🎯⭐ INTERACTIVE LESSON

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

Cardiovascular 🎯

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 & Action Potential

Resting potential: ~ $-70$ mV (Na $^+$ /K $^{}$ ATPase maintains gradient)

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)

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

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

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 Bohr Effect (MCAT FAVORITE)

Conditions that RIGHT-shift the O $_2$ -Hb dissociation curve (promote O $_2$ unloading):

Increased CO $_2$ (metabolically active tissue)
Decreased pH (acidic — more CO $_2$ )
Increased temperature
Increased 2,3-BPG

Mnemonic: Right shift = Release O $_2$ to tissues

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

Respiratory 🎯

Key Takeaways — Part 2

Gas exchange driven by partial pressure gradients
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)
CO $_2$ transport: mostly as bicarbonate (70%)

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!

Renal System 🎯

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

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

Digestive System 🎯

Key Takeaways — Part 3

Know all digestive enzymes with their sources and substrates
Stomach: HCl (parietal cells), pepsinogen (chief cells), gastrin (G cells)
Bile emulsifies fat (produced by liver, stored in gallbladder)
Most absorption occurs in the jejunum; B $_{12}$ absorbed in ileum

Negative feedback (most hormones): Product inhibits further production

Example: T3/T4 inhibits TSH release from pituitary

Positive feedback (rare):

Oxytocin during labor (contractions → more oxytocin → stronger contractions)
LH surge during ovulation

Key Takeaways — Part 5

Hypothalamus → Anterior pituitary → Target gland (3-level axis)
Negative feedback is the default; positive feedback is rare
Insulin: lowers glucose (fed state). Glucagon: raises glucose (fasting).
PTH raises Ca $^{2+}$ ; Calcitonin lowers Ca $^{2+}$ (opposite effects)

Depolarization: Na

^+

channels open → Na

^+

rushes in → membrane potential rises

Repolarization: Na

^+

channels close, K

^+

channels open → K

^+

flows out

Hyperpolarization: Slight overshoot below

-70

Refractory period: Na

^+

channels inactivated → cannot fire again immediately

Synaptic Transmission

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

Key Neurotransmitters

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

Nervous System 🎯

Key Takeaways — Part 6

Sympathetic: fight-or-flight. Parasympathetic: rest-and-digest.
Action potential: resting → depolarization (Na $^+$ in) → repolarization (K $^+$ out)
Myelin enables saltatory conduction (faster signaling)
Know major neurotransmitters and their functions

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

Immune System 🎯

Organ Systems — Complete! ✅

From cardiovascular to immune, organ systems make up the bulk of MCAT biology. Know the key structures, functions, and regulatory mechanisms for each system. Integration between systems (e.g., kidney + endocrine, nervous + cardiovascular) is frequently tested.

Organ Systems

Oxygen Transport

The Bohr Effect (MCAT FAVORITE)

CO $_2$ Transport

Key Takeaways — Part 2

Hormonal Regulation

GFR

Key Takeaways — Part 4

Absorption

Key Takeaways — Part 3

Feedback Loops

Key Takeaways — Part 5

Synaptic Transmission

Key Neurotransmitters

Key Takeaways — Part 6

MHC Molecules

Organ Systems — Complete! ✅

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

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 & Action Potential

Part 7: Review & MCAT Practice

Organ Systems for the MCAT

Innate vs. Adaptive Immunity

Adaptive Immunity

Oxygen Transport

The Bohr Effect (MCAT FAVORITE)

CO2_22​ Transport

Key Takeaways — Part 2

Hormonal Regulation

GFR

Key Takeaways — Part 4

Absorption

Key Takeaways — Part 3

Feedback Loops

Key Takeaways — Part 5

Synaptic Transmission

Key Neurotransmitters

Key Takeaways — Part 6

MHC Molecules

Organ Systems — Complete! ✅

CO $_2$ Transport