$ reabsorption, less vitamin-D activation → serum $Ca^{2+}$ falls (hypocalcemia). ✓
2. Low $Ca^{2+}$ increases neuromuscular excitability → tetany (muscle cramps/spasms).

High-yield connection: PTH is the dominant minute-to-minute $Ca^{2+}$ regulator. Without it, calcitonin cannot compensate (calcitonin only lowers $Ca^{2+}$), so hypocalcemia results. Hyperparathyroidism does the opposite: bone pain, kidney stones, "stones, bones, groans."

• Bedside signs of hypocalcemic tetany: Chvostek's sign (facial twitch when the facial nerve is tapped) and Trousseau's sign (carpal spasm under an inflated blood-pressure cuff).

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)

• Low-yield extras: Chvostek and Trousseau signs as the bedside tests for hypocalcemic tetany

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:

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

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 $E_K$ less negative → resting cells partially depolarize → dangerous cardiac arrhythmias.

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

Solution:

1. Acetylcholinesterase normally clears ACh from the synapse.
2. Inhibit it → ACh accumulates → receptors are continuously activated → sustained depolarization.
3. 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 a cholinergic crisis (excess parasympathetic activity) plus respiratory muscle failure.

• The cholinergic crisis is remembered as SLUDGE: salivation, lacrimation, urination, defecation, GI upset, emesis.

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:

1. Net change = $2(+8) + 1(-5) = +16 - 5 = +11$ mV.
2. New $V_m = -70 + 11 = -59$ mV. ✓
3. $-59$ mV has NOT reached the $-55$ mV threshold → no action potential.

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.

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 $V_m$ ($-70$ mV) 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: $Ca^{2+}$ influx → vesicle fusion → NT release; EPSP/IPSP summation decides firing

• Low-yield extras: the SLUDGE mnemonic for the signs of a cholinergic crisis

Organ Systems for the MCAT

Part 7 of 7 — Immune System

Innate vs. Adaptive Immunity

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)

MHC Molecules

• 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 from the antigen-presenting cell. Signal 1 without signal 2 → anergy (tolerance) — a built-in brake against autoimmunity.

• The classic costimulatory pair is B7 (on the APC) binding CD28 (on the T cell).

Primary vs. Secondary Response (Figure)

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

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:

1. First curve = primary response: naïve B cells need time to activate; IgM appears first (slow, low). ✓
2. 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.

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

Solution:

1. CD4+ Th cells provide cytokine "help" that activates B cells AND boosts CD8+ cytotoxic T cells.
2. Lose CD4+ → B-cell antibody responses weaken AND CD8+ activation falters → both arms collapse. ✓
3. 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.

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:

1. IgG and C3b are opsonins — they coat ("butter") the pathogen surface.
2. Macrophages bear Fc receptors (for IgG) and complement receptors (for C3b) → they bind the coated bacterium far more avidly.
3. 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.

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.

• Low-yield extras: the costimulatory pair is B7 (APC) binding CD28 (T cell)